The effect of hypoxia on the ventilatory and cerebral blood flow (CBF) responses to CO2

Abstract

ObjectivesThis study examines the proposed approach to a standard measurement regime for characterizing the respiratory chemoreflexes as detailed by Duffin (2007) and develops a similar approach to characterize cerebrovascular responsiveness to hypoxia and CO2. We measured the effect of hypoxia on the isoxic modified rebreathing ventilatory and CBF responses to CO2 in terms of sensitivity, ventilatory recruitment threshold and wakefulness ventilation at 2 levels of isoxia (PO2 = 50 mmHg & 150 mmHg). We also measured the effect of hypercapnia on the isocapnic steady‐state (SS) ventilatory and CBF responses to hypoxia at 3 levels of hypercapnia (PCO2 = 4, 7 & 10 mmHg above resting). Finally we compared the isoxic rebreathing measurements with the SS isocapnic measurements.MethodCustom designed software controlling a specialized gas blending device (RespirAct™; Thornhill Research) was used to implement the steady state and modified rebreathing protocols. Ventilation (VE), PetCO2 and PetO2, middle cerebral artery velocity (MCAv), and blood pressure (BP) were recorded.Results9 subjects completed the tests (6 male).This graph displays the SS response of VE, MCAv and BP to CO2 (PCO2 = 50 mmHg) first in hyperoxia (PO2 = 150 mmHg) and then in hypoxia (PO2 = 50 mmHg). We compared the SS and rebreathing ventilation and CBF responses.Rebreathing analysis (figures above) gives us interesting results. Ventilation shows a higher slope (increase of sensitivity) as well as a lower threshold during the Hypoxic phase as expected. For the MCAv, the interindividual variability is larger than expected. This example shows same slope, as expected. Comparison with SS method is more complex. Some subjects might have limitations of the MCAv increase. Further analysis still need to be done.