Altered cardiac? 1 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 mecha-nisms 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 degrees 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, gas-trointestinal 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 chrono-tropic 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 81-adrenergic mechanisms while hyperthermic.