Comparative respiratory system mechanics in rodents

Because of the wide utilization of rodents as animal models in respiratory research and the limited data on measurements of respiratory input impedanc e (Zrs) in small animals, me measured Zrs between 0.25 and 9.125 Hz at diff erent levels (0-7 hPa) of positive end-expiratory pressure (PEEP) in mice, rats, guinea pigs, and rabbits using a computer-controlled small-animal ven tilator (Schuessler TF and Bates JHT, IEEE Trans Biomed Eng 42: 860-866, 19 95). Zrs was fitted with a model, including a Newtonian resistance (R) and inertance in series with a constant-phase tissue compartment characterized by tissue damping (Gti) and elastance (Hti) parameters. Inertance was negli gible in all cases. R, Gti, and Hti were normalized to body weight, yieldin g normalized R, Gti, and Hti (NHti), respectively. Normalized R tended to d ecrease slightly with PEEP and increased with animal size. Normalized Gti h ad a minimal dependence on PEEP. NHti decreased with increasing PEEP, reach ing a minimum at similar to 5 hPa in all species except mice. NHti was also higher in mice and rabbits compared with guinea pigs and rats at low PEEPs , which we conclude is probably due to a relatively smaller air space volum e in mice and rabbits. Our data also suggest that smaller rodents have prop ortionately wider airways than do larger animals. We conclude that a detail ed, comparative study of respiratory system mechanics shows some evidence o f structural differences among the lungs of various species but that, in ge neral, rodent lungs obey scaling laws similar to those described in other s pecies.