Cerebrovascular and blood pressure responses during voluntary apneas are larger than rebreathing

Both voluntary rebreathing (RB) of expired air and voluntary apneas (VA) elicit changes in arterial carbon dioxide and oxygen (CO₂ and O₂) chemostimuli. These chemostimuli elicit synergistic increases in cerebral blood flow (CBF) and sympathetic nervous system activation, with the latter increasing systemic blood pressure. The extent that simultaneous and inverse changes in arterial CO₂ and O₂ and associated increases in blood pressure affect the CBF responses during RB versus VAs are unclear. We instrumented 21 healthy participants with a finometer (beat-by-beat mean arterial blood pressure; MAP), transcranial Doppler ultrasound (middle and posterior cerebral artery velocity; MCAv, PCAv) and a mouthpiece with sample line attached to a dual gas analyzer to assess pressure of end-tidal (P_ET)CO₂ and P_ETO₂. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as P_ETCO₂/P_ETO₂ during RB. For VA, where P_ETCO₂ and P_ETO₂ could not be measured throughout, SI values were calculated using interpolated end-tidal gas values before and at the end of the apneas. MAP reactivity (MAPR) was calculated as the slope of the MAP/SI, and cerebrovascular reactivity (CVR) was calculated as the slope of MCAv or PCAv/SI. We found that compared to RB, VA elicited ~ fourfold increases in MAPR slope (P < 0.001), translating to larger anterior and posterior CVR (P ≤ 0.01). However, cerebrovascular conductance (MCAv or PCAv/MAP) was unchanged between interventions (P ≥ 0.2). MAP responses during VAs are larger than those during RB across similar chemostimuli, and differential CVR may be driven by increases in perfusion pressure.