The effect of nitrous oxide on the measurement of single-breath transfer factor

One hour after a bone marrow biopsy and inhalation of Entonox gas (50% nitrous oxide (N₂O) and 50% oxygen), a patient had a markedly reduced transfer factor of the lung for carbon monoxide (TL,CO). Three hours after Entonox, the patient had a normal TL,CO. Since carbon monoxide (CO) and N₂O have similar spectral wavelengths, it was proposed that residual N₂O in the lungs was interfering with the infra-red analysers used to detect CO concentrations. Experiments were performed to verify the 'interference' effect and its duration. Five healthy volunteers performed serial triplicate TL,CO measurements over 3 h on two randomized days (Control vs N₂O). The first triplicate TL,CO on each day served as a baseline measurement. Following the baseline measurement on the N₂O day, each subject inhaled Entonox for 10 min. To serve as a control for the infrared effect, the identical protocol was repeated using a gas chromatography method for TL,CO determination. The infra-red method showed a marked reduction (> 50%) in TL,CO 30 min after N₂O inhalation. This reduction did not return to baseline levels for at least 2 h. In comparison, the gas chromatography method showed no significant reduction in TL,CO. In a group of healthy nonsmoking subjects, N₂O markedly affected the measurement of the transfer factor of the lungs for carbon monoxide using infra-red analysers. The time course over which the measurement was reduced was at least 2 h for a 10 min inhalation period. The effect was entirely due to a measurement error associated with infra-red technology.