Joel Grube PhD, MS, AB

• Adjunct Professor, Health and Social Behavior

https://publichealth.berkeley.edu/people/joel-grube/

In fact pregnancy myths estrace 2 mg order overnight delivery, laparoscopy is now the gold standard approach for cholecystectomy and bariatric surgery menstruation quotes estrace 1 mg buy online. The growth of the specialty has been fueled women's health big book of 15 minute workouts review discount estrace amex, in large part menopause underarm odor discount 2 mg estrace fast delivery, by the benefits of "minimally invasive" surgery (Table 44-1) menstruation ovulation period buy estrace 1 mg lowest price. Improved surgical cosmesis, reduced postoperative pain, faster return to work, and lower surgical-related complications continue to make laparoscopy, in many cases, preferable to open surgery. Today, a large number of surgeries that once required prolonged hospital stays are now performed in outpatient surgery centers and short-stay facilities. Technological advances have now introduced robotics to laparoscopic surgery to address many of its technical issues that affect all laparoscopic surgeons. Disadvantages exist in laparoscopy for patients and medical providers alike (Table 44-2). A significant source of intraoperative and postoperative issues during laparoscopy stems from the creation of pneumoperitoneum. Physiologic derangements, particularly affecting the cardiopulmonary system, are common during pneumoperitoneum, and are further aggravated by steep positioning changes common in laparoscopy. In robotic surgery, long operative time and limited access to the patient, due to prominent robotic equipment, can further complicate management of urgent conditions. As the application of laparoscopy and robotic-assisted surgery continues to expand to more complex patients and diseases, the anesthetist must be increasingly attentive to avoid or minimize serious patient harm. In this chapter, a general overview is provided regarding the anesthetic management of laparoscopic and robotic-assisted surgery for the adult patient undergoing abdominal and pelvic exploration. For additional discussion on 3144 their application in other areas of surgery, we refer the reader to other relevant chapters within this textbook. Laparoscopic Surgery Surgical Approach and Positioning Laparoscopic surgery is a minimally invasive surgical technique where specialized tubes are inserted for surgical access. Small skin incisions are made, approximately 1 cm in length, to facilitate insertion of rigid tubes, called trocars. Trocars are sharp, multiport, one-way conduits used to insufflate gas and to guide various specialized surgical instruments. Intraperitoneal viewing is conducted using a video-capable telescopic camera, called a laparoscope. Exposure of the intraperitoneal space can be achieved either by intraperitoneal pressurization, called pneumoperitoneum, or by external abdominal wall retraction. Intraperitoneal insufflation is generally established by creating a small subumbilical incision, through which a stainless steel, spring-loaded, blunt needle, called a Veress needle, is inserted. Several other incisions are then made through which trocars are sequentially inserted under direct laparoscope visualization and transillumination to avoid inadvertent intra-abdominal injury. The surgery is conducted using a laparoscope for video monitoring, and various long, handheld surgical instruments. This technique requires a specialized horizontal lifting apparatus inserted into the abdominal wall for suspension of the anterior abdominal wall away from the abdominal viscera. Despite the benefits of avoiding gas insufflation and its side effects, abdominal wall lift is generally believed to be inferior to pneumoperitoneum laparoscopy due to longer operative times and an unclear safety profile. Long, rigid laparoscopic instruments facilitate minimal access, but limit the ease of tissue manipulation. Bed tilting is usually needed to passively optimize surgical exposure with minimal surgical retraction. The lateral jackknife position is used to expose the retroperitoneal space during radical nephrectomy surgery. Leftward tilting exposes the appendix, whereas rightward tilting exposes the left colon. An in-depth discussion on patient positioning and potential injuries is discussed elsewhere (see Chapter 29). The earliest reports of outpatient laparoscopy date back to the 1970s, when it was first used in gynecological surgeries. Laparoscopic cholecystectomy for symptomatic cholelithiasis is now the most commonly performed outpatient laparoscopic surgery. Common laparoscopic weight loss procedures include gastric bypass, sleeve gastrectomy, and adjustable gastric band. Due to its low rates of complications, and readmissions, and predictably short operative time, gastric banding is the most commonly performed outpatient bariatric surgery. Nonetheless, postoperative complications, unanticipated admissions, and readmission rates for these procedures remain concerns for suitability and safety. Based on a comprehensive retrospective review of ambulatory laparoscopic gastric bypass surgery, unplanned admission and readmission rates were 16% and 1. More serious complications include unexpected gastric leaks after sleeve gastrectomy, gastrointestinal bleeding, and pulmonary embolism. Ultimately, well-supported recommendations for weight limits in obese patients being screened for ambulatory surgery are 3147 lacking, and may only be based on expert opinion. An in-depth review on the practice of ambulatory anesthesia (see Chapter 31) and anesthesia in obesity are presented elsewhere (see Chapter 45). Robotic Laparoscopic Surgery Robotic laparoscopic surgery is a highly sophisticated, technologic variation on conventional laparoscopic surgery, requiring modification of both surgical and anesthetic management. The advanced technology has transformed technically challenging procedures into feasible ones via a minimally invasive approach. Although it was first popularized in urology for radical prostatectomies, robotic-assisted surgery has since gained ground in other fields, in part, due to reports of improved surgical outcomes, lower complication rates, shortened lengths of hospital stay, and improved surgeon ergonomics. Table 44-3 Examples of Robotic-assisted Laparoscopic Surgery Robotic surgery is performed most commonly employing the Da Vinci Surgical System (Intuitive Surgical Inc. Similar to conventional 3148 laparoscopy, robotic laparoscopic surgery entails the creation of a pneumoperitoneum, insertion of a video laparoscope, and insertion of trocars for surgical access. The surgical robot is positioned near the patient with robotic arms inserted into the insufflated cavity. The robotic arms are controlled remotely by a surgeon seated at an ergonomically designed control console. Remote handling of instruments via a specialized control allows for movements that mimic natural maneuvers, improved degrees of freedom, and optimal surgical instrument rotation and pivoting. An assistant near the surgical field provides surgical support, such as robotic arm adjustments, tissue retraction, and suctioning. Anesthetic management during laparoscopic robotic surgery requires preparing for patient accessibility limitations and adjusting for patient positioning challenges. Prominent surgical robotic equipment near the patient can greatly limit anesthesia provider access to the patient in case of an emergency. Though newer surgical robotic systems are becoming more compact, with thinner robotic arms, and improved motorized maneuverability functions for steering and engaging, the robotic systems in use today have a large footprint near and above the operating room bed. In the rare event of an airway or cardiopulmonary emergency, the robotic surgical arms must first be carefully disengaged from the trocars, before the robot can be removed safely and the patient positioned in a manner consistent with that needed for airway management or cardiopulmonary resuscitation, respectively. Surgeon at the console (left), robot and patient (center), and video tower (right). Steep Trendelenburg positioning used in many robotic surgeries requires greater vigilance of the patient. Ocular injury risk in steep Trendelenburg during robotic surgery may be greater than in conventional laparoscopic 3149 surgery (see Complications Related to Surgery). Physiologic Impact of Laparoscopy Laparoscopic surgery induces complex physiologic changes that impact multiple organ systems. Direct mechanical stress placed on the patient, as well as neuroendocrine stimulation during laparoscopy are the primary forces responsible for much of the physiologic derangement observed. Cardiovascular System the cardiovascular system is exquisitely challenged during laparoscopy by multiple stressors on preload, inotropy, rhythm, and afterload (Table 44-4). Modifiable factors that affect hemodynamics during laparoscopy include the intravascular volume status of the patient, positioning, baseline comorbidities, and surgical technique. Carbon dioxide gas is highly soluble and, during insufflation, rapidly moves from the peritoneal cavity into the circulation. Further complicating the response of the myocardium to transient hypercarbia is the potential for acute elevations in right ventricular afterload from hypercarbia-induced pulmonary vasoconstriction. Stimulation of these autonomic pathways during pneumoperitoneum, typically results in sympathetic nervous system activation, catecholamine release, activation of the renin­angiotensin system, and release of the neurohypophysial hormone vasopressin. Mechanical stretch on the peritoneum and abdominal viscera can result in parasympathetic stimulation through the vagus nerve, but sympathetic tone usually predominates. Intravascular volume status is an important modifier of the mechanical effects of pneumoperitoneum. Steep Trendelenburg positioning during pneumoperitoneum may augment venous return and cardiac filling. Morbidly obese patients undergoing laparoscopic gastric bypass surgery show similar hemodynamic changes as nonbariatric patients. However, complex hemodynamic changes in elderly patients with cardiovascular disease may be significant during pneumoperitoneum, despite a lack of observable myocardial ischemia by electrocardiogram. Hypercarbia and acidosis can lead to increased pulmonary vasoconstriction and increased right ventricular afterload, in addition to impaired inotropy. Significant volume loading of a dilated right ventricle can in turn compress the left ventricle through the mechanism of ventricular interdependence leading to reduced global ventricular function. Respiratory System Laparoscopic abdominal surgery exerts changes on the pulmonary system by 3152 mechanically displacing thoracic structures, altering lung mechanics. An early effect of insufflation on the pulmonary system is the displacement of the diaphragm into the thorax, which can be further aggravated by Trendelenburg positioning. However, abnormally low levels of oxygen are rarely observed in patients with normal preoperative pulmonary function. Ventilator adjustments may be needed to minimize peak airway pressure, while maintaining acceptable minute ventilation. Despite speculation that a ventilation­perfusion mismatching contributes to hypercapnia during laparoscopy, minimal changes occur in alveolar dead space and pulmonary shunting during prolonged and steep Trendelenburg positioning with pneumoperitoneum used in robotic-assisted hysterectomy and prostatectomy. Furthermore, steep Trendelenburg positioning with pneumoperitoneum can produce close to a 50% reduction in lung compliance while simultaneously producing in an unexpected improvement in oxygenation by mechanisms that remain unclear. Physiologic changes induced during pneumoperitoneum and extreme positioning can reduce the number of ventilated alveolar units being perfused. Nonetheless, this ventilation­perfusion relationship during laparoscopy is still unclear, given the observation in the porcine animal model of hypoxic pulmonary vasoconstriction­mediated improvements in arterial oxygenation (PaO2) after pneumoperitoneum, possibly due to perfusion redistribution away from atelectatic areas. Compensating for hypercarbia, managing inspiratory resistance, and maintaining normoxia in morbidly obese patients are common intraoperative dilemmas. The degree of instituted compensatory hyperventilation is a balance between the detrimental effects of extreme hypercarbia and the potential benefits of mild hypercapnia, which can improve tissue oxygenation, vasodilatation, and rightward shift of the oxyhemoglobin dissociation curve. Splanchnic blood flow during laparoscopy may decrease both from external compression during pneumoperitoneum, and systemic vasoconstriction from released neuroendogenous hormones. Both intracranial pressure and cerebral perfusion increase during Trendelenburg positioning and pneumoperitoneum,51 likely due to diminished cerebral venous outflow and hypercarbia-induced cerebral hyperperfusion. Although the cerebral venous blood fraction is increased from a reduction in cerebral venous drainage, regional cerebral tissue oxygen saturation appears to increase in this setting, possibly due to an increase in cerebral oxygen delivery from an elevated cerebral perfusion pressure and cerebral hyperperfusion. Rare cases of postoperative blindness after prolonged steep Trendelenburg positioning in laparoscopic prostatectomy56 and colorectal surgery57 have been reported. Pre-existing diseases, such as atherosclerotic disease, diabetes, and glaucoma, may lower the threshold of physiologic tolerance to acute intraocular derangements during laparoscopic surgery. Some factors that make laparoscopic surgery best suited for general anesthesia versus other anesthetic techniques include extreme patient positioning, discomfort from pneumoperitoneum, prolonged operative times, and induced cardiopulmonary derangements. Regional anesthesia, if considered, may be suited for brief laparoscopy procedures with minimal positioning changes. Mechanical ventilation settings and pulmonary mechanics should be modified and measured via the anesthesia machine. Invasive or advanced noninvasive monitoring, such as arterial catheter, pulse contour analysis, pulmonary artery catheter, or echocardiography, may be considered if significant pre-existing cardiopulmonary disease is present. Anesthesia Maintenance Inhaled Anesthetics and Propofol the induction agent of choice in laparoscopy is propofol given its predictable pharmacokinetic profile, and its antiemetic properties. Anesthesia maintenance with inhaled volatile anesthetics remains the standard anesthetic approach in laparoscopic surgery. Desflurane and sevoflurane are inhaled anesthetics with short-acting and easily titratable properties, ideally suited for ambulatory surgery. N2O during anesthesia is believed to diffuse into air spaces, such as intestinal lumina, leading to adverse pressurization. However, detectable intestinal distension and disruption of laparoscopic surgical conditions does not appear to occur during an N2O-based anesthetic. During an N2O-based anesthetic, N2O has been shown to accumulate in the peritoneal cavity to combustion levels as early as 30 minutes64 to as late as 2 hours. This may be explained by the mechanical circulation of peritoneal gases during pneumoperitoneum. Pharmacologic Adjuncts Lower postoperative pain is a benefit of laparoscopic surgery over conventional open surgery. A number of pharmacologic adjuncts are available for use during a balanced anesthetic to minimize intraoperative sympathetic stimulation, while optimizing postoperative recovery. Remifentanil significantly suppresses sympathetic stimulation and the neuroendocrine stress response during pneumoperitoneum65 without the prolonged respiratory effects of longer-acting opioids. Continuous local anesthetic wound infiltration may have a role in laparoscopic-assisted surgical procedures with longer incisions. Neuromuscular Blockade Neuromuscular blocking agents are routinely used to improve surgical exposure during pneumoperitoneum. Surrounding these concerns remains the topic of how surgeons and anesthetists define "optimal surgical working conditions. Both modes of 3159 ventilation are suitable to handle the transient effects of laparoscopy on lung mechanics and to control minute ventilation during pneumoperitoneum. Hypercapnia during laparoscopic cholecystectomies in healthy patients may be normalized by increasing the minute ventilation about 25% above baseline. Ultimately, caution should be used when using compensatory ventilatory maneuvers because increased peak airway pressure may occur, especially during steep Trendelenburg positioning in morbidly obese patients.

Immediate administration of O2 women's health clinic epworth buy 2 mg estrace mastercard, which is required for all burn victims women's health clinic port adelaide purchase estrace mastercard, may be lifesaving for this complication breast cancer zazzle purchase estrace with mastercard. Fluid Replacement Immediately after a serious burn microvascular permeability increases women's health issues research paper estrace 2 mg buy online, causing the loss of a substantial amount of protein-rich fluid into the interstitial space birth control dangerous women's health purchase 2 mg estrace otc. A major burn, a delay in initiation of resuscitation, or an inhalation injury increases the size of the leak. In addition, cardiac contractility may decrease because of circulating mediators, a diminished response to catecholamines, decreased coronary blood flow, and increased systemic vascular resistance. Smaller burns can be managed with oral or intravenous replacement at 150% of the calculated maintenance rate and careful monitoring of fluid status. Intravascular volume should be restored 3794 with utmost care to prevent excessive edema formation in both damaged and intact tissues resulting from the generalized increase in capillary permeability caused by the injury. Edema from overaggressive resuscitation has many deleterious and potentially life-threatening effects. Mention has already been made of the facilitation of upper airway edema after rapid fluid infusion in large cutaneous burns with or without smoke inhalation. Abdominal edema may also occur, and when resuscitation volume exceeds 300 mL/kg/24 hours, increased intra-abdominal pressure may produce abdominal compartment syndrome with impedance of venous return. This, together with decreased tissue oxygen tension, may produce necrosis of damaged but viable cells, increasing the extent of injury and the risk of infection. Crystalloid solutions are preferred for resuscitation during the first day following a burn injury; leakage of colloids during this phase may increase edema. Some centers use plasma with crystalloid routinely and partly attribute the good outcomes of their patients to this practice. Avoidance of early overresuscitation, routine use of colloids, and adherence to protocols are recommended to prevent this problem. Albumin 5% may be administered after the first day following injury at a rate of 0. These formulas are guidelines only, and none can be expected to provide adequate restoration of intravascular volume in all burn victims, especially small children and patients with inhalation injuries. An increase in Hct during the first day suggests inadequate fluid resuscitation because hemolysis and sequestration are actually expected to cause a decrease in this parameter. Acute anemia, as may occur during excision and grafting of burns, is usually well tolerated. Blood replacement is usually not initiated until the Hct is decreased to 20% to 24% in healthy patients requiring limited operations, to approximately 25% in those who are healthy but need extensive procedures, and to 30% or more when there is a history of pre-existing cardiovascular disease. Dopamine in small doses (5 g/kg/min) and/or -adrenergic agents may improve urine output without further need for fluids. In contrast, aqueous topical agents such as 5% silver nitrate solution may cause hyponatremia and its consequences of cerebral edema and seizure secondary to electrolyte leaching. Central pontine demyelination may occur if the hyponatremia is corrected rapidly with salt solutions. Indeed there is some evidence to suggest that hourly monitoring of urine output as an end point of resuscitation compared to sophisticated hemodynamic monitoring provides similar outcomes in terms of mortality, organ function, length of hospital or intensive care stay, duration of mechanical ventilation, and burn-related complications such as pulmonary edema, compartment syndromes, or infection. They may not be readily available in some burn units, may lack proof of accuracy, and may be invasive, presenting a risk to patients who are already in a critical clinical state. Not using the Lund­Browder chart is considered one of the reasons for wrong estimation of burn size. Several measures are suggested to improve the accuracy of computing fluid requirement, such as a nomogram or an electronic calculator. For every 10 kg above 80 kg of body weight, 100 mL/hr is added to the calculated rate. The Rule of Ten appears accurate for patients weighing between 40 and 140 kg, overestimating the rate for patients below 40 kg and underestimating for those above 140 kg of body weight. If actual body weight is used, the calculated fluid need is lower than that determined for normal-weight patients. If ideal body weight is used, which is rare in clinical practice, the patient may receive a higher resuscitation volume. Morbidly obese patients are likely to have higher acidosis, organ dysfunction, and mortality. It has been shown that fluid intake in the second 24 hours correlates with that of the first 24-hour period; the higher the resuscitation volume during the first period, the higher the fluid intake during the second 24 hours. Generally the actual fluid volume administered exceeds the amount calculated by this formula by a factor of 1. An ultrasound-guided technique or a surgical cutdown may be necessary to facilitate access. The radial artery is the vessel of choice in abdominal or chest trauma in which the aorta may be cross-clamped, making a femoral or dorsalis pedis cannula nonfunctional. The right radial artery is preferred in cases of chest trauma in which cross-clamping of the descending aorta might result in occlusion of the left subclavian artery. In mechanically ventilated patients, the magnitude of systolic pressure variation (the difference between the maximum and minimum systolic pressures over the respiratory cycle) and its delta down component (the difference between systolic pressures at end-expiration and the lowest value during the respiratory cycle) can provide reliable information about the intravascular volume status and predict responsiveness to fluid loading. A systolic pressure variation over 5 mmHg and a delta down over 2 mmHg suggest hypovolemia and responsiveness to fluid. Measuring the right ventricular volume alone can provide information about the adequacy of the intravascular volume. This technique also allows visualization of fat and air entry into the right heart, or into the left heart through a patent foramen ovale during internal fixation of lower extremity fractures. Other qualitative findings to be looked for during evaluation of heart function with the parasternal short axis view at the level of the papillary muscles are inward motion of the endocardium, myocardial thickening, longitudinal motion of the mitral annulus, and geometry of the left ventricle. Urine Output Urine output is routinely monitored as an indicator of organ perfusion, hemolysis, skeletal muscle destruction, and urinary tract integrity after trauma. Its reliability in monitoring perfusion is decreased by prolonged shock prior to surgery and osmotic diuresis caused by administration of mannitol or radiopaque dye. Dark, cola-colored urine in the trauma patient suggests either hemoglobinuria resulting from incompatible blood transfusion or myoglobinuria caused by massive skeletal muscle destruction after blunt or electrical trauma. Although the definitive diagnosis is made by serum electrophoresis, rapid differential diagnosis can be made by centrifugation of a blood specimen. Pink-stained serum suggests hemoglobinuria, whereas unstained serum indicates myoglobinuria. Prevention involves inducing diuresis with fluids and mannitol and, in myoglobinuria, although controversial, additional alkalinization of the urine with sodium bicarbonate to pH greater than 5. Red-colored urine usually is caused by hematuria, which, in the traumatized 3802 patient, suggests urinary tract injury. Oxygenation Trauma patients frequently develop hypoxemia (O2 saturation <90%), hypothermia, hypotension, and/or decreased peripheral perfusion. Of the available O2 saturation (SpO2) devices, finger or earlobe pulse oximeters are more affected by decreased perfusion than forehead probes, probably because the latter senses the pulsation of the supraorbital artery, a branch of the carotid artery, which is presumably less affected by shock or hypothermia. However, SpO2 results with the forehead monitor may be affected by venous pulsation, especially in patients receiving positive-pressure ventilation or in any situation that distends the tributaries of the superior vena cava. They can also measure noninvasive continuous hemoglobin concentration with reasonable accuracy. Organ Perfusion and Oxygen Utilization As discussed previously, unrecognized hypoperfusion may lead to splanchnic ischemia with resulting acidosis in the intestinal wall, permitting the passage of luminal microorganisms into the circulation and release of inflammatory mediators, causing sepsis and multiorgan failure. Subsequent organ failure may occur if it decreases below a value of 170 mL/min/m2, indicating a flow-dependent phase of O2 utilization. However, it is possible that dysoxia may be present in an individual organ in the presence of a normal overall O2 extraction ratio. The maximum amplitude is the widest portion of the curve and indicates the absolute strength of the fibrin clot. The -angle is the slope of the external divergence of the tracing from the R-value point, indicating the speed of clot formation and fibrin crosslinking. The value of this parameter is determined by both coagulation factors and platelets. Hypothermia can cause coagulopathy by interfering with both platelets and coagulation factors. Results of coagulation parameters are obtained within 10 to 15 minutes because of activation with specific materials for each of clotting, platelet, and fibrinogen function. The varying contribution of these conditions to the clinical picture of a given patient necessitates priority-oriented planning. If a rapid-sequence induction is contemplated, ketamine and etomidate may confer advantages over propofol. In equipotent doses in normovolemic patients, they produce less cardiovascular depression. Contrary to findings of increased mortality with prolonged etomidate infusion, a single induction dose (0. Its longer duration of action may be disadvantageous and may lead to hypoxia, if both ventilation and intubation prove to be impossible. Under these circumstances, one of the available videolaryngoscopes or other aids can be employed to overcome the problem. Sugammadex can also be utilized to encapsulate rocuronium or vecuronium and recover spontaneous breathing in a reasonably short period. Bradycardia, dysrhythmias, and cardiac arrest may occur after succinylcholine in the presence of hypoxia and hypercarbia. Some of these complications may also follow an apparently uneventful intubation performed without succinylcholine. Hypovolemia In the absence of controlled human studies of anesthetic drug effects in hemorrhage and hemorrhagic shock, our current knowledge in this area is based on the results of experimental work, mostly in swine, and clinical 3807 experience from managing trauma victims. Our planning process for deciding how to use anesthetic agents is further complicated by the damage control resuscitation principle, specifically permissive hypotension. The facts that should drive decision making in this regard are as follows: First, anesthetic agents not only have direct cardiovascular depressant effects but also inhibit compensatory hemodynamic mechanisms such as central catecholamine output and baroreflex (neuroregulatory) mechanisms, which maintain systemic pressure in hypovolemia. Second, hemorrhage and hypovolemia alter the pharmacokinetics and pharmacodynamics of almost all anesthetic agents and often lead to a higher than normal blood concentration of intravenous agents and increased sensitivity of the brain and heart. Preferential distribution of the cardiac output to the brain and the heart, cerebral hypoxia, dilutional hypoproteinemia producing a larger free fraction of intravenous drugs, and acidosis all seem to be responsible for these effects. Third, hemorrhage and hypovolemia have different hemodynamic effects in the absence and presence of trauma. In the presence of trauma pain and a catecholamine surge, maintain blood pressure despite significant intravascular volume depletion and ischemia of vital organs such as the brain and the heart. Based on this knowledge, reducing or eliminating anesthesia to avoid abolishing the hemodynamic balance is a natural and often utilized practice, especially when permissive hypotension to limit bleeding is used. Dutton252 recently suggested that proper management under these circumstances may be aggressive titrated administration of anesthetics and of blood products to produce a high-flow and low-pressure hemodynamic state with vasodilation to improve organ flow and oxygenation, which may reduce fibrinolytic activity and inflammation. Further research is needed to help the clinician in selection of either of the approaches. The pharmacokinetic and pharmacodynamic responses of intravenous agents to experimental hemorrhagic shock vary depending on the severity of the hemorrhage, the specific agent, and whether the effect analyzed is hypnosis or immobility to noxious stimuli. For example, in swine with compensated hemorrhage, when administered as a continuous infusion, blood propofol concentration increased by less than 20%, while during uncompensated shock. It is possible that hydrolysis by tissue esterases is more intense than by their blood counterparts because the decreased tissue blood flow in uncompensated shock is able to produce a 3808 major reduction in remifentanil metabolism. Another example is that hemorrhage has a similar potentiating effect on the production of hypnosis and immobility by propofol. Although etomidate pharmacodynamics are unchanged,257 a significant increase in the sensitivity of the brain and heart to propofol is noted in animals, even after fluid resuscitation. Although he calculated that etomidate dose should not require adjustment for shock, the authors decrease the dose by at least 25% to 50% when hypovolemia is suspected. As to the opioids, the calculated dose for fentanyl and remifentanil is approximately one-half that for healthy patients. Calculation is based on pharmacokinetic and pharmacodynamic studies performed in experimental hemorrhagic shock. For example, the baroreceptor depression produced by intravenous agents is usually milder than that of 3809 inhalational agents. Opioid agents have little direct cardiovascular or baroreflex depressant effect; however, these agents can cause hypotension by inhibiting central sympathetic activity, especially in the hypovolemic trauma patient whose apparent hemodynamic stability is maintained by hyperactive sympathetic tone. Two important principles in the use of anesthetic agents are accurate estimation of the degree of hypovolemia and reduction of doses accordingly. The presence of hypotension suggests uncompensated hypovolemia, in which case anesthetics almost invariably produce further deterioration of systemic blood pressure and sometimes cardiac standstill. When time constraints or continuing hemorrhage prevent restoration of blood volume, the airway may be secured without the benefit of anesthesia (perhaps using only rapidly acting muscle relaxants and small doses of opioids, etomidate, or ketamine), even though this approach may result in recall of induction and intraoperative events in up to 40% of patients, and, as mentioned before, vital organ ischemia. Intraoperative use of the bispectral index monitor and, whenever possible, titrating anesthetics to bispectral index levels lower than 60 may prevent recall in trauma patients. In normotensive but hypovolemic patients, restoration of volume and selection of an agent with the least cardiovascular depressant effect appears logical. Ketamine and etomidate are the preferred induction agents,257 although at low doses other intravenous anesthetics are also unlikely to produce hypotension. Therefore, the use of any of these drugs in reduced doses is probably more important than the particular agent chosen. Maintenance of anesthesia in the hypovolemic trauma patient raises concerns similar to those pertaining to induction. Under these circumstances, patients are unlikely to respond to the sympathetic effect of N2O, and the cardiovascular depressant properties of the gas are unmasked. In addition, by reducing FiO2, use of N2O incurs a risk of hypoxemia in patients with reduced cardiac output or pulmonary compromise.

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For the second stage of labor and delivery women's health big book of exercises free download order estrace 2 mg with mastercard, the nerve block should be extended to include the S2 to S4 segments in order to block pain from vaginal and perineal distension and trauma menopause heart palpitations buy generic estrace 2 mg online. Long-acting amides such as bupivacaine or ropivacaine are most frequently used because they produce excellent sensory analgesia while sparing motor function menstruation after miscarriage buy generic estrace on line, particularly at low concentrations (<0 menstrual inflammation discount 1 mg estrace otc. Although some studies have found that ropivacaine is associated with less motor blockade than equipotent doses of bupivacaine women's health center southern pines order estrace 2 mg without prescription, there was no difference in the rate of instrumental vaginal delivery among women randomized to receive epidural levobupivacaine, bupivacaine, or ropivacaine for maintenance of labor analgesia. There is controversy regarding the need for an epidural test dose when using dilute solutions of local anesthetic. Because catheter aspiration is not always diagnostic, particularly when using singleorifice epidural catheters, some experts believe that a test dose should be administered to improve detection of an intrathecally or intravascularly placed catheter. Analgesia may be maintained with a continuous infusion (8 to 12 mL/hr) of bupivacaine (0. Data are conflicting as to whether a background infusion improves analgesia; however, a background infusion may be helpful in selected parturients. Thirty percent to 50% of the hourly dose is often administered as a background infusion. The timed, or programmed intermittent epidural bolus technique is a new method for maintaining epidural analgesia. In this technique, the pump is programmed to deliver a bolus dose at regular intervals. Presumably, the bolus administration of drugs into the epidural space results in better distribution of the drug solution. Before ambulation, women should be observed for 30 minutes after initiation of neuraxial blockade to assess maternal and fetal well-being. During delivery, the sacral dermatomes may be blocked with 10 mL of bupivacaine (0. Many parturients have adequate analgesia for delivery without an additional bolus dose, particularly if epidural analgesia has been maintained for a long interval (hours). However, instrumental vaginal delivery may require a denser block than that obtained with dilute local anesthetic solutions. Spinal analgesia with fentanyl (15 to 25 g) or sufentanil (2 to 5 g) in combination with plain bupivacaine (1. A potential disadvantage of single-shot spinal analgesia is that the duration of labor, even in a rapidly progressing multiparous woman, may be longer than anticipated. Furthermore, if the woman requires an urgent cesarean delivery, a new anesthetic will need to be initiated. However, spinal anesthesia (a "saddle block") is a safe and effective alternative to general anesthesia or pudendal nerve block for instrumental delivery in parturients without pre-existing epidural analgesia. After identification of the epidural space using a conventional (or specialized) epidural needle, a longer (127 mm), pencilpoint spinal needle is advanced into the subarachnoid space through the epidural needle. After intrathecal injection, the spinal needle is removed and an epidural catheter is inserted. Intrathecal injection of fentanyl (10 to 25 g) or sufentanil (2 to 5 g) alone or more commonly in combination with bupivacaine (1. Spinal opioid alone provides complete analgesia for the early latent phase of labor. However, the addition of bupivacaine is necessary for satisfactory analgesia during advanced labor. The most common side effects of intrathecal opioids are pruritus, nausea, vomiting, and urinary retention. The incidence of pruritus is lower if opioid is coadministered with local anesthetic. Presumably, uterine 2859 tachysystole and decreased uteroplacental perfusion occur as a result of rapid decrease in circulating maternal epinephrine levels after initiation of analgesia or as a result of hypotension after sympatholysis. Spinal opioid provides complete analgesia without the need for local anesthetic in early labor, thus avoiding an acute decrease in preload, and almost always allowing motivated women to ambulate because there is no motor block. The onset of sacral analgesia is accomplished significantly faster with much less drug than initiation of lumbar epidural analgesia. Five to ten milliliters of dilute local anesthetic solution is injected submucosally via a needle guide in the vagina into the left and right lateral vaginal fornices. Although paracervical block effectively relieves pain during the first stage of labor, the technique has fallen out of favor during childbirth because it is associated with a high incidence of fetal asphyxia and poor neonatal outcome, particularly with the use of bupivacaine. Performing the block with dilute local anesthetic solutions, allowing 5 to 10 minutes to elapse between injections on the left and right sides, and limiting the block to women with less than 8 cm cervical dilation, may decrease the incidence of complications. Pudendal Nerve Block the pudendal nerves, derived from the sacral nerve roots (S2 to S4), supply the vaginal vault, perineum, rectum, and parts of the bladder. The nerves are easily anesthetized transvaginally where they loop around the ischial spines. Ten milliliters of dilute local anesthetic solution deposited behind each sacrospinous ligament can provide adequate anesthesia for outlet forceps delivery and episiotomy repair. Inhalation Analgesia and General Anesthesia Inhalation labor analgesia is uncommon in the United States, although its use is more common in other parts of the world (see Chapter 18). Nitrous oxide, 50% by volume, is the most commonly used inhalation agent for analgesia during labor. The mother is trained to intermittently self-administer the gas at the onset of a contraction. Studies are conflicting as to whether nitrous oxide provides benefit to the parturient; its safety for the fetus and the neonate has also not been well studied. General anesthesia is rarely used for vaginal delivery, and precautions against gastric aspiration must always be observed (see General Anesthesia in the section Anesthesia for Cesarean Delivery). General anesthesia may be required when time constraints prevent induction of regional anesthesia. However, in current practice, intravenous nitroglycerin (50 to 250 g) has largely replaced the need for general anesthesia for uterine relaxation. Anesthesia for Cesarean Delivery the most common indications for cesarean delivery include arrest of dilation, nonreassuring fetal status, cephalopelvic disproportion, malpresentation, prematurity, prior cesarean delivery, and prior uterine surgery involving the corpus. A 2001 survey of obstetric anesthesia practices in the United States revealed that most patients undergoing cesarean delivery do so under spinal or epidural anesthesia. Neuraxial Anesthesia Blockade to the T4 dermatome is necessary to perform cesarean delivery without maternal discomfort. The most common complication of neuraxial anesthesia is hypotension and the attendant risk of decreased uteroplacental perfusion (see Hypotension in the section on Anesthetic Complications). Measures to decrease the incidence and severity of hypotension include left uterine displacement, intravenous fluid administration, and the liberal use of vasopressors to prevent and treat hypotension. Most anesthesiologists administer a nonparticulate antacid before induction of anesthesia for pulmonary aspiration prophylaxis. Intraoperative monitoring mimics that for all anesthetics, although blood pressure should be measured frequently (every several minutes) for the first 20 minutes after initiation of anesthesia. Although supplemental oxygen is frequently administered, there is no evidence of benefit to the mother, the fetus, or the neonate. Although postcesarean delivery analgesia should take the nursing infant into account, very small amounts of drugs administered to the mother actually cross into breast milk, and even smaller amounts are absorbed from the neonatal gut. Prolonged (12 to 24 hours) postoperative pain relief in the postpartum patient can be provided by intrathecal morphine (100 to 150 g)74 or epidural morphine (3. Delayed respiratory depression is a rare but potentially devastating complication; therefore, patients who receive neuraxial opioids must be monitored carefully in the postoperative period. Spinal Anesthesia Subarachnoid block is probably the most commonly administered neuraxial anesthetic for cesarean delivery because of its simplicity, speed of onset, and reliability. It is an alternative to general anesthesia for almost all but the most emergent of cesarean deliveries. Despite an adequate dermatomal level for surgery, women may experience varying degrees of visceral discomfort and nausea and vomiting, particularly during exteriorization of the uterus and traction on abdominal viscera. Improved perioperative anesthesia and analgesia can be provided with the addition of fentanyl (10 to 20 g), sufentanil (2. Fentanyl has a rapid onset, but is short acting and provides little additional postoperative analgesia. In contrast, morphine has a longer latency than fentanyl, but will also provide anesthesia for 12 to 18 hours after delivery. Lumbar Epidural Anesthesia In contrast to spinal anesthesia, epidural anesthesia is associated with a slower onset of action and a larger drug requirement to establish adequate sensory block. The major advantages of epidural compared with single-shot spinal anesthesia are the ability to titrate the extent and duration of anesthesia. To avoid unintentional intrathecal or intravascular injection, correct placement of the epidural needle and catheter is essential. This is especially true because epidural anesthesia for cesarean delivery necessitates the administration of large doses of local anesthetic. Aspiration of the epidural catheter for blood or cerebrospinal fluid is not reliable for detection of catheter misplacement, particularly with singleorifice catheters. Thus, most anesthesiologists administer a test dose before the initiation of surgical anesthesia. Addition of epinephrine (15 g) with careful hemodynamic monitoring may signal intravascular injection if followed by a transient increase in heart rate and blood pressure. The use of an epinephrine test dose (15 g) in obstetrics is controversial because false positive results do occur (10% increase in heart rate), especially in laboring women. Rapid 2863 injection of 1 mL of air with simultaneous precordial Doppler monitoring appears to be a reliable indicator of intravascular catheter placement. The most commonly used agents for obstetric epidural anesthesia are 2% lidocaine with epinephrine, 5 g/mL (1:200,000) and 3% 2-chloroprocaine. Adequate anesthesia is usually achieved with 15 to 25 mL of local anesthetic solution, administered in divided doses over 5 to 10 minutes. However, 2% lidocaine with epinephrine and sodium bicarbonate (1 mEq/10 mL lidocaine) and fentanyl may also be used when the rapid conversion of pre-existing epidural labor analgesia to surgical anesthesia is required for urgent cesarean delivery. Lidocaine should be administered with epinephrine, as lidocaine without epinephrine does not consistently provide satisfactory surgical anesthesia. Unintentional intravascular injection of bupivacaine is associated with a high incidence of maternal mortality. A metaanalysis of studies comparing different anesthetic solutions for extension of labor epidural analgesia for cesarean delivery concluded that ropivacaine provided denser anesthesia compared with bupivacaine or levobupivacaine. The standard technique uses the same spinal dose of local anesthetic as one would use for standard spinal anesthesia. After 15 minutes, if anesthesia is inadequate, the block is extended by injecting supplemental local anesthetic via the epidural catheter. A third technique is also associated with a lower incidence of hypotension without prolonging onset time. A small dose of spinal local anesthetic is followed by the routine injection of additional anesthetic through the epidural catheter approximately 5 minutes after the intrathecal dose. General Anesthesia General anesthesia may be necessary when absolute or relative contraindications exist to neuraxial anesthesia. General anesthesia should be used cautiously in women with asthma, upper respiratory tract infection, obesity, or a history of difficult tracheal intubation. Preoperative airway evaluation is particularly important in pregnant women because the inability to intubate the trachea and provide effective ventilation is the leading cause of maternal death related to anesthesia. Pulmonary aspiration prophylaxis should be administered and the patient should be positioned with left lateral tilt to prevent aortocaval compression. To minimize the risk of hypoxemia during induction, denitrogenation for 3 to 5 minutes with a tight-fitting mask is essential. Although somewhat controversial,87 a rapid-sequence induction is usually performed. A trained assistant applies cricoid pressure until the airway is properly secured with a cuffed endotracheal tube. Once correct placement of the endotracheal tube is confirmed with capnography and auscultation, the obstetrician may proceed with incision. Although some experts advise attempting to maintain cricoid pressure throughout, this practice may actually make visualization of the glottis and mask ventilation more difficult in some patients. However, if the fetus is in extremis, airway management with a mask or supraglottic airway device may be an acceptable alternative. However, the incidence of intraoperative awareness appears to be unacceptably high with this technique. After delivery, the nitrous oxide concentration can be increased and/or an intravenous amnestic. General anesthesia for cesarean delivery is associated with lower neonatal Apgar scores at 1 minute compared with neuraxial anesthesia93; however, the Apgar scores at 5 minutes are comparable. Therefore, an individual trained in neonatal resuscitation should be present at delivery of the infant. After delivery, prophylactic intravenous oxytocin is administered to decrease the risk of uterine atony and anesthesia may be deepened with an opioid and benzodiazepine, as necessary. The usual blood loss during cesarean delivery is 750 to 1,000 mL; transfusion is rarely necessary. During the most recent period, 2867 case-fatality rates from general anesthesia fell, whereas those for neuraxial anesthesia rose. Anesthesia-related mortality was most often associated with cesarean delivery (86%). The leading causes of death were intubation failure or induction problems (23%), respiratory failure (20%), and high spinal or epidural block (16%). Women who have recently eaten, are laboring, received systemic opioids, or have frequent heartburn are of greatest concern. Comprehensive airway evaluation, prophylactic administration of nonparticulate antacids, and use of regional anesthesia decrease the risk of aspiration. General anesthesia may be unavoidable occasionally; therefore, awake intubation may be indicated in women in whom airway difficulties are anticipated. Labor lowers the risk of hypotension in term pregnant women compared with nonlaboring women. Blood pressure should be monitored frequently (every 2 to 3 minutes) after the induction of neuraxial anesthesia.

Scoring systems that quantify physical status or establish thresholds for vital signs are useful for assessment but cannot replace individual evaluation women's health issues symptoms estrace 2 mg amex. A plan for the continued management of likely postdischarge symptoms such as pain pregnancy jokes cartoons cheap estrace 2 mg free shipping, nausea the australian women's health big book of exercises discount estrace 1 mg line, headache menstrual when to see a doctor order estrace 1 mg line, dizziness women's health center chelsea mi order 1 mg estrace, drowsiness, and fatigue must be made prior to discharge. The policy for postanesthesia follow-up requires a written follow-up that is performed by an individual that is qualified to administer anesthesia no later than 48 hours post procedure. The evaluation should be performed only after the patient has sufficiently recovered from anesthesia to be able to participate, such as answer questions or perform simple tasks. Perhaps the two most common types of patients to encounter troubles will be the patient with coronary artery disease and the patient with congestive heart failure. That does not mean that all tachycardia heralds myocardial ischemia, but in a patient who seems at risk for coronary artery disease, new-onset tachycardia that is not caused by pain should be taken seriously. So, the clinician must be especially suspicious of a series of hemodynamic changes in a person at risk for coronary artery disease. Early intervention with nitrates, opioids, blockers, and even anticoagulants may save a life. Cardiology should be involved to gain immediate and timely access to the cardiac catheterization laboratory or for anxiolytic drug therapy. Involvement and communication with the surgical service must be immediate and decisions, especially as to anticoagulation and lytic therapy, should be made among several services in consultation. The outpatient cardiology services have an expanding armamentarium of new inotropic/vasodilator therapy, devices, and interventions that allow patients to compensate for their congestive heart failure. It is helpful to know not only the ejection fraction but also the activities of daily living, exercise tolerance, and other risk indices. The ejection fraction is only an estimate of the fractional shortening of the myocardial actin and myosin fibrils. Although it is a useful estimate of severity of impairment, one is struck by how stable some patients may be with a large dilated heart contracting at a 15% ejection fraction. There are also no absolute numbers with regard to fluid restriction but precaution should be taken when giving fluid challenges. Within a very few minutes a puzzling hypotensive situation might be explained by an echocardiogram. The echocardiogram allows rapid viewing of myocardial contractility, regional wall motion, volume status, and valvular dysfunction. In a study of 85 prospective patients29 undergoing "off-pump" coronary artery bypass graft procedures, the patients were extubated in 12 ± 2 minutes after the chest was closed. Bradycardia was the cause for failure in three cases; the cause for the fourth 3871 failure was myocardial infarction. Although this study seems quite favorable, the two groups of patients were not comparable. Nursing reviews are available to give input as to how to structure such new units. Invasive cardiology suites are used for ablation techniques for dysrhythmias, and automated implantable defibrillators are placed in hybrid suites, operating rooms, or catheterization laboratories; these facilities may also be the sites of percutaneous valve replacements as well as some hybrid and percutaneous coronary revascularization procedures. Postoperative Pulmonary Dysfunction Mechanical, hemodynamic, and pharmacologic factors related to surgery and anesthesia impair ventilation, oxygenation, and airway maintenance. Inadequate ventilation should be suspected when (1) respiratory acidemia occurs coincident with tachypnea, anxiety, dyspnea, labored ventilation, or increased sympathetic nervous system activity; (2) hypercarbia reduces the arterial pH below 7. Inadequate Respiratory Drive During early recovery from anesthesia, residual effects of intravenous and inhalation anesthetics blunt the ventilatory responses to both hypercarbia and hypoxemia. Sedatives augment depression from opioids or anesthetics and reduce the conscious desire to ventilate (a significant component of ventilatory drive). Coincident depression of medullary centers that regulate the sympathetic nervous system can blunt signs of acidemia or hypoxemia such as hypertension, tachycardia, and agitation, concealing hypoventilation. Patients might communicate lucidly and even complain of pain while experiencing significant opioid-induced hypoventilation. A balance must be struck between an acceptable level of postoperative ventilatory depression and a tolerable level of pain or agitation. Children with active or recent upper respiratory infection are more prone to breath-holding, severe cough, and arterial desaturations below 90% during recovery, especially if they have a history of reactive airway disease or secondhand smoke exposure or have undergone intubation and/or airway surgery. Intracranial hemorrhage or edema sometimes presents with hypoventilation, especially after posterior fossa craniotomy. In postoperative patients, increased upper airway resistance is caused by obstruction in the pharynx (posterior tongue displacement, change in anteroposterior and lateral dimensions from soft tissue collapse), in the larynx (laryngospasm, laryngeal edema), or in the large airways (extrinsic compression from hematoma, tumor, or tracheal stenosis). Weakness from residual neuromuscular relaxation,39 myasthenia gravis or myasthenic syndromes can contribute, but it is seldom the primary cause of airway compromise. If the airway is clear of vomitus or foreign bodies, simple maneuvers such as improving the level of consciousness, lateral positioning, chin lift, mandible elevation, or placement of an oropharyngeal or nasopharyngeal airway may relieve obstruction. A nasopharyngeal airway may be better tolerated when the patient has functional gag reflexes. During emergence, stimulation of the pharynx or vocal cords by secretions, blood, foreign matter, or extubation can generate laryngospasm. Patients who smoke or are chronically exposed to smoke have irritable airway conditions, have copious secretions, or have undergone upper airway surgery are at higher risk. Severe laryngeal obstruction can 3874 occur secondarily because of acute hypocalcemia after parathyroid excision. Nebulized vasoconstrictors like epinephrine help somewhat, but steroids have little effect acutely. Patients with C1 esterase inhibitor deficiency can develop severe angioneurotic edema after even slight trauma to the airway. Judgment by the individual anesthesiologist regarding timing, patient status, available equipment along with airway management skills all play a part of the decision as to where, when, and how to intubate. Equipment and personnel necessary for emergency cricothyroidotomy or tracheostomy should be available. Needle cricothyroidotomy using a 14-gauge intravenous catheter or a commercially available kit permits oxygenation and marginal ventilation until the airway is secured, especially if jet ventilation with 100% oxygen is used. Reduction of cross-sectional area in small airways increases overall airway resistance because resistance varies inversely with the fourth power of radius 3875 during turbulent flow. Pharyngeal or tracheal stimulation from secretions, suctioning, aspiration, or a tracheal tube can trigger a reflex constriction of bronchial smooth muscle in emerging patients with reactive airways. Histamine release precipitated by medication or allergic reactions also increases airway smooth muscle tone. Preoperative spirometric evidence of increased airway resistance predicts an increased risk of postoperative bronchospasm. Prolonged expiratory time or audible turbulent air flow (wheezing) during forced vital capacity expiration often unmasks subclinical airway resistance. Resistance is higher during expiration because intermediatediameter airways are compressed by positive intrathoracic pressure. High airway resistance does not always cause wheezing because flow might be so impeded that no sound is produced. Spontaneously breathing patients exhibit accessory muscle recruitment, labored ventilation, and increased work of breathing with either condition. Levalbuterol or metaproterenol nebulized in oxygen resolves postoperative bronchospasm with minimal tachycardia. Nebulized racemic epinephrine effectively relaxes smooth muscle, but side effects of tachycardia and flushing can be seen. Administration of steroid therapy offers little acute improvement, but may prevent later recurrence. Bronchospasm that is resistant to 2-sympathomimetic medication may improve with an anticholinergic medication such as atropine or ipratropium. If bronchospasm is life-threatening, an intravenous epinephrine infusion yields profound bronchodilation. Restoration of lung volume with incentive spirometry or deep tidal ventilation increases radial traction on small airways. Reducing left ventricular filling pressures might relieve airway resistance caused by increased lung water, although interstitial fluid accumulation can persist. Also, extended contraction of airway smooth muscle obstructs venous and lymphatic flow, leading to airway wall edema that 3876 resolves slowly. Decreased Compliance Reduced pulmonary compliance increases the elastic work of breathing. In the extreme, low compliance causes progressive respiratory muscle fatigue, hypoventilation, and respiratory acidemia. Pulmonary contusion or hemorrhage interferes with lung expansion, as do restrictive lung diseases, skeletal abnormalities, intrathoracic lesions, hemothorax, pneumothorax, or cardiomegaly. Obesity affects pulmonary compliance, especially when adipose tissue compresses the thoracic cage or increases intra-abdominal pressure in supine or lateral positions. Extrathoracic factors such as tight muscles of the chest or abdominal dressings and gas in the stomach or bowel reduce chest wall compliance. An intra-abdominal tumor, hemorrhage, ascites, bowel obstruction, or pregnancy impairs diaphragmatic excursion and reduces compliance. Allowing patients to recover in a semi-sitting (semi-Fowler) position reduces work of breathing. Neuromuscular and Skeletal Problems Postoperative airway obstruction and hypoventilation are accentuated by incomplete reversal of neuromuscular relaxation. Residual paralysis compromises airway patency, ability to overcome airway resistance, airway protection, and ability to clear secretions. Intraoperative use of shorteracting relaxants might decrease the incidence of residual paralysis but does 3877 not eliminate the problem. Marginal reversal can be more dangerous than near-total paralysis because a weak, agitated patient exhibiting uncoordinated movements and airway obstruction is more easily identified. A somnolent patient exhibiting mild stridor and shallow ventilation from marginal neuromuscular function might be overlooked, allowing insidious hypoventilation and respiratory acidemia or regurgitation with aspiration to occur. Even without relaxant administration, these patients can exhibit postoperative ventilatory insufficiency. Diaphragmatic contraction is compromised in some postoperative patients, forcing more reliance on intercostal muscles and reducing the ability to overcome decreased compliance or increased ventilatory demands. Impairment of phrenic nerve function from interscalene block, trauma, or thoracic and neck operations can "paralyze" one or rarely both diaphragms. However, with high work of breathing, muscle weakness, or increased ventilatory demands, a nonfunctional diaphragm impairs minute ventilation. The ability to sustain head elevation in a supine position, a forced vital capacity of 10 to 12 mL/kg, an inspiratory pressure more negative than -25 cm H2O, and tactile train-of-four assessment imply that strength of ventilatory muscles is adequate to sustain ventilation and to take a large enough breath to cough. However, none of these clinical end points reliably predicts recovery of airway protective reflexes,44 and failure on these tests does not necessarily indicate 3878 the need for assisted ventilation. By using these noninvasive techniques, patients can often overcome some of the above discussed issues interfering with normal respiration, thus reducing the risk of remaining intubated or reintubation. Occasionally, a clinical picture suggests ventilatory insufficiency when ventilation is adequate. Voluntary limitation of chest expansion to avoid pain (splinting) causes labored, rapid, shallow breathing characteristic of inadequate ventilation. Splinting seldom causes actual hypoventilation and usually improves with analgesia and repositioning. Ventilation with small tidal volumes due to thoracic restriction or reduced compliance seems to generate afferent input from pulmonary stretch receptors, leading to dyspnea, labored breathing, and accessory muscle recruitment in spite of appropriate minute ventilation. Finally, spontaneous hyperventilation to compensate for a metabolic acidemia might generate tachypnea or labored breathing, which is mistaken for ventilatory insufficiency. Patients with high V·D/V·T are at greater risk for postoperative ventilatory failure. Occasionally, an acute increase in deadspace contributes to respiratory acidemia in postoperative patients. Decreased cardiac output can transiently increase V·D/V·T by decreasing perfusion to well-ventilated, nondependent lung and is the most common cause of acute increase in 3879 deadspace in the acute care setting. Deadspace may appear high if an inhalation interrupts the previous exhalation and spent alveolar gas is retained. This "gas trapping" occurs when high airway resistance lengthens the time required to exhale completely, or if improper inspiration/expiration ratios or high ventilatory rates are used during mechanical ventilation. Increased Carbon Dioxide Production Carbon dioxide production varies directly with metabolic rate, body temperature, and substrate availability. Inadequate Postoperative Oxygenation Systemic arterial partial pressure of oxygen (PaO2) is the best indicator of pulmonary oxygen transfer from alveolar gas to pulmonary capillary blood. Arterial hemoglobin saturation monitored by pulse oximetry yields less information on alveolar-arterial gradients and is not helpful in assessing impact of hemoglobin dissociation curve shifts or carboxyhemoglobin. Adequate arterial oxygenation does not mean that cardiac output, arterial perfusion pressure, or distribution of blood flow will maintain tissue oxygenation. Sepsis, hypotension, anemia, or hemoglobin dissociation abnormalities can generate tissue ischemia despite adequate oxygenation. In postoperative patients, the acceptable lower limit for PaO2 varies with 3880 individual patient characteristics. A PaO2 below 65 to 70 mmHg causes significant hemoglobin desaturation, although tissue oxygen delivery might be maintained at lower levels. Maintaining PaO2 between 80 and 100 mmHg (saturation 93% to 97%) ensures adequate oxygen availability. Little benefit is derived from elevating PaO2 above 110 mmHg because hemoglobin is saturated and the amount of additional oxygen dissolved in plasma is negligible. Distribution of Ventilation Loss of dependent lung volume commonly causes V·/Q· mismatching and hypoxemia. Right upper lobe collapse secondary to partial right main stem intubation is a frequently overlooked cause. During one-lung anesthesia, the weight of unsupported mediastinal contents, pressure from abdominal contents on the dependent diaphragm, and lung compression all reduce dependent lung volume.

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