The Effect of Heat Exposure on Myocardial Blood Flow and Cardiovascular Function

Background: Heat extremes are associated with greater risk for cardiovascular death. The pathophysiologic mechanisms mediating this association are unknown. Objective: To quantify the myocardial blood flow (MBF) requirements of heat exposure. Design: Experimental study. (ClinicalTrials.gov: NCT04549974) Setting: Laboratory-based. Participants: 61 participants, comprising 20 healthy young adults (mean age, 28 years), 21 healthy older adults (mean age, 67 years), and 20 older adults with coronary artery disease (CAD) (mean age, 70 years). Intervention: Participants were heated until their core temperature increased 1.5 degrees C; MBF was measured before heat exposure and at every increase of 0.5 degrees C in core temperature. Measurements: The primary outcome was MBF measured by positron emission tomography-computed tomography. Secondary outcomes included heart rate, blood pressure, and body weight change. Results: At a core temperature increase of 1.5 degrees C, MBF increased in healthy young adults (change, 0.8 mL/min/g [95% CI, 0.5 to 1.0 mL/min/g]), healthy older adults (change, 0.7 mL/min/g [CI, 0.5 to 0.9 mL/min/g]), and older adults with CAD (change, 0.6 mL/min/g [CI, 0.3 to 0.8 mL/min/g]). This represented a 2.08-fold (CI, 1.75- to 2.41-fold), 1.79-fold (CI, 1.59- to 1.98-fold), and 1.64-fold (CI, 1.41- to 1.87-fold) change, respectively, from preexposure values. Imaging evidence of asymptomatic heat-induced myocardial ischemia was seen in 7 adults with CAD (35%) in post hoc analyses. Limitations: In this laboratory-based study, heating was limited to about 100 minutes and participants were restricted in movement and fluid intake. Participants refrained from strenuous exercise and smoking; stopped alcohol and caffeine intake; and withheld beta-blockers, calcium-channel blockers, and nitroglycerin before heating. Conclusion: Heat exposure that increases core temperature by 1.5 degrees C nearly doubles MBF. Changes in MBF did not differ by age or presence of CAD, but some older adults with CAD may experience asymptomatic myocardial ischemia. Primary Funding Source: Canadian Institutes of Health Research.