Cerebral blood flow sensitivities to CO2 measured with steady-state and modified rebreathing methods

It is well established that the ventilatory response to carbon dioxide (CO2) measured by modified rebreathing (SrVE) is closer to that measured by the steady-state method (SsVE) than is the response measured by Read's rebreathing method. It is also known that the value estimated by the steady-state method depends upon the combination of data points used to measure it. The aim of this study was to investigate if these observations were also true for cerebral blood flow (CBF), as measured by steady-state (SsCBF) and modified rebreathing (SrCBF) tests. Six subjects undertook two protocols: (a) steady state: PETCO2 was held at 1.5 mmHg above normal (isocapnia) for 10 min, then raised to three levels of hypercapnia, (8 min each; 6.5, 11.5 and 16.5 mmHg above normal, separated by 4 min isocapnia). End-tidal PO2 was held at 300 mmHg; (b) modified rebreathing: subjects underwent 6 min of voluntary hyperventilation to PETCO2∼20 mmHg, and then rebreathed via a 6 l bag filled with 6.5% CO2 in O2. We confirmed that the value for SsVE depended upon the combination of data points used to calculate it, and also confirmed that SsVE and SrVE were similar. However, this was not the case with CBF. Estimates of SsCBF were the same, regardless of the data points used in calculation, and SsCBF was 89% greater than SrCBF (P < 0.05). We interpret these findings as consistent with the notion that the specific CO2 stimulus differs for CBF and ventilatory control. The data also indicate that prior hypocapnia in the modified rebreathing protocol may have a persistent effect on both cerebral vessels and central ventilatory control.