Fast and slow components of cerebral blood flow response to step decreases in end-tidal PCO2 in humans

This study examined the dynamics of the middlecerebral artery (MCA) blood flow response to hypocapnia in humans(n = 6) by using transcranial Dopplerultrasound. In a control protocol, end-tidalPCO₂(PET_CO2) was heldnear eucapnia (1.5 Torr above resting) for 40 min. In ahypocapnic protocol, PET_CO2was held near eucapnia for 10 min, then at 15 Torr below eucapnia for20 min, and then near eucapnia for 10 min. During both protocols,subjects hyperventilated throughout andPET_CO2 and end-tidalPO₂ were controlled by using thedynamic end-tidal forcing technique. Beat-by-beat values werecalculated for the intensity-weighted mean velocity (_IWM),signal power (), and theirinstantaneous product(_IWM).A simple model consisting of a delay, gain terms, time constants(_f,on, _f,off) and baseline levels offlow for the on- and off-transients, and a gain term(g_s) and time constant(_s) for a second slower component was fitted to the hypocapnic protocol. The cerebral bloodflow response to hypocapnia was characterized by a significant (P < 0.001) slowprogressive adaptation in_IWM, with g_s = 1.26 %/Torr and_s = 427 s, that persistedthroughout the hypocapnic period. Finally, the responses at the onsetand relief of hypocapnia were asymmetric(P < 0.001), with_f,on (6.8 s) faster than_f,off (14.3 s).