Deciphering the nitric oxide to carbon monoxide lung transfer ratio:: physiological implications

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 (mu) 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 mu. 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.