Exponential analysis of the pressure-volume characteristics of ovine lungs

Static pressure-volume curves were generated from data obtained from 18 normal anaesthetized adult sheep. Lung volumes were determined by helium dilution. An exponential curve of the form V = Vmax - Ae-KP was fitted to the pressure-volume data from each sheep where P is the static recoil pressure, Vmax represents the volume asymptote, A is the difference between Vmax and the intercept on the volume axis and K defines the slope and hence the shape of the P-V curve. Quality of fit of the data was assessed visually, by means of a sign test and a runs test and by the coefficient of determination (r2). Exponential equations were found to adequately describe the shape of the pressure-volume curve in sheep. The exponent K was not correlated with effective alveolar volume (VAeff) (rs = 0.183; P > 0.05). Static lung compliance was determined over a volume range from the end-expiratory level (VEEL) to VEEL plus 400 ml. Measurements of static lung compliance were significantly correlated with measurements of effective alveolar volume (VAeff) (rs = 0.505; P < 0.025). In the ovine, the exponent K, an index of distensibility, is independent of lung volume and offers a means of assessing lung distensibility in this species.