Effects of incremental levels of continuous positive airway pressure on cerebral blood flow velocity in healthy adult humans

Sleep disordered breathing is common in patients with cerebrovascular disease, and could exacerbate the cerebral damage in acute stroke. Data about the effects of continuous positive airway pressure (CPAP) upon cerebral perfusion are conflicting. We investigated whether increasing levels of CPAP may affect cerebral haemodynamics, assessed by transcranial Doppler (TCD) in normal humans. A group of 25 healthy young volunteers were evaluated before (CPAP0-pre), during (CPAP5, CPAP10 and CPAP15, denoting CPAP at 5, 10 and 15 cmH²O respectively) and after (CPAP0-post) application of incremental levels of CPAP delivered through a mouthpiece. The mean cerebral blood flow velocity (CBFV) and the pulsatility index (PI; an indirect measure of cerebrovascular resistance) in the middle cerebral artery were measured with TCD. Respiratory rate, heart rate, end-tidal carbon dioxide pressure (PETCO²), transcutaneous haemoglobin oxygen saturation (SpO²), mean arterial blood pressure and anxiety score were also recorded. Compared with CPAP0-pre, CBFV was significantly decreased as higher levels of CPAP were applied (P<0.0001). CPAP15 increased PI (P<0.05), PETCO² was reduced by CPAP10 and CPAP15 (P<0.0001), and anxiety score and SpO² increased at all levels of CPAP (P<0.05). Heart rate, respiratory rate and mean arterial pressure did not change. The decrease in CBFV was correlated with the fall in PETCO² (CPAP15) and the increase in PI (CPAP10, CPAP15) (P<0.05). In conclusion, even low levels of CPAP delivered through a mouthpiece in awake, young volunteers led to a decrease in CBFV, measured by TCD. This fall in CBFV was associated with hypocapnia and with an increase in both cerebrovascular resistance and anxiety due to breathing against positive pressure. As the negative consequences of a fall in CBFV may outweigh the therapeutic effects of CPAP in the post-stroke setting, further studies of the cerebrovascular effects of CPAP with different interfaces in elderly patients with and without stroke are needed before intervention trials can be performed safely.