The Influence of Halothane and Isoflurane on Pulmonary Collateral Ventilation

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Abstract
The effects of halothane and isoflurane on hypocapnic increases in pulmonary collateral resistance were studied in dogs. A bronchoscope with a double lumen catheter in the suction port obstructed a peripheral airway and allowed gas to flow out of the isolated segment of lung only via collateral channels. The collateral gas flow (.ovrhdot.Vcoll) was measured with a flowmeter and delivered through one lumen of the catheter, wile the other lumen measured distal pressure (Pb). At FRC [functional residual capacity], the resistance to collateral ventilation (Rcoll) was calculated as Rcoll = Pb/.ovrhdot.Vcoll. The rest of the lung was ventilated with air, while air (hypocapnia), 10% CO2 in air, or air and halothane or isoflurane were delivered to the isolated segment. A measurement of resistance was made after 4 min of test gas flow. For each segment, when air replaced 10% CO2, the average increase in Rcoll was calculated and called Rmax. When 10% CO2 in air was infused into segments the mean Rcoll (n = 50) was 0.0196 .+-. 0.0022 cmH2O .cntdot. ml-1 .cntdot. min. This increased to 0.0285 .+-. 0.0031 cmH2O .cntdot. ml-1 .cntdot. min (mean .+-. E) when air was infused, a mean increase in resistance of 52 .+-. 3%. When halothane or isoflurane was added to air the hypocapnic increase in Rcoll was attenuated with a 50% decrease at 1.3% (1.4 MAC [minimum alveolar concentration] and 0.8 MAC, respectively). These 2 inhalational anesthetics reduce active changes in the flow resistance to collateral ventilation. When collateral resistance acts to adjust ventilation perfusion deviations, this action of halothane and isofluorane may make this regulation less effective.

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