There are three “degrees” of atrioventricular block, and within the three degrees, there are two subcategories in second-degree AV block. This page will describe the types of AV block and their ECG characteristics. In describing the ECG characteristics, I hope to make clear the reasons for the particular ECG findings in each type of block, as the ECG represents the depolarization and repolarization of cardiac tissue throughout the heart.
The “P” wave, which occurs immediately before the contraction of the atria, represents the indication on ECG of the depolarization of the atria. The PR interval describes the time the impulse from the SA node travels through the internodal atrial pathways and through the specialized cells of the AV nodal tissue. As mentioned previously, the time for an impulse to transit from the sinoatrial node to the ventricles is composed of three subsections.
The electrical impulse first travels through the internodal atrial pathways, depolarizing the adjacent atrial myocardium, and this allows the impulse to reach the AV node normally in 0.03 seconds. At the AV node, due to the cellular characteristics of the transitional fibers, there is a further delay of 0.09 seconds before the impulse is transmitted to the penetrating section of the atrioventricular bundle of His.
This section of the conduction pathway travels through fibrous tissue separating the atria from the ventricles and the impulse here is delayed 0.04 seconds before reaching the ventricular Purkinje fibers. (The repolarization of the atria is represented by an atrial “T wave” which is buried in the QRS wave due to its length and low voltage.) The QRS complex represents depolarization of the ventricles.
AV block, in this discussion, infers a normal sinoatrial pacemaker, with conduction delay for various reasons along this brief pathway. In sinus tachycardia, the PR interval will shorten, and will increase in sinus bradycardia. Assuming normal sinus rhythm, first degree AV block is noted in ECGs with a PR interval of greater than or equal to 0.20 seconds. That is to say that in first degree AV block, the electrical impulse from the AV node is slowing down somewhere along the conduction pathway, in a uniform fashion. There are no dropped beats, however. This is a conduction delay, and not a complete conduction block.
A progressive slowing of conduction through the AV nodal pathway characterizes second degree AV block, until eventually the action potential is not large enough to reliably pass through the AV node each time. The PR interval is usually 0.25-0.45 seconds. In second-degree AV block, some atrial impulses are not conducted to the ventricles. The two different types of second degree AV block must be understood, as they involve different prognostic implications.
Second-degree Mobitz Type I, or Wenkebach AV block, is characterized by a progressive lengthening of the PR interval until a sinoatrial beat fails to be conducted to the ventricular conduction system, and a dropped beat occurs. The site of this type of block is almost always within the AV node. Mobitz Type I block does not usually lead to development of complete heart block. Etiology and management will be discussed in a later section.
Mobitz Type II second-degree AV block is characterized by a dropped beat that is not preceded by any lengthening of the QRS complex. It represents an abrupt stop in conduction. This block occurs most often in the region between the Bundle of His and the Purkinje system. The ECG will reveal uniform intervals between regularly occurring QRS complexes, with a sudden pause, a p wave without a subsequent QRS complex.
2:1 AV block may be either Mobitz Type I or Type II, but it may be difficult to initially identify on ECG. Sometimes a 2:1 AV block may accompany first-degree heart block, with prolonged PR intervals and regular QRS complexes interrupted by a dropped beat.
In Wenkebach 2:1 rhythms, an increase in the sinus rate may result in correction of conduction, in the event of increased sympathetic discharge. In Mobitz Type II, the ECG often reveals other conduction abnormalities in the ventricular system, such as bundle branch block. Increase in sinus rhythm may worsen the 2:1 block in Mobitz Type II, producing a 3:1 or greater block.
Third-degree AV conduction block represents complete failure of SA nodal impulses to propagate through the AV node. The result is AV dissociation, or an ECG where the P waves and QRS waves have no evident connection, with the SA node firing at 60 -100 BPM and the ventricles depolarizing at a typically slower ventricular rate, i.e. 40 BPM.
This type of heart block, depending upon its etiology (acquired or congenital) may occur at the AV node or in the Bundle of His-Purkinje system. The ventricular escape rate is regular and dependent upon the automaticity of the ventricular myocardial cells. The rate of the recurring P waves and QRS waves, although independent of each other, should be regular.