Edward M. Copeland, III, MD, FACS

• Emeritus Distinguished Professor of Surgery (Surgical Oncology)
• Department of Surgery
• Attending Physician, Shands
• University of Florida
• College of Medicine
• Gainesville, Florida

Natriuresis occurs to a degree similar to that in normotension treatment vitamin d deficiency order 600mg praziquantel, so as to maintain a stable body water volume symptoms bronchitis discount praziquantel 600 mg without prescription, but requires a higher arterial pressure to do so pretreatment order 600mg praziquantel amex. Long-term ingestion of excess salt combined with low potassium ingestion contributes to hypertension treatment junctional tachycardia buy 600 mg praziquantel with mastercard, a condition not seen in populations with daily salt intake less than 50 mmol treatment genital warts cheap praziquantel 600mg with mastercard. The mechanism involves renal salt retention and initial extracellular volume expansion (later mitigated by pressure natriuresis), with release of an Chapter 59: Perioperative Fluid and Electrolyte Therapy 1775 endogenous digitalis-like factor and stimulation of renal Na+ pumps, furthering renal Na+ retention. Daily requirements reflect age and growth, with more K+ required in higher metabolic rates. Transmembrane potentials particularly depend on K+ permeability, with K+ egress occurring through ion channels down its concentration gradient. This leaves behind intracellular anions, with a resultant negative transmembrane potential. The resting value of this potential is achieved when the tendency of K+ to move extracellularly as a result of its concentration gradient is matched by the tendency of K+ to move intracellularly because of the electrical gradient. K+ is freely filtered at the glomerulus, then undergoes extensive unregulated reabsorption along the proximal tubule, with only 10% to 15% reaching the distal nephron, where its reabsorption or secretion is tightly controlled. Principal cell behavior is influenced by the following: · Aldosterone, synthesized and released by the adrenal glands in response to raised K+ concentrations. Increased distal tubular Na+ content leads to a steeper Na+ concentration gradient and increased principal cell reabsorption of Na+. To maintain electroneutrality of the tubular fluid, K+ efflux into the tubule increases; this is partly responsible for the hypokalemia associated with diuretics that increase delivery of Na+ to the cortical collecting ducts (thiazides and loop diuretics). In contrast, amiloride blocks the principal cell luminal Na+ channel and therefore does not affect K+ efflux here. Low K+ settings lead to up-regulation of this luminal antiporter, reabsorbing more K+ at the expense of renal acid loss. Intracellular Ca2+ entry may have direct effects-for example, leading to neurotransmitter release or inducing further large-scale release of Ca2+ from intracellular stores (Ca2+-induced Ca2+ release), in cardiac and skeletal muscle contraction. Increases in cytoplasmic free Ca2+ concentration occurring as a result of cellular energetic failure and impaired Ca2+ transport are a key mediator of cell death pathways. For example, Ca2+ levels remain normal after loss of the calcitonin-secreting parafollicular (C-) cells during thyroidectomy. An increase in the (calcium × phosphate) product may be seen in advanced chronic kidney disease and is associated with ectopic bone deposition. Approximately 50% of circulating Ca2+ is in the biologically active ionized form (normal range 2 to 2. Hypoalbuminemia decreases the total serum Ca2+ but has less effect on the biologically important ionized form. The degree of albumin-protein binding is affected by pH, with acidemia reducing protein binding and increasing the ionized fraction. Specimens should ideally be taken without tourniquet (uncuffed), because local acidosis increases the ionized fraction. It is primarily an intracellular anion, although most is sequestered within organelles, bound to phospholipids, proteins, and nucleic acids. Of total body Mg2+, 50% is within bone, 20% within muscle, and the rest in liver, heart, and other tissues. In addition, effects on ion channels underlie one of the core functions of Mg2+, namely physiologic competitive antagonism of Ca2+. These effects result in inhibition of a diverse array of excitable tissue cellular actions, including neurotransmitter release, muscular contraction, cardiac pacemaker and action potential activity, and pain signal transmission. Seventy-five percent is freely filtered at the glomerulus, and proximal tubule reabsorption is minimal, with 60% to 70% being reabsorbed at the thick ascending loop of Henle and 10% reabsorbed under regulation in the distal tubule. Inhibition of neuronal Ca2+ influx reduces catecholamine release from adrenal medulla and adrenergic nerve endings. Pharmacologic use of Mg2+ in obtunding pressor response to intubation or during surgery for pheochromocytoma. Mg2+ administration typically leads to a minor reflex increase in inotropy despite the direct action of Mg2+ on reducing cardiac contractility. Increases atrioventricular nodal conduction time and refractory periods, suppresses accessory pathway transmission, and inhibits early and delayed afterdepolarizations. Clinical use is in supraventricular tachycardias, atrial fibrillation rate control and postoperative prophylaxis, and tachyarrhythmias associated with dyskalemia, digoxin, bupivacaine, or amitriptyline. Ca2+ antagonism­related smooth muscle relaxation Pharmacologic doses of magnesium sulfate reduce monocyte inflammatory cytokine production. Other influences may alter the intracellular-extracellular balance of magnesium distribution. Catecholamines, acting by both -adrenoreceptors and -adrenoreceptors, and glucagon lead to extrusion of magnesium from intracellular stores. Although experimental models have shown that adrenergic stimulation may increase serum Mg2+ concentrations, decreases in serum Mg2+ concentrations actually occur after stressors such as cardiac, orthopedic, and abdominal surgery, trauma, burns, and sepsis. Of total body phosphorus, 80% to 90% is stored in bone, with the remainder in the intracellular (soft tissues and erythrocytes) and extracellular fluid compartments. Normal plasma values are 97 to 107 mEq/L; Cl- is therefore responsible for nearly a third of plasma osmolality and two thirds of plasma negative charge. Cl- excretion is primarily renal, largely in the proximal tubule by passive reabsorption or cotransport. Administration of fluid with a Cl- concentration higher than that of plasma will in sufficient quantities. Similar changes are not seen with solutions containing other anions, which are metabolized after infusion, such as lactated Ringer solution. Saline-induced hyperchloremic acidosis is recognized to have a variety of potentially deleterious physiologic effects. A meta-analysis of studies comparing saline with balanced perioperative fluid regimens confirmed the presence of hyperchloremia and acidosis postoperatively in the saline groups, but typically these biochemical abnormalities had cleared by the first or second postoperative day. However, the available trials were relatively small, and higher risk surgical groups (those with preexisting impairment of acid-base status or emergency and major surgery) were underrepresented. Interestingly, in one trial of patients undergoing renal transplant, saline administration was associated with significant hyperkalemia, presumably caused by cellular K+ extrusion resulting from extracellular acidosis. In many other situations, clinical benefit is not apparent, a finding that highlights an important pathophysiologic concept. Acidosis Acid-Base Disturbances and Fluid Therapy Acid-base balance in general is discussed in Chapter 60; however, the two key areas in which intravascular fluid therapy may affect acid-base balance are iatrogenic acidosis caused by the administration of Cl-rich fluids and administration of sodium bicarbonate to correct acidosis. Chapter 59: Perioperative Fluid and Electrolyte Therapy 1779 in itself may not be physiologically deleterious; indeed it is a normal event during strenuous exercise, in which it may aid O2 offloading to tissues. Rather, perhaps acidosis serves as a marker for the severity of underlying disease processes, such as hypoxia, ischemia, or mitochondrial dysfunction, which cause morbidity without adequate correction. They may be classified by their tonicity after infusion or their overall composition; crystalloids containing a range of electrolytes also found in plasma and a buffer such as lactate or acetate may be referred to as balanced solutions. Crystalloids are indicated for replacement of free water and electrolytes but also may be used for volume expansion. This is challenged by large clinical trials and current knowledge of microvascular fluid handling (see section on vascular endothelium), which suggest that isotonic crystalloids may have a larger intravascular volume expanding effect than this, particularly in patients with low capillary hydrostatic pressures. The study of volume kinetics has quantified the redistribution of crystalloids from the central (intravascular) volume to the larger peripheral (total extracellular) volume. Perhaps up to 70% of a crystalloid infusion remains in the intravascular compartment at the end of a 20-minute continuous infusion, decreasing to 50% after 30 minutes. Large-volume crystalloid infusion also may be associated with a hypercoagulable state caused by dilution of circulating anticoagulant factors; the clinical significance of this is not currently known. Although many of the crystalloids being examined for in vivo clinical usage during the 1800s had a composition much closer to that of plasma, Hamburger ascertained using in vitro red cell lysis experiments that 0. Many of the fluids available currently were developed several decades ago and entered clinical practice without rigorous analysis of their clinical benefits or knowledge of their effects at an organ or cellular level. Newer colloid solutions have been approved by regulatory authorities and entered widespread clinical usage based on relatively small trials of efficacy. In some cases, safety concerns such as the impact of colloidrelated renal dysfunction have been highlighted only by much later adequately powered trials. Plasma-Lyte, PlasmaVolume, Baxter International, Deerfield, Ill; Gelofusine, Gelaspan, Venofundin, Sterofundin, and Tetraspan, B Braun (Melsungen, Germany); Plasmion, Geloplasma, Voluven, and Volulyte, Fresenius-Kabi, Bad Homburg, Germany; Hextend, BioTime, Berkeley, Calif; Pentaspan from Bristol-Myers Squibb, Canada; Hemosol, Hosptal, Rugby, U. The NaCl content and osmolarity of albumin solutions varies dependent on formulation. Although important differences in clinical outcomes in the surgical populations are not clear,38 in the wider critical care population an increased incidence of kidney injury and requirement for renal replacement therapy are seen when compared with the use of lower Cl- solutions. These side effects mean that the volume of saline administered perioperatively should be limited, unless there are compelling indications such as the following: · Situations in which increased plasma Na+ may be beneficial, such as in the presence of cerebral edema. Although it has not been studied extensively in the perioperative phase, use of hypertonic saline for trauma resuscitation, particularly in the prehospital phase, has been considered, with no convincing benefit. The increased plasma osmolality reduced cerebral edema and increased intracranial pressure. The reduction in anionic content is compensated for by the addition of stable organic anionic buffers such as lactate, gluconate, or acetate. The measured osmolality of balanced solutions (265 mOsm/kg) is slightly lower than that of plasma, and they are therefore mildly hypotonic. Fluid compartment distribution of balanced solutions is similar to that of other crystalloids. The metabolism of gluconate is less well characterized in terms of location and kinetics, but it is converted to glucose with subsequent entry into the citric acid cycle. The excretion of the excess water and electrolyte load with balanced crystalloids is more rapid than with isotonic saline. Some potential negative effects have been identified with balanced crystalloid solutions. Lactated Ringer solutions contain racemic (d- and l-) lactate, although d-lactate is only found in trace quantities in vivo. Concerns that large doses of d-lactate may be associated with encephalopathy and cardiac toxicity in patients with renal failure63,64 have not been confirmed in human studies at plasma levels achievable by the use of racemic lactated Ringer solution; the metabolism of d-lactate appears to be nearly as rapid as that of l-lactate. Concerns over the effects of excess exogenous acetate have been raised by the well-recognized syndrome of acetate intolerance experienced by patients undergoing hemodialysis with acetate-based dialysate. The proinflammatory, myocardial depressant, vasodilatory, and hypoxemia-promoting effects of high acetate levels manifest as nausea, vomiting, headaches, and cardiovascular instability and have led to the removal of acetate from contemporary dialysis fluids. It is therefore possible that critically ill patients or those with advanced kidney disease may exhibit biochemical acetate intolerance, although this possibility has not been explored in patients receiving acetate-based balanced crystalloids. Unlike acetate, much less is known about the effects of increased gluconate levels, which occurs with the infusion of fluids containing this as an anion. Dextrose solutions have the following two main indications in the perioperative setting: 1. As a source of free water: An infusion of 5% dextrose effectively represents administration of free water. In vitro osmolality is similar to that of plasma so that infusion does not lead to hemolysis, but soon after infusion the dextrose is taken up into cells in the presence of insulin, leaving free water. These solutions are therefore hypotonic with respect to the cell membrane and in excess can dilute plasma electrolytes and osmolality. Nevertheless, in carefully controlled volumes and with regular monitoring of serum electrolytes they are a useful source of free water for maintenance requirements postoperatively, particularly if combined with a low concentration of NaCl. Dextrose solutions are less suitable for intravascular plasma volume expansion, because water is able to move between all fluid compartments and a very small volume therefore remains in the intravascular space. Source of metabolic substrate: Although the caloric content of 5% dextrose is inadequate to maintain nutritional requirements, higher concentrations are adequate as a metabolic substrate, such as 4000 kCal/L for 50% glucose. Colloids Colloid is defined as large molecules or ultramicroscopic particles of a homogeneous noncrystalline substance dispersed in a second substance, typically isotonic saline, or a balanced crystalloid (see also Chapter 61). Although not all solutions are available in all countries, those in production include semisynthetic colloids and Chapter 59: Perioperative Fluid and Electrolyte Therapy 1783 human plasma derivatives. Semisynthetic colloids have a range of molecular sizes (polydispersed) in contrast to human albumin solution, which contains more than 95% albumin molecules of a uniform size (monodispersed). Colloid molecules above 70 kDa are too large to pass through the endothelial glycocalyx and are excluded from the subglycocalyx layer, with their initial volume of distribution as the plasma (rather than the entire intravascular) volume (see discussion of vascular endothelium). However, at normal or supranormal capillary pressures, hydrostatic pressure will be increased and transcapillary filtration will occur. Colloids alter blood rheology, typically improving blood flow by hemodilution effects, reductions in plasma viscosity, and red cell aggregation effects. In an attempt to limit these toxicities, maximum recommend doses are produced for most colloids, but adverse effects may still occur with smaller administered doses. As the potential clinical relevance of toxicity is highlighted by large clinical trials, the use of colloids, at least in critical care, is increasingly cautious. No known cases of variant Creutzfeldt-Jakob disease transmission have occurred involving pharmaceutical gelatin preparations. Gelatins are commonly used in perioperative practice in Europe but are not approved by the U. The pattern of substitution may vary because hydroxyethylation can occur at carbon positions 2, 3, or 6 of the glucose unit. The substitution type is defined by the C2/C6 hydroxyethylation ratio, and a higher ratio leads to slower starch metabolism. A proportion of larger molecules, particularly those resistant to hydrolysis, is taken up by the mononuclear phagocyte (reticuloendothelial) system, where they may persist for several weeks or more. Despite this, study populations with critical illness, particularly sepsis, cannot be compared directly with elective perioperative patients. However, the accumulation may persist for several years, and a larger degree of tissue deposition is associated with pruritus.

In these cases medicine hat news discount 600 mg praziquantel, the right internal jugular vein joins the persistent left superior vena cava by a bridging innominate vein medications you cant take with grapefruit cheap 600mg praziquantel fast delivery. The air-filled balloon tends to float to nondependent regions as it passes through the heart into the pulmonary vasculature symptoms 1 week after conception discount 600mg praziquantel mastercard. On occasion medicine garden 600mg praziquantel buy amex, a catheter may be floated to proper position when stiffened by injecting 10 to 20 mL of ice-cold solution through the distal lumen medicine man pharmacy generic praziquantel 600mg line. Finally, a catheter that is initially difficult to place may be positioned easily when hemodynamic conditions change, as commonly occurs after induction of general anesthesia and initiation of positive-pressure ventilation. Arrhythmias are the primary complication observed during pulmonary artery catheterization. Shah and associates observed transient premature ventricular contractions in 68% and atrial dysrhythmias in 1. However, the position of the catheter tip should always be checked by observation of the pressure waveform and chest radiograph to identify catheters that have migrated back into the right ventricle. In these patients, complete heart block may be precipitated, although this is rare. Shah and associates catheterized 113 patients with preexisting left bundle branch block; only 1 patient developing complete heart block (0. Although gross structural defects in the catheter itself should be recognized by inspection of the catheter before insertion, more subtle manufacturing problems may escape detection. Knots may be untied by radiologists using intravascular snares and fluoroscopic guidance. Procedural errors include unnecessary catheter manipulation, excessive insertion depth, unrecognized persistent wedge pressure, prolonged balloon inflation, or improper balloon inflation with liquid rather than air. This problem is more common during cardiopulmonary bypass, owing to the repeated cardiac manipulations and temperature changes that alter the stiffness of the catheter. If the visceral pleura fails to contain the bleeding, free rupture into the pleural space produces a large hemothorax. Although its initial appearance may be confused with catheter-related pulmonary infarction, the pattern of resolution and clinical course differentiate these diagnoses. The first priority is ensuring adequate oxygenation and ventilation and may require endobronchial intubation with either a single- or double-lumen endotracheal tube to selectively ventilate and protect the unaffected lung. A bronchial blocker may be guided into the involved bronchus to tamponade the bleeding and prevent contamination of the uninvolved lung. In 1990, Iberti and colleagues reported the results of a 31-question multiple-choice examination given to 496 resident and staff physicians in medicine, surgery, and anesthesiology departments, who practiced in 13 North American medical centers. Premature ventricular beats are common during this period as the balloon-tipped catheter strikes the right ventricular infundibular wall. Entry into the pulmonary artery is heralded by a step-up in diastolic pressure and a change in waveform morphology. However, careful observation of the pressure waveforms, focusing on the diastolic pressure contours, allows differentiation. As a practical matter, though, the pulmonary and systemic arterial pressure waveforms appear to overlap on the bedside monitor. As noted earlier, the wedge pressure is an indirect measurement of pulmonary venous pressure and left atrial pressure and should therefore resemble these venous waveforms with characteristic a and v waves and x and y descents. Additionally, atrial depolarization originates in the sinoatrial node located at the junction of the superior vena cava and the right atrium, and therefore the left-sided a wave appears slightly later than the right-sided a wave. Tall left atrial a or v waves will distort the normal pulmonary 1 sec R artery pressure waveform appearance, with the a wave inscribed at the onset of the systolic upstroke and the v wave distorting the dicrotic notch290,291. However, pulmonary capillary pressure must not be confused with wedge pressure or left atrial pressure, nor should the term pulmonary capillary wedge pressure be used at all. This is the pressure that must exceed left atrial pressure in order to maintain antegrade blood flow through the lungs. Although the magnitude of the difference between pulmonary capillary pressure and wedge pressure is generally small, it can increase markedly when resistance to flow in the pulmonary veins is elevated. However, rare conditions like pulmonary venoocclusive disease may cause a marked increase in postcapillary resistance to flow. Similar situations arise in conditions that disproportionately increase pulmonary venous resistance, such as central nervous system injury, acute lung injury, hypovolemic shock, endotoxemia, and norepinephrine infusion. Artifactual pressure spikes may be distinguished from the underlying physiologic pressure waveform by their unique morphology and timing. If the monitor detects this inappropriate pressure nadir, it may be erroneously designated as the pulmonary artery diastolic pressure. This phenomenon is termed overwedging and usually is caused by distal catheter migration and eccentric balloon inflation that forces the catheter tip against the vessel wall. The catheter now records a gradually rising pressure as the continuous flush system builds up pressure against the obstructed distal opening. For a catheter that has migrated to a more distal position, it is possible for overwedging to occur without balloon inflation. Note that the overwedged pressure is devoid of pulsatility, is higher than expected, and increases continuously because of the continuous flush pressure. The catheter should be withdrawn before overwedging results in vascular injury or pulmonary infarction. Pathophysiologic conditions involving the left-sided cardiac chambers or valves produce characteristic changes in the pulmonary artery and wedge pressure waveforms. One of the most easily recognized patterns is the tall v wave of mitral regurgitation. Unlike a normal wedge pressure v wave produced by late systolic pulmonary venous inflow, the prominent v wave of mitral regurgitation begins in early systole. Mitral regurgitation causes fusion of c and v waves and obliteration of the systolic x descent, as the isovolumic phase of left ventricular systole is eliminated owing to the retrograde ejection of blood into the left atrium. Although mean wedge pressure exceeds left ventricular end-diastolic pressure in patients with severe mitral regurgitation, it remains a good approximation for mean left atrial pressure and the subsequent risk of hydrostatic pulmonary edema. When large v waves are present in the wedge pressure trace, it is critically important to recognize them and be able to distinguish the wedged from the unwedged pressure waveform. After the catheter is withdrawn slightly, balloon inflation allows proper wedge pressure measurement (third arrow). The pulmonary artery pressure upstroke is steeper and slightly precedes the systemic arterial pressure upstroke, whereas a wedge tracing with a prominent v wave has a more gradual upstroke that begins after the radial artery pressure upstroke. Another distinguishing feature in patients with severe mitral regurgitation is the unusual morphology of the pulmonary artery waveform itself. The larger the regurgitant v wave, the more it distorts the pulmonary artery waveform, giving it a bifid appearance and obscuring the normal end-systolic dicrotic notch291. Given that the left atrial pressure-volume relation is not linear, the same volume of regurgitation will result in a variable increment in systolic pressure, depending on the preexisting atrial volume at onset of systole. Although the total regurgitant volume of blood entering the left atrium will influence the height of the v wave, this clearly is not the only determinant of v wave magnitude. This may explain why patients with acute mitral regurgitation tend to have tall wedge pressure v waves-they have smaller, stiffer left atria with poorer compliance compared with those of patients with longstanding disease. It is not surprising that wedge pressure v waves are neither sensitive nor specific indicators of mitral regurgitation severity, and the height of these waves should not be used in such a manner. In this condition, the holodiastolic pressure gradient across the mitral valve results in an increased mean wedge pressure, a slurred early diastolic y descent, and a tall end-diastolic a wave. Similar hemodynamic abnormalities are seen in patients with left atrial myxoma or whenever mitral flow is obstructed. Left atrial pressure-volume curves describe the three factors that determine v wave height. For the same regurgitant volume (x), the left atrial v wave will be taller if baseline atrial volume is greater (point B versus point A). For the same regurgitant volume (x), the left atrial v wave will be taller if baseline atrial compliance is reduced (point B versus point A). Beginning at the same baseline left atrial volume (points A and B), if regurgitant volume increases (X versus x), the left atrial pressure v wave will increase (V versus v). In these conditions, mean wedge pressure is increased and the trace displays a prominent a wave, but the y descent remains steep, because there is no obstruction to flow across the mitral valve during diastole. Because patients with advanced mitral stenosis often have coexisting atrial fibrillation, the a wave will not be present in many of these cases226. Ischemia itself impairs left ventricular relaxation resulting in diastolic dysfunction, a pattern particularly characteristic of demand ischemia associated with tachycardia or induced by rapid atrial pacing. Not only does this, in turn, increases left atrial and wedge pressures, but the morphology of these waveforms changes as well, with the phasic a and v wave components becoming more prominent as diastolic filling pressure increases. In patients with left ventricular ischemia, the tall wedge pressure a wave is produced by end-diastolic atrial contraction into a stiff, incompletely relaxed left ventricle. Myocardial ischemia also produces a characteristic pattern of left ventricular systolic dysfunction. Systolic dysfunction is the hallmark of supply ischemia, caused by a sudden reduction or cessation of coronary blood flow to a region of the myocardium. As ejection fraction falls significantly, left ventricular end-diastolic volume and pressure rise and systemic arterial hypotension and elevated pulmonary diastolic and wedge pressures develop. Although patients with left ventricular ischemia are likely to have higher mean wedge pressures than those without ischemia, these differences are small and may be difficult to detect clinically. In restrictive cardiomyopathy and right ventricular infarction, diastolic dysfunction impairs ventricular relaxation and decreases chamber compliance, whereas in constrictive pericarditis cardiac filling is limited by the rigid, often calcified pericardial shell. Impaired venous return decreases end-diastolic volume, stroke volume, and cardiac output. Despite reduced cardiac volumes, cardiac filling pressures are markedly elevated and equal in all four chambers of the heart at end-diastole. In tamponade, the venous pressure waveform appears more monophasic and is dominated by the systolic x pressure descent. The diastolic y pressure descent is attenuated or absent, because early diastolic flow from right atrium to right ventricle is impaired by the surrounding compressive pericardial fluid collection321,325,326. Clearly, other clinical and hemodynamic clues help distinguish these diagnoses, such as the presence of pulsus paradoxus, an almost invariable finding in cardiac tamponade327. Coexisting abnormalities such as tachycardia, arrhythmias, and atrial contractile failure may complicate interpretation of these waveforms. On occasion, localized pericardial constriction may simulate valvular stenosis, and hypovolemia may lower cardiac filling pressures to within the normal range and confound the diagnosis. During positive-pressure ventilation, inspiration increases pulmonary artery and wedge pressures. By measuring these pressures at end-expiration, the confounding effect of this inspiratory increase in intrathoracic pressure is minimized328. Forceful inspiration during spontaneous ventilation has the opposite effect, but again, measurement of these pressures at end-expiration eliminates this confounding factor. Bedside monitors are designed with algorithms that aim to identify and report the numeric values for end-expiratory pressures but are often inaccurate. Because resistance in the large pulmonary veins is negligible, pulmonary artery wedge pressure provides an indirect measurement of both pulmonary venous pressure and left atrial pressure. This is acceptable under normal circumstances because when pulmonary venous resistance is low, the pressure in the pulmonary artery at end of diastole will equilibrate with downstream pressure in the pulmonary veins and left atrium. The central venous pressure waveform shows an increased mean pressure (16 mm Hg) and attenuation of the y descent. Influence of positive-pressure mechanical ventilation on pulmonary artery pressure. Pulmonary artery pressure should be measured at end expiration (1, 15 mm Hg) in order to obviate the artifact caused by positive-pressure inspiration (2, 22 mm Hg). At the microcirculatory level, this channel consists of pulmonary capillaries that are subject to external compression by surrounding alveoli. West and associates described a three-zone model of the pulmonary vasculature based on the gravitationally determined relationships between relative pressures in the pulmonary arteries, pulmonary veins, and surrounding alveoli. In most clinical settings, the supine position of the patient favors zone 3 conditions, a finding that has been confirmed by radiographic studies. Even when surrogate pressures such as pulmonary artery diastolic pressure and wedge pressure accurately estimate left ventricular end-diastolic pressure, many factors can influence the relationship between end-diastolic pressure and end-diastolic chamber volume, which is the true preload. For example, a pulmonary artery wedge pressure of 20 mm Hg is somewhat higher than normal, but depending on its interpretation and the clinical setting, different treatments would be indicated. Proper interpretation of filling pressures requires assessment of juxtacardiac pressure and ventricular compliance. However, different conclusions are reached if juxtacardiac pressure is increased, for example, as a result of cardiac tamponade, pericardial constriction, or positive-pressure ventilation. Ventricular interdependence and pericardial constraint couple changes in right and left ventricular function, such that a primary change in right ventricular filling may produce a secondary and opposite change in left ventricular filling by altering its diastolic pressure-volume relation. Conversely, primary changes on the left side can adversely affect the right heart structures in similar ways. Under these two situations, a wedge pressure of 20 mm Hg can coexist with a small, hypovolemic left ventricle. An intravenous bolus of crystalloid or colloid solution (250 to 500 mL) is given over 15 minutes, and the change in wedge pressure is measured. Note that these calculations of systemic and pulmonary vascular resistance are based on a hydraulic fluid model that assumes continuous, laminar flow through a series of rigid pipes. A more physiologic model of the systemic circulation considers the vasculature to be a series of collapsible vessels with intrinsic tone. A detailed consideration of these issues is beyond the scope of this discussion and is available in other sources. Additional problems arise in considering the pulmonary vasculature and using the formulas as a measure of resistance to flow through the lung. On occasion, the systemic and pulmonary vascular resistances are indexed as well (systemic vascular resistance index = systemic vascular resistance × body surface area; pulmonary vascular resistance index = pulmonary vascular resistance × body surface area). In theory, normalizing hemodynamic values through "indexing" should help clinicians determine appropriate normal physiologic ranges to help guide therapy. Unfortunately, there is little evidence that these additional calculations provide valid normalizing adjustments.

Recently symptoms pregnancy praziquantel 600 mg order on line, interest in chloroprocaine has increased for use in spinal anesthesia for ambulatory surgery (see Chapter 89) treatment quadriceps tendonitis order 600mg praziquantel visa. Modern 5 medications for hypertension order 600mg praziquantel mastercard, preservative-free preparations of chloroprocaine administered in small doses (30 to 60 mg) produce reliable medications zanaflex cheap praziquantel uk, short-duration spinal anesthesia medicine zolpidem buy praziquantel 600mg overnight delivery,126 with a faster recovery time than procaine, lidocaine, and bupivacaine. Articaine is a relatively novel amide local anesthetic that also has an ester linkage. It has been widely used since 1973 for dental nerve blocks with a good safety profile. These in turn are dictated in part by the pKa, lipid solubility, and protein binding of the local anesthetic solution. The choice and dose of local anesthetic depend on both the expected duration and the nature (location, ambulatory) of surgery. Table 56-4 shows a range of local anesthetics used for spinal anesthesia with corresponding doses, onset times, and durations of action. Note that duration depends on how the regression of the block is measured, which varies widely between studies. Chapter 56: Spinal, Epidural, and Caudal Anesthesia 1697 provide rapid-onset spinal anesthesia for about 1 hour, with a recovery profile faster than bupivacaine. Lidocaine is a hydrophilic, relatively poorly protein-bound amide local anesthetic. It has a rapid onset and intermediate duration and is used in doses of 50 to 100 mg for shorter procedures that can be completed in 1. Despite efforts to reduce the concentration of both the drug and dextrose,150,151 the use of intrathecal lidocaine declined and has not yet recovered. Prilocaine was introduced in 1965 and has an intermediate duration of action that may lend itself to use in the ambulatory surgery setting. This should not be an issue with doses used for spinal anesthesia, but it has been reported after epidural infusions. It was first introduced for spinal anesthesia in 1962 and was initially prepared as a hyperbaric solution. Recovery profiles using small doses appear to be similar to that of lidocaine168-170 and thus low-dose bupivacaine is used in ambulatory procedures. A recent systematic review171 concluded that 4 to 5 mg of hyperbaric bupivacaine combined with unilateral positioning was adequate for short knee arthroscopy procedures. Although it is used in similar doses to bupivacaine and has a similar onset and duration, levobupivacaine potency appears to be slightly less than bupivacaine. Ropivacaine was introduced in 1996 and is another highly protein-bound amide local anesthetic. Compared to bupivacaine, the proposed advantages of spinal ropivacaine were less cardiotoxicity and greater motor-sensory block differentiation, resulting in less motor block. As such, the coadministration of these agents often allows for a reduction in the required dose of local anesthetic, with the advantage of motor block sparing and faster recovery while still producing the same degree of analgesia. The effect at each of these sites depends on both the dose administered and the physicochemical properties of the opioid, particularly lipid solubility. In addition to increasing uptake into neural tissue, greater lipid solubility results in rapid uptake into both blood vessels (with a resultant systemic effect) and fatty tissue. Tetracaine is an ester local anesthetic with a rate of metabolism one tenth that of chloroprocaine. It is packaged either as niphanoid crystals (20 mg) or as an isobaric 1% solution (2 mL, 20 mg). When niphanoid crystals are used, a 1% solution is obtained by adding 2 mL of preservative-free sterile water to the crystals. Tetracaine is usually combined with a vasoconstrictor additive because the duration of tetracaine alone can be unreliable. Bupivacaine was introduced in 1963 and is a highly protein-bound amide local anesthetic with a slow onset because of its relatively high pKa. As a result, hydrophilic opioids have a greater risk of late respiratory depression, which is one of the rare but most serious consequences of intrathecal opioid administration. The extent of neural tissue and vascular uptake also affects the potency of intrathecal opioids. For example, the relative intrathecal to intravenous potency of morphine is 200 to 300 to 1, whereas for fentanyl and sufentanil it is only 10 to 20 to 1. Preservative-free morphine is the most widely used hydrophilic opioid in spinal anesthesia. Doses as high as 500 g may be used for major abdominal surgery or thoracotomies, where it is becoming increasingly common to administer spinal opioids alone as a simple alternative to epidural local anesthetic­based analgesia. Overall, the beneficial effects of intrathecal morphine seem most marked in abdominal surgery, and within the first 24 hours in particular. Once in the dorsal horn of the spinal cord, it is converted to morphine and 6-monoacetyl morphine, both of which are -agonists with a relatively long duration of action. There are only limited data related to the use of hydromorphone for spinal analgesia. Limited data suggest that intrathecal hydromorphone 50 to 100 g provides comparable analgesia with similar side effects to 100 to 200 g of morphine, with a similar duration of action. However, it has not undergone full neurotoxicity screening and does not provide any advantage compared with morphine. Both 10 mg and 20 mg improve analgesia compared with placebo after cesarean delivery,193 although side effects were more frequent with the larger dose. However, this drug is used infrequently because of the availability of other opioids and its unknown neurotoxicity profile. Fentanyl and sufentanil are used frequently in obstetrics for labor analgesia and cesarean delivery as discussed elsewhere (also w 77). Sufentanil 2 to 10 g and fentanyl 25 g provide comparable analgesia in early labor. Although the local anesthetic dose can be reduced and analgesia prolonged,199 the addition of fentanyl to bupivacaine may increase side effects and delay discharge. Vasoconstrictors, such as epinephrine and phenylephrine, prolong the duration of sensory and motor blockade when added to local anesthetics. The mechanism of action is reduced systemic local anesthetic uptake caused by an 1-mediated vasoconstriction. However, lidocaine spinal anesthesia can be prolonged by epinephrine when measured by both two-dermatome regression in the lower thoracic dermatomes and by occurrence of pain at the operative site for procedures carried out at the level of the lumbosacral dermatomes. There is a concern that potent vasoconstrictive action places the blood supply of the spinal cord at risk. However, there are no human data supporting this theory, and in animal studies,164,202-204 administering either subarachnoid epinephrine (0. Phenylephrine 2 to 5 mg prolongs both lidocaine and tetracaine spinal anesthesia to a similar extent as epinephrine. Caldwell and associates163 used larger doses of vasoconstrictors, epinephrine at 0. Clonidine, dexmedetomidine, and epinephrine all act on prejunctional and postjunctional 2 receptors in the dorsal horn of the spinal cord. Activation of presynaptic receptors reduces neurotransmitter release, whereas postjunctional receptor activation results in hyperpolarization and reduction of pulse transmission. A systematic review concluded that the hypotension associated with spinal clonidine was not dose-related and that the risk of bradycardia with clonidine was not increased. Neostigmine in doses of 10 to 50 g has analgesic effects after intrathecal administration. Neostigmine inhibits the breakdown of acetylcholine, therefore increasing acetylcholine concentration, which itself is antinociceptive. Its benefits, however, are limited by nausea, vomiting, bradycardia, and, in higher doses, lower extremity weakness,222,223 and is therefore not in widespread use. Early work raised concerns of spinal cord toxicity, but more recent studies suggest that it is safe. Scanning electron micrographs of spinal needle tip designs: Quincke (left), Sprotte (middle), and Whitacre (right). Informed consent must be obtained, with adequate documentation of the discussion of risk (see Complications, discussed later). Resuscitation equipment must always be readily available whenever a spinal anesthetic procedure is performed. The patient should have adequate intravenous access and be monitored with pulse oximetry, noninvasive arterial blood pressure, and electrocardiogram. Pre-prepared packs are now commonly used and often contain fenestrated drapes, swabs and towels, syringes, needles, filters, spinal needles, sterilizing solution, and local anesthetic for skin infiltration. When the local anesthetic for subarachnoid injection is chosen, the duration of block should be matched with both the surgical procedure and patient variables (see Table 56-4). Needle tip shapes fall into two main categories: those that cut the dura and those with a conical, pencil-point tip. The former include the Pitkin and the Quincke-Babcock needle, and the Whitacre and Sprotte needles belong to the latter group. If a continuous spinal technique is chosen, use of a Tuohy or other thin-walled needle can facilitate passage of the catheter. The use of small needles reduces the incidence of post­dural puncture headache from 40% with a 22-G needle to less than 2% with a 29-G needle. The use of larger needles, however, improves the tactile sense of needle placement, and so although 29-G needles result in a very low rate of post­dural puncture headache, the failure rate is increased. Pencil-point needles of 25, 26, and 27 G probably represent the optimal needle choice. An introducer needle can assist with guidance of smaller-gauge spinal needles in particular. These have been designed to prevent inadvertent intrathecal injection but still rely on the correct drug being drawn up into the "special" connector syringe. One of the most common organisms responsible for postspinal bacterial meningitis is Streptococcus viridans, which is an oral commensal, emphasizing the purpose of wearing a mask as part of a full aseptic technique. A variety of solutions may be used to clean the back, such as chlorhexidine or alcohol (alone or in combination), or iodine solutions. In the obstetric population, there have been small studies demonstrating that block operator performance was faster in the sitting position, albeit this benefit was offset by a slower onset time compared with the lateral decubitus position232 (see Chapter 77). Current consensus guidelines state that neuraxial blocks should be undertaken with the patient awake,76 except in those circumstances where the physician and patient conclude that benefit outweighs the risk. General anesthesia or heavy sedation can prevent a patient from recognizing warning signs of pain or paresthesia if the needle is in close proximity to nervous tissue. A patient in the lateral decubitus position facilitates the administration of sedative medication if required, is less dependent on a well-trained assistant than for a patient in the sitting position, and is arguably more comfortable. Patients are placed with their back parallel to the edge of the operating table nearest the anesthesiologist, thighs flexed onto the abdomen, with the neck flexed to allow the forehead to be as close as possible to the knees in an attempt to "open up" the vertebral spaces. The assistant may still be invaluable during this positioning by encouraging and assisting the patient in assuming the ideal lateral decubitus position. Because of the differing proportional sizes of hips and shoulders, the spine may slope down toward the head in females, with the opposite occurring in males. Identification of the midline may be easier when the patient is placed in the sitting position, especially when obesity or scoliosis renders midline anatomy difficult to examine. When placing patients in this position, a stool can be provided as a footrest and a pillow placed in the lap, or a specially designed stand may be used. The prone position is rarely used but may be chosen when the patient is to be maintained in that position (often with the jack-knife modification) during the surgical procedure. The midline approach relies on the ability of patients and assistants to minimize lumbar lordosis and allow access to the subarachnoid space between adjacent spinous processes, usually at the L2-L3, L3-L4, or the L4-L5 space. The spinal cord ends at the level of L1-L2 and so needle insertion above this level should be avoided. The intercristal line is the line drawn between the two iliac crests and traditionally corresponds to the level of the L4 vertebral body or the L4-L5 interspace, but the reliability of this landmark is questionable as demonstrated by recent ultrasonography studies. The needle, with its bevel parallel to the midline, is advanced slowly to heighten the sense of tissue planes traversed and to prevent skewing of nerve roots, until the characteristic change in resistance is noted as the needle passes through the ligamentum flavum and dura. The paramedian approach exploits the larger "subarachnoid target" that exists if a needle is inserted slightly lateral to the midline. The most common error when using the paramedian technique is that the needle entry site is placed too far off midline, which makes the vertebral laminae barriers to insertion of the needle. In the paramedian approach, a skin wheal is raised 1 cm lateral and 1 cm caudad to the corresponding spinous process. The spinal introducer and needle are next inserted 10 to 15 degrees off the sagittal plane in a cephalomedial plane. Similar to the midline approach, the most common error is to angle the needle too far cephalad on initial insertion. Nevertheless, if the needle contacts bone, it is redirected slightly in a cephalad direction. A, the palpating fingers are "rolled" in a side-to-side and a cephalad-to-caudad direction to identify the interspinous space. B, During needle insertion, the needle should be stabilized in a tripod fashion while placed in the hand, similar to a dart being thrown. Vertebral anatomy of the midline and paramedian approaches to centroneuraxis blocks. The paramedian approach shown in the inset and in the posterior view requires an additional oblique plane to be considered, although the technique may be easier in patients who are unable to cooperate in minimizing their lumbar lordosis. The paramedian needle is inserted 1 cm lateral and 1 cm caudad to the caudad edge of the more superior vertebral spinous process. As in the midline approach, the characteristic feel of the ligaments and dura is possible, but only once the ligamentum flavum is reached because the needle is this time not passing through the supraspinous and interspinous ligaments. In obstetrics, it may also be used in patients with morbid obesity and where previous spinal surgery may hinder epidural spread. A midline or paramedian approach may be used, with some experts suggesting that use of the paramedian approach facilitates insertion of the catheter.

Despite the display of the waveform on a vertical axis medications such as seasonale are designed to praziquantel 600mg buy on line, remembering that the arterial pressure is not a transverse wave (such as a wave on the ocean) but a longitudinal wave (such as a sound wave or a pulse transmitted through a coiled spring) is important symptoms umbilical hernia praziquantel 600 mg purchase on line. For slowly changing pressures medicine ball exercises purchase praziquantel 600 mg mastercard, a water or mercury manometer is simple and dependable medicine kim leoni purchase 600mg praziquantel. The manometer cannot quickly respond to rapid changes in pressure because of its inertia; that is treatment 4 sore throat order cheap praziquantel line, the mass of the liquid column resists rapid changes in height. A, A peaked arterial waveform indicates some resonance with an overestimation of the systolic blood pressure. In arterial systems, this illustration clarifies the fact that energy transfer (blood pressure) does not mass transfer (blood flow). Systolic pressure is defined as the instantaneous maximal pressure; diastolic pressure is defined as the instantaneous minimum pressure; and the mean pressure is defined as the average pressure over a cycle. The mean pressure is estimated as the diastolic plus 1/3 the pulse pressure (systolic-diastolic) when only the systolic and diastolic pressures are known. The variable transducer electrical resistance is placed in a circuit involving three known resistances-Wheatstone bridge. The damper, shown schematically as a piston moving in oil, represents the friction generated by the fluid moving to and fro in the tubing. A more commonly encountered harmonic oscillator is that of a car driving down a bumpy dirt road. In this case, the bumps in the road provide the oscillating driving pressure, which forces the car wheels to oscillate up and down. The frequency of the driving force that causes maximal amplification of the signal is called the natural or resonant frequency. The degree of amplification is directly related to the mass and inversely related to the amount of friction present; for large amounts of friction, attenuation rather than amplification occurs (see Appendix 44-4). To visualize this concept intuitively, hang a weight on the end of a rubber band while holding the upper end of the band in your hand. If you move your hand up and down slowly, the weight follows your hand movements almost exactly. As you increase the frequency of your hand oscillations, the weight begins to lag behind your hand, and the amplitude of the weight movement begins to increase. As you approach the natural frequency of this simple system, you will observe the phenomenon of resonance when the amplitude of the weight motion becomes extremely large. If you try different rubber bands and weights, you will find that stiffer bands or smaller weights yield higher natural frequencies. Pressure measured in an invasive arterial catheter can actually overshoot or amplify the real blood pressure. This phenomenon is referred to as the dynamic frequency response of the fluid-filled arterial line and transducer system. This phenomenon has a physical model, which can generate an equation to predict the output pressure response, depending on the frequency of the input pressure and several physical parameters of the system. Depending on the input frequency, the output may go through an amplification as it reaches a specific frequency, known as the resonant frequency of the system. In the top row, a common phenomenon is noted when a car drives along a bumpy dirt road. In this situation, the driving forces are the bumps in the road, which act on the tire. The car spring is equivalent to the compliance of the pressure tubing, and the shock absorber corresponds to the resistance of fluid moving back and forth in the arterial line. You may have experienced the phenomenon in which you reach a certain speed as you are driving along a bumpy road when the front of the car starts to oscillate with increasing amplitude. The car bounces highest when you have reached the resonant frequency of this harmonic oscillator (see Appendix 44-4 for detailed mathematical description of this process). As the frequency increases, the amplification can increase to a maximum, and then the signal becomes attenuated. In most clinical systems, the natural resonant frequency is 10 to 15 Hz, which is significantly higher than the primary frequency of the arterial waveform (the heart rate is 60 to 120 bpm or 1 to 2 Hz). The higher frequency components of the arterial waveform (higher harmonics) are those that are closer to the natural frequency of the system and are therefore amplified. This is why a whip is seen in the waveform when the peak systolic pressure and the initial upstroke are significantly amplified above the true systolic pressure, Amplitude Chapter 44: Fundamental Principles of Monitoring Instrumentation 200 Cuff pressure (mm Hg) 1325 150 Systolic pressure Mean pressure 100 50 Diastolic pressure are not related to arterial pressure and may result in an erroneous reading, most commonly a higher diastolic pressure. Mercury sphygmomanometers are being phased out of use in most countries and hospitals, which leads to questions regarding the accuracy and precision of alternative devices such as the aforementioned automated noninvasive blood pressure monitors and aneroid sphygmomanometers. Unlike electromagnetic waves such as light (see "Measurement Using Light Energy" later in this chapter), sound cannot propagate in a vacuum. In an active examination, acoustic energy is transmitted into the patient, and the resulting interaction of this energy with the patient is analyzed for information. In 1842, Christian Johann Doppler first described the apparent change in pitch of a sound that occurs when either the source of the sound or the listener is moving. This Doppler effect now has several applications in patient monitoring, including precordial and esophageal Doppler ultrasound monitoring of local blood velocities or cardiac output. When a sound source is moving toward the listener, the apparent pitch increases, and vice versa. The exact amount of frequency shift depends on whether the listener or the sound source is moving. Because changes in the frequency of sine waves can be precisely measured, the Doppler principle provides an accurate method of measuring the velocities of moving sound reflectors. Using the signal from the arterial pulse, oscillometric blood pressure measurements are obtained by determining the point at which the signal is first detected, its maximal amplitude, and the signal decay rate. In theory, mean arterial pressure should be the same because this amplification of systolic pressure also produces a reduction in diastolic pressure (see Appendix 44-4). Signal-Processed Pressure Measurement (Noninvasive Blood Pressure Monitor) Systolic pressure can be estimated by noting the return of the flow pulse after occlusion of the brachial artery by a cuff. The return of flow can be detected by (1) simple palpation of the radial artery, (2) recording with a Doppler device over the radial artery, or (3) the use of a pulse oximeter. Most anesthesiologists are familiar with the loss of pulse oximeter signal when the noninvasive blood pressure monitor is cycling. The automated noninvasive blood pressure monitoring devices in surgical units use a more sophisticated application of this principle. These devices monitor the oscillating signal generated in the cuff by the arterial pressure changes. The cuff first inflates to above systolic pressure, at which point the signal and oscillations are abolished. The pressure at which the oscillating pressure signal first appears is interpreted as the systolic pressure. The point at which the signal is at maximal amplitude is interpreted as mean arterial pressure. Thus bronchial breath sounds are better heard when the bronchi are surrounded by lung consolidation. A, When a listener is moving toward a stationary sound source, the frequency increases because the listener transverses more waves per unit time than a stationary listener. B, When a sound source is moving toward a stationary listener, the wavefronts "stack up," causing an apparent increased frequency. Some simple facts about sound waves can facilitate an understanding of the reflection and scattering process in the body. First, all sound waves can be represented as a summation of sinusoidal waves of various frequencies and amplitudes. The fundamental frequency describes the pitch of the tone-middle C is standardized at 256 Hz, for example. Fortunately for our ears, all of these many frequencies propagate at the same speed, the speed of sound, called a. For ideal gases, the speed of sound is proportional to the square root of temperature. The speed of sound in air at room temperature is 344 m/sec, or 1129 feet per second (ft/sec), or 770 miles per hour (mph). At an altitude of 13,000 m (40,000 ft), where the standard air temperature is -57° C, the speed of sound is only 295 m/sec or 661 mph. This value also approximates the speed of sound through most of the solid parts of the human body. In other solids, the speed of sound greatly varies, with a range of 54 m/sec in rubber to 6000 m/sec in granite. Reflection of sound occurs at interfaces where the product of the density and the speed of sound (× a) suddenly changes. Larger changes in this acoustic impedance result in greater reflection and less transmission. In the human body, the largest changes in acoustic impedance occur at gas-tissue boundaries: the lungs and the gastrointestinal tract. Reflection of sound by the lungs thus makes it difficult to auscultate heart tones through an air-filled, emphysematous chest. For the same reason, a transthoracic echocardiograph provides less detail than the transesophageal technique; in the former case, the lungs are in the way. Although this procedure had many limitations, it led to the development of the modern stethoscope, which is based on the physical principles of sound transmission. The stethoscope uses a large diaphragm to transmit and concentrate the sound energy. The bell acts as both an amplifier and a low-pass filter to transmit low-frequency diastolic rumbles. Because it is a nonpowered, nontechnologic device (the energy levels come from the phenomena themselves), an esophageal or precordial stethoscope has unique value in being a continuous monitor during power outages. Physical limits to the technique include air space disease (in itself an informational finding), an inability to place the monitor appropriately, and a lack of quantifiable data. Active Sound Examination (Percussion, Echo, Doppler) the earliest active acoustic diagnostic technique was percussion of the chest wall. A skilled clinician can use this method to detect consolidation of the lungs, pleural effusion, and a few other chest pathologic conditions. Although based on transmission and the reflection of sound, percussion is purely qualitative and is unable to localize pathologic changes accurately. Modern ultrasound improves on percussion by using shorter-wavelength sound waves and quantitative detection of their reflections (echoes). With the use of esophageal transducers, echocardiography has become a popular intraoperative monitoring technique. After each pulse, the transducer passively listens to the reflected echoes from various tissues. The ability to place the transducer in the esophagus is advantageous because sound does not then have to pass through air spaces or bone on its way to and from the heart. The speed of sound through the heart and surrounding soft tissues is a nearly constant 1540 m/sec. Thus the exact measurement of the elapsed time between transmission of the pulse and receipt of the echo provides the distance to the reflecting structure. The sound beam from the transducer Chapter 44: Fundamental Principles of Monitoring Instrumentation 1327 is projected in a narrow searchlight pattern; therefore the exact direction of reflecting structures is also known. The Doppler effect is used in echocardiography to determine the presence and degree of valvular regurgitation by converting the Doppler shift of sound waves reflected from erythrocytes into a color display (see Appendix 44-7). Cardiac output can also be estimated from descending thoracic aortic blood velocity by using a Doppler technique. These devices estimate the blood flow in the descending aorta and ignore flow to the head and arms. They calibrate descending aortic flow to cardiac output by assuming a constant proportional relationship between the two flows. The quantity of charge on an electron is determined by balancing the electric force on an oil drop against the gravitational force on the same drop. Nearly all transducers use some form of electrical energy as their output, and the subsequent data processing and display are entirely electrical. Some basic electromagnetic principles are reviewed in this section using examples from medical equipment. Electromagnetic waves, including light, are transverse waves, meaning that the electric and magnetic field vectors are oriented perpendicular to the direction of wave propagation. Direct Current Just as mechanical energy may be stored as potential energy, electrical energy can be stored as a potential difference. A common analogy is to compare electrical potential difference with water pressure. The potential difference between points A and B is defined as the work required to move a unit charge from A to B. Charges can easily move through conductors, but they do not move well through insulators, also called dielectrics. If a potential difference (V) exists between A and B and a conductor connects these two points, then charges will flow between them and produce an electrical current (I). If an insulator separates points A and B, then no current will flow until the potential difference becomes so great that a breakdown of the insulator occurs. For example, if dry air separates A and B, then no current will flow until the potential difference reaches 3000 V/mm. At this very large potential gradient, air will ionize and become a conductor, and a current flows in the form of a visible (and audible) spark. To generate a spark between two electrodes 1 cm apart, a potential difference of 30,000 V must be created. The battery can provide a continuous source of electrons, or current, to flow through any conducting circuit connected between its electrodes. Flow of electricity is opposed by resistance, analogous to resistance in water pipes. Static electricity involves charges at rest; like charges repel one another, whereas opposite charges attract. Usually the meaning of the word electricity involves the flow of charges, or electrical current.

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