Second-degree AV block (Mobitz II): Identifying ECG features

Atrial: Regular

Ventricular: Usually regular but may be irregular if AV conduction ratios vary Atrial: That of the underlying sinus rhythm Ventricular: Varies depending on number of impulses conducted through AV node (will be less than the atrial rate)

Sinus; two or three P waves (sometimes more) before each QRS complex May be normal or prolonged; remains consistent Normal if block located at level of bundle of His; wide if block located in bundle branches

Or T wave (Figure 8-25). The PR interval of the conducted beat may be normal or prolonged, but remains consistent. The ventricular rhythm is usually regular unless the AV conduction ratio varies (alternating among 2:1, 3:1, and 4:1). The location of the conduction disturbance is below the AV node in the bundle of His or bundle branches. As a result, the QRS complex may be narrow (if located in the bundle of His) or wide (if located in the bundle branches). The most common location is the bundle branches.

Mobitz II is usually associated with an anterior-wall MI and, unlike Mobitz I, is not the result of increased vagal tone or drug toxicity. Other causes include acute myocarditis and degeneration of the electrical conduction system seen in the elderly.

The patient’s response to Mobitz II is usually related to the ventricular rate. If the ventricular rate is within normal

Figure 8-24. Second-degree AV block, Mobitz II. Rhythm: Regular atrial and ventricular rhythm

Rate:  Atrial: 82 beats/minute

Ventricular: 41 beats/minute

P waves:  Two sinus P waves to each QRS complex

PR interval:  0.16 second (remains constant)

QRS complex: 0.14 second.

Figure 8-25. Second-degree AV block, Mobitz II. Rhythm: Regular atrial and ventricular rhythm

Rate:  Atrial: 123 beats/minute

Ventricular: 41 beats/minute

P waves:  Three sinus P waves to each QRS complex

PR interval:  0.24 to 0.26 second (remains constant)

QRS complex: 0.12 second.

Limits (rare), the patient may be asymptomatic. More commonly, the ventricular rate is extremely slow, cardiac output is decreased, and symptoms are present (hypotension, shortness of breath, heart failure, chest pain, or syncope). The syncopal episodes (called Stokes-Adams attacks or Stokes-Adams syncope) are caused by a sudden slowing or stopping of the heartbeat.

Mobitz II is less common but more serious than Mobitz I. Mobitz II has the potential to progress suddenly to third-degree AV block or ventricular standstill (asystole) with

I  ittle or no warning. Treatment is required immediately for symptomatic Mobitz II and for asymptomatic Mobitz

II  with wide QRS complexes in the setting of acute anterior-wall MI. An external pacemaker should be applied while preparations are made for insertion of a temporary transvenous pacemaker. Atropine is usually not effective in reversing Mobitz II second-degree AV block and may actually worsen the conduction disturbance. A dopamine infusion may be used to increase blood pressure. Unresolved Mobitz II will require a permanent pacemaker.

Figure 8-26. Mobitz I. This strip shows a typical Wenckebach pattern during the first part of the strip changing to a 2:1 conduction ratio at the end of the strip. Even though 2:1 conduction is seen (common with Mobitz II), the presence of a Wenckebach pattern confirms the diagnosis of Mobitz I.

Rhythm:  Atrial (regular); ventricular (irregular)

Rate:  Atrial (100 beats/minute); ventricular (60 beats/minute)

P waves:  Sinus

PR interval:  Progressively lengthens from 0.24 to 0.36 second

QRS complex: 0.06 to 0.08 second.

A comment about 2:1 conduction: A 2:1 conduction ratio is common with Mobitz II (two P waves to one QRS complex). A 2:1 conduction ratio may also occasionally occur with Mobitz I. In Mobitz I with 2:1 conduction, every other impulse is not conducted and the ECG shows two P waves to one QRS complex. The only difference on the ECG would be a narrow QRS (seen in Mobitz I) and a wide QRS (seen more commonly, but not exclusively, with Mobitz II). Typically, if Mobitz I with 2:1 conduction is present, an occasional Wenckebach pattern will usually assert itself when a longer rhythm strip is viewed, thus confirming the diagnosis of Mobitz I. Figure 8-26 shows such an example.

The AV block strips with consistent 2:1 AV conduction and a narrow QRS complex have been interpreted in the answer keys as Mobitz II with a notation that clinical correlation may be necessary to determine a definite diagnosis.

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