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Middle cerebral artery blood velocity is reduced with hyperthermia during prolonged exercise in humans
Author(s) -
Nybo Lars,
Nielsen Bodil
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.t01-1-00279.x
Subject(s) - hyperthermia , middle cerebral artery , medicine , blood pressure , vo2 max , cardiology , anesthesia , mean arterial pressure , heart rate , ischemia
1 In the present study we examined the effect of hyperthermia on the middle cerebral artery mean blood velocity (MCA V mean ) during prolonged exercise. We predicted that the cerebral circulation would be impaired when hyperthermia is present during exercise and assumed that this could be observed as a reduced MCA V mean . 2 Eight endurance trained men (maximum oxygen uptake ( V̇ O2,max ) 70 ± 1 ml min −1 kg −1 (mean ± s.e.m .)) performed two exercise trials at 57 % of V̇ O2,max on a cycle ergometer in a hot (40 °C; hyperthermic trial) and in a thermoneutral environment (18 °C; control trial). In the hyperthermic trial, the oesophageal temperature increased throughout the exercise period reaching a peak value of 40.0 ± 0.1 °C at exhaustion after 53 ± 4 min of exercise. In the control trial, exercise was maintained for 1 h without any signs of fatigue and with core temperature stabilised at 37.8 ± 0.1 °C after ≈15 min of exercise. 3 Concomitant with the development of hyperthermia, MCA V mean declined by 26 ± 3 % from 73 ± 4 cm s −1 at the beginning of exercise to 54 ± 4 cm s −1 at exhaustion ( P < 0.001). In contrast, MCA V mean remained unchanged at 70‐72 cm s −1 throughout the 1 h control trial. 4 When individually determined regression lines for MCA V mean and arterial carbon dioxide pressure (P a,CO2 ) obtained during preliminary exercise tests were used to account for the differences in P a,CO2 between the hyperthermic and control trial, it appeared that more than half of the reduction in MCA V mean (56 ± 8 %) was related to a hyperventilation‐induced drop in P a,CO2 . Declining cardiac output and arterial blood pressure accounted for the remaining part of the hyperthermia‐induced reduction in MCA V mean . 5 The present results demonstrate that the development of hyperthermia during prolonged exercise is associated with a marked reduction in MCA V mean .