PaCO₂ measurement in cerebral haemodynamics: face mask or nasal cannulae?

PaCO₂ affects cerebral blood flow (CBF) and its regulatory mechanisms, but the effects of CO₂ measurement technique on cerebrovascular parameters are unknown. In order to determine if the two most commonly used approaches, face mask (FM) or nasal cannulae (NC), are interchangeable or not, we tested the hypothesis that the use of FM versus NC does not lead to significant differences in CO₂-related systemic and cerebrovascular parameters.

Recordings of CBF velocity (CBFV), blood pressure (BP), heart rate, and end-tidal CO₂ (EtCO₂) were performed in 42 subjects during normocapnia (FM or NC) and 5% CO₂ inhalation (FM) or hyperventilation (NC). Dynamic cerebral autoregulation was assessed with the autoregulation index (ARI), derived by transfer function analysis from the CBFV response to a hypothetical step change in BP.

Significant differences in physiological parameters were seen between FM and NC: EtCO₂ (37.40 versus 35.26 mmHg, p  =  0.001) and heart rate (69.6 versus 66.7 bpm, p  =  0.001) respectively. No differences were observed for mean BP, CBFV or the ARI index.

Use of FM or NC for measurement of EtCO₂ leads to physiological changes and differences in parameter values that need to be taken into consideration when interpreting and/or comparing results in studies of cerebral haemodynamics.