Extremes of cerebral blood flow during hypercapnic squat-stand maneuvers

Squat-stand maneuvers (SSMs) are a popular method of inducing blood pressure (BP) oscillations to reliably assess dynamic cerebral autoregulation (dCA), but their effects on the cerebral circulation remain controversial. We designed a protocol whereby participants would perform SSMs under hypercapnic conditions. Alarmingly high values of cerebral blood flow velocity (CBFV) were recorded, leading to early study termination after the recruitment of a single participant. One healthy subject underwent recordings at rest (5 min sitting, 5 min standing) and during two SSMs (fixed and random frequency). Two sets of recordings were collected; one while breathing room air, one while breathing 5% CO₂ . Continuous recordings of bilateral CBFV (transcranial Doppler), heart rate (ECG), BP (Finometer), and end-tidal CO₂ (capnography) were collected. Peak values of systolic CBFV were significantly higher during hypercapnia (p < 0.01), and maximal values exceeded 200 cm.s^-1 . Estimates of dCA (ARI) during hypercapnia were impaired relative to poikilocapnia (p = 0.03). The phase was significantly reduced under hypercapnic conditions (p = 0.03). Here we report extremely high values of CBFV in response to repeated SSMs during induced hypercapnia, in an otherwise healthy subject. Our findings suggest that protocols performing hypercapnic SSMs are potentially dangerous. We, therefore, urge caution if other research groups plan to undertake similar protocols.