Altered cardiac β1 responsiveness in hyperthermic older adults

Compared with younger adults, passive heating induced increases in cardiac output are attenuated by ~50% in older adults. This attenuated response may be associated with older individuals’ inability to maintain stroke volume through ionotropic mechanisms and/or through altered chronotropic mechanisms. The purpose of this study was to identify the interactive effect of age and hyperthermia on cardiac responsiveness to dobutamine-induced cardiac stimulation. Eleven young (26 ± 4 yr) and 8 older (68 ± 5 yr) participants underwent a normothermic and a hyperthermic (baseline core temperature + 1.2^◦C) trial on the same day. In both thermal conditions, after baseline measurements, intravenous dobutamine was administered for 12 min at 5 mg/kg/min, followed by 12 min at 15 mg/kg/min. Primary measurements included echocardiography-based assessments of cardiac function, gastrointestinal and skin temperatures, heart rate, and mean arterial pressure. Heart rate responses to dobutamine were similar between groups in both thermal conditions (P > 0.05). The peak systolic mitral annular velocity (S^'), i.e., an index of left ventricular longitudinal systolic function, was similar between groups for both thermal conditions at baseline. While normothermic, the increase in S^' between groups was similar with dobutamine administration. However, while hyperthermic, the increase in S^' was attenuated in the older participants with dobutamine (P < 0.001). Healthy, older individuals show attenuated inotropic, but maintained chronotropic responsiveness to dobutamine administration during hyperthermia. These data suggest that older individuals have a reduced capacity to increase cardiomyocyte contractility, estimated by changes in S^', via β1-adrenergic mechanisms while hyperthermic.