Physiological determinants of the pulmonary filtration coefficient

Abstract

Current emphasis on translational application of genetic models of lung disease has renewed interest in the measurement of the gravimetric filtration coefficient (K_f) as a means to assess vascular permeability changes in isolated perfused lungs. The K_f is the product of the hydraulic conductivity and the filtration surface area, and is a sensitive measure of vascular fluid permeability when the pulmonary vessels are fully recruited and perfused. We have observed a remarkable consistency of the normalized baseline K_f values between species with widely varying body weights from mice to sheep. Uniformity of K_f values can be attributed to the thin alveolar capillary barrier required for gas exchange and the conserved matching of lung vascular surface area to the oxygen requirements of the body mass. An allometric correlation between the total lung filtration coefficient (K_f,t) and body weight in several species (r² = 1.00) had a slope that was similar to those reported for alveolar and pulmonary capillary surface areas and pulmonary diffusion coefficients determined by morphometric methods in these species. A consistent K_f is dependent on accurately separating the filtration and vascular volume components of lung weight gain, then K_f is a consistent and repeatable index of lung vascular permeability.

the current emphasis in translational application of genetic modifications and signaling pathways in the study of lung diseases has renewed interest in the measurement of the gravimetric filtration coefficient (K_f) in isolated lungs to evaluate changes in vascular permeability. In a recent editorial focus, Dr. Bhattacharya (5) elegantly summarized four basic assumptions for using K_f to assess lung microvascular injury, as well as conditions that could alter these assumptions and lead to errors in interpretation. In this perspective, we focus on the basis of the extremely consistent values obtained for K_f between species when normalized to lung or body weight.

The K_f is defined as the product of the hydraulic conductivity (Lp) and filtration surface area (SA), and essentially measures the endothelial barrier permeability to convective water transport (7). As a measure of vascular permeability, K_f is inherently more sensitive than protein flux because convective transport through paracellular pathways is a function of the fourth power of the pathway radius (r⁴), compared with r² for protein flux (20, 26, 27). We have previously reviewed the merits of K_f for measuring lung microvascular permeability and the most reliable method for extracting the filtration component from the rate of lung weight gain (20).

K_f normalized to lung or body mass is remarkably consistent across species. This consistency can largely be attributed to the matching of the microvascular surface area for filtration to the gas exchange surface area, the latter designed to achieve a gas exchange that will support oxidative metabolism of a given body mass (31). K_f is normalized to 100 g of lung weight for comparison of values between experiments, but a total lung filtration coefficient (K_f,t) can be calculated for the total lung weight and filtration surface area. K_f,t then represents a hydraulic conductance equivalent of the capillary oxygen conductance embodied in the pulmonary diffusion capacity for oxygen for the whole animal. As shown in Fig. 1, K _f,t varies as a function of body weight (BW) for mouse (11, 16, 32), rat (2, 3, 18), rabbit (1, 4), dog (21, 29), and sheep (8, 22) as well as a predicted value for a 70-kg human. The relationship of K_f,t to body mass is similar to that for alveolar (S_ALV) or pulmonary capillary (S_CAP) surface areas and pulmonary diffusion coefficient for oxygen (D_LO₂) determined by Weibel and associates using morphometric methods (31). Weibel has previously shown that these variables have a relationship of the form Y = A·BW^B, where A is the intercept and B is the slope of the logarithmic plot. Relative to BW, the total lung K_f,t had a slope of 0.98 ml·min⁻¹·cmH₂O⁻¹·g⁻¹ (r² = 1.00), which was comparable to slopes of 0.95 m²/g (r² = 0.98), 0.90 m²/g (r² = 0.98), and 0.94 ml·s⁻¹·mBar⁻¹·g⁻¹ (r² = 0.98) for S_ALV, S_CAP, and D_LO₂, respectively. The average S_CAP/S_ALV ratio determined by Weibel for the species shown was 0.87 ± 0.04. However, small extra-alveolar arterioles and venules (20- to 50-μm diameter) likely filter fluid and contribute an additional surface area of ∼13% (25). This additional surface area for filtration and the 20-fold greater hydraulic conductance of these conduit vessels compared with alveolar capillary endothelium undoubtedly account for the greater slope of K_f,t compared with S_CAP normalized to BW (17).

Fig. 1.

Allometric relationship of total lung filtration coefficient (K_f,t), alveolar surface area (S_ALV), pulmonary capillary surface area (S_CAP), and pulmonary diffusion coefficients (D_LO₂) for oxygen to body mass of several species. S_ALV, S_CAP, and D_LO₂ data are from Weibel (31).

The excellent correlation of K_f,t with body weight suggests that the baseline hydraulic conductivity is approximately the same in mouse and dog lungs. In contrast to the large proportional increase in pulmonary surface area with body mass, Weibel observed that the mean harmonic thickness of the alveolar capillary barrier only increased from 0.32 μm in mouse to 0.5 μm in human, with a slope of only 0.05 μm/g relative to body weight (10). Thus, the thinnest possible septal barrier is conserved between species to minimize diffusion distances and results in consistency of fluid exchange. Although normalized baseline K_f values are similar in adult lungs of various species, a thinner alveolar capillary barrier could account for the slightly higher baseline K_f and lower threshold for vascular pressure injury in mice compared with dogs and for the lungs of younger compared with older animals (1, 9, 13, 28).

We can conclude that K_f,t measures the total transvascular fluid exchange across a perfused surface area that is proportional to that available for gas exchange, as also suggested by lung mass resection studies (30). A further conclusion is that an increase in K_f from a baseline value can be attributed to an increase in endothelial hydraulic conductance rather than recruitment of additional surface area, provided that the pulmonary vascular surface area is fully recruited at baseline. In previous studies in isolated dog lungs, changes in blood flow from 0.03 to 2.5 l·min⁻¹·100 g⁻¹ lung weight had no significant effect on baseline K_f in uninjured lungs (24). Increasing the perfusion flow in these same lungs after injury caused significant increases in K_f due to recruitment of injured and vasoconstricted lung segments. In this situation, the magnitude of the increase in hydraulic conductance is partially offset by a decrease in surface area, unless equivalent perfusion rates are maintained between K_f measurements in the baseline and experimental injury states (24). In smaller species such as mouse and rat, some decrease in baseline K_f was observed at very low perfusion rates, presumably due to derecruitment, so a baseline perfusate flow of at least 300 ml·min⁻¹·100 g⁻¹ lung weight in rats was recommended (20). Conceivably, very high perfusion and shear rates could activate endothelial cells and increase K_f, but previous isolated lung baseline K_f measurements over a wide range of flows do not support a shear-induced permeability response in uninjured lungs (24, 30).

We have previously emphasized the necessity of maintaining the increased weight transient for 18–20 min to minimize errors due to vascular stress relaxation during baseline K_f measurements and completely separate the vascular volume and filtration components of lung weight gain (19). A wide range of baseline K_f values has been published for lungs of the same species using methods that include a variable amount of the vascular stress relaxation component of lung weight gain to the filtration measurement. These other methods produce K_f values that do not correlate as well with body weight as the K_f,t data we show here. Only during severe lung vascular injury when the filtration component is large compared with the vascular volume component of lung weight gain can valid filtration rates be measured using much shorter periods of weight gain. Since alveolar surface tension forces and interstitial fluid pressures increase as edema accumulates, a modest increase in K_f occurs after a lung weight gain of ∼40–50% due to initiation of alveolar flooding (6). Thus, only the K_f measurements in uninjured, fully recruited lungs without edema can be used as a reliable indicator of filtration surface area.

The possibility of endothelial signaling induced by the increased vascular pressure during a K_f measurement was recently emphasized by Bhattacharya (5). Endothelial responses to pressure were not considered during original applications of K_f because the endothelium was considered to be an inert physical barrier. However, seminal intravital microscopy studies by Bhattacharya, Kuebler, and associates (12, 14, 15) demonstrated increases in both endothelial calcium and endothelial P-selectin expression after a microvascular pressure increase of only 10 cmH₂O. Although a 10 cmH₂O pressure elevation is commonly used for K_f measurement, a smaller vascular pressure increase would minimize potential endothelial activation. However, previous studies indicated no acute increase in K_f at microvascular filtration pressures lower than 42 cmH₂O in dog lungs or 30 cmH₂O in mouse lungs (13, 23, 28). Baseline K_f measured using a 10 cmH₂O increase in microvascular pressure were not significantly different between transient receptor potential-4 (TRPV4) channel knockout and wild-type mice even though these mechanogated channels were essential for high vascular or airway pressure injury to the lung (11, 13). These results suggest that modest transient microvascular pressure increases do not cause endothelial activation sufficient to induce an acute vascular permeability increase, but when more subtle proinflammatory endothelial signaling and longer term inflammatory cell effects must be considered, then the increases in vascular pressure should be kept at a minimum.

In summary, K_f is determined by the hydraulic conductivity of the thin endothelial layer and the large pulmonary vascular surface area of the blood gas barrier. These anatomic features are optimized for efficient gas exchange to meet the oxygen requirements of the animals' body mass. The relationship of alveolar capillary surface area for gas exchange to body mass leads to the remarkable consistency of K_f across species when normalized to lung or body weight. When the pulmonary vascular surface area is fully recruited, the K_f measurement is then a reliable and repeatable method used to track vascular permeability changes in isolated lungs.

GRANTS

This work was supported by National Heart, Lung, and Blood Institute Grant P01-HL-66299.