The aim of the present study was to calculate reference equations for carbon monoxide and nitric oxide transfer, measured in two distinct populations.The transfer factor of the lung for nitric oxide (TL,NO) and carbon monoxide (TL,CO) were measured in 303 people aged 18-94 yrs. Measurements were similarly made in two distant cities, using the single-breath technique. Capillary lung volume (Vc) and membrane conductance, the diffusing capacity of the membrane (Dm), for carbon monoxide (Dm,CO) were derived.The transfer of both gases appeared to depend upon age, height, sex and localisation. The rate of decrease in both transfers increased after the age of 59 yrs. TL,NO/alveolar volume (VA) and TL,CO/VA were only age-dependent. The mean TL,NO/TL,CO was 4.75 and the mean Dm/Vc was 6.17 min ; these parameters were independent of any covariate. Vc and Dm,CO calculations depend upon the choice of coefficients included in the Roughton-Forster equation. Values of 1.97 for Dm,NO/Dm,CO ratio and 12.86 min?kPa -1 for 1/red cell CO conductance are recommended.The scatter of transfer reference values in the literature, including the current study, is wide. The present results suggest that differences might be due to the populations themselves and not the methods alone.KEYWORDS: Ageing, capillary lung volume, carbon monoxide, diffusion, nitric oxide, pollution T he measurement of the transfer of gases through the lung is one of the few tests aimed at investigating alveolar function. The 1957 model and equation of ROUGHTON and FORSTER [1] permitted the transfer of carbon monoxide through the aveolocapillary structure to be split into two resistances, one for the alveolar membrane (1/membrane conductance, the diffusing capacity of the membrane (Dm), for carbon monoxide (Dm,CO)) and the other for the blood reacting with the gas (1/HCOVc), where HCO is the red cell conductance at a concentration, set by the pioneers of the method, of 14.9 g?dL -1 [2] and Vc the capillary lung volume:where TL,CO is the transfer factor of the lung for carbon monoxide. The first technique used to solve this equation with two unknowns, Dm and Vc, was to measure two transfers of CO, one under conditions of normoxia the other under hyperoxia. Breathing O 2 , by reducing HCO, lowers the TL,CO. GUENARD et al.[3] first published measurements of Dm and Vc using transfer factor of the lung for nitric oxide (TL,NO) and TL,CO and assuming HNO to be infinity, i.e. TL,NO5Dm,NO.The transfer of CO is dependant upon both Dm and Vc with HCO as a finite value.The relationship between Dm for nitric oxide (Dm,NO) and Dm,CO introduces a constant a: Dm,NO5aDm,CO. Therefore, the measurement of NO transfer alone permits the calculation of Dm,CO and, by introducing the latter into the CO transfer equation, of Vc.Most published reference values for Dm and Vc have been derived from the first two-step technique; one used the NO/CO method in a population of 127 healthy adults with a mean¡SD age of ,40¡12 yrs [4] and another focused on NO transfer in a population of 1...

Using simultaneous nitric oxide and carbon monoxide lung transfer measurements (T LNO and T LCO ), the membrane transfer capacity (D m ) and capillary lung volume (V c ) as well as the dimensionless ratio T LNO /T LCO can be calculated. The significance of this ratio is yet unclear. Theoretically, the T LNO /T LCO ratio should be inversely related to the product of both lung alveolar capillary membrane (μ) and blood sheet thicknesses (K ). NO and CO transfers were measured in healthy subjects in various conditions likely to be associated with changes in K and/or μ. Experimentally, deflation of the lung from 7.4 to 4.8 l decreased the T LNO /T LCO ratio from 4.9 to 4.2 (n = 25) which was consistent mainly with a thickening of the blood sheet. Compared with continuous negative pressure breathing, continuous positive pressure breathing increased this ratio suggesting a thinning of the capillary sheet. It was also observed with 12 healthy subjects that slight haemodilution that may thicken the blood sheet decreased the T LNO /T LCO ratio from 4.85 to 4.52. In conclusion, the T LNO /T LCO ratio is related to the thickness of the alveolar blood barrier. This ratio provides novel information for the analysis of the diffusion properties. Diffusion of gases between the alveolar space of the lung and blood is usually described with Roughton and Forster's model (Forster et al. 1957). In this model, key factors affecting gas transfer include two components: (a) the membrane which is supposed to be homogenous and characterized by its conductance (D m ) and (b) the product of red cell conductance for a given gas (θ ) and pulmonary capillary volume (V c ). D m is considered to be an independent variable to V c .In order to calculate D m and V c the NO/CO transfer method (T LNO /T LCO ) was introduced in the lung function testing of humans in 1987 (Guénard et al. 1987). As the in vivo conductance of NO in blood is very high, the only limitation to its transfer through the barrier is the membrane. CO transfer (T LCO ) depends on D m , V c and haemoglobin concentration. T LCO also varies with pulmonary capillary oxygen tension since θ CO is inversely proportional to this pressure. The ratio of NO to CO transfer (T LNO /T LCO ) should therefore provide some insight into the relative properties of the membrane and capillaries.The surface area of the alveolar membrane and capillary are identical or closely related and by consequence, D m and V c should be directly correlated. The hypothesis of this study is based on the assumption that V c and D m are dependent parameters.Under this assumption, this study is designed to identify the significance of the T LNO /T LCO ratio through both theoretical and experimental approaches with the expectations that the new model would be relevant for physiological or clinical purposes. Specifically, we have shown that the T LNO /T LCO ratio is independent of membrane surface area and inversely proportional to the product of alveolar membrane and capillary blood layer thicknesses. Methods Theoretica...

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