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Automaticity of cardiac myocytes is increased when the slope of phase 4 depolarization increases allergy symptoms of amoxicillin purchase astelin 10 ml visa, with a shift of threshold potentials to more negative values allergy symptoms chest congestion order cheapest astelin and astelin, or in the presence of more positive maximal diastolic potentials. The sinus node is the primary intrinsic pacemaker, and spontaneous depolarization leads to action potential generation, with normal resting rates of 60 to 100 beats per minute. Cardiac myocytes are joined by electrical synapses called gap junctions, which permit the flow of intracellular current from cell to cell. Chapter 9 CardiacArrhythmias 111 ClassificationofArrhythmias Mechanistically, cardiac arrhythmias can be broadly divided into disorders of action potential formation and disorders of impulse conduction. Clinically, arrhythmias are classified as bradycardias and tachycardias, with further categorization according to arrhythmia origin. ElectrophysiologicMechanisms ofArrhythmias Automaticity is a normal function of pacemaker cells, occurring during phase 4 depolarization. Enhanced automaticity occurs when pacemaker cells depolarize at a faster rate due to an increased slope of phase 4 depolarization, a shift of threshold potential to a more negative value, or a shift of the maximal diastolic potential to a more positive value. Spontaneous depolarization occurring in nonpacemaker cardiac myocytes is called abnormal automaticity. Conditions as ischemia, electrolyte abnormalities, and sympathetic stimulation may produce abnormal automaticity. Triggered activity occurs when secondary cardiac depolarizations are initiated by prior depolarizations. If these secondary depolarizations reach threshold potential, they may generate action potentials during or immediately after phase 3 of the action potential. Reentry describes the reexcitation of a localized region of cardiac tissue by the same impulse, requiring bifurcating conduction pathways with different velocities and refractory periods. To permit reentry, unidirectional block in one pathway and slowed conduction in the other are required. Reentry is further categorized as anatomic, circling around a fixed anatomic obstacle, or functional, in which the inexcitable center of a reentrant circuit is not fixed but functionally refractory. A normally timed impulse enters the two pathways through the proximal common pathway, conducting rapidly down A and slowly down B. As the impulse from pathway A reaches the distal common pathway, while continuing distally, it may also turn around to activate B retrogradely. This impulse collides with the slowly conducting antegrade impulse in pathway B, extinguishing the impulse. However, a sufficiently premature stimulus may enter the proximal common pathway, finding pathway A with its long refractory period inexcitable, traveling slowly down pathway B, and finally reaching the distal common pathway. Due to the slow conduction velocity in pathway B, pathway A may no longer be refractory, and the impulse may successfully travel retrograde up pathway A, potentially repeatedly activating the circuit. For a deeper discussion on this topic, please see Chapter 61, "Principles of Electrophysiology," in Goldman-Cecil Medicine, 25th Edition. Ambulatory recording devices permit electrocardiographic monitoring over longer periods to establish symptom-rhythm correlations. External event monitors or loop recorders, which can be worn for 30 days, store electrograms when triggered by patients for symptoms or are autoactivated based on heart rate detection above or below a programmed threshold value. External loop recorders are intended to identify cardiac rhythm disturbances underlying infrequent symptoms. For patients with arrhythmia symptoms occurring less than once per month, implantable loop recorders may be useful. With a 3-year anticipated battery longevity, implantable loop recorders are valuable in establishing the cause of recurrent infrequent syncope. Electrophysiologic Testing To perform electrophysiologic studies, temporary transvenous pacing catheters are positioned in multiple locations in the heart, permitting pacing and recording of intracardiac electrograms. Catheters are typically placed in the right atrium, the right ventricle, close to the bundle of His, and in the coronary sinus for left atrial recording and pacing. Electrophysiologic studies can define the mechanism of tachyarrhythmias and guide therapy. A, In normal rhythm, the circuit is activated in an antegrade direction down both pathways.

Many pregnant women with known cardiac disease can complete a normal pregnancy and delivery without significant harm to the mother or fetus allergy medicine benadryl purchase 10 ml astelin mastercard. However allergy shots list cheap astelin 10ml visa, certain cardiac conditions, including irreversible pulmonary hypertension, cardiomyopathy associated with severe heart failure, and Marfan syndrome with a dilated aortic root, are associated with a high risk for cardiovascular complications and death. If pregnancy occurs, a first-trimester therapeutic abortion should be strongly recommended. CongestiveHeartFailure Several studies have shown that decompensated heart failure is associated with increased perioperative cardiac complications. In these patients, surgery should be postponed until appropriate treatment is instituted and symptoms have been stabilized. If planned surgery is associated with large blood loss or fluid shifts, a pulmonary artery catheter may be helpful. During the postoperative period, congestive heart failure most commonly occurs in the first 24 to 48 hours, when fluid administered during surgery is mobilized from the extravascular space. However, heart failure may also result from myocardial ischemia and new arrhythmias. In addition, intravenous diuretics usually provide rapid relief of pulmonary congestion. If heart failure is complicated by hypotension or poor urine output, insertion of a pulmonary artery catheter may be helpful to guide additional therapy (see Chapter 6). ValvularHeartDisease In regard to valvular heart disease the greatest risk for complications after noncardiac surgery is in those with aortic or mitral stenosis. Patients with symptomatic, severe aortic stenosis should have valve replacement before noncardiac surgery. In patients with mild to moderate mitral stenosis, careful attention to volume status and heart rate control are necessary to optimize left ventricular filling and avoid pulmonary congestion. Patients with severe mitral stenosis should be considered for percutaneous valvuloplasty or mitral valve replacement before high-risk surgery. In patients with valve disease or prosthetic heart valves, prophylactic antibiotics are recommended if appropriate. Atrial arrhythmias such as atrial fibrillation are common after surgery and usually are not associated with significant complications if the ventricular rate is well controlled. Ventricular premature beats and nonsustained ventricular tachycardia are also common after noncardiac surgery and do not require specific therapy unless they are associated with myocardial ischemia or heart failure. The physiologic increases in heart rate and cardiac output during pregnancy result in a significant increase in the gradient across the mitral valve and a rise in left atrial and pulmonary venous pressures. Congestive heart failure may develop as the pregnancy progresses through the second and third trimesters, or it may occur more acutely with the onset of atrial fibrillation. In general, patients with severely symptomatic mitral valve stenosis should have percutaneous or surgical correction of the valve before conception. Management includes salt restriction, diuretic therapy, and aggressive treatment of pulmonary infections. Patients who develop refractory heart failure during pregnancy should be considered for mitral balloon valvuloplasty because surgical commissurotomy or valve replacement may be associated with fetal demise. CardiacDiseaseinPregnancy Pregnancy is associated with dramatic changes in the cardiovascular system that may result in significant hemodynamic stress to the patient with underlying heart disease. During a normal pregnancy, plasma volume increases an average of 50%, beginning in the first trimester and peaking between the 20th and 24th weeks AorticStenosis Aortic stenosis in a pregnant woman is usually congenital in origin. Patients with significant outflow obstruction may develop angina or heart failure during the later portion of the pregnancy as cardiac output increases. If these measures fail to control symptoms and the fetus is not near term, balloon valvuloplasty, transaortic valve replacement, or aortic valve surgery should be considered to reduce the risk for maternal death. Chapter 11 OtherCardiacTopics 157 delivery, anticoagulation therapy is interrupted to avoid bleeding complications. Hypertension is not an uncommon problem during pregnancy and is defined as a consistent increase in blood pressure of 30/15 mm Hg or an absolute blood pressure greater than 140/90 mm Hg. The three major forms of hypertension that may develop during pregnancy are chronic hypertension, gestational hypertension, and toxemia. Toxemia is a form of hypertension that develops during the second half of pregnancy and is associated with proteinuria, edema, and, in severe forms, seizures. This problem is primarily managed by the obstetrician and is not discussed in this text. Gestational hypertension is an elevation in blood pressure that occurs late in the pregnancy, during delivery, or during the first postpartum days.

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