In 11 anesthetized (chloralose 50 mg/100 gr. i.p.), paralyzed (cis-atracurium 1mg/100gr. i.p.), positive-pressure ventilated rats, we measured static respiratory system compliance (Cst,rs) by inflating the respiratory system using a constant flow pump, set to deliver 3 ml air (V). Static elastic pressures (Pel,rs) were measured by a pressure transducer, and Cst,rs values were calculated as Csr,rs = V / Pel,rs.

Measurements were taken in basal condition (mean rectal temperature 36.9 ± 0.06C°) and after slowly warming the rats by an infrared lamp (150 W) positioned at about 50 cm (mean rectal temperature 39 ± 0.02 C°).

At both the temperatures, the respiratory system was inflated by a precision glass syringe for five 1 ml steps, and then deflated in a similar manner. Respiratory sytem static elastic pressures in inflation and deflation were measured for each volume, and the hysteresis surface area (Hy,rs) calculated for each rat by electronic digital integration.

The mean value of Cst,rs at 36.9 C° resulted lower than at 39 C° (0.44 ± 0.02 vs. 0.525 ± 0.02 ml/cmH2O respectively, p < 0.01, Student t-test for paired data), while the decrease of the mean value of Hy,rs with increasing rectal temperature resulted almost significant (4.99 ± 0.43 vs. 4.16 ± 0.31 cmH2Oml, p = 0.08), probably due to a lack of statistical power.

According to literature suggestions, it is concluded that body warming increases Cst,rs and almost significantly decreases Hy,rs, due to an increased alveolar surfactant activity.