A microcomputer program of pulmonary and tissue gas exchange

A microcomputer program written in BASIC for the IBM-PC and compatibles has been developed to analyze the effects of many parameters on the gas exchange and transport phenomena in the lungs and the tissues. The program is designed for use by medical students and residents concerned with gas exchange (anesthesiology, pulmonary diseases, critical care, etc.) to study the steady state effects on blood and tissue oxygen and carbon dioxide levels. The present program consists of two main subroutines: Pulmonary Gas Exchange and Tissue Gas Exchange. Steady state gas exchange at the lungs can be studied using either a three-compartment model or V/Q relationships. The V/Q subroutine uses single or multiple populations of V/Q distributions to determine gas exchange using a log-normal distribution of V/Q ratios. Other variables can be adjusted which determine the arterial and mixed-venous blood gases. These values are then fed into the second part of the program to analyze factors which determine tissue O₂ tension. The Tissue Oxygen Tension subroutine is also subdivided into a modified Krogh-Erlang model, which provides a three-dimensional plot of theoretical capillary and tissue O₂ tensions, and a Piiper model which includes the effect of diffusion shunt on O₂ tensions and treats the tissue as a well-stirred compartment. Minimal and maximal tissue O₂ tensions are calculated using the Piiper model since the intercapillary distance is allowed to vary depending on the O₂ delivery by diffusion. Estimates of blood and tissue O₂ tensions, diffusion/perfusion coefficients, amount of O₂ delivered and the size of the active capillary bed are summarized in a table.