Effects of halothane on respiratory mechanics and lung histopathology in normal rats

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.