An animated, non-compensating pulmonary model for teaching ventilation and perfusion relationships to medical students

Abstract

Medical students, in their study of gas exchange within the lung, are typically challenged by the complexities and interactions resulting from the non-uniform ventilation, diffusion, blood flow maldistribution, and the mechanisms of blood gas transport. The model uses simple animated graphics to show how a non-compensating, alveolus / capillary lung model would respond to changes in systemic and environmental parameters. The alveolus acts like a well-mixed compartment and has a non-cyclical bulk-flow input that is determined by frequency and depth of respiration. The resulting volume and partial pressures of the gas compartment are obtained by integrating the difference between this inflow and the outflow caused by simple diffusion into the blood compartment. Diffusion of oxygen from alveolus to blood is influenced by the mean-capillary oxygen partial pressure that is obtained by expressing Fick's first law of diffusion in terms of blood O2 concentration and integrating between pulmonary artery and end-capillary values of concentration and time. This procedure also provides the end-capillary oxygen concentration which varies as diffusion characteristics and blood flow influence gas exchange. The student can observe the effect on arterial oxygen by changing frequency or depth of breathing, dead space volume, diffusion coefficient, rate of blood flow, and the hemoglobin content of blood. After perturbing the system from steady-state, the student can try to return to normal values by altering blood flow, rate and depth of breathing, or metabolism. Clinical interventions, such as O2 supplementation or transfusion, may also be used. Students observe the clinical effects of diffusion impairment, increased dead space, venous admixture, hyper- and hypoventilation, altered cardiac output, anemia, and high altitude. Our goal is to facilitate the medical student's understanding of a complex system and to emphasize the benefits and limitations of physiologic compensation and clinical interventions.