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 VC2 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 is 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 is less than 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 is less than 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.