The clinical presentation of first-degree and Mobitz type I block is typically benign and rarely produces symptoms. Normal, physiologic block of this type occurs in response to increases in parasympathetic output. This is commonly seen during sleep, with carotid sinus massage, or in well-trained athletes. It may also occur as a medication effect (calcium channel blockers, beta-blockers, digitalis, or antiarrhythmics). Pathologic causes, including myocardial ischemia or infarction (discussed earlier), inflammatory processes (ie, Lyme disease), fibrosis, calcification, or infiltration (ie, amyloidosis or sarcoidosis), should be excluded.
Mobitz type II block and complete (third-degree) heart block are almost always due to pathologic disease involving the infranodal conduction system, and symptoms including fatigue, dyspnea, presyncope or syncope are common. With complete heart block, where no atrial impulses reach the ventricle, the ventricular escape rate is usually slow (less than 50 beats/min) and severity of symptoms may vary depending on the rate and stability of the escape rhythm (eFigure 10–58). Exercise does not increase the rate. The first heart sound varies in intensity; wide pulse pressure, a changing systolic BP level, and cannon venous pulsations in the neck are also present. As for lesser degrees of AV block, pathologic causes should be explored.
Admission electrocardiogram demonstrating complete heart block. (Reproduced with permission from Dolbec KWD, Higgins GL, Saucier JR. Lyme carditis with transient complete heart block. West J Emerg Med. 2010;11(2):211–2.)
Intraventricular conduction block is relatively common and may be transient (ie, related to increases in heart rate) or permanent. Right bundle branch block is often seen in patients with structurally normal hearts. The left bundle is composed of two components (anterior and posterior fascicles) and left bundle branch block is more often a marker of underlying cardiac disease, including ischemic heart disease, inflammatory or infiltrative disease, cardiomyopathy, and valvular heart disease. Below the AV node and bundle of His, the conduction system trifurcates into a right bundle and anterior and posterior fascicles of the left bundle. Conduction block in each of these fascicles can be recognized on the surface ECG (eFigure 10–59) (eFigure 10–60). Bifascicular block is present when two of these—right bundle, left anterior, and left posterior fascicle—are involved. Trifascicular block is defined as right bundle branch block with alternating left hemiblock, alternating right and left bundle branch block, or bifascicular block with documented prolonged infranodal conduction (long His-ventricular interval). In asymptomatic patients with bifascicular block (block in two of three infranodal components—right bundle, left anterior, and left posterior fascicle), the incidence of occult complete heart block or progression to it is low (1% annually).
Left anterior fascicular block. Note the qR complexes in leads I and aVL and rS complexes in II, III, and aVF. Also note the prolonged R-wave peak time in aVL (the time from onset of the QRS to the peak of the R wave > 45 msec). (Reproduced, with permission, from Alemzadeh-Ansari MJ. Electrocardiography. In: Maleki M, Alizadehasl A, Haghjoo M (editors). Practical Cardiology. Elsevier, 2018. Copyright © Elsevier.)
Left posterior fascicular block. Note the rS complexes in leads I and aVL and qR complexes in II, III, and aVF. Also note the prolonged R-wave peak time in aVF (the time from onset of the QRS to the peak of the R wave > 45 msec). (Reproduced, with permission, from Alemzadeh-Ansari MJ. Electrocardiography. In: Maleki M, Alizadehasl A, Haghjoo M (editors). Practical Cardiology. Elsevier, 2018. Copyright © Elsevier.)