Pulmonary diffusing capacity for carbon monoxide by rebreathing in mechanically ventilated patients.

Determination of pulmonary diffusing capacity is a routine method in pulmonary function laboratories for spontaneous breathing patients. However, it is not used in intensive care medicine for controlled ventilated patients with severe respiratory failure. We describe a rebreathing method for determination of pulmonary diffusing capacity for carbon monoxide (DCO) during mechanical ventilation based on an improved mathematical approach by Piiper and coworkers. The theoretical two-compartment model and the mathematical analysis are described. Factors (such as functional inhomogeneities, central blood volume, haemoglobin concentration, oxygen partial pressure, etc.) affecting this complex parameter of gas exchange are discussed. As the rebreathing technique for DCO seems to be influenced least by functional inhomogeneities in the diseased lung, it is advantageously qualified for measurements in intensive care patients. By adding an insoluble inert gas (for instance argon), functional residual capacity (FRC) can be determined at the same time. The method is well reproducible (+/- 3.8% for DCO and +/- 2.1% for FRC in duplicate determinations). During mechanical ventilation, the borderline towards pathological values determined by this method proved to be about 10 ml . min-1 . mmHg-1. First experimental and clinical results are presented which demonstrate DCO to be a qualified parameter for evaluating the pulmonary gas exchange function, indicating a progression of respiratory failure.