Rebuttal from Drs. Hughes and West

Abstract

POINT-COUNTERPOINTRebuttal from Drs. Hughes and WestPublished Online:01 May 2008https://doi.org/10.1152/japplphysiol.01092.2007bMoreSectionsPDF (59 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat The areas of agreement in this debate are twofold: 1) mean pulmonary blood flow per horizontal slice [per unit volume or, more accurately, per unit density or “alveolus” (1,6)] increases in the gravity direction, independent (for the most part) of postural orientation, 2) there is considerable inhomogeneity of blood flow within a slice of minimal vertical height. Heterogeneity, whether random or ordered, is inevitable as the spatial resolution increases, just as the length of the shoreline increases indefinitely as the sample size gets smaller. Suffice to say that there are two extremes, 1) a well-expanded, vasodilated lung with low PVR and a mean Ppa level below the top of the lung, and 2) a low volume vasoconstricted lung with high PVR and a Ppa level greater than the lung's vertical height. The effects of gravity will be prominent in the former and largely obscured in the latter.The core of the debate is about the mechanisms responsible for pulmonary blood flow heterogeneity. There has been acceptance (for more than 40 yr) that mean pulmonary blood flow per horizontal slice is influenced by the relations between Ppa, Palv, and Pv (zones I, II, and III) (7), provided the lung is well expanded and PVR is normal. But what determines heterogeneity at the same horizontal level? Between acini, or larger domains, the Seattle group maintains that the architecture of the vascular tree (presumably, arterial) determines local vascular resistance and blood flow by, for example, uneven diameters of the two daughter vessels at a bifurcation. Glenny (5) maintains that this is not random, but forms part of the fractal design of the lung (4). Against this hypothesis is 1) the arterial system contributes only 40% to total PVR (2), and 2) the compliance of the arterial tree means diameters are not fixed, but respond to changes of smooth muscle tone or intra- or extravascular pressures (themselves influenced by gravity from distortion of the lung by its own weight and by other intrathoracic structures).Distal to the arterial tree, at the alveolar septal level, recruitment and distension of vessels occurs (3), independent of arteriolar domains (8), as input pressure increases and exceeds critical opening pressure (9); in the lung periphery, input pressures are a function of vertical height and gravity. So, in subtle ways, gravity may interact with compliant vascular structures to influence local PVR within as well as between horizontal lung slices.REFERENCES1 Anthonisen NR, Milic-Emili J. Distribution of pulmonary perfusion in erect man. J Appl Physiol 21: 760–766, 1966.Link | ISI | Google Scholar2 Dawson CA. Dynamics of blood flow and pressure-volume relationship. In: The Lung: Scientific Foundations, edited by Crystal RG, West JB, et al. Philadelphia: Lippincott-Raven, 1997, p. 1503–1522.Google Scholar3 Glazier JB, Hughes JMB, Maloney JE, West JB. Measurements of capillary dimensions and blood volume in rapidly frozen lungs. J Appl Physiol 26: 65–76, 1969.Link | ISI | Google Scholar4 Glenny RW, Robertson HT. Fractal modelling of pulmonary blood flow inhomogeneity. J Appl Physiol 70: 1024–1030, 1991.Link | ISI | Google Scholar5 Glenny RW. Spatial correlation of regional pulmonary perfusion. J Appl Physiol 72: 2378–2386, 1992.Link | ISI | Google Scholar6 Hughes JMB, 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 Scholar7 West JB, Dollery CT, Naimark A. Distribution of blood flow in isolated lung; relation to vascular and alveolar pressures. J Appl Physiol 19: 713–724, 1964.Link | ISI | Google Scholar8 Warrell DA, Evans JW, Clarke RO, Kingaby JP, West JB. Pattern of filling in the pulmonary vascular bed. J Appl Physiol 32: 346–356, 1972.Link | ISI | Google Scholar9 West JB, Schneider AM, Mitchell MM. Recruitment in networks of pulmonary capillaries. J Appl Physiol 39: 976–984, 1975.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 104Issue 5May 2008Pages 1535-1535 Copyright & PermissionsCopyright © 2008 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.01092.2007bHistory Published online 1 May 2008 Published in print 1 May 2008 Metrics