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POINT-COUNTERPOINT COMMENTS
University of California, San Diego
The following letters are in response to Point:Counterpoint "Gravity is/is not the major factor determining the distribution of blood flow in the human lung."
To the Editor: Glenny (1) states that the foundation of the debate should be set on two issues: 1) methods for measuring perfusion distribution and 2) lung tissue redistribution between different body positions. The former is obvious, but the latter appears not so simple.
Glenny asserts that sagging of parenchyma near the base and corresponding stretching at the apex (in the upright lung) accounts for much of the total vertical perfusion gradient, because, due to sagging, there are more alveoli in a given field of view in basal than apical regions. He claims that this must be corrected for so as to obtain the effect of gravity independent of this (gravitationally induced) tissue movement. Thus, after tissue-sagging contributions to the vertical gradient are removed, what is left as a gradient is implied to be the real effect of gravity, and this is what Glenny considers.
I have great trouble with this logic. Gravitationally induced tissue sagging is a fundamental part of how gravity affects perfusion (and, for that matter, ventilation) distribution. Deducting that component is tantamount to eliminating a major, real gravitational influence on perfusion distribution. Of course this "correction" must diminish the residual apex to base gradient. My point is that Glenny's removal of the sagging-related gravitational component is inappropriate in this debate because it is an important effect of gravity. What Hughes and West (2) describe is the total effect of gravity without such "corrections," and that is what Glenny should be comparing his data with and basing his conclusions on.
REFERENCES
Department of Pharmacology
University of South Alabama
To the Editor: Hughes and West (2) and Glenny (1) agree that gravity has an effect on pulmonary blood flow distribution. In the West zone model, gravity is the predominate determinant of blood flow distribution. Glenny uses a higher-resolution microsphere technique to measure flow in 2-cm3 pieces of lung. When the flows in the cubes are averaged into 19 isogravitational planes, the data of West and Glenny agree (1, 2). However, when Glenny reports the individual flows from all 1,265 cubes, all three zones exist within each cross-lung isogravitational plane. Recently, results from a much higher resolution technique have been reported (3). Videomicroscopic recordings were made of alveolar capillary blood flow patterns in three adjoining alveolar walls located in the same horizontal plane, the ultimate isogravitational plane. The flow patterns in these three alveolar networks, fed by the same arteriole and drained by the same venule, showed no correlation with each other. During the same 4-s time interval, alveolus A might be completely recruited (zone III), alveolus B next-door partially recruited (zone II), while alveolus C might have no flow (zone I). These patterns switched rapidly during each of the 4-s intervals in 16-min-long video recordings [r2 between all alveolar combinations, i.e., A-B, A-C, B-C, in each of five animals averaged 0.06 ± 0.06 (SD)]. Thus each alveolus could momentarily and independently be in any zone. Glenny (1) emphasizes that as higher resolution techniques are developed, the view of pulmonary perfusion will likely change, as demonstrated by videomicroscopy, which shows that all three zones can exist simultaneously even in adjoining alveolar walls (3).
REFERENCES
University of California, San Diego
To the Editor: Our colleagues are arguing at right angles to one another literally and figuratively. Drs. Hughes and West (4) focus on vertical gradients. Dr. Glenny (2), while acknowledging vertical gradients, focuses on heterogeneity within an iso-gravitational plane.
It is clear that gravitational deformation of the lung tissue (1, 3, 6) (the so-called Slinky effect), occurs because of the weight of the lung itself and also because of the weight of the blood in the pulmonary vasculature, which comprises approximately one-half of the total weight of the lung (1). This necessarily affects measurements of perfusion: at one extreme, when low-resolution external imaging fails to take this effect into account, this leads to overestimating the effect of gravity. At the other extreme, high-resolution microsphere studies with the associated postprocessing of the lung (exsanguination, washing, inflation, drying) tends to minimize the effect of in situ lung deformation, underestimating the effect of gravity.
Studies that take deformation into account and report density-normalized perfusion (3, 5, 6) show persisting, but smaller, gravitational gradients than external counter studies, and show significant in-plane heterogeneity. Furthermore, regional lung density data provide a measure of lung compression, which necessarily affects not only the distribution of pulmonary capillaries and thus perfusion, but also alveolar size and consequently ventilation. Thus considering density-normalized perfusion provides a more physiologically relevant descriptor of pulmonary perfusion as it more closely describes the perfusion inhomogeneity as it is likely to affect gas exchange, which after all is the primary function of the lung.
REFERENCES
University of Oxford
To the Editor: An interesting point that has not yet been considered in this debate is the origin of the so-called zone 4 (4), which is characterized by a reduction of flow in the most gravitationally dependent region of the (upright) lung. The earliest iteration of the zonal theory did not include this flow reduction (6), but it was later postulated to occur through compression of extra-alveolar vessels (4). This region of reduced flow is present in measurements from the groups on each side of this debate (2, 3, 5). Interestingly, a reduction in flow can also be reproduced by computational models that do not include interaction between the tissue and the vasculature (the original theory), but do include a realistic representation of the vascular branching topology (1). That is, compression of the extra-alveolar vessels is not a necessity for zone 4 flow. Although these physics-based models could be criticized for not coupling the interaction of the large vessel circulation with the—perhaps more gravity-influenced—capillary bed, they undoubtedly show an intrinsic regional pattern of perfusion that is solely attributable to their branching geometry and its interplay with hydrostatic pressure gradients. Further model development to include tissue interaction and to couple the microcirculation with perfusion of the arterial and venous trees will provide a means to study some of the questions that have been raised in this debate.
REFERENCES
This article has been cited by other articles:
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  M. Hughes and J. B. West Last Word on Point:Counterpoint: Gravity is/is not the major factor determining the distribution of blood flow in the human lung J Appl Physiol, May 1, 2008; 104(5): 1539 - 1539. [Full Text] [PDF]
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R. Glenny Last Word on Point:Counterpoint: Gravity is/is not the major factor determining the distribution of blood flow in the human lung J Appl Physiol, May 1, 2008; 104(5): 1540 - 1540. [Full Text] [PDF]
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