Second Degree Atrioventricular Block

If some, but not all, impulses are conducted from the sinus node to the ventricles, the atrioventricular block is described as second degree.

There are two distinct types of second degree atrioventricular block (as originally classified by the German Cardiologist Woldermar Mobitz):

Conduction: Second Degree Block

Mobitz Type I (Wenckebach)

The Wenckebach phenomenon (named after the Dutch Internist Karl Wenckebach) describes a disturbance in conduction where the atrioventricular node conducts each successive impulse earlier and earlier. Eventually, an impulse arrives when the node is not able to conduct it. The following impulse will then be conducted normally, causing the cycle to begin again.

The QRS complex and the P wave are normal, but not all P waves are followed by a QRS complex. The PR interval gets longer until an impulse is not conducted.

The ECG trace below shows sinus rhythm at a rate of 67 bpm (arrowed), with a lengthening PR interval (shaded) preceding a nonconducted P wave.

Electrocardiogram: Second Degree Block — Mobitz Type I (Wenckebach)
Electrocardiogram: Second Degree Block — Mobitz Type I (Wenckebach)

Mobitz Type II (Constant)

With Mobitz type II, there may be alternate conducted and nonconducted sinus impulses, with the ECG showing twice as many P waves as QRS complexes. This is termed 2:1 conduction.

As with other rhythms, the P wave may only show itself as a distortion of the preceding T wave.

Mobitz type II is more serious than Mobitz type I because the frequency and severity of the block is unpredictable. This form of block may degenerate to third degree atrioventricular block.

The ECG trace below shows P waves at a rate of 71 bpm (arrowed), with only every second impulse resulting in a QRS complex.

Electrocardiogram: Second Degree Block — Mobitz Type II (Constant)
Electrocardiogram: Second Degree Block — Mobitz Type II (Constant)

Mobitz Type II (Periodic)

The periodic version of Mobitz type II atrioventricular block consists of an interruption of electrical conduction, originating in the atrioventricular node, in which dropped beats occur without any pattern.

Most beats are conducted with a constant PR interval, but occasionally there is an atrial contraction (P wave) without a subsequent ventricular contraction (QRS complex).

The ECG trace below shows sinus rhythm with a constant PR interval. The fifth P wave does not conduct to the ventricles, leading to an RR interval of 1.76 seconds.

Electrocardiogram: Second Degree Block — Mobitz Type II (Periodic)
Electrocardiogram: Second Degree Block — Mobitz Type II (Periodic)
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