Dharmakumara M, Prisk GK, Royce SG, Tawhai M, Thompson BR. The effect of gas exchange on multiple-breath nitrogen washout measures of ventilation inhomogeneity in the mouse. J Appl Physiol 117: 1049 -1054, 2014. First published September 11, 2014 doi:10.1152/japplphysiol.00543.2014.-Inert-gas washout measurements using oxygen, in the lungs of small animals, are complicated by the continuous process of oxygen consumption (V O2). The multiplebreath nitrogen washout (MBNW) technique uses the alveolar slope to determine measures of ventilation inhomogeneity in the acinar (S acin) and conducting (S cond) airway regions, as well as overall inhomogeneity, as determined by the lung clearance index (LCI). We hypothesized that measured ventilation inhomogeneity in the mouse lung while it is alive is in fact an artifact due to the high V O2 in proportion to alveolar gas volume (VA), and not ventilation inhomogeneity per se. In seven male C57BL/6 mice, MBNW was performed alive and postmortem to derive measures with and without the effect of gas exchange, respectively. These results were compared with those obtained from an asymmetric multibranch point mathematical model of the mouse lung. There was no statistical difference in S acin and LCI between alive and postmortem results (S acin alive ϭ 0.311 Ϯ 0.043 ml Ϫ1 and Sacin postmortem ϭ 0.338 Ϯ 0.032 ml Ϫ1, LCI alive ϭ 7.0 Ϯ 0.1 and LCI postmortem ϭ 7.0 Ϯ 0.1). However, there was a significant decrease in S cond from 0.086 Ϯ 0.005 ml Ϫ1 alive to 0.006 Ϯ 0.002 ml Ϫ1 postmortem (P Ͻ 0.01). Model simulations replicated these results. Furthermore, in the model, as V O2 increased, so did the alveolar slope. These findings suggests that the MBNW measurement of S cond in the mouse lung is confounded by the effect of gas exchange, a result of the high V O2-to-VA ratio in this small animal, and not due to inhomogeneity within the airways. multiple-breath nitrogen washout; ventilation inhomogeneity; mouse lung; oxygen consumption; alveolar slope THE DISTRIBUTION OF VENTILATION is inhomogeneous, even in the healthy lung. In lung disease, however, ventilation inhomogeneity increases. The degree of inhomogeneity, especially within the small airways, has been demonstrated as an important feature in a number of common respiratory conditions (33, 34), in terms of both pathophysiology and patient-centered outcomes, such as symptoms and disease control (10, 33).One of the most widely used noninvasive methods for measuring ventilation inhomogeneity in humans is the multiple-breath nitrogen washout (MBNW) technique. This test consists of inspiring several consecutive breaths of 100% oxygen (O 2 ) until the nitrogen (N 2 ) concentration in the lungs is reduced to Ͻ2%. Each breath generates an alveolar slope, the evolution of which has been used to measure ventilation inhomogeneity that is a result of convective gas flow (termed S cond ) the site of origin which is thought to be proximal to the so-called diffusion front. The technique also generates a measure that results from inhomogeneity in...