Sex differences in cerebral haemodynamics across the physiological range of PaCO₂

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.

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.

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: y _min, y _max, k (exponential coefficient) and x (EtCO₂ level) across the haemodynamic variables. Significant differences included the CBFV-EtCO₂ and ARI-EtCO₂ relationship; ARI_min (p  =  0.036) and CBFV_max (p  =  0.001), respectively. Furthermore, significant differences were observed for both CrCP_min (p  =  0.045) and CrCP_max (p  =  0.005) and RAP_min (p  <  0.001) and RAP_max (p  <  0.001).

This is the first study to examine sex individually within the context of a multi-level CO₂ protocol. The demonstration that the logistic curve parameters are influenced by sex, highlights the need to take into account sex differences between participants in both physiological and clinical studies.