Staying Awake to Evade Death
ANSWER
The correct answer includes sinus rhythm with second-degree atrioventricular (AV) block (Mobitz I) and a right bundle branch block (RBBB).
Sinus rhythm is evidenced by the consistent P-P intervals (best measured in the lead I rhythm strip). Mobitz I block is distinguished from Mobitz II block by the varying PR interval.
Remember that in Mobitz I (Wenckebach) block, the PR interval becomes progressively longer as the AV node fatigues, until there is blocked conduction between the atria and ventricles resulting in a P wave with no associated QRS complex. This is evident in the pause between the 6th and 7th QRS complexes. Notice also that the very next PR interval following the dropped QRS complex is much shorter than the PR interval immediately prior to that complex. In this example, the increasing PR interval before the pause is subtle compared to the PR intervals following the pause.
The skipped beats the patient feels are these pauses. Increased parasympathetic (vagal) activity occurs when a person is falling asleep, resulting in a reduction in heart rate and AV nodal conduction variability.
Finally, don’t forget the RBBB. It is indicated by a QRS duration > 120 ms, rSR’ complexes in precordial leads V1 to V3, and slurred S waves in leads I and aVF.
