Sinus rate in acute myocardial infarction.

The clinical course of sinus bradycardia and sinus tachycardia has been studied in 735 patients owith acute myocardial infarction admitted to a coronary care unit. Sinus bradycardia was considered to have occurred when two or more records showed sinus rhythm with a rate below 6o a minute, and sinus tachycardia when a sinus rate over 0OO a minute occurred in two or more records. Hospital mortality of patients having sinus bradycardia was significantly lower (6%) and of those with sinus tachycardia was significantly higher (26%) than of those who had neither bradycardia nor tachycardia (i5%). Major ventricular arrhythmias were no more common in patients having sinus bradycardia and when they did occur did not usually do so at the time ,'that the bradycardia was present. Sinus bradycardia was more common in cases ofposterior and subendocardial infarction without radiological evidence of cardiac failure, while sinus tachycardia was frequent in patients with anterior transmural infarction complicated by heart failure. The benign course of sinus bradycardia is surprising because experimental evidence suggests that slow heart rates after infarction favour the occurrence of ventricular arrhythmias. Further work is necessary to correlate heart rate with prognosis, particularly in patients seen at the onset ,.f infarction, and before admission to a coronary care unit.