The effect of body temperature on the dynamic respiratory system compliance-breathing frequency relationship in the rat.

The mechanical inhomogeneity of the respiratory system is frequently investigated by measuring the frequency dependence of dynamic compliance, but no data are currently available describing the effects of body temperature variations. The aim of the present report was to study those effects in vivo. Peak airway pressure was measured during positive pressure ventilation in eight anesthetized rats while breathing frequency (but not tidal volume) was altered. Dynamic compliance was calculated as the tidal volume/peak airway pressure, and measurements were taken in basal conditions (mean rectal temperature 37.3 °C) as well as after total body warming (mean rectal temperature 39.7 °C). Due to parenchymal mechanical inhomogeneity and stress relaxation-linked effects, the normal rat respiratory system exhibited frequency dependence of dynamic lung compliance. Even moderate body temperature increments significantly reduced the decrements in dynamic compliance linked to breathing rate increments. The results were analyzed using Student's and Wilcoxon's tests, which yielded the same results (p < 0.05). Body temperature variations are known to influence respiratory mechanics. The frequency dependence of dynamic compliance was found, in the experiments described, to be temperature-dependent as temperature variations affected parenchymal mechanical inhomogeneity and stress relaxation. These results suggest that body temperature variations should be taken into consideration when the dynamic compliance-breathing frequency relationship is being examined during clinical assessment of inhomogeneity of lung parenchyma in patients.