Theoretical basis of single breath gas absorption tests

Absorption of gas from alveoli is examined in a simplified model of the respiratory system during a stylized single breath consisting of constant inspiratory flow, constant expiratory flow, and breathholding. The equations describing gas behavior are general since they are based upon conservation of mass. The equations simplify considerably when gases that are not soluble in pulmonary tissue and/or blood are utilized. In a three-compartment model, diffusing capacity of the lung for carbon monoxide (D_CO) will be underestimated except when both uneven distribution of lung volume andD_CO are present; under most circumstances, the standard clinical 10-s method [9] is at least as accurate as any other. When pulmonary capillary blood flow is calculated by the one point method [2] in a one-compartment lung, it is underestimated; in the three-compartment model, it is underestimated except when both uneven distribution of. and lung volume are present. The multiple single breath method [2] accurately measuresD_CO and. Measurement of pulmonary tissue volume is improved by correcting the value of the intercept of acetylene absorption to the time when carbon monoxide apparently began rather than utilizing the beginning of inspiration.Nomenclature D_CO diffusing capacity of the lung for CO (ml CO, STPD/min/mm Hg) - pulmonary capillary blood flow rate (L/min) - V_t pulmonary tissue volume (L) - V_A alveolar compartment volume (L) - V_Ao alveolar compartment volume at conclusion of inspiratory flow (L) - inspiratory flow rate (L/sec) - expiratory flow rate (L/sec) - Bunsen coefficient of pulmonary tissue for test gas (ml test gas/ml tissue/atm) - Bunsen coefficient of pulmonary tissue for test gas (ml test gas/ml blood/atm) - F_A fractional pressure of test gas in the alveolar compartment (atm)