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CLINICAL STUDIES

Attenuation of myocardial ischemia with repeated exercise in subjects with chronic stable angina

Relation to myocardial contractility, intensity of exercise and the adenosine triphosphate–sensitive potassium channel

^a Quebec Heart Institute/Laval Hospital, Laval University, Ste-Foy, Quebec, Canada

Manuscript received April 8, 1998; revised manuscript received July 8, 1998, accepted July 29, 1998.

Address for correspondence: Dr. Peter Bogaty, Quebec Heart Institute/Laval Hospital, 2725 Chemin Ste-Foy, Ste-Foy, Quebec, Canada G1V 4G5
peter.Bogaty{at}med.ulaval.ca

Objectives. This study characterized the attenuation of myocardial ischemia observed with re-exercise to determine whether: 1) a differing exercise intensity modifies this attenuation; 2) it could be explained by contractile down-regulation or stunning; 3) it is mediated by activation of ATP-sensitive potassium channels (K⁺-ATP).

Background. Subjects with ischemic heart disease (IHD) frequently note less angina with re-exercise after a brief rest. Potential mechanisms of this ‘warm-up’ phenomenon have been little explored.

Methods. IHD subjects with a positive exercise test were studied. Groups I and II (12 subjects each) underwent 2 successive Naughton protocol exercise echocardiography tests (with 1 min instead of 2 min stages for Group II). Group D (10 subjects) had type II diabetes, were on 10 mg daily of the K⁺-ATP blocker, glibenclamide, and underwent the group I exercise protocol. The ischemic threshold or rate-pressure product at 1 mm ST segment depression, ST depression corresponding to the peak rate-pressure product of the first exercise (maximum ST depression equivalent), and left ventricular wall motion indexes before and immediately after each exercise were analyzed.

Results. Exercise-induced myocardial ischemia with re-exercise was similarly attenuated in groups I, II, and D. The ischemic threshold was raised by nearly 20% with re-exercise (p = 0.001, p = 0.02, and p = 0.02, respectively) and the maximum ST depression equivalent was nearly halved on re-exercise (p = 0.005, p = 0.006, and p = 0.001, respectively). Exercise-induced wall motion dysfunction was attenuated with re-exercise. In group I, wall motion returned to the initial baseline score prior to exercise 2, whereas in the more intense protocol of group II, wall motion dysfunction persisted prior to exercise 2.

Conclusions. Thus, the attenuation of myocardial ischemia observed with re-exercise appears to be independent of the intensity of the exercise protocol and is not explained by down-regulation of myocardial contractility induced by the initial ischemic stimulus. Since results were similar in diabetic subjects on robust doses of glibenclamide, this phenomenon does not appear to be mediated by K⁺-ATP activation.

Abbreviations and Acronyms   ECG = electrocardiogram or electrocardiographic   ET = exercise test   IHD = ischemic heart disease   K+-ATP = adenosine triphosphate–sensitive potassium channel(s)   LV = left ventricular   RPP = rate–pressure product   STD = ST-segment depression   VD = vessel disease

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