Integrative Physiology of Brain Perfusion During Extreme Chemoreflex Manoeuvers

Both cerebral perfusion pressure (CPP) and vascular conductance (CVC) contribute to blood flow. Yet, rarely are these factors considered in cerebrovascular reactivity studies, in favour of the change in mean flow velocity (MFV) for a given dilatory stimulus such as carbon dioxide, an approach that can ignore the potential contributions of CPP. We tested the hypothesis that both CVC and CPP facilitate the increase cerebral perfusion during progressive chemoreflex stress. In healthy free divers (n=7; 1 female; age range = 22 to 34 years) middle cerebral artery MFV (transcranial Doppler ultrasound; Multigon), mean arterial pressure (MAP; Finometer), central venous pressure (CVP; dependent antecubital vein), end tidal CO 2 (ETCO 2 ; ADInstruments), and hemoglobin saturation (HbSat, pulse oximetry) were measured while CPP (MAP‐CVP) and CVC (MFV/CPP) were calculated. These indices were obtained at baseline, during a 2‐min preparatory phase of rebreathing to establish early effects on CVC and CPP, and then during i) extended rebreathe (RB), ii) apnea at total lung capacity (TLC), or iii) apnea at functional residual capacity (FRC); each stress assigned randomly and continued to volitional tolerance. ETCO 2 increased 12–18 mmHg in all conditions (Effect of Time, P<0.001; no Condition effect). The 59% HbSat level attained during RB was lower than FRC (78%) or TLC (80%) (Condition x Time interaction P<0.001). MFV was similar at baseline (TLC, 52±11; FRC, 49±8; RB, 51±11 cm/s) and increased similarly (Time effect, P<0.001) in TLC (111±17 cm/s), FRC (106±18 cm/s) and RB (108±11 cm/s; No Condition effect). CVC increased similarly in each condition (TLC, 49±11; FRC, 57±31; RB, 61±23%; Effect of Time P<0.001; No Condition effect). Also, CPP increased similarly in each condition (TLC, 46±16%; FRC, 42±14%; RB, 36±16%; Time effect P<0.001). The CVC response typically plateaued following the preparatory 2‐min rebreathe phase of each protocol whereas the majority of the CPP response occurred during the Apnea and extended rebreathe phases. Therefore, the determinants of the linear increase in MFV across the preparatory rebreathe phase, and the subsequent RB, TLC, or FRC condition phases, switched from a largely CVC mechanism at lower levels of stress, to increased CPP during the extended Apnea or RB manoeuvres, regardless of differences in HbSat. Support or Funding Information NSERC discovery grant # 217916‐2013 awarded to J.K.S.

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