Bradyarrhythmias arise from (1) failure of impulse initiation (sinoatrial [SA] node dysfunction) or (2) impaired electrical conduction (e.g., AV conduction blocks).
SINOATRIAL NODE DYSFUNCTION
Etiologies are either intrinsic (degenerative, ischemic, inflammatory, infiltrative [e.g., amyloid], or rare mutations in sodium channel or pacemaker current genes) or extrinsic (e.g., drugs [beta blockers, Ca++ channel blockers, digoxin], autonomic dysfunction, hypothyroidism).
Symptoms are due to bradycardia (fatigue, weakness, lightheadedness, syncope) and/or episodes of associated tachycardia (e.g., rapid palpitations, angina) in pts with sick sinus syndrome (SSS).
Examine ECG for evidence of sinus bradycardia (sinus rhythm at <60 beats/min) or failure of rate to increase with exercise (chronotropic incompetence), sinus pauses, or exit block. In pts with SSS, periods of tachycardia (i.e., atrial fibrillation/flutter) also occur. Prolonged ECG monitoring (24–48 h Holter, 30-day loop recorder, or long-term implanted monitor) aids in identifying these abnormalities. Invasive electrophysiologic testing is rarely necessary to establish diagnosis.
TREATMENT Sinoatrial Node Dysfunction
Remove or treat extrinsic causes such as contributing drugs or hypothyroidism. Otherwise, symptoms of bradycardia respond to permanent pacemaker placement (Table 124-1). In SSS, treat associated atrial fibrillation or flutter as indicated in Chap. 125.
TABLE 124-1Summary of Guidelines for Pacemaker Implantation in SA Node Dysfunction |Favorite Table|Download (.pdf) TABLE 124-1 Summary of Guidelines for Pacemaker Implantation in SA Node Dysfunction
|Class I |
SA node dysfunction with symptomatic bradycardia or sinus pause
Symptomatic SA node dysfunction as a result of essential long-term drug therapy with no acceptable alternatives
Symptomatic chronotropic incompetence
Atrial fibrillation with bradycardia and pauses >5 s
|Class IIa |
SA node dysfunction with heart rates <40 beats/min without a clear and consistent relationship between bradycardia and symptoms
SA node dysfunction with heart rates <40 beats/min on an essential long-term drug therapy with no acceptable alternatives, without a clear and consistent relationship between bradycardia and symptoms
Syncope of unknown origin when major abnormalities of SA node dysfunction are discovered or provoked by electrophysiologic testing
|Class IIb |
Mildly symptomatic pts with waking chronic heart rates <40 beats/min
|Class III |
SA node dysfunction in asymptomatic pts, even those with heart rates <40 beats/min
SA node dysfunction in which symptoms suggestive of bradycardia are not associated with a slow heart rate
SA node dysfunction with symptomatic bradycardia due to nonessential drug therapy
Impaired conduction from atria to ventricles may be structural and permanent, or reversible (e.g., autonomic, metabolic, drug-related)—see Table 124-2.
TABLE 124-2Etiologies of Atrioventricular Block