Propranolol therapy alters estimation of potential cardiovascular risk derived from submaximal postinfarction exercise testing

Abstract

We studied the effect of propranolol administration on risk assessment based on submaximal exercise testing performed early after myocardial infarction. A total of 70 patients with recent infarction underwent modified Bruce treadmill testing with simultaneous measurement of expired gases in the absence of antianginal agents including β-antagonists. Among these, 31 patients who had at least one of the following abnormalities—ST depression ⩾ 1 mm (22 patients), chest pain (four patients), or treadmill time < 360 seconds (12 patients)—were studied in a randomized double-blind fashion and received either placebo or 240 mg of propranolol/day. A total of 28 patients completed the randomized phase and were able to undergo repeat exercise testing an average of 3.4 ± 1.8 days later. Randomized groups were equivalent at baseline except for a higher peak oxygen consumption and carbon dioxide production ( p < 0.05) in the propranolol compared with the placebo group; these differences were taken into account in statistical analyses of the study data. Resting heart rate (59 ± 1.2 versus 82 ± 4.2 beats/min) and peak heart rate X systolic blood pressure (14,208 ± 496 versus 20,075 ± 1,062) were both significantly less ( p < 0.01) after propranolol than after placebo. Eight of nine patients treated with placebo maintained ST depression ⩾ 1 mm from the initial to the randomized exercise test, compared with only 4 of 13 receiving propranolol ( p < 0.01). In those with continued ST depression, time to positivity was significantly longer in those receiving propranolol compared with those taking placebo (538 ± 73 versus 318 ± 44 seconds, p < 0.05). In contrast, the peak ratio between carbon dioxide production and oxygen consumption was higher in those receiving propranolol compared with those receiving placebo (0.93 ± 0.04 versus 0.81 ± 0.03, p < 0.05). We conclude that propranolol therapy reduces evidence of ischemia and changes traditional estimates of potential cardiac risk derived from submaximal postinfarction exercise testing.