Little carbon dioxide diffusional shunting in coronary circulation

The magnitude of CO₂ countercurrent diffusional trapping (net diffusion from coronary venous to coronary arterial vessels) in the myocardium was determined. The left main coronary artery was pump perfused with a modified Gregg cannula in open-chest, heart-blocked, anesthetized dogs. Coronary venous PCO₂, pH, and PO₂ were simultaneously measured with a rapidly responding continuous electrode-cuvette system. Coronary venous CO₂ concentration was calculated from the electrode outputs and a corresponding hemoglobin determination. After a step change in coronary flow, the amount of CO₂ washed out or retained by the perfused myocardium was determined from the resulting coronary venous CO₂ concentration changes. Analysis of the data with a compartmental mass balance model indicated that CO₂ washouts and retentions were best fit with a model in which 80% of the myocardial CO₂ resides in the interstitial and intracellular compartments and the remaining 20% in vascular compartments. The maximal diffusional trapping compatible with the data resulted in a modest end-capillary-to-end-venous CO₂ concentration ratio of 1.02 to 1.04 at resting coronary flows. It is concluded that diffusional trapping of myocardial CO₂ was below the detection limits ofthe experiment and that at normal coronary blood flow rates there is little CO₂ diffusional trapping.