STANDARDIZATION OF GAS-MIXTURES FOR ESTIMATING CARBON-MONOXIDE TRANSFER-FACTOR

Background-The American Thoracic Society recommends that the inspired concentration used for the estimation of carbon monoxide transfer fact or (TLCO) mixture should be 0.25-0.35% carbon monoxide, 10-14% helium, 17-21% oxygen, balance nitrogen. Inspired oxygen influences alveolar oxygen and hence carbon monoxide uptake, such that transfer factor inc reases by 0.35% per mm Hg decrease in alveolar oxygen. To aid in the s tandardisation of TLCO either a known inspired oxygen concentration sh ould be used, or TLCO should be corrected to a standard inspired oxyge n concentration. The range of gas mixtures used in practice and the im plications for cost and accuracy have been investigated. Methods-A que stionnaire was sent to 185 respiratory units in the UK requesting info rmation on (1) the method used to estimate TLCO, (2) the manufacturer of the equipment, (3) the mixture used, (4) whether ''medical quality' ' gas was ordered, and (5) the level of satisfaction with supplier ser vice. Results-Replies were received from 106 units. Most used the sing le breath breath holding method for which 17 different test mixtures w ere ordered. One unit also used the single breath exhalation method. I nspired oxygen ranged from 17.94% to 25%, giving a wide variation in a lveolar oxygen and hence TLCO. Forty seven units ordered a specific in spired oxygen, the rest ordering ''air'' as balance. The cost per litr e of gas varied greatly, with the mixture 14% helium, 0.28% carbon mon oxide, balance air (17.9% oxygen) and 10% helium, 0.28% carbon monoxid e, balance air (18.8% oxygen) being cheapest to produce. Ordering a sp ecific inspired oxygen concentration increased the cost. Large cylinde rs of gas were cheaper for the same mixture. The mixture for the exhal ation method was the most expensive. Sixty seven units ordered ''medic al quality'' gas and six assumed this was supplied. Twenty nine (27%) were dissatisfied with their supplier due to (1) poor service, (2) lon g delivery times, (3) costs, or (4) wrongly labelled cylinders. Conclu sions-It is recommended that two mixtures be available: (a) 14% helium , 0.28% carbon monoxide, balance air for a helium analyser reading up to 15%, and (b) 10% helium, 0.28% carbon monoxide, balance air for low er reading helium analysers. The mixture should be produced under a me dical product licence. The advantage of the single exhalation method f or routine clinical use needs to be investigated in view of the higher cost of the mixture.