AJP - Regulatory, Integrative and Comparative Physiology, Vol 245, Issue 5 701-R705, Copyright © 1983 by American Physiological Society
Cerebrospinal fluid transients induced by hypercapnia
M. J. Fisher, S. R. Heisey, T. Adams and D. L. Traxinger
Ventriculocisternal perfusion studies using tracers have shown that
hypercapnia causes a transient increase in cerebrospinal fluid (CSF)
outflow rate (displaced CSF volume, Vd) and a decrease in CSF effluent
tracer concentration (tracer-free CSF, CSFtf). This dilution could be due
to an increase in CSF formation rate (Vf) and/or to displacement of
unequilibrated CSFtf sequestered in poorly mixed compartments. To
facilitate convection in the subarachnoid spaces, we used a "stop-flow"
procedure (by clamping the cisternal outflow tube while infusion was
constant) in anesthetized cats during ventriculocisternal perfusion with
mock CSF containing [14C]dextran. Each animal spontaneously breathed air,
then 5% CO2 both before and after stopflow. Although Vd and the times over
which Vd and CSFtf were defined were unaffected, CSFtf was decreased by 50%
after stop-flow. We conclude that during ventriculocisternal perfusion,
mixing is incomplete in CSF spaces, and that unequilibrated CSF contributes
significantly to the reduced tracer concentration in Vd during acute
hypercapnia. To determine whether Vf transiently increases in response to
CO2 breathing, or to any perturbation causing craniospinal fluid
redistribution, homogeneity in CSF spaces must be verified.
