Capillary Recruitment of the Cerebral Cortex during CO2 Inhalation: Fact or Illusion?

Abstract

To the Editor: A recent article by Gobel et aI. (1989) reported data suggesting no capillary recruit­ ment in the rat brain during the high-flow conditions of hypercapnia. They employed 'V-globulin-coupled fluorescein isothiocyanate injected intravenously for the detection of perfused capillaries. This situ­ ation is just opposite to our own observations. CO2 was found to produce a gradual increase in red cell volume of the cerebral cortex when detected with our photoelectric apparatus (Tomita et aI., 1978) at­ tached in situ to the parietotemporal cortex of the cat. The increase by -1.0 vol% agreed well with that reported previously in the literature (Smith et aI., 1971). In addition, tissue hemodilution curves detected with the same apparatus following intraca­ rotid injection of a small amount of saline revealed marked shortening when compared with that of the control. Moment analysis of the transfer functions calculated from the tissue hemodilution curves (To­ mita et aI., 1983) indicated that with CO2 inhalation, the tissue microvasculature became more equally perfused in association with shortening of the mean transit time (MTT) of blood by 20%. Opening of thoroughfare channels or an arteriovenous di­ rect shunt (if any) in the microvasculature that may shorten the MTT is unlikely, since such open­ ing would increase the heterogeneity of perfusion as shown in our model analysis (Tomita and Gotoh, 1989). The combined evidence of an increase in red cell volume in the tissue, shortening of the MTT, and more homogeneous perfusion strongly suggests that capillary recruitment takes place in the cat ce­ rebral cortex. The discrepancy between our conclu­ sion and that of Gobel et al. might be attributable to methodological differences. Their technique might equally detect plasma capillaries with no or few red cells perfusing as well as normal capillaries full of red cells, while our method detects only cap­ illaries with red cells in the microvasculature. The term recruitment, which originally meant open­ ing of closed capillaries on the basis of the all­ or-none principle, may no longer be appropriate to describe relative changes in the density of red blood cells. Minoru Tomita and Fumio Gotoh Department of Neurology School of Medicine Keio University Tokyo, Japan