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Effects of Increasing Carboxyhemoglobin on the Single Breath Carbon Monoxide Diffusing Capacity

Division of Respirology, Department of Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Correspondence and requests for reprints should be addressed to Brian L. Graham, Division of Respirology, Room 546 Ellis Hall, Department of Medicine, University of Saskatchewan, Saskatoon, SK, S7N OW8 Canada. E-mail: brian.graham{at}sk.lung.ca

Although carboxyhemoglobin (COHb) is often increased in smokers, American Thoracic Society recommendations for adjusting the single breath carbon monoxide (CO) diffusing capacity (DL_CO^SB) for COHb remain optional. Using a previously described 3-equation technique, we measured DL_CO^SB and an index of diffusion inhomogeneity (DI) in 10 healthy, nonsmoking subjects who performed DL_CO^SB maneuvers both before and after increasing COHb. CO backpressure (FA_CO) was measured from the exhaled gas of a standardized deep breath of room air that immediately preceded each DL_CO^SB and was validated by measurement of FA_CO from an identical "sham" maneuver without inhaling CO. Without adjustments for FA_CO or COHb, DL_CO^SB decreased with increasing COHb. This effect persisted when DL_CO^SB was adjusted only for FA_CO, but it was eliminated with further adjustment for the anemia effect of increasing COHb. The anemia adjustment was proportional to the fractional COHb. DI, adjusted for FA_CO, was unaffected by increasing COHb. We conclude that DL_CO^SB must be adjusted for both the buildup of CO backpressure and the anemia effect of increasing COHb. Adequate corrections of DL_CO^SB can be implemented using FA_CO measured during a standardized deep breath immediately preceding the DL_CO^SB maneuver. Current American Thoracic Society recommendations for DL_CO^SB standardization do not adequately compensate for COHb.

Key Words: pulmonary diffusing capacity • carbon monoxide • carboxyhemoglobin • smoking • diffusion index

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