Electrocardiography of SA Node Disease PDF

# ELECTROCARDIOGRAPHY OF SA NODE DISEASE The electrocardiographic manifestations of SA node dysfunction include sinus bradycardia, sinus pauses, sinus arrest, sinus exit block, tachycardia (in SSS), and chronotropic incompetence. It is often difficult to distinguish pathologic from physiologic sinu...

# ELECTROCARDIOGRAPHY OF SA NODE DISEASE The electrocardiographic manifestations of SA node dysfunction include sinus bradycardia, sinus pauses, sinus arrest, sinus exit block, tachycardia (in SSS), and chronotropic incompetence. It is often difficult to distinguish pathologic from physiologic sinus bradycardia. * **By definition, sinus bradycardia is a rhythm driven by the SA node with a rate of <60 beats/min; sinus bradycardia is very common and typically benign.** * Resting heart rates <60 beats/min are very common in young healthy individuals and physically conditioned subjects. * A sinus rate of <40 beats/min in the awake state in the absence of physical conditioning generally is considered abnormal. * Sinus pauses and sinus arrest result from failure of the SA node to discharge, producing a pause without P waves visible on the ECG (Fig. 239-3). * Sinus pauses of up to 3 s are common in awake athletes, and pauses of this duration or longer may be observed in asymptomatic elderly subjects. * Intermittent failure of conduction from the SA node produces sinus exit block. * The severity of sinus exit block may vary in a manner similar to that of AV block (Chap. 240). * Prolongation of conduction from the sinus node will not be apparent on the ECG; second-degree SA block will produce intermittent conduction from the SA node and a regularly irregular atrial rhythm. * **Type I second-degree SA block results from progressive prolongation of SA node conduction with intermittent failure of the impulses originating in the sinus node to conduct to the surrounding atrial tissue.** * Second-degree SA block appears on the ECG as an intermittent absence of P waves (Fig. 239-4). * In type II second-degree SA block, there is no change in SA node conduction before the pause. * Complete or third-degree SA block results in no P waves on the ECG. * **Tachycardia-bradycardia syndrome is manifest as alternating sinus bradycardia and atrial tachyarrhythmias.** * Although atrial tachycardia, atrial flutter, and atrial fibrillation may be observed, the latter is the most common tachycardia. * Chronotropic incompetence is the inability to increase the heart rate in response to exercise or other stress appropriately and is defined in greater detail below.