Nitric oxide (NO) is commonly thought to reveal more precise values of pulmonary gas uptake through alveolar-capillary membranes (DL) than the normally used carbon monoxide (CO). Since such measurements are influenced by a significant endogenous NO delivery within human airways, we propose the use of the naturally occurring (15)N-labelled stable nitric oxide isotope (15)NO. It occurs with a relative abundance of 0.37% of the dominating isotope (14)NO. Therefore, the endogenous (15)NO production can be neglected. In the present pilot study we demonstrate the workability of (15)NO in determining DL in healthy individuals. In seven female and 15 male volunteers, averaged values of DL increase with increasing mean alveolar volume as well as individual body height ( P=0.000001). Due to the very high significance level obtained from the multiple regression analysis, we conclude that the application of (15)NO establishes a novel approach to calculate standard values of DL. Such calculations can be employed to predict a reference for patients who suffer from pulmonary diffusion limitation.

Apart from the current teaching of spirometric methods in laboratory courses on respiratory physiology, we have included an experiment in which medical students determine their own residual volume by applying the indicator-dilution technique. For hygienic reasons we used a bag-in-the-box system to dilute helium within alveolar space by performing the single-breath method. Although each participant independently underwent only one single-breath maneuver, we gained a reliable relationship between residual volume and subjects' height and body weight in 68 female (r = 0.6, P < 0.0001) and 99 male (r = 0.42, P < 0.0001) students. From this successful outcome and with the opportunity to discuss the limitations of the single-breath method as well, we inferred that this experiment affords a transparent and instructive approach to interpreting the determination of lung volumes on the basis of the indicator-dilution technique.