Cardiovascular drift and maximal oxygen uptake during heat stress
Wingo, Jonathan Eric
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The mechanisms causing cardiovascular drift (CV drift), the temporal rise in heart rate (HR) and fall in stroke volume (SV) during prolonged, constant -rate exercise, are well studied. Whether or not CV drift has any consequences is unknown. The research for this dissertation was designed to determine if the CV drift that occurs during prolonged exercise in hot conditions is associated with decreased maximal oxygen uptake (V . O2max), and further, whether this relationship is cause and effect. Three studies were completed. In each, CV drift was measur ed during 45 min of cycling at 60% V . O2max in 35°C, immediately followed by measurement of V . O2max. V . O2max also was measured after 15 min of cycling in 35°C. The purpose of the 15- and 45-min trials was to measure V . O2max during the same time interval in wh ich CV drift occurred. In Study 1, the 15- and 45-min trials were performed with and without fluid ingestion. Fluid ingestion did not affect CV drift or change in V . O2max. A 19% V . O2max decrease accompanied a 12% HR increase a nd 16% SV decrease, while %V. O2max increased from 63% to 78% between 15 and 45 min. In Study 2, the 45-min trials occurred either with HR held constant by lowering exercise inte nsity by a magnitude su fficient to attenuateCV drift or with unaltered exercise intensity. Power output decreased 37%, V . O2max decreased 7.5%, and %V . O2max decreased from ~60% to ~50% from 15 to 45 min when HR was held constant. When exercise in tensity was unaltered, HR increased 13%, SV decreased 10%, and V . O2max decreased 15%. In Study 3, the 45-min trials were performed with and without body cooli ng via fan airflow. V . O2max fell 18%, HR rose 16%, and SV fell 12% from 15–45 min without cooling, whereas V . O2max declined 5.7% with fan cooling, and HR and SV remained more stable across time. In conclusion, CV drift that occurs during prolonged exerci se in hot conditions is associated with decreased V . O2max, and the relationship appears to be cause and effect. Additionally, the HR– %V . O2max relationship is not preserved over time during prolonged exercise in heat stress in the absence of CV drift.