CARBON DIOXIDE AND OXYGEN TENSIONS OF THE MIXED VENOUS BLOOD OF MAN AT REST

Abstract

For the investigation of mixed venous blood gases in man at rest, 2 normal subjects were studied. The methods employed were simple, involving the rebreathing of gas mixtures from a bag, and the taking of instantaneous samples at intervals of 5 sec. during the rebreathing period. The latter lasted usually about 30 sec., sometimes as long as 50 sec. Thus it was found that an equilibrium can be regularly established, for CO2 with certain high O mixtures, and for both CO2 and O with certain low O mixtures. These 2 equilibria represent essentially the same CO2 content levels in the (venous) blood of the subject. The mixtures of rebreathed gases that can be used to establish such equilibria can be varied within small but fairly well defined limits. To accept these equilibria as representing the state of the mixed venous blood at rest involves certain assumptions; these have been described in the 1st part of the present study. Previous workers have shown that (a) with proper technique complete mixture of lung-bag gases is attainable; (b) "true" and "oxygenated" mixed venous CO2 levels are the same. The authors'' experiments have confirmed the above and have further shown that (c) the error due to progressive O absorption during the oxygenated mixed venous equilibration is small, (d) lung gases and blood are in approximate equilibrium at the end of a "true" mixed venous equilibration, (e) the method of intermittent rebreathings gives false "mixed venous" equilibria in 1 of the subjects studied. No fur-ther evidence has been obtained as to the presence or extent of error due to (a) altered respiratory move-ments during the procedure, or (b) recirculation within the time of the procedure, of small amounts of blood; but the comparison of the authors'' arteriovenous differ-ences with those recently obtained by Lauter with direct cardiac puncture, suggests that the error involved is small.