Donald M. Yealy and Joshua M. Kosowsky

Cardiac Cellular Electrophysiology

The function of individual cells in the conductive and contractile tissues of the heart depends on an intact resting membrane potential. Na⁺, K⁺, and Ca²⁺ ions create the membrane potential and regulate conduction and contractility. The membrane potential is the result of a differential concentration of Na⁺ and K⁺ on either side of the cell membrane, measuring approximately −90 mV in normal resting, nonpacemaker cells. This electrical gradient exists mainly because of the Na⁺-K⁺ exchange pump and the natural concentration-dependent flow of K⁺ out of the cell. Adenosine triphosphate (ATP) fuels Na⁺ transport out to the extracellular fluid, with Mg²⁺ used as a cofactor (Fig. 79-1). This process creates an osmotic gradient, allowing Ca²⁺ to be passively exchanged for Na²⁺ and promoting conduction as well as myofibril contraction. The resting membrane potential is generated from the flow of K⁺ down a concentration gradient toward the extracellular fluid. The cell membrane is far more permeable to potassium than sodium ions, resulting in a net loss of intracellular positive charge.

In normal nonpacemaker cells, the application of an electrical stimulus causes the membrane potential to become less negative, termed depolarization. When the membrane potential reaches −70 mV, specialized Na²⁺ channels open, causing a rapid influx of positive charge into the cell. This “fast” channel activity further decreases the membrane potential and is augmented at 30 to 40 mV by a second “slow” channel that allows Ca²⁺ influx. When these channels close, resting potential is restored by the sodium-potassium pump, an event known as repolarization (Fig. 79-2).

In nonpacemaker cells, additional depolarization from a second electrical stimulus is not possible when the membrane potential is more positive than −60 mV. This period is termed the effective refractory period (Fig. 79-3). At a membrane potential of −60 to −70 mV, a strong impulse can cause a response that is likely to be propagated, although abnormally; this response represents the relative refractory period. At a membrane potential of −70 mV or less, virtually all fast channels are ready for activity if properly stimulated (see Fig. 79-3).

Pacemaker cells differ from non–impulse-generating cells in two ways: their resting membrane potential is less negative, and they can spontaneously depolarize via slow Na⁺ influx . Pacemaker cells exist normally within the sinoatrial (SA) and atrioventricular (AV) nodes and can also be found on the atrial surfaces of the AV valves and within the His-Purkinje system. Nonpacemaker cells may undergo spontaneous depolarization under pathologic conditions, especially during ischemia.

Afterdepolarizations are fluctuations in membrane potential that occur as the resting potential is restored. These fluctuations may precipitate another depolarization, either just before full resting potential is reached (early afterdepolarizations) or after full resting potential is reached (delayed afterdepolarizations). Early afterdepolarizations are associated with high resting membrane potentials and are more likely with slower heart rates. Delayed afterdepolarizations can arise from ischemia, catecholamine excess, or electrolyte disturbances (especially K⁺, Mg²⁺, and Ca²⁺) and are enhanced by faster heart rates.

Anatomy and Conduction

The SA node is an area of specialized impulse-generating tissue located at the junction of the right atrium and the superior vena cava. Its blood supply is from the right coronary artery (RCA) in 55% of patients and the left circumflex artery (LCA) in 45%. The normal SA node produces spontaneous depolarizations at a faster rate than other pacemakers and functions as the dominant pacemaker. The SA node maintains a rate of 60 to 90 beats/min in most adults. Hypothermia and vagal stimulation slow the sinus rate, whereas hyperthermia and sympathetic stimulation increase the rate. In the setting of low or absent parasympathetic tone—for example, with certain drugs or after heart transplantation—the resting sinus rate tends to be faster.

In the absence of normal SA node activity, other myocardial tissues may assume the role of pacemaker. The AV node has an intrinsic impulse-generating rate of 45 to 60 beats/min. Infranodal pacemakers, found within the His bundle, the Purkinje system, and the bundle branches, maintain intrinsic rates ranging from 30 to 45 beats/min. Other atrial and ventricular tissues may assume pacemaker-like activity under conditions of ischemia or metabolic derangement. Rates of ectopic pacemaker activity vary widely depending on the underlying pathology.

Figure 79-4 correlates the surface ECG tracings with tissue electrical events. Impulses generated from within the SA node itself, imperceptible on the surface ECG, are propagated through the atrial tissue to the AV node. Atrial depolarization is characterized by the P wave on the surface ECG.

The AV node is an area of specialized tissue between the atria and the ventricles of the heart, located in the posterior-inferior region of the interatrial septum. Its blood supply is from a branch of the RCA in 90% of patients (right dominant) and from the LCA in the remaining 10% (left dominant). Transmission of impulses within the AV node is slower than in parts of the conducting system (Table 79-1) because of a dependence on slow-channel ion influx for membrane depolarization. In some patients, pathologic “accessory pathways” connect atrial and ventricular tissues. These accessory pathways do not share the normal conduction delay of the AV node and may allow for rapid ventricular rates. Preexcitation refers to the early depolarization of ventricular myocardium when accessory paths are used instead of the normal conduction system.

On the surface ECG, the PR interval (normally 0.10 to 0.20 second) represents the time it takes for conduction of a sinus impulse through the atria and AV node and into the ventricles. Impulses originating in lower atrial tissues are associated with a shortened PR interval, as are impulses conducted to the ventricles via accessory pathways outside the AV node. PR prolongation is usually a result of nodal or supranodal conduction system disease.

After passing through the AV node, impulses propagate down the His bundle to the three main bundle branch fascicles: the right bundle branch (RBB), the left anterior-superior bundle (LASB), and the left posterior-inferior bundle (LPIB). The RBB and LASB are typically supplied by the left anterior descending (LAD) artery, whereas the LPIB may be supplied by either the RCA or the LCA. After conduction down the three main bundle branches, impulses are delivered to the Purkinje fibers, which propagate impulses to myocardial tissues in a swift and orderly fashion, allowing for coordinated ventricular contraction. If an impulse arrives prematurely, it may be conducted aberrantly (if bundles are relatively refractory), or it may be blocked (if the bundles are completely refractory).

On the surface ECG, the QRS complex represents ventricular depolarization, and the normal QRS interval is 0.09 seconds or less, with a duration of 0.12 seconds or greater being clearly abnormal. The T wave corresponds to ventricular repolarization and varies in duration depending, among other things, on the length of the cardiac cycle. The total time of ventricular depolarization and repolarization is represented by the QT interval. Shorter cycle lengths (i.e., shorter R-R intervals) result in a shorter repolarization time. Conversely, longer cardiac cycles are associated with longer repolarization times.

Mechanisms of Dysrhythmia Formation

There are three electrophysiologic mechanisms: enhanced automaticity, triggered activity, and reentry. Enhanced automaticity includes spontaneous depolarization of nonpacemaker cells or a lower threshold for depolarization in normal pacemaker cells (Fig. 79-5). Both types of enhanced automaticity can occur in the setting of ischemia, electrolyte disturbances, or drugs. Examples of enhanced automaticity include idioventricular rhythms in the setting of acute myocardial infarction, and atrial tachycardias or junctional tachycardias (JTs) seen with digitalis toxicity.

Triggered activity occurs as the result of early or delayed afterdepolarizations. The classic dysrhythmia associated with early afterdepolarization is torsades de pointes. Triggered dysrhythmias caused by delayed afterdepolarizations are frequently found in the setting of intracellular Ca²⁺ overload as occurs with digitalis toxicity or with reperfusion therapy for acute myocardial infarction. Many ectopic atrial, junctional, and ventricular rhythms fall into this category and are amenable to treatment with medications that slow the heart rate or interfere with calcium entry into the cell.

Reentry dysrhythmias occur as a consequence of abnormal conduction (Fig. 79-6). For a reentry mechanism, two alternate pathways for conduction must be present and one path must have a longer refractory period. The unequal responsiveness of the limbs creates a functional unidirectional block such that when the impulse exits one limb, it may then reenter the other in retrograde fashion. The cycle is then repeated, creating a self-sustaining or “circus movement” tachycardia that can appear orderly or disorderly (i.e., fibrillatory).

Reentry mechanisms are responsible for most narrow-complex tachycardias and many ventricular tachycardias (VTs). Treatment is predicated on altering conduction in one or both limbs of the reentry circuit.

Classification of Antidysrhythmic Drugs

Medications used to treat dysrhythmias are classified into four major categories on the basis of their electrophysiologic effects (Box 79-1). Other agents fall outside this classification system and are discussed separately.

Class I agents exert their major effects on the fast Na⁺ channels, resulting in membrane stabilization. The subclasses IA, IB, and IC are distinguished on the basis of differential effects on depolarization, repolarization, and conduction. Class II agents are the beta-adrenergic antagonists, which depress SA node automaticity and slow AV node conduction. Beta-blockers can also suppress conduction in ischemic myocardial tissue. Class III agents prolong repolarization and refractory period duration, predominantly via their effects on K⁺ channels. Class IV agents are the Ca²⁺ channel blockers, which slow conduction through the AV node and suppress other calcium-dependent dysrhythmias. Other agents important in the emergency treatment of dysrhythmias include magnesium sulfate, digitalis, and adenosine.

All antidysrhythmics can cause “prodysrhythmic effects.” This occurs most often in patients with existing structural heart disease and in patients receiving new or higher doses of antidysrhythmic agents. The class I and III agents are associated with prodysrhythmic effects in up to 15% of patients.1,2

Approach to Dysrhythmia Recognition and Management

Dysrhythmias are classified according to their electrophysiologic origin, ECG appearance, and underlying ventricular rate. Although overlap exists, the following categorization is useful:

Classically, the approach to any specific dysrhythmias is defined on the basis of the clinical stability. Unstable patients demonstrate evidence of severe or multiple end-organ features of hypoperfusion, such as altered sensorium, respiratory distress, hypotension, syncope, or chest pain suggestive of myocardial ischemia. Stable patients may be asymptomatic or have mild symptoms, such as light-headedness, dyspnea on exertion, palpitations, or mild anxiety. In practice, clinical stability is a continuum; in the absence of profound altered sensorium or hypotension, a clear line distinguishing stable and unstable patients is often not present.

It is important to consider whether a dysrhythmia is the cause or an effect of a clinical presentation; for example, rapid atrial fibrillation may be causing hypotension or resulting from profound volume depletion. Failure to consider the clinical situation can lead to an inappropriate focus on diagnosing and treating the rhythm to the detriment of the patient. With recognition of this need to incorporate the overall clinical picture, treatment of those with a dysrhythmia and clear instability is empirical and assumes the rhythm is the cause, whereas stable patients can be approached in a more systematic and thoughtful manner to identify the cause and choose the most appropriate therapy.