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Determine the following:

Rhythm: _______________________________________

Figure. No caption a... - Click to enlarge in new windowFigure. No caption available.

Rate: ___________________________________________


P waves: _______________________________________


PR interval: ____________________________________


QRS complex: __________________________________


What's your interpretation?

(Answers on next page)


Name that strip: Answers

Rhythm: Regular atrial rhythm; irregular ventricular rhythm


Rate: Atrial: 72 beats/minute; Ventricular: 50 beats/minute


P waves: Sinus P waves present


PR interval: Progressively lengthens from 0.20 to 0.32 second


QRS complex: 0.06 to 0.08 second


Comment: ST-segment depression is present


Interpretation: Second-degree AV block: Mobitz type I (Wenckebach block)


Mobitz I can be confused with a nonconducted premature atrial contraction (PAC). Both rhythms have episodes where P waves are not followed by a QRS complex, but instead by a pause. To differentiate between the two rhythms, one must examine the configuration of the P waves and measure the P-P regularity. The nonconducted PAC will have an abnormal P wave and will occur prematurely. In Mobitz I, the P wave is normal and occurs on schedule, not prematurely.


Mobitz I is common following acute inferior wall myocardial infarction owing to atrioventricular (AV) node ischemia. Other causes include increased parasympathetic (vagal) tone, effects of medications (digitalis, beta-blockers, calcium channel blockers), and hyperkalemia. Mobitz I may also occur as a normal variant in athletes because of a physiologic increase in vagal tone. Mobitz I, under certain conditions, may progress to a higher degree of AV block, but generally this is not the case. This type of AV block is usually temporary and resolves spontaneously.


Mobitz I is usually asymptomatic because the ventricular rate is often normal and cardiac output is usually not affected. If the ventricular rate is slow and the patient develops serious signs and symptoms related to the slow rate, Advanced Cardiovascular Life Support guidelines for clinically unstable bradycardia with a pulse should be followed. Conduction usually improves in response to the administration of I.V. atropine. Drugs causing AV block should be discontinued as prescribed.


Second-degree AV block: Mobitz type I (Wenckebach block): Identifying ECG features

Rhythm: Regular atrial rhythm; irregular ventricular rhythm


Rate: 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)


P waves: Sinus


PR interval: Progressive prolongation of the PR interval until a P wave is not conducted (P wave occurs without the QRS complex); a pause follows the dropped QRS complex.


QRS complex: Usually normal (0.10 second or less)