The Effect of Acute Hypocapnia and Respiratory Alkalosis on Neurovascular Coupling Magnitude

Abstract

The brain requires well‐regulated cerebrovascular perfusion to match local metabolic demand. Regional coupling of perfusion to metabolic rate is termed neurovascular coupling (NVC). NVC remains stable during blood gas perturbations upon ascent and acclimatization to altitude, where individuals are exposed to antagonistic effects of hypoxia (vasodilation) and hypocapnia (vasoconstriction), where acid‐base variables are likely well‐compensated. Few studies have addressed the effects of acute hypocapnia and respiratory alkalosis independently. The aim of this study was to assess the specific effects of acute steady‐state hypocapnia on NVC magnitude in a laboratory setting. We recruited 10 healthy participants and instrumented the posterior cerebral artery (PCA) with a transcranial Doppler ultrasound. NVC was elicited using a standardized strobe light stimulus (6 Hz; 5×30sec on/off), and both peak and mean absolute responses from baseline (BL) were quantified. Participants were coached to hyperventilate to reach steady‐state hypocapnic steps of Δ‐5 and Δ‐10 Torr end‐tidal (PET)CO2 from baseline levels. PETCO2 levels were significantly decreased from 36.7±4.2 (BL) to 31.0±5.1 and 25.9±5.4 Torr (P<0.05), with estimated arterial pH(a) values of 7.43±0.05 (BL), 7.47±0.08 (Δ‐5) and 7.59±0.11 (Δ‐10), respectively (P<0.05). There was a significant reduction in NVC magnitude from BL (10.2±2.5 cm/s) during controlled hypocapnia at both Δ‐5 (8.3±2.6 cm/s) and Δ‐10 (8.1±2.7 cm/s) in peak PCA velocity (PCAv) (P<0.05), but no significant decrease in mean PCAv (P>0.05). Our study demonstrates that acute respiratory alkalosis attenuates peak NVC magnitude with a floor effect at Δ‐5 Torr PETCO2 (~7.47 pHa), without further attenuation at Δ‐10 (~7.59 pHa). This is one of few studies demonstrating a change in NVC with acute blood gas challenges. However, the effect is small and only observed in peak PCAv responses, with no differences between mild and more severe respiratory alkalosis.Support or Funding InformationMRU Faculty of Science and Technology, University College Cork, University of Calgary