Mobitz type I (Wenckebach type): PR interval progressively prolongs until QRS drops, i.e., until a regularly occurring P wave is not followed by a QRS. This contrasts with SA block, where both P and QRS drop. To make the diagnosis, compare the PR that follows the blocked P wave (the shortest PR) with the PR that immediately precedes the blocked P wave (the longest PR).
Mobitz type II: QRS suddenly drops without a preceding PR change. The baseline QRS is usually wide. It may present as intermittently non-conducted P waves or as one non-conducted P wave that is not preceded by progressive PR prolongation and not followed by PR shortening. It is more ominous than Mobitz type I and is almost always a distal infranodal AV block. It progresses to a complete infranodal AV block commonly and suddenly.
2:1 AV block (alternative drop of one QRS) could be equivalent to Mobitz I or Mobitz II AV block. If QRS is wide, the block is likely a Mobitz II equivalent. If QRS is narrow, the block is likely a Mobitz I equivalent. Also, look for periods of 3:2 conduction on a rhythm strip, as this may elucidate the Mobitz type of block.
A junctional or ventricular escape rhythm takes over. The ventricular rate is regular and unrelated to P waves (AV dissociation). The PR distance is variable yet the R–R interval is regular, providing evidence that none of these P waves is conducted. There are more P waves than there are QRS complexes.
High-grade or advanced second-degree AV block is a block in which the AV conduction ratio is 3:1 or worse. In one form, P waves and the escape QRS complexes are dissociated but P wave occasionally conducts: R–R intervals are mostly regular, with occasional R–R intervals that are shorter as they relate to the few conducted P waves.
In patients with AF, a slow and regular ventricular response implies complete AV block. A long AF pause, >3 seconds, generally implies high-grade AV block and is concerning when it occurs during wakefulness.
The escape is ventricular with a wide QRS complex if the block is infranodal (rate 20–40 bpm). The escape is junctional with a narrow QRS complex if the block is at the nodal level (rate 40–60 bpm); the junctional escape may be wide if a bundle branch block is present on the baseline ECG. Patients with complete AV block at the infranodal level may have preserved VA conduction, and retrograde P waves may be seen.
B. Causes of a pause on the rhythm strip
Outside the pause that follows an obvious PAC or PVC, a pause on a rhythm strip may be secondary to:
Sinus pause: no P wave is seen within the pause.
Second-degree AV block (Mobitz I or Mobitz II): a blocked P wave is seen within the pause. The blocked P wave marches out with the regularly occurring sinus P wave. Occasionally, however, if sinus arrhythmia is present, the blocked P wave of the AV block may not perfectly march out with the other P waves. A particular form of sinus arrhythmia seen with AV block (especially 2:1 AV block) is ventriculophasic sinus arrhythmia, in which the P–P interval containing a QRS is shorter than the P–P interval not containing a QRS (the QRS complex leads to a stroke volume which leads to reflex slowing of the P–P interval).
Blocked PAC (the most benign pause): the blocked P wave is a very premature P wave that falls in the AV nodal refractory period and does not get conducted. As opposed to AV block, the blocked P wave does not march out with the sinus P waves and often has a different morphology.
Concealed premature junctional complex (less common): a premature junctional complex (His complex) is rare, much less common than a PAC or a PVC. A blocked premature junctional complex prevents the conduction of the next sinus impulse through the His (still in refractory period), creating the impression of a Mobitz II AV block. The presence of conducted premature junctional complexes elsewhere on the rhythm monitor is a hint to this phenomenon.
In patients with intermittent AF, a long pause may occur at the junction between AF and sinus rhythm (the sinus node is “sleeping” during AF and needs time to wake up); this pause is a common cause of syncope in intermittent AF (Figure 13.10).
C. Location of the AV block
AV block may occur at the level of the AV node or at the infranodal level, i.e., at the His or the infra-His/Purkinje level (Figure 13.13). Infranodal AV block is ominous and leads to a slow ventricular escape rhythm, or no escape at all. Usually, in infranodal AV block, the baseline QRS is wide as the fascicles have abnormal baseline conduction. Occasionally, the block is at the His level and the baseline QRS may not be wide.
Nodal AV block is less ominous and leads to a faster, junctional, narrow escape rhythm. Location of the block in each type of AV block:
First-degree AV block is usually a nodal block, particularly if QRS is narrow. A PR interval >250 ms with a narrow QRS ~always implies a nodal block.
Mobitz I is often a nodal block, especially when QRS is narrow. When the QRS is wide, 75% of Mobitz I blocks are still nodal blocks, while 25% are infranodal blocks. A wide-QRS Mobitz I block with a maximal PR change of <50 ms is likely an infranodal Mobitz I block.
Mobitz II is an infranodal block, with a QRS that is often wide on the baseline ECG. If the baseline QRS is narrow, the block is likely infranodal at the level of the His bundle (~20% of Mobitz II block). Occasionally, what seems to be a narrow-QRS Mobitz II can be an overlooked Mobitz I with small overlooked increments in PR intervals, or can result from a sudden burst of vagal tone blocking one or several consecutive P waves.
2:1 AV block: analyze the QRS width and the PR interval to define the site of the block:
if QRS is wide, the block is likely infranodal; if QRS is narrow, the block is likely nodal.
PR interval has two components: nodal component (AH interval, normally <125 ms) and infranodal component (HV interval, nor- mally <55 ms and shorter than the AH component). A PR interval >250 ms more likely results from a delay of the nodal com- ponent, i.e., an increase in AH interval; infranodal HV interval rarely prolongs enough to cause such PR prolongation. On the other hand, a PR interval that is <200–250 ms argues against a block at the AV nodal level.
Two additional modalities help elucidate the type of 2:1 AV block: exercise testing (Table 13.1) or prolonged rhythm monitoring.
Third-degree AV block or high-grade AV block (e.g., 3:1, 4:1) may be nodal or infranodal. As such, it may be less ominous than Mobitz II AV block. Determine the location of the AV block by the width and the rate of the escape. An escape rhythm that is wide usually implies an infranodal block; the rate is usually, but not necessarily, <40 bpm (Figure 13.14). VA conduction (retrograde P waves) implies an infranodal block as well.
Table 13.1 Effect of atropine and exercise on AV block.
AV nodal block
Conduction ratio improves (AV node has rich autonomic innervations and is affected by cholinergic and sympathetic effects)
AV infranodal block
Conduction ratio remains unchanged or worsens His and Purkinje conduction is not substantially affected by the cholinergic system. On the other hand, the increase in sinus rate leads to more atrial depolarizations reaching the infranodal area. Many of these atrial depolarizations partially penetrate the infranodal area without getting conducted all the way; as such, they extend the refractory period and not only they do not conduct, they prevent subsequent beats from getting conducted. This is concealed conduction= blocked, partial conduction that does not show up on the ECG yet modifies the expected behavior of subsequent beats (blocked P waves prevent the conduction of subsequent P waves). A slower atrial rate is more likely to get conducted
Degenerative, fibrotic disease is the most common cause of AV block (Lenegre); it may extend to the cardiac skeleton, such as the mitral annulus and the aortic valve (Lev). It is related to aging and atherosclerotic risk factors but may have an inherited premature component.
AV block may result from acute anterior or inferior ischemia, or chronic ischemic cardiomyopathy with diffuse myocardial fibrosis.
Calcific valvular disease.
Any cardiomyopathy can affect the conduction system. Varying degrees of AV blocks are seen in up to 15% of dilated cardiomyopathies.
Drugs, electrolytes (hypo- or hyperkalemia, hypermagnesemia).
High vagal tone (sleep, vomiting, cough, athlete’s heart) may lead to AV block at the nodal level. Vasovagal syncope may lead to a vagally-mediated AV block, in which case AV block is part of the vagal reaction, not the sole cause of syncope (Figure 13.15).
AV block after cardiac surgery (~5–10% of valvular surgeries and congenital heart disease surgeries, in which AV block resolves in 2/3 of the cases). AV block usually resolves within 7 days, if at all.
Lyme disease leads to cardiac involvement in 1–5% of the cases, mostly high-degree AV block (80–90%), that is nodal and reversible. The degree of AV block can abruptly fluctuate. This AV block typically resolves within 1–2 weeks of antibiotic therapy but may persist for months; it is a nodal AV block that paradoxically worsens with exercise, leading to significant exercise limitation. Diagnosis is suggested by the context (outdoor exposure in endemic areas, tick bites, erythema migrans in the last 1–2 months) and established by serology.