First-degree atrioventricular block

Abstract

Marked first-degree AV block (PR> or =0.30 s) can produce a clinical condition similar to that of the pacemaker syndrome. Clinical evaluation often requires a treadmill stress test because patients are more likely to become symptomatic with mild or moderate exercise when the PR interval cannot adapt appropriately. Uncontrolled studies have shown that many such symptomatic patients with normal left ventricular (LV) function improve with conventional dual chamber pacing (Class IIa indication). In contrast, marked first-degree AV block with LV systolic dysfunction and heart failure is still a Class IIb indication, a recommendation that is now questionable because a conventional DDD(R) pacemaker would be committed to right ventricular pacing (and its attendant risks) virtually 100% of the time. It would seem prudent at this juncture to consider a biventricular DDD device in this situation. Patients with suboptimally programmed pacemakers may develop functional atrial undersensing because the P wave tends to migrate easily into the postventricular atrial refractory period (PVARP). Retrograde vetriculoatrial conduction block is uncommon in marked first-degree AV block so a relatively short PVARP can often be used at rest with little risk of endless loop tachycardia. The usefulness of a short PVARP may be negated by special PVARP functions in some pulse generators designed to time out a long PVARP at rest and a gradually shorter one with activity. First-degree AV block during cardiac resynchronization therapy (CRT) predisposes to loss of ventricular resynchronization during biventricular pacing because it favors the initiation of electrical "desynchronization" especially in association with a relatively fast atrial rate and a relatively slow programmed upper rate. Patients with first-degree AV block have a poorer outcome with CRT than patients with a normal PR interval, a response that may involve several mechanisms. (1) The long PR interval may be a marker of more advanced heart disease. (2) Patients with first-degree AV block may experience more episodes of undetected "electrical desynchronization". (3) "Concealed resynchronization" whereupon ventricular activation in patients with a normal PR interval may result from fusion of electrical wavefronts coming from the right bundle branch and the impulse from the LV electrode. The resultant hemodynamic response may be superior because the detrimental effects of right ventricular stimulation (required in the setting of a longer PR interval) are avoided.

Leadless Cardiac Pacemaker as a Novel Intervention Modality for Atrioventricular Conduction Disturbance in Hypertrophic Cardiomyopathy

“…To solve this controversy, uncontrolled trials of pacing in marked first-degree AVB were carried out. The results showed that the patients' condition was improved with dual chamber pacing, specifically in patients with normal ejection fraction [18].…”

“…Some researchers suggest that pacing is only recommended for symptomatic patients; but others argue that pacing should be started earlier, regardless of symptoms, to improve survival especially for AVB that occurs diurnally [11]. Another study showed that marked first-degree AVB (PR ≥ 0.30s) can cause similar symptoms to the pacemaker syndrome without any existence of higher degree AVB [18]. To solve this controversy, uncontrolled trials of pacing in marked first-degree AVB were carried out.…”

III SBC Guidelines on the Analysis and Issuance of Electrocardiographic Reports - Executive Summary

“…Delayed atrioventricular conduction - 18,19 Occurs when the atrial impulses experience a delay or fail to reach the ventricles.…”

The Prescription of Minimal Ventricular Pacing

“…The adverse hemodynamic effects may manifest clinically as a constellation of symptoms similar to pacemaker syndrome. Indeed, a pacemaker‐like syndrome is well recognized in patients with markedly prolonged PR interval, particularly if the prolonged PR interval fails to shorten on exercise 26 . Mabo et al demonstrated in a study of 17 patients with AAIR pacemaker, failure of the AV interval to shorten in six patients, and prolongation of the AV interval in five patients on exercise, resulting in symptoms of pacemaker syndrome in two patients 27 .…”