Changes in lung membrane diffusing capacity for oxygen produced by halothane.

The effect of halothane on membrane diffusing capacity for O2 (DMO2) was measured in isolated left lower lobes of dog lungs using the sodium dithionite method. At 25 degrees C, halothane reduced DMO2 according to the regression equation: per cent control DMO2 = -4.85(per cent halothane) + 97.5 (r = -0.55, P = 0.0007). Although DMO2 was reduced from control by halothane administration, lung volume (VL) increased at higher halothane concentrations and tended to restore DMO2 by increasing surface area. There was a better correlation between the DMO2/VL ratios and per cent halothane: per cent (DMO2/VL) = -5.76 (per cent halothane) + 95.6 (r = -0.65, P = 0.00003). Effects of halothane on DMO2 and VL were reversible and were not influenced by gas mixing efficiency since argon dilution half-times over two decades were unchanged by halothane. It is unlikely that altered vascular recruitment affected the measured DMO2 since resistance to blood flow was unchanged. We conclude that halothane decreases DMO2 by either decreasing the physical diffusion coefficient (D') for O2 or decreasing the effective O2 solubility (alpha), or both, in the alveolar-capillary membrane.