The Effect Of Enhanced Cardiac Contractility On Vertebral Artery Blood Flow In Heat-stressed Individuals

Blood supply to the brain originates from the internal carotid arteries (ICA) and vertebral arteries (VA). The responsiveness of the ICA and VA blood flows to various stresses, such as changes in partial pressure of carbon dioxide (PCO2), orthostatic stress, exercise, and others is not always identical. Thus, changes in flow through these arteries to changes in intravascular pressures and/or temperature may differ, which may affect cerebral circulation in hyperthermia. PURPOSE: To characterize ICA and VA blood flow responses to a beta-1 agonist, dobutamine, under normothermia and during heat stress. METHODS: Nine healthy volunteers participated in this study. Intravenous dobutamine (DOB) was administered for 12 min at a dose of 5 μg/kg/min, followed by 12 min at a dose of 15 μg/kg/min, both under normothermia and during heat stress conditions. In addition to the blood flow in the ICA and VA, blood flow through the external carotid artery (ECA) was measured via vascular ultrasound. RESULTS: DOB infusion increased the peak systolic mitral annular velocity (S'), i.e., an index of left ventricular longitudinal systolic function, and systolic blood pressure. Despite these responses, ICA and VA blood flows decreased during the DOB challenge in normothermia, while ECA blood flow increased (see Table). During the DOB challenge while heat stressed, only VA blood flow decreased, with ICA and ECA blood flows remaining unchanged. Interestingly, at the high dose of DOB, retrograde flow was observed in the VA but not in the ICA, which remained antegrade flow. CONCLUSION: These data suggest that flow through the VA is more susceptible to the combined effects of pressure and temperature compared to blood flows through the ICA and ECA. - normothermia heat stress rest low DOB high DOB rest low DOB high DOB Heart rate, bpm 65±12 67±12 102±19 †‡ 99±15 * 113±21 *† 145±19 *†‡ Systolic blood pressure, mmHg 116±10 139±14 † 174±19 †‡ 120±19 132±22 † 140±31 *†‡ Diastolic blood pressure, mmHg 67±7 67±6 62±5 56±10 * 50±9 * 45±9 *† Mean blood pressure, mmHg 83±7 91±7 † 99±9 †‡ 78±8 * 77±11 * 77±16 * S’, cm/s 9.4±0.9 13.9±1.7 † 19.5±4.4 †‡ 13.9±2.1 * 21.5±2.8 *† 27.7±5.2 *†‡ ICA blood flow, cm/min 318.8±64.7 293.3±51.6 † 301.8±53.2 † 292.3±48.4 * 286.1±56.8 273.9±58.5 * ECA blood flow, cm/min 104.4±43.5 123.7±45.7 † 167.4±69.4 †‡ 262.7±114.8 * 272.2±118.5 * 267.3±104.2 * VA blood flow, cm/min 154.6±46.5 141.3±40.7 131.9±43.6 † 140.4±36.4 128.0±35.5 101.6±23.9 *†‡ End-tidal PCO2, mmHg 47.9±2.8 43.9±4.1 † 43.2±2.5 † 44.1±3.4 * 40.9±5.4 † 36.4±5.6 *†‡ *:p < 0.05 from normothermia,†:p < 0.05 from rest for that thermal state,‡:p < 0.05 from low DOB for that thermal state Supported by: 18H03166

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