Sex differences in cerebral haemodynamics across the physiological range of PaCO2

Objective: Cerebral blood flow (CBF) is influenced by changes in arterial CO2 (PaCO2). Recently, cerebral haemodynamic parameters were demonstrated to follow a four parameter logistic curve offering simultaneous assessment of dCA and CO2 vasoreactivity. However, the effects of sex on cerebral haemodynamics have yet to be described over a wide range of PaCO2. Approach: CBF velocity (CBFV, transcranial Doppler), blood pressure (BP, Finometer) and end-tidal CO2 (EtCO2, capnography) were measured in healthy volunteers at baseline, and in response to hypo- (−5 mmHg and −10 mmHg below baseline) and hypercapnia (5% and 8% CO2), applied in random order. Main results: Forty-five subjects (19 male, 26 female, mean age 37.5 years) showed significant differences between males and females in CBFV (50.9 ± 10.4 versus 61.5 ± 12.3 cm · s−1, p = 0.004), EtCO2 (39.2 ± 2.8 versus 36.9 ± 3.0 mmHg, p = 0.005), RAP (1.16 ± 0.23 versus 0.94 ± 0.40 mmHg cm · s−1, p = 0.005) and systolic BP (125.2 ± 8.0 versus 114.6 ± 12.4 mmHg, p = 0.0372), respectively. Significant differences between sexes were observed in the four logistic parameters: ymin, ymax, k (exponential coefficient) and x (EtCO2 level) across the haemodynamic variables. Significant differences included the CBFV-EtCO2 and ARI-EtCO2 relationship; ARImin (p = 0.036) and CBFVmax (p = 0.001), respectively. Furthermore, significant differences were observed for both CrCPmin (p = 0.045) and CrCPmax (p = 0.005) and RAPmin (p