Muscle metaboreflex and cerebral blood flow regulation in humans: implications for exercise with blood flow restriction

We investigated the effect of activating metabolically sensitive skeletal muscle afferents (muscle metaboreflex) on cerebral blood flow and the potentially confounding influence of concomitant changes in the partial pressure of arterial carbon dioxide. Eleven healthy males (25±4 years) performed submaximal leg cycling exercise on a semi-recumbent cycle ergometer (heart rate ~120 b∙min(-1)), and assessments made of the partial pressure of end-tidal carbon dioxide (PETCO2), internal carotid artery blood flow (ICAQ) and conductance (ICACVC), middle cerebral artery mean blood velocity (MCAVm) and conductance index (MCACVCi).The muscle metaboreflex was activated during cycling with leg blood flow restriction (BFR) or isolated with post exercise ischemia (PEI). In separate trials, PETCO2 was either permitted to fluctuate spontaneously (control trial) or was clamped at 1 mmHg above resting levels (PETCO2 clamp trial). In the control trial, leg cycling with BFR decreased PETCO2 (Δ-4.8±0.9 mmHg vs. leg cycling exercise) secondary to hyperventilation, while ICAQ, ICACVC, and MCAVm were unchanged, and MCACVCi decreased. However, in the PETCO2 clamp trial, leg cycling with BFR increased both MCAVm (Δ5.9±1.4 cm∙s(-1)) and ICAQ (Δ20.0±7.8 ml∙min(-1)), and attenuated the decrease in MCACVCi, while ICACVC was unchanged. In the control trial, PEI decreased PETCO2 (Δ-7.0±1.3 mmHg vs. rest), MCAVm and MCACVCi, whereas ICAQ and ICACVC were unchanged. In contrast, in the PETCO2 clamp trial both ICAQ(Δ18.5±11.9 ml∙min(-1)) and MCAVm (Δ8.8±2.0 cm∙s(-1)) were elevated, while ICACVC and MCACVCi were unchanged. In conclusion, when hyperventilation-related decreases in PETCO2 are prevented the activation of metabolically sensitive skeletal muscle afferent fibres increases cerebral blood flow.