Intermittent heart block related to treatment of hypertension in a patient with acute myocardial infarction

Adverse effects due to morphine sulfate. Challenge to previous clinical doctrine

Despite a lack of clinical data in this area, conventional wisdom holds that morphine sulfate induces vagally mediated conduction defects, especially in patients with inferior myocardial infarction. To assess the accuracy of this "clinical pearl," the records of 244 patients admitted to the Barnes Hospital Cardiac Care Unit with suspected acute myocardial infarction were reviewed to determine the frequency of deleterious cardiovascular effects related to the administration of morphine sulfate. Of 184 patients (156 subsequently documented to have infarction) who received morphine sulfate, four patients had symptomatic hypotension temporally associated with morphine sulfate administration. This represented a frequency of 2.2 percent for all patients treated with morphine sulfate and a frequency of 2.6 percent in those with proved infarction. In each instance, the heart rate response was inappropriate, i.e., decreased or less markedly accelerated than might be expected given the reduced blood pressure, suggesting a vagal mechanism for the adverse effects. Only one of the four patients had inferior infarction, and in three of four instances, the adverse effect occurred after the first dose. All patients subsequently received morphine sulfate without evidence of toxicity. No case of narcotic-induced conduction abnormality was identified. This series, which is the most extensive evaluation of the topic, documents that adverse cardiovascular effects due to morphine sulfate are rare and do not conform to preconceived clinical doctrine. They consist of inappropriate heart rate responses to hypotension rather than conduction defects and are not particularly associated with inferior myocardial infarction.

Atrioventricular conduction in patients with clinical indications for transvenous cardiac pacing

Eighty patients with clinical indications for cardiac pacing had atrioventricular conduction analysed by His bundle study. The indications for cardiac pacing included high grade atrioventricular block, sick sinus node syndrome without tachycardia, bradycardia-tachycardia syndrome, unstable bilateral bundle-branch block, and uncontrolled ventricular irritability. Complete heart block, Wenckeback block (Mobitz I), and 2:1 block were noted proximal and distal to the His bundle. Mobitz II block only occurred distal to the His bundle. Of special interest were the high incidence of distal conduction abnormalities by His bundle analysis (40/80, 50%), the re-establishment of normal atrioventricular conduction in acutely ill patients with recent evidence of heart block, and the high incidence of intraventricular conduction disturbances on standard electrocardiogram (48/80, 60%).

The His bundle electrogram

The His bundle electrogram is discussed with respect to its rationale, methods for its recording and evaluation, findings with its use in the various forms of heart block and arrhythmias, its clinical value, and its limitations.