Delineating the Role of Cerebral Blood Flow and Hypocapnia on Neuromuscular Function: 1250 Board #43 May 28, 9

Neuromuscular fatigue, defined as the inability of a muscle to maintain a given level of force regardless of whether or not a task can be sustained, has been attributed, in part, to central mechanisms. Although reductions in cerebral blood flow (CBF) may be implicated in the development of central fatigue, the contribution from hypocapnic-reductions (i.e., PETCO2) in CBF versus reductions in CBF per se has yet to be isolated. PURPOSE: To examine the hypothesis that hypocapnia, independent of concomitant reductions in CBF, cause impairments in neuromuscular function. METHODS: Neuromuscular function, as indicated by motor evoked potentials (MEP), M-waves and cortical voluntary activation (cVA) of the flexor carpal radialis muscle during isometric wrist flexion, was assessed in 8 males (31±12 y) during 3 separate conditions: 1) cyclooxygenase inhibition using Indomethacin (Indo) to selectively reduce CBF; 2) controlled hyperventilation-induced hypocapnia (Hypocapnia); and 3) isocapnic hyperventilation (Isocapnia). CBF was estimated using transcranial Doppler ultrasound velocity measurements of the middle cerebral artery. Change from baseline and comparison of the 3 conditions was assessed using two-way repeated measures ANOVA. Relationships between variables were assessed using Pearson‘s correlations. RESULTS: The experimental conditions successfully isolated CBF and PETCO2. MEP amplitude (% M-wave amplitude) increased in hypocapnia (14.6±9.5%, p < 0.01) in comparison to Indo (-2.0±2.8%) and Isocapnia (0.6±1.8%); however, no significant changes were observed in M wave (p = 0.666) or cVA (p = 0.117). Correlation analysis revealed that MEP amplitude was inversely associated with PETCO2 (r = -0.54, p < 0.01) and cVA was weakly associated with CBF (r = 0.30, p < 0.05). CONCLUSIONS: Increased motor cortex excitability appears to be exclusive to hypocapnia but not reductions in CBF per se. However, a weak relationship between reductions in CBF and cVA exists, such that further research is required to confirm this finding and explore the potential implications. Supported by NSERC Discovery 227912-2012.