Little carbon dioxide diffusional shunting in coronary circulation

Area of Science:

• Cardiovascular Physiology
• Respiratory Gas Exchange
• Myocardial Metabolism

Background:

• Understanding carbon dioxide (CO2) transport in the myocardium is crucial for assessing cardiac metabolism and blood flow.
• Previous studies have not precisely quantified CO2 diffusional trapping within the heart muscle.

Purpose of the Study:

• To determine the magnitude of CO2 countercurrent diffusional trapping in the myocardium.
• To quantify the extent of CO2 diffusion from coronary venous to coronary arterial vessels.

Main Methods:

• Utilized a pump-perfused open-chest dog model with a modified Gregg cannula.
• Simultaneously measured coronary venous partial pressure of CO2 (PCO2), pH, and partial pressure of O2 (PO2) using a continuous electrode-cuvette system.
• Employed a compartmental mass balance model to analyze CO2 washout and retention after step changes in coronary flow.

Main Results:

• A compartmental model indicated 80% of myocardial CO2 is in interstitial/intracellular spaces and 20% in vascular compartments.
• Maximal diffusional trapping yielded a modest end-capillary-to-end-venous CO2 concentration ratio of 1.02–1.04 at resting coronary flows.
• CO2 diffusional trapping was below experimental detection limits.

Conclusions:

• Diffusional trapping of CO2 in the myocardium is minimal and below experimental detection limits.
• At normal coronary blood flow rates, there is little evidence of significant CO2 diffusional trapping in the heart muscle.