Reverse nitrogen gradients in the study of phase III and cardiogenic oscillations of the single-breath nitrogen test

The slope of phase III, phase IV, the slope of phase IV, and cardiac oscillations were measured on tracings obtained by both the regular single-breath N₂ test (Tech I) and by a reverse technique (Tech II) in 9 healthy volunteers. Tech II consisted of 3 consecutive vital capacities (VC) of 100% O₂ followed by one VC of room air. Theoretically, this should create a reversed apicobasal N₂ gradient quantitatively similar to that of Tech I. From the total lung capacity following the VC of air, we monitored N₂ concentration continuously at the mouth during a slow expiration in a manner similar to that of the single-breath N₂ test. With Tech II, it was possible to preserve phase IV and its reversed slope in the presence of an almost flat slope of phase III and markedly blunted cardiac oscillations. When compared to Tech I, the slope of phase III with Tech II decreased from 0.66 ± 0.20% N₂/L (mean ± SD) to 0.19 ± 0.12 (p<0.001), and cardiac oscillations decreased from a mean % N₂ change with each heart beat of 0.87 ± 0.37 to 0.24 ± 0.21 (p<0.005), whereas phase IV, although reversed in direction, remained quantitatively unchanged (0.35 ± 0.15 L with Tech I and 0.37 ± 0.14 L with Tech II), and the slope of phase IV tended to increase (2.7 ± 1.9% N₂ with Tech I and 3.4 ± 2.1% N₂ with Tech II, p=NS). We conclude that the N₂ gradients within the lungs responsible for the slope of phase III and cardiac oscillations are largely independent of the gradients that give rise to phase IV and the slope of phase IV.