VentSim: a simulation model of cardiopulmonary physiology.

VentSim is a quantitative model that predicts the effects of alternative ventilator settings on the cardiopulmonary physiology of critically ill patients. VentSim is an expanded version of the physiologic model in VentPlan, an application that provides ventilator-setting recommendations for patients in the intensive care unit. VentSim includes a ventilator component, an airway component, and a circulation component. The ventilator component predicts the pressures and airflows that are generated by a volume-cycled, constant-flow ventilator. The airway component has anatomic and physiologic deadspace compartments, and two alveolar compartments that participate in gas exchange with two pulmonary blood-flow compartments in the circulatory component. The circulatory component also has a shunt compartment that allows a fraction of blood flow to bypass gas exchange in the lungs, and a tissue compartment that consumes oxygen and generates carbon dioxide. The VentSim model is a set of linked first-order difference equations, with control variables that correspond to the ventilator settings, dependent variables that correspond to the physiologic state, and one independent variable, time. Because the model has no steady state solution, VentSim solves the equations by numeric integration, which is computation intensive. Simulation results demonstrate that VentSim predicts the effects of a variety of physiologic abnormalities that cannot be represented in less complex models such as the VentPlan model. For a ventilator-management application, the time-critical nature of ventilator-setting decisions limits the use of complex models. Advanced ventilator-management applications may include a mechanism to select patient-specific models that balance the trade-off of benefit of model detail and cost of computation delay.