Fast and slow components of cerebral blood flow response to step decreases in end-tidal P CO 2 in humans

Abstract

This study examined the dynamics of the middle cerebral artery (MCA) blood flow response to hypocapnia in humans ( n = 6) by using transcranial Doppler ultrasound. In a control protocol, end-tidal[Formula: see text]([Formula: see text]) was held near eucapnia (1.5 Torr above resting) for 40 min. In a hypocapnic protocol, [Formula: see text]was held near eucapnia for 10 min, then at 15 Torr below eucapnia for 20 min, and then near eucapnia for 10 min. During both protocols, subjects hyperventilated throughout and[Formula: see text] and end-tidal[Formula: see text] were controlled by using the dynamic end-tidal forcing technique. Beat-by-beat values were calculated for the intensity-weighted mean velocity ([Formula: see text] IWM), signal power ([Formula: see text]), and their instantaneous product ([Formula: see text] IWM). A simple model consisting of a delay, gain terms, time constants (τf,on, τf,off) and baseline levels of flow for the on- and off-transients, and a gain term (gs) and time constant (τs) for a second slower component was fitted to the hypocapnic protocol. The cerebral blood flow response to hypocapnia was characterized by a significant ( P < 0.001) slow progressive adaptation in[Formula: see text] IWM, with gs = 1.26 %/Torr and τs = 427 s, that persisted throughout the hypocapnic period. Finally, the responses at the onset and relief of hypocapnia were asymmetric ( P < 0.001), with τf,on (6.8 s) faster than τf,off (14.3 s).