Effect of coronary artery disease on myocardial blood flow during passive heat stress

INTRODUCTION. Extreme heat is associated with a greater risk of cardiovascular mortality in older adults. However, the pathophysiology that links extreme heat and cardiovascular mortality remains unknown. One possibility is that myocardial oxygen requirements during heat exposure surpass the capacity for myocardial oxygen delivery, thereby predisposing to myocardial ischemia. This may notably occur in individuals with coronary artery disease (CAD), because it reduces the capacity of the coronary arteries to dilate (i.e., coronary flow reserve). However, it is unknown to what extent myocardial blood flow (MBF) increases during heat exposure and whether this increase is altered by CAD. OBJECTIVE. This study tested the hypothesis that passive heat stress increases myocardial blood flow in older adults and that this increase is attenuated in older adults with CAD. METHODS. Healthy older adults (n=19, 68 ± 7 years) and older adults with CAD (n=12, 72 ± 6 years) were passively heated (water-perfused suit) until rectal temperature increased 1.5°C above baseline values. Myocardial blood flow was measured at baseline and at each 0.5°C increment in rectal temperature during heating by positron emission tomography following intravenous infusion of 82-Rubidium. Heart rate (ECG) and blood pressure (automated auscultation) were also measured to calculate rate pressure product (heart rate × systolic blood pressure). MBF reserve (MFR) was calculated by dividing the MBF at 1.5°C by baseline MBF. Adults with CAD were asked to withhold β-blockers, calcium channel blockers and nitroglycerin for 24h prior to the study. RESULTS. At a 1.5°C increase in core temperature, myocardial blood flow increased from 0.85 ± 0.20 to 1.51 ± 0.40 mL/min/g in healthy older adults and from 0.87 ± 0.19 to 1.37 ± 0.48 mL/min/g in adults with CAD. The change in myocardial blood flow at a 1.5°C increase in rectal temperature did not differ between groups (healthy: 0.66 ± 0.31 mL/g/min, CAD: 0.49 ± 0.42 mL/g/min, p=0.25. Furthermore, healthy adults and adults with CAD displayed a similar MFR at an increase in rectal temperature of 1.5°C (healthy: 1.8 ± 0.4 vs. CAD: 1.6 ± 0.6, p=0.25). CONCLUSION. These results demonstrate that passive heat stress increases myocardial blood flow nearly 1.6 times in older adults, and that this increase is not affected by CAD. These results will help determine if the increase in myocardial blood flow during passive heat exposure is sufficient to predispose to myocardial ischemia in individuals with low coronary flow reserve. Canadian Institutes for Health Research, Montreal Heart Institute Foundation. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.