Capnography and the Bain circuit I: A computer model.

The Mapleson D anesthesia breathing system has no valves and allows rebreathing of carbon dioxide. Its coaxial version is known as the Bain system. The interpretation of capnograms obtained during its use requires an understanding of the interrelationships of patient and system variables. Toward that end, a systematic description of mechanical ventilation with the Bain circuit was undertaken based on the physical laws of gas transport. The mathematical formulation of the model contains the relations between pressure, flow, and volume in the tube, alveolar space, and ventilator. The flows, calculated from these relations, are used to determine the CO2 concentrations in the different parts of the model. Two sets of data are used--patient and system. The patient data, used to solve the equations numerically, are lung-thorax compliance, CO2 inflow into alveolar space (CO2 production), functional residual capacity, dead space volume, airway resistance, and respiratory quotient. The ventilation system data comprise the dimensions and volumes of the Bain circuit, ventilator, connectors, and tubes; spill valve pressure; resistances to flow in the individual tube parts; ventilator settings; and fresh-gas flow rates. After incorporation of a volunteer's respiratory variables into the model, capnograms obtained from the model compared well with those obtained from the volunteer. The structure of the model is such that it permits easy introduction or changes of patient and system variables to obtain individual results or model specific circumstances. This flexibility makes it a useful tool for understanding the properties of the Bain circuit under a variety of clinical circumstances. The results may be displayed in a number of different ways.