A 6-year-old girl is admitted for appendectomy. After noting an irregular heart rate on preoperative examination, the nurse hands you this rhythm strip. What is the next most appropriate step?

A Reassurance—this is sinus arrhythmia

B Reassurance—this is second-degree Mobitz type I/Wenckebach atrioventicular (AV) block, a normal variant

C Consult cardiology—this is second-degree Mobitz type II AV block and could deteriorate to complete heart block

D Consult cardiology—this is type III/complete heart block and may be very problematic for safe anesthesia

E Check electrolytes—this is a junctional escape rhythm with a narrow QRS complex

Explanation:

Answer. B

This figure shows second-degree, Mobitz type I/Wenckebach AV block. The tracing shows 2 Wenckebach cycles with progressive lengthening of the PR interval until the atrial beat is not conducted to the ventricles. The first atrial impulse of a group of beats is conducted normally, but the next atrial impulse reaches the AV node while it is still partly refractory and thus is conducted more slowly, resulting in a longer PR interval. The next atrial impulse arrives even earlier in the AV nodal relative refractory period, resulting in an even longer PR interval. Eventually, the atrial impulse reaches the AV node in its absolute refractory period and is blocked so that no QRS complex follows. Mobitz type I second-degree AV block may occur in healthy people, particularly athletes. In this setting, it occurs at night and is caused by increased vagal tone. If it is asymptomatic in someone with no structural or functional heart disease, it does not require aggressive evaluation or treatment.

In contrast, Mobitz type II second-degree AV block occurs when a QRS complex drops out without prior lengthening of the PR intervals. It is less common but more serious than type I second-degree AV block and is more likely to lead to complete AV block. It usually results from disease in the His-Purkinje system below the level of the compact AV node and always requires careful evaluation; therefore, reassurance would be incorrect. Third-degree AV block occurs if no atrial beats are conducted and ventricular contraction is stimulated by a junctional or ventricular escape focus. There are normal P waves at one rate and QRS complexes at a slower rate, with no fixed relationship between P waves and QRS complexes (AV dissociation). Phasic sinus arrhythmia is a normal variant in which the sinus rate varies with increases with inspiration and slows with expiration secondary to changing right heart filling with negative intrathoracic pressure with inspiration. During expiration, the sinus rate may be sufficiently slowed to allow escape beats from an atrial or junctional pacemaker. An escape beat is recognized by its late appearance (R-R interval longer than normal) and evidence of a nonsinus focus (abnormal P-wave axis and morphology for an atrial ectopic focus, or no P wave, very short PR, or retrograde P wave for junctional focus).

Section 26: Disorders of the Cardiovascular System

Question is from Rudolph's Pediatrics, 23e, Self-Assessment and Board Review (C) 2023 McGraw Hill.