The Relation Between Cardiorespiratory Fitness And Cerebral Blood Flow Regulation

Abstract

Contradictory relationships have been reported between cardiorespiratory fitness levels and cerebral blood flow regulation. Some studies report an inverse relationship between cardiorespiratory fitness and cerebrovascular reactivity to carbon dioxide (CO2), and an inverse relationship between cardiorespiratory fitness and dynamic cerebral autoregulation. Other studies have found a positive relationship. PURPOSE: The purpose of this study was to assess the relation between objectively measured cardiorespiratory fitness and cerebrovascular reactivity to CO2 and to dynamic cerebral autoregulation. METHODS: Twenty-three healthy, normotensive adults (13M/10F; age: 26±4 yrs; BMI: 24±5 kg/m2; BP: 105±9/58±6 mmHg, mean±SD) participated in this study. Mean VO2 max was 40.0±9.7 ml/kg/min (range, 17.9-62.3 ml/kg/min). Each participant completed a maximal graded exercise test on a treadmill until volitional fatigue. Heart rate was measured using a heart rate monitor (Polar H7, Polar, USA). Oxygen consumption and CO2 production were measured and averaged in 15-second intervals using indirect calorimetry via an automated open circuit system (Parvo Medics, Sandy, UT) throughout the exercise test. Transcranial Doppler of the right middle cerebral artery was measured. We measured cerebrovascular reactivity to two minutes of hypercapnia (via 8% CO2, 21% oxygen, balance nitrogen). We assessed dynamic cerebral autoregulation during eight minutes of supine rest. The relation between cerebral blood flow regulation and cardiorespiratory fitness were analyzed using Pearson’s correlations. RESULTS: Cerebrovascular reactivity to hypercapnia was not significantly correlated with cardiorespiratory fitness (r=0.35, P=0.10). There was no correlation with cardiorespiratory fitness in very low frequency gain (r=-0.22, P=0.92) or phase (r=-0.03, P=0.87). There was no correlation with cardiorespiratory fitness and low frequency gain (r=0.3, P=0.13). Interestingly, low frequency phase was inversely correlated with cardiorespiratory fitness (r=-0.4, P=0.04). CONCLUSION: These preliminary data suggest that cardiorespiratory fitness may not impact cerebrovascular reactivity to hypercapnia. However, a relation may exist between cardiorespiratory fitness and dynamic cerebral autoregulation.