We applied the end-inflation occlusion method to measure the flow (F) and volume (V) dependances of rat respiratory system resistance. Ohmic and visco-elastic additional resistances (R_ohm and R_visc respectively) were measured for two different respiratory system V and inspiratory F.

As expected, R_ohm exhibited both V and F dependance, significantly decreasing with increasing respiratory system volume at constant inspiratory flow (0.11±0.009 vs. 0.085±0.008 cmH₂O/ml sec^-1 for inspiratory flow 4 ml/sec and inflation volumes 1 and 5 ml respectively) and increasing with inspiratory flow (0.092±0.006 vs. 0.11±0.009 cmH₂O/ml sec^-1 for inflation volumes 1 ml and inspiratory flows 1 and 4 ml/sec respectively).

According to previously published results pertaining to human subjects, R_visc also showed both V and F dependance, decreasing with increasing inspiratory flow (0.34±0.08 vs. 0.14±0.07 cmH₂O/ml sec^-1 for inflation volume 1 ml and inspiratory flows 1 and 4 ml/sec respectively), and increasing with increasing respiratory system volume (0.14±0.07 vs. 0.25±0.08 cmH₂O/ml sec^­1 for inspiratory flow 4 ml/sec and inflation volumes 1 and 5 ml respectively).

All the data are expressed as mean±SE, n=7.

Our data allowed to solve Rohrer equation, which describes the flow dependence of the ohmic resistance: R_ohm(cmH₂O/ml sec^­1) = 0.085+0.0065 F(ml/sec). This result , and the description of the V and F dependances of Rvisc, were never obtained before for rat respiratory system.

Our results confirm that the additional visco-elastic pressure dissipation during inflation contributes substantially to the overall mechanical work of breathing, and that both ohmic and additional visco-elastic resistances to inflation exhibit volume and flow dependance in the rat respiratory system.