The TL,NO/TL,CO ratio in pulmonary function test interpretation

The transfer factor of the lung for nitric oxide (T(L,NO)) is a new test for pulmonary gas exchange. The procedure is similar to the already well-established transfer factor of the lung for carbon monoxide (T(L,CO)). Physiologically, T(L,NO) predominantly measures the diffusion pathway from the alveoli to capillary plasma. In the Roughton-Forster equation, T(L,NO) acts as a surrogate for the membrane diffusing capacity (D(M)). The red blood cell resistance to carbon monoxide uptake accounts for ~50% of the total resistance from gas to blood, but it is much less for nitric oxide. T(L,NO) and T(L,CO) can be measured simultaneously with the single breath technique, and D(M) and pulmonary capillary blood volume (V(c)) can be estimated. T(L,NO), unlike T(L,CO), is independent of oxygen tension and haematocrit. The T(L,NO)/T(L,CO) ratio is weighted towards the D(M)/V(c) ratio and to α; where α is the ratio of physical diffusivities of NO to CO (α=1.97). The T(L,NO)/T(L,CO) ratio is increased in heavy smokers, with and without computed tomography evidence of emphysema, and reduced in the voluntary restriction of lung expansion; it is expected to be reduced in chronic heart failure. The T(L,NO)/T(L,CO) ratio is a new index of gas exchange that may, more than derivations from them of D(M) and V(c) with their in-built assumptions, give additional insights into pulmonary pathology.