It is generally accepted that halothane reduces airway and tissue resistanc
e in lungs with pre-existing airway tone. However, under conditions of rest
ing airway tone, pulmonary resistance remains unaltered. In this study, we
have determined the effects of halothane on respiratory system, pulmonary a
nd chest wall resistive, elastic and viscoelastic mechanical properties, an
d related the results to findings from lung histology in intact normal rats
. Sixteen adult male Wistar rats were allocated randomly to one of two grou
ps (n=8 in each group): control or halothane group. In the control group, a
nimals were sedated with diazepam 5 mg i.p. and anaesthetized with pentobar
bital 20 mg kg(-1) i.p. In the halothane group, the anaesthetic was adminis
tered at an end-tidal concentration of 1 MAC throughout the study. Rats wer
e paralysed and underwent mechanical ventilation. Halothane decreased airwa
y resistance but increased the tissue component of resistance (caused by vi
scoelastic elements and lung inhomogeneity). Static and dynamic elastance a
lso increased with halothane anaesthesia. Pulmonary resistance remained unc
hanged. Lung histopathology demonstrated airway dilatation and a greater de
gree of lung collapse and hyperinflation in the halothane group. We conclud
e that halothane anaesthesia acts both on airway and lung tissue. In airway
tissue, dilatation occurs but the lung periphery stiffens. Consequently, t
hese opposing effects result in no overall apparent change in mechanical pr
operties, although changes are observed during halothane anaesthesia in nor
mal animals and subjects.