A Double Dose of Trouble

ANSWER
This ECG demonstrates marked sinus bradycardia with a second-degree atrioventricular (AV) block (Mobitz I). Other findings include left-axis deviation, an old inferior MI, and poor R-wave progression.

Second-degree Mobitz I block is present in a 3:1 pattern of progressive prolongation of the PR interval until the third beat, where there is block in the AV node preventing conduction of the P wave to the ventricles. This is typically caused by progressive fatigue within the AV node until block occurs, then the cycle repeats.

Left-axis deviation is evidenced by a QRS axis of –78°. An old inferior MI is signified by the significant Q waves in leads II, III, and aVF. Finally, poor R-wave progression is demonstrated by small R waves in all of the precordial leads.

This ECG represented a significant change from one obtained three months earlier, during a routine outpatient visit. Careful review of the records at the Alzheimer facility revealed that the patient had received twice his usual dose of propranolol on three consecutive days. His rhythm returned to a baseline sinus rhythm (at 68 beats/min) after his ß-blocker was withheld for two days, and no further intervention was needed.