Technical note Correction of the single breath carbon monoxide transfer factor in exercise for variations in alveolar oxygen pressure

BackgroundCarbon monoxide transfer factor (TLco) varies inversely with the partial pressure of alveolar oxygen (PAo2). During exercise the PAo2 in the alveolar gas sample bag decreases so the TLCO increases more than would be expected from the effects of exercise alone. The effects of PAo2 on the estimation of TLCO during exercise have been investigated and studies have been performed to determine whether it is appropriate to standardise to a PAo2 of 16 kPa. Methods TLCO was estimated at rest and at a single level of exercise in six normal subjects using test gas mixtures of 03% carbon monoxide, 14% helium, and oxygen in three different percentages (17%, 21%, and 27%), remainder nitrogen. In three of the subjects an incremental exercise test with estimates of oxygen consumption (Vo2) and cardiac frequency (fc) was also performed using a mixture containing 18% oxygen. Results TLCO decreased as levels of inspired oxygen increased. When standardised to a PAo2 of 16 kPa TLCO became independent of the inspired oxygen concentration. The significance of the curvilinear relations of TLCO and transfer coefficient to V02 and fc improved. Conclusion The single breath breath holding TLCO should be standardised to a PAo2 of 16 kPa when estimated during exercise. (Thorax 1994;49:82-84) Respiratory Department, Bristol Royal Infirmary, Bristol BS2 8HW A H Kendrick G Laszlo Correspondence to: Dr A H Kendrick. Reprints will not be available Received 30 March 1993 Returned to authors 1 September 1993 Revised version received 14 September 1993 Accepted for publication 22 September 1993 The single breath breath holding transfer factor for carbon monoxide (TLCO) is inversely related to the alveolar partial pressure of oxygen (PAo2), the reaction rate of carbon monoxide with oxyhaemoglobin being dependent on the concentration of oxygen.Y13 During measurements on exercise the PAO2 decreases, as estimated from the expired gas sample, as the level of exercise increases from rest. The increases in TLCO seen on exercise will therefore not only reflect changes in the volume of blood in the alveolar capillaries and diffusing capacity of the alveolar-capillary membrane,4 but also decreases in PAO. Kanner and Crapo5 have recently investigated the effect of varying PAO2 on TLCO at rest and have derived an equation which standardises TLCO to a PAO2 of 16 kPa. This equation would permit standardisation ofTLCO and overcome variations in the inspired level of oxygen and the effects of altitude. The purpose of this study was to determine the effect of applying this equation to estimates of TLCO made during exercise.