Mobitz Type I and Type II Second Degree Atrioventricular Blocks

The terms atrioventricular (AV) block or heart block, by definition, refer to an abnormality in conduction of the atrial impulse to the ventricular tissue. The AV blocks may be classified according to incomplete or complete AV block. First and second degree forms of AV block are classified as incomplete, indicating that all or some of the atrial impulses are conducted to the ventricles. Third degree AV block falls under the complete heading, indicating none of the atrial impulses are conducted to the ventricles.1 Second degree AV blocks can be further divided into 3 categories. The first category, 2:1 second degree AV block, is a constant block where every other atrial impulse is conducted to the ventricles. The next 2 categories include Mobitz type I and Mobitz type II, which are the other forms of second degree AV block. The term Mobitz type I is used interchangeably with Wenckebach second degree AV block. Generally, Wenckebach is used more often than Mobitz type I. Wenckebach is associated with progressively increasing PR intervals until a QRS complex is dropped in a cyclic fashion. Mobitz type II may also present with cycles, but the PR intervals are constant until a P wave abruptly fails to be conducted to the ventricles. The last type of second degree AV block is high degree, or advanced, second degree AV block; it is characterized by fewer than half of the atrial impulses being conducted to the ventricles. Regardless of the specific type, second degree AV blocks may be intermittent or continuous. Additionally, second degree AV blocks may have any ratio of P waves to QRS complexes.1The purpose of this column is to focus on the etiology, characteristics, significance, and management of Mobitz type I and Mobitz type II second degree AV blocks.In 1888, Karel Wenckebach graduated medical school and began his medical career in physiology at the same university. He became interested in finding out more about irregular pulses. After a few years in private practice, he returned to the university to resume researching and studying arrhythmias. A 40-year old woman consulted him about her irregular pulse. He used a sphygmogram and a tuning fork to study her radial artery pulse tracings, identifying the presence of groups of beats with repeating changing pulse intervals. He called these groups of beats Luciani periods after a physiologist who had described similar groups of beats 20 years earlier.2 Finally, in 1906, he was able to use the electrocardiogram (ECG) to document the classic progressive prolongation of the PR interval before a dropped ventricular complex, which is commonly referred to as Wenckebach second degree heart block.3In 1906 John Hay, a physician in Liverpool, England, used recordings of the a-c interval from the jugular venous pulse and identified a new form of second degree AV block. The a-c interval corresponds to timing of the interval between the atrial and ventricular contractions. He noticed that the a-c interval remained constant except in instances where the a wave would not be followed by the c wave, indicating the absence of a ventricular contraction.4Woldemar Mobitz’s work in the 1920s laid the groundwork for the classification of AV blocks. In 1924, he used the ECG to classify the Wenckebach second degree AV block as Mobitz type I and the newer form identified in 1906 by Hay as Mobitz type II. Mobitz believed that the A-V conduction in the type II block was an all-or-none phenomenon. The atrial impulse was either conducted within the same time interval or completely blocked. Interestingly, later studies revealed that there is a minuscule prolongation of the PR interval until the atrial impulse fails to be conducted. This prolongation cannot be picked up on the standard ECG. Therefore, clinical practice applies the all-or-none phenomenon to Mobitz type II today.4 It is interesting to note that problems have been identified not only with Mobitz’s definition but also with those of others who have put forth criteria over time. Barold described the contemporary definition of Mobitz type II second degree AV block as a “single non-conducted P wave associated with consistency of all the PR intervals of the conducted beats before and after the blocked P wave.”5The definition and classic characteristic of Wenckebach is the progressive prolongation of the PR interval until a P wave fails to be conducted to the ventricles, resulting in a dropped QRS complex. The initial PR interval may be within normal limits or be prolonged. With each successive beat, the PR gradually prolongs until the P wave fails to conduct to the ventricular tissue. This occurs because each successive P wave falls earlier in the repolarization phase of the AV node and therefore takes longer to be conducted through the AV node.1 At some point, the AV node fails to repolarize, which causes the P wave to be blocked and results in a dropped QRS complex. After the dropped QRS, the cycle may be repeated, or it could be shorter or longer and variable.1There are 3 classic ECG features that were described by Dr Marriott as the “footprints of Wenckebach.” The first is the presence of clusters of beats in small groups commonly in 3:2 ratios (3 P waves to 2 QRS complexes); these clusters are often described as “group beating.” Other ratios may occur, such as 4:3 or 5:4, but are less common. Secondly, the P-P intervals tend to be regular and the R-R intervals tend to shorten within the group. Lastly, the long cycle containing the dropped beat is always less than 2 of the conducted cycles.1 Figure 1 is an example of these classic features.It is important to recognize that the common characteristics of Wenckebach may not always be present. For example, progressive prolongation of the PR interval may not occur. Within a group of beats, the second PR interval may be longer than the first, but subsequent PR intervals may be constant until the last conducted atrial impulse in the group, where the PR interval may lengthen and the following P wave will be blocked.1 Figures 2 and 3 are examples of this.Wenckebach commonly occurs when there is impairment of conduction through the AV node.1 Wenckebach may develop during inferior myocardial ischemia or infarction due to blockage of the right coronary artery, which supplies the inferior wall of the left ventricle and also supplies the AV node in 80% of the population. Other situations include hyperkalemia, Chagas disease, cardiomyopathy, or following mitral valve surgery.3 Common medications that can cause this are those that slow conduction through the AV node, including the nondihydropyridine calcium channel blockers (diltiazem, verapamil), β blockers, and digitalis preparations, as well as several antiarrhythmic agents.There are reports that Wenckebach is “generally benign.”3 However, the “dropped” QRS complexes result in a reduction of the heart rate and subsequently a fall in cardiac output and perhaps in blood pressure. Therefore, the cause needs to be identified. Medications should be assessed and withheld, as appropriate. Administration of atropine may be helpful. If the condition does not respond to atropine, the cardiology team may determine the patient requires a temporary transvenous pacemaker. In the setting of inferior infarction, the onset of Wenckebach is usually transient and the pacemaker may be removed. However, there are rare circumstances where a permanent pacemaker may be required.3Mobitz type II second degree AV block represents a true block in the transmission of impulses; it is not a delay in transmission of impulses. Mobitz type II falls into the category of infranodal block, which means the block is below the AV node versus in the distal part of the conduction system. It is often preceded by the development of a bundle-branch block. It has been reported that Mobitz type II is almost always due to bilateral bundle-branch block rather than a His-bundle block.1Mobitz type II block is identified by a non-conducted P wave that occurs at the expected time (not prematurely) when the patient is in a sinus rhythm. The PR intervals of the beats preceding and immediately following the blocked P wave are constant.4–6 There is no ECG evidence of progressive lengthening of the PR intervals as seen in Mobitz type I AV block (Wenckebach).The ECG changes associated with Mobitz type II include a single nonconducted P wave preceded by at least 2 or more consecutively conducted sinus beats with consistent PR intervals. In addition, the PR interval of the beat after the nonconducted P wave must be the same as the PR intervals before the nonconducted P wave. There is no forewarning of the dropped QRS complex. The sinus rate should be stable and regular. The PR interval is usually within normal limits, however, it may also be prolonged. It is important to assess all PR intervals before and after the dropped beats; they all should be constant.4–6 If there is only 1 sinus beat before a nonconducted P wave, the rhythm could be Mobitz type I (Wenckebach) without the typical PR prolongation, 2:1 AV block, or Mobitz type II.4–6 Figure 4 is an example of classic Mobitz type II block. Two P waves are conducted with fixed PR intervals and abruptly a P wave fails to be conducted. Figure 5 shows variable cycles of Mobitz type II, including 2:1. Figure 6 is another example of Mobitz type II episodes associated with a 2:1 block. The presence of 2:1 in the context of Mobitz II may be a common occurrence. Note that if more than one P wave in a row is blocked, the dysrhythmia is not called Mobitz type II but is classified as high grade, or advanced, second degree AV block, examples of which would be 3:1 or 4:1 block (Figure 7).5 It is common for the patient to display a right bundle-branch block plus a left anterior hemiblock because this is an infranodal problem, as indicated previously.1Mobitz type II is found in patients who have experienced an anteriorseptal myocardial infarction or patients who have myocardial fibrosis or sclerosis leading to chronic degeneration of the conduction system. Refer to the Table for other less common causes.4,7,8The diagnosis of Mobitz type II is considered to be a medical emergency even in patients experiencing minimal symptoms, with short runs of the block,4 because the condition may progress to complete heart block or prolonged asystole and sudden death. The dropped beats can contribute to bradycardia or a fall in cardiac output and can cause hypotension, vertigo, and syncope. Administration of atropine will have little or no effect. Cardiology should be notified immediately and consulted for temporary transvenous pacemaker insertion. If a cardiologist or other provider is not available, transcutaneous pacing should be implemented.4 The patient is at high risk for a cardiac arrest due to ventricular tachycardia or ventricular fibrillation.1,4Mobitz type I and type II second degree AV blocks create challenges in identifying rhythm disturbances, in part because of varying definitions. Experts have held this widespread definitional confusion to be responsible for misinterpretations of AV blocks.5 Using the simple definitions for Wenckebach and Mobitz type II will assure correct identification of the block.