Age related changes in the rate of stress relaxation within the rat respiratory system

Stress relaxation within the respiratory system was examined in young rats at different ages, 0-1 day old (GPI), 4-5 days old (GPII) and 30-40 days old (GPIII). Each rat was anesthetized, tracheostomized and placed inside a saline filled plethysmograph with the tracheal cannula projecting through the wall of the plethysmograph. Volume history was standardized by three inflations to a mean transrespiratory system pressure (P) of 20-25 cm H2O, then P was set to zero and lung volume was abruptly increased by rapidly withdrawing fluid from the plethysmograph. Following the volume step, lung volume was maintained constant and changes in P due to stress relaxation were recorded for 30 sec. The rate of stress relaxation was obtained by calculating the slope (R) of the normalized change in P per unit of time on a semi-log plot. GPIII rats exhibited the slowest rate (R = 0.068 +/- 0.004 SD) whereas GPII rats demonstrated the fastest rate (R = 0.092 +/- 0.011 SD). Stress relaxation in GPI was intermediate (R = 0.076 +/- 0.005 SD). Values of dynamic (Cdyn) and static (Cstat) compliance were determined for rats similar in age to the three age groups used to determine R. The difference between Cstat and Cdyn increased with R and was greatest in rats between 0 and 6 days old. We conclude that: (1) stress relaxation within the rat respiratory system at 30-40 days is less than that found in rats during the first week after birth, and (2) changes in the viscoelastic properties of the respiratory system may contribute to age related variation in the difference between Cstat and Cdyn.