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

It is generally accepted that halothane reduces airway and tissue resistance in lungs with pre-existing airway tone. However, under conditions of resting airway tone, pulmonary resistance remains unaltered. In this study, we have determined the effects of halothane on respiratory system, pulmonary and chest wall resistive, elastic and viscoelastic mechanical properties, and related the results to findings from lung histology in intact normal rats. Sixteen adult male Wistar rats were allocated randomly to one of two groups (n=8 in each group): control or halothane group. In the control group, animals were sedated with diazepam 5 mg i.p. and anaesthetized with pentobarbital 20 mg kg(-1) i.p. In the halothane group, the anaesthetic was administered at an end-tidal concentration of 1 MAC throughout the study. Rats were paralysed and underwent mechanical ventilation. Halothane decreased airway resistance but increased the tissue component of resistance (caused by viscoelastic elements and lung inhomogeneity). Static and dynamic elastance also increased with halothane anaesthesia. Pulmonary resistance remained unchanged. Lung histopathology demonstrated airway dilatation and a greater degree of lung collapse and hyperinflation in the halothane group. We conclude that halothane anaesthesia acts both on airway and lung tissue. In airway tissue, dilatation occurs but the lung periphery stiffens. Consequently, these opposing effects result in no overall apparent change in mechanical properties, although changes are observed during halothane anaesthesia in normal animals and subjects.