Influence of halothane and enflurane on respiratory airflow resistance and specific conductance in anaesthetized man.

We have developed a method for the measurement of respiratory resistance and specific airways conductance (s.Gaw) using the forced airflow oscillation method, and have used it to study the effects of halothane and enflurane on airway mechanics in anaesthetized patients. Resistance (i?ra) was determined over a range of lung volumes and s.Gaw was obtained by computer-aided analysis of the hyperbolic relationship between Rn and lung volume. Patients received diazepam orally, followed by thiopentone and pancuronium. The trachea was intubated and the lungs ventilated with 70% nitrous oxide in oxygen. After obtaining three baseline measurements of s.Gaw, 1.3% halothane (10 patients) or 2.5% enflurane (10 patients) was added to the inspired gas. Halothane caused an increase in s.Gaw (bronchodilatation) of 47% (P< 0.05; paired t test) at 3 min and a non-significant increase of 72% at 15 min. Enflurane produced no significant increase in s.Gaw at 3 and 8 min, but a 56% increase (P<0.02) at 15 min. One patient responded to halothane with an increase in bronchomotor tone, manifest by a significant reduction in s.Gsw (P<0.01; two-sample t test). Enflurane did not cause bronchoconstriction in any patient. There was a significant reduction in resistance with halothane (P<0.05; paired t test) and enflurane (P<0.01). Expiratory reserve volume (ERV) was found to be small, and contributed to the high resistances observed: mean resistance 0.59 kPa litre" s, range 0.15-1.71. Small changes in ERV were also shown to produce changes in resistance independent of changes in bronchomotor tone. Neither halothane nor enflurane produced significant mean changes in ERV.