Increased Diastolic Time

Diastolic time (DT) calculated as the cycle length minus electromechanical systole (QS2) has a nonlinear relationship to heart rate (HR), increasing rapidly as rates fall below 75. The effect of propranolol on DT was studied in 150 patients with coronary artery disease. Patients were divided into three groups. Group I included patients with stable angina pectoris: propranolol (2.5 mg, i.v.) significantly increased DT from 411 +/- 18 to 527 +/- 22 msec (p less than 0.001) in 23 patients of group I; therapy with propranolol (mean daily dose 200 +/- 15 mg) increased DT from 446 +/- 29 to 766 +/- 26 msec (p less than 0.001) in 15 patients with stable angina. Group II was made up of patients with acute myocardial infarction: Propranolol (2.5 mg, i.v.) increased DT from 379 +/- 16 to 458 +/- 24 (p less than 0.001) in 18 of these patients. Group III included patients with recent coronary bypass surgery: propranolol (2.5 mg, i.v.) increased DT from 323 +/- 9 to 468 +/- 24 msec (p less than 0.001) in 14 patients 7 days after surgery. In addition, DT at 15 hr and 2 weeks after surgery was compared in 30 patients maintained on propranolol (mean daily dose, 155 +/- 11 mg preoperative and 68 +/- 9 mg postoperative) and 50 other patients who underwent coronary bypass surgery not on propranolol. DT was greater in propranolol patients (546 +/- 21 vs. 388 +/- 16 msec, p less than 0.001), preoperative and 396 +/- 15 vs. 320 +/- 12 msec, p less than 0.001, postoperative). Changes in DT after propranolol are mainly attributed to decreased HR. Changes in QS2 were much less profound and always less (p less than 0.01) than changes in DT. Thus propranolol significantly increased DT per beat in patients with coronary artery disease, which allowed more time for coronary perfusion; this effect of propranolol could well be as important as the reduction of myocardial oxygen consumption.