Effective airway resistance: a reliable variable from body plethysmography.

Since the introduction of whole-body plethysmography in pulmonary function routines, airway resistance measurements by this method have been plagued by a side-effect of the ventilation process. Warming up and humidifying of inspired air and the cooling and condensation of expired air lead to a change in volume of the gas in the system and thus to a change in pressure in the body box which is superimposed on the regular box pressure. One generally applied solution is to compensate electronically for the disturbing pressure change. However, the required adjustment of the size of the compensating signal is difficult or impossible to obtain with patients who have obstructive lung diseases. In this paper we show that if no correction at all is applied the measurement error in the effective airway resistance remains relatively small. Moreover the magnitude of the error appears to depend on certain symmetrical and asymmetrical properties of the mouth flow curve. The maximum error in the effective resistance, as computed from a number of measured flow patterns, was found to be -18% for the normal persons and -9% for patients with obstructive lung diseases. We conclude that the uncorrected effective airway resistance is reasonably accurate. Alternatively, a good accuracy can be obtained with only a coarse adjustment of the correction circuit.