Rebuttal from Dr. Glenny

Abstract

POINT-COUNTERPOINTRebuttal from Dr. GlennyPublished Online:01 May 2008https://doi.org/10.1152/japplphysiol.01092.2007cMoreSectionsPDF (56 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Drs. Hughes and West (6) argue convincingly that a vertical gradient of perfusion exists in human lungs. We agree completely and have never stated otherwise. The controversy, however, is about whether gravity is the most important determinant of regional pulmonary blood flow. They reason that gravity must be the single most important determinant of regional perfusion because numerous prior studies have observed a gradient vertical. This logic parallels the argument that the sun revolves around the earth because it is repeatedly seen traversing from one horizon to the other. While the observation is consistent with the hypothesis, it is not sufficient proof. When it was discovered that the earth spins about its axis, prior observations were not invalidated but rather reinterpreted. The realization that blood flow within horizontal planes is present, well structured, and stable over time, suggests a reinterpretation of prior studies of pulmonary blood flow distribution.None of the studies cited in their Point (6) can determine if gravity is the most important determinant of regional pulmonary blood flow. To settle this issue, high spatial resolution measurements of perfusion must be obtained under varying conditions of posture or gravity while tracking the same lung regions across conditions. Although this kind of study is not currently possible in humans, the requisite study has been performed in baboons (2). The baboon is a biped that has similar lung anatomy, muscularization of pulmonary vessels, hydrostatic gradients, and pulmonary arterial pressures found in humans and therefore meets the criteria set by Drs. Hughes and West (4, 6). Baboons were studied in the upright, supine, prone, and head-down postures. Blood flow to each lung region was described by its vertical position in the lung and a structural component that remained stable across changes in posture. Vertical gradients of perfusion were observed in all postures, corroborating prior observations in humans (5, 8). Multiple stepwise linear regression was used to estimate the relative importance of the gravitational and structural components of blood flow. As expected, gravity had its largest influence in the upright posture. However, structure was more than twice as important as gravity in determining regional pulmonary blood flow (59 vs. 25%).The perspective that the geometry of the vascular tree is more important than gravity in determining regional blood flow requires that other concepts of the gravitational model be reconsidered. While the zones of the lung must exist due to physical laws, they do not need to be vertically stacked in the lung (3). The fractal model predicts that all three zones can exist within the same horizontal plane. The concept that the shared effects of gravity cause matching of regional ventilation and perfusion, however, cannot explain efficient gas exchange in the face of large heterogeneities of ventilation (1) and perfusion within isogravitational planes or during sustained microgravity (7).REFERENCES1 Altemeier WA, McKinney S, Glenny RW. Fractal nature of regional ventilation distribution. J Appl Physiol 88: 1551–1557, 2000.Link | ISI | Google Scholar2 Glenny RW, Bernard S, Robertson HT, Hlastala MP. Gravity is an important but secondary determinant of regional pulmonary blood flow in upright primates. J Appl Physiol 86: 623–632, 1999.Link | ISI | Google Scholar3 Glenny RW, Robertson HT. Regional differences in the lung. In: Complexity in Structure and Function of the Lung, edited by Hlastala MP and Robertson HT. New York: Marcel Dekker, 1998, p. 461–481.Google Scholar4 Hughes JM. Pulmonary blood flow distribution in exercising and in resting horses. J Appl Physiol 81: 1049–1050, 1996.Link | ISI | Google Scholar5 Hughes JM, Glazier JB, Maloney JE, West JB. Effect of lung volume on the distribution of pulmonary blood flow in man. Respir Physiol 4: 58–72, 1968.Crossref | Google Scholar6 Hughes JM, West JB. Counterpoint: Gravity is not the major factor determining the distribution of blood flow in the healthy human lung. J Appl Physiol; doi:10.1152/japplphysiol.01092.2007a.Link | ISI | Google Scholar7 Prisk GK, Guy HJ, Elliott AR, West JB. Inhomogeneity of pulmonary perfusion during sustained microgravity on SLS-1. J Appl Physiol 76: 1730–1738, 1994.Link | ISI | Google Scholar8 West JB, Dollery CT. Distribution of blood flow and ventilation-perfusion ratio in the lung, measured with radioactive carbon dioxide. J Appl Physiol 15: 405–410, 1960.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Collections More from this issue > Volume 104Issue 5May 2008Pages 1535-1536 Copyright & PermissionsCopyright © 2008 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.01092.2007cHistory Published online 1 May 2008 Published in print 1 May 2008 Metrics