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University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, United Kingdom
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
PCO2
(PETCO2) was held
near eucapnia (1.5 Torr above resting) for 40 min. In a
hypocapnic protocol, PETCO2
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
PETCO2 and end-tidal
PO2 were controlled by using the
dynamic end-tidal forcing technique. Beat-by-beat values were
calculated for the intensity-weighted mean velocity (
IWM),
signal power (
), and their
instantaneous product
(
IWM).
A simple model consisting of a delay, gain terms, time constants
(
f,on,
f,off) and baseline levels of
flow for the on- and off-transients, and a gain term
(gs) and time constant
(
s) 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
IWM, with gs = 1.26 %/Torr and
s = 427 s, that persisted
throughout the hypocapnic period. Finally, the responses at the onset
and relief of hypocapnia were asymmetric
(P < 0.001), with
f,on (6.8 s) faster than
f,off (14.3 s).
transcranial Doppler; hypocapnia; modeling
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