The effect of rate on calculated alveolar surface tension

Isolated rat and rabbit lungs were inflated and deflated sequentially with air, saline and a liquid fluorocarbon (DC200) at various rates. In separate studies the saline lavage fluid was used as a source of surfactant and the effect of cycling rate was determined on a modified Cahn Surface Balance. DC200 has a low and constant surface tension at a saline interface and the pressure difference between the saline pressure volume (P-V) curve and DC200 P V curve is due only to changes in the alveolar surface area. Since surface tension is known for DC200 curves the mean air space diameter can be calculated using LaPlace's law. Assuming that mean air space diameter is the same during DC200 and air P V curves it is then possible to solve the LaPlace equation for the surface tension during the air inflation and deflation cycles. Results from these experiments reveal that calculated surface tension in the alveoli exhibits a hysteresis loop which closely resembles that of surfactant measured on a surface balance. During air P V curves the hysteresis was rate dependent while with saline hysteresis was markedly reduced and was independent of rate for comparable cycle durations. The rate dependent hysteresis for air cannot be due to resistance effects since both density and viscosity of air are less than 10^-3 that of saline. For air P V loops of 20 min duration the minimal and maximal surface tensions averaged 15.9 ± 2.3 and 42.7 ± 4.2 dynes/cm, respectively (mean ± S.E.).