Dynamics of the cerebral blood flow response to step changes in
end-tidal pco2 and po2 in humans.
Poulin, M. J., P.-J. Liang, and P. A. Robbins.
University Laboratory of Physiology, Parks Road, Oxford OX1 3PT,
United Kingdom
APStracts 3:0221A, 1996.
This study examined the dynamics of the cerebral blood flow response
to hypoxia and hypercapnia in humans. Middle cerebral arterial blood
flow (MCAF) was assessed continuously using transcranial Doppler
ultrasound. MCAF was calculated on a beat-by-beat basis as the
product of the intensity-weighted mean velocity and the total power
of the reflected signal. End-tidal PCO2 (PETCO2) and PO2 (PETO2) were
controlled using a dynamic end-tidal forcing system. Six repeats of
each of four protocols were administered to six subjects. The first
was a control protocol with PETO2 held at 100 mmHg and PETCO2 held 1
-2 mmHg above eucapnia throughout. The second was a hypoxic step
protocol with PETO2 lowered from control values to 50 mmHg for 20
min. The third was a hypercapnic step protocol with PETCO2 elevated
from control by 7.5 mmHg for 20 min. The fourth was a combined
hypoxic and hypercapnic step protocol lasting 20 min. The total power
of the Doppler signal remained relatively constant, suggesting that
the cross sectional area of the vessel changed little. After the
initial transient in MCAF at the onset of the stimulus, no adaptation
or progressive increase was observed over the remaining 20 min. A
simple model consisting of a single pure delay, gain terms, time
constants and offsets for the on- and off- transients was fitted to
the hypoxic and hypercapnic protocols. For hypercapnia, all of the
parameters were significantly different for the onset compared with
the relief of the stimulus. The asymmetry was characterized by a
slower on-transient than off-transient, and also by a degree of
undershoot following the relief of hypercapnia. Finally, the results
from this study show that the cerebral blood flow response to hypoxia
and hypercapnia in humans is much faster than has previously been
thought.
Received 29 November 1995; accepted in final form 9 April 1996.
APS Manuscript Number A1238-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 May 96