INFLUENCE OF HALOTHANE AND ENFLURANE ON RESPIRATORY AIRFLOW RESISTANCE AND SPECIFIC CONDUCTANCE IN ANAESTHETIZED MAN

We have developed a method for the measurement of respiratoryresistance and specific airways conductance (s.G_aw) using theforced airflow oscillation method, and have used it to studythe effects of halothane and enflurane on airway mechanics inanaesthetized patients. Resistance (R_rs) was determined overa range of lung volumes and s.G_aw was obtained by computer-aidedanalysis of the hyperbolic relationship between R_rs and lungvolume. Patients received diazepam orally, followed by thiopentoneand pancuronium. The trachea was intubated and the lungs ventilatedwith 70% nitrous oxide in oxygen. After obtaining three baselinemeasurements of s.G_aw, 1.3% halothane (10 patients) or 2.5%enflurane (10 patients) was added to the inspired gas. Halothanecaused an increase in s.G_aw (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.G_awat 3 and 8 min, but a 56% increase (P<0.02) at 15 min. Onepatient responded to halothane with an increase in bronchomotortone, manifest by a significant reduction in s.G_aw (P<0.01;two-sample t test). Enflurane did not cause bronchoconstrictionin any patient. There was a significant reduction in resistancewith halothane (P<0.05; paired t test) and enflurane (P<0.01).Expiratory reserve volume (ERV) was found to be small, and contributedto the high resistances observed: mean resistance 0.59 kPa litre^–1s, range 0.15–1.71. Small changes in ERV were also shownto produce changes in resistance independent of changes in bronchomotortone. Neither halothane nor enflurane produced significant meanchanges in ERV.