Estimation of pulmonary interstitial fluid space compliance in isolated perfused rabbit lung

Edemagenic gain and interstitial fluid volume regulation

Under physiological conditions, interstitial fluid volume is tightly regulated by balancing microvascular filtration and lymphatic return to the central venous circulation. Even though microvascular filtration and lymphatic return are governed by conservation of mass, their interaction can result in exceedingly complex behavior. Without making simplifying assumptions, investigators must solve the fluid balance equations numerically, which limits the generality of the results. We thus made critical simplifying assumptions to develop a simple solution to the standard fluid balance equations that is expressed as an algebraic formula. Using a classical approach to describe systems with negative feedback, we formulated our solution as a "gain" relating the change in interstitial fluid volume to a change in effective microvascular driving pressure. The resulting "edemagenic gain" is a function of microvascular filtration coefficient (K(f)), effective lymphatic resistance (R(L)), and interstitial compliance (C). This formulation suggests two types of gain: "multivariate" dependent on C, R(L), and K(f), and "compliance-dominated" approximately equal to C. The latter forms a basis of a novel method to estimate C without measuring interstitial fluid pressure. Data from ovine experiments illustrate how edemagenic gain is altered with pulmonary edema induced by venous hypertension, histamine, and endotoxin. Reformulation of the classical equations governing fluid balance in terms of edemagenic gain thus yields new insight into the factors affecting an organ's susceptibility to edema.

Alveolar barrier function assessed by hydrophobic and hydrophilic fluorescent solutes in rabbit lung

Loss of alveolar barrier function is important in the development of pulmonary edema, but quantitation of its integrity has been difficult in the intact lung. We report a new non-radioactive method to assess paracellular and transcellular permeability of alveolar barrier in buffer-perfused rabbit lungs. Changes in alveolar barrier parameters were then correlated with different types of lung edema formation. The paracellular and transcellular barrier function was quantified by calculating the apparent epithelial permeability-surface area products (PS) for a fluorescent hydrophilic solute, FITC-dextran (FD-4), and a hydrophobic solute, rhodamine B, respectively. In control lungs, the apparent epithelial PS for FD-4 and rhodamine B were 0.7+/-0.05 x 10(-4) and 40.0+/-4.1 x 10(-4) ml/sec, respectively. The apparent epithelial PS of FD-4 could be increased 25-fold by inhibition of epithelial anion exchange with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) without affecting the PS of rhodamine B. The apparent epithelial PS of FD-4 could be increased 6- and 1.7-fold by disrupting microtubules with nocodazole and colchicine respectively, but microtubule agents decreased PS for rhodamine B. A pattern similar was produced when ATP production in the lung was inhibited by 2-deoxyglucose or when oxidative injury was induced by ischemia-reperfusion. Neither DIDS nor nocodazole altered endothelial permeability to albumin. DIDS, but not nocodazole, increased transcapillary liquid filtration and calculated interstitial compliance of the lung during hydrostatic challenge. We conclude that epithelial permeability in the intact lung can be assessed using fluorescent solutes, and that increased permeation of hydrophilic solutes may enhance lung edema formation.

Inhibition of PMN- and HOC1-induced vascular injury in isolated rabbit lungs by acetylsalicylic acid: a possible link between neutrophil-derived oxidative stress and eicosanoid metabolism?

Neutrophils are involved in the pathogenesis of acute lung injury. The neutrophil-derived enzyme myeloperoxidase (MPO) catalyzes the formation of the oxidant hypochlorous acid (HOCl). This study characterizes the effects of (A) continuous HOCl infusion, and (B) stimulated neutrophils on pulmonary circulation in an isolated rabbit lung model. Furthermore, the effect of cyclooxygenase inhibition by acetylsalicylic acid (ASA, 0.5 mM) on these effects was investigated. (A) Infusion of HOCl (in nmol min-1, groups: 0, 0+ASA, 1000, 1000+ASA, 2000, and 2000+ASA) into the isolated organ was started after a 45-min steady-state period (t=0). (B) Neutrophils (PMN group: 1480+/-323 and ASA group 1294+/-320 microliter-1) were added into the perfusate between (t=-45 min) and stimulated with FMLP (1 microM) after two 45-min steady-state periods (t=0). Perfusate MPO activity was measured at t=-90, -45, 0, 1, 2, 3, 5, 10, 15, 30, 60, and 90 min. For both groups, pulmonary artery pressure (PAP) and lung weight were continuously recorded and the capillary filtration coefficient (Kf,c in 10(-4) cm(3) s(-1) cm H2O(-1) g(-1) was calculated from the slope of weight gain after a hydrostatic challenge at t=-45, -15, 30, 60 and 90 min. (A) Continuous HOCl infusion (1000/2000 nmol min-1) evoked a significant increase in DeltaPAP and an up to 10-fold increase in Kf,c reaching the maximum extent of the observed effects significant earlier in the 2000 nmol min-1 group. ASA reduced DeltaPAPmax significantly to about 50% in corresponding groups and the increase in PAP and Kf,c occurred later in the ASA groups. (B) Neutrophil stimulation (PMN group/ASA group) evoked a rapid increase in DeltaPAP and MPO activity, while the changes in vascular permeability were rather moderate, but still significant. The release of MPO activity was similar in both groups. ASA significantly reduced the increase in DeltaPAP without affecting the release of MPO activity. Compared to baseline values, the preventive effects on vascular permeability increase reached level of significance as well. In summary, the described changes in pulmonary circulation caused by HOCl infusion or by neutrophil stimulation are significantly reduced by ASA. An involvement of cyclooxygenase products in the mediation of neutrophil-derived oxidative stress could be concluded.

Determination of fluid extraction and osmotic conductance sigma K in the lung with hypertonic NaCl infusion. I. Theory

A dispersion and extraction model of the lung is developed to assess how the infusion of hypertonic saline into the pulmonary artery changes the gravimetric density of pulmonary venous blood. The dispersion analysis is built on the indicator dilution curve measured for the pulmonary circulation. The extraction model consists of microvascular and interstitial compartments separated by a permeable pulmonary endothelium. Because the density of fluid extracted by the hypertonic disturbance is lower than the blood density, the extraction leads to a decrease in blood density. Two cases of fluid extraction are analyzed, a hypertonic infusion to elevate the osmotic pressure in the pulmonary arterial blood in the form of a step function and an infusion performed over a period of 1 sec. Both cases show that the dispersion significantly attenuates the changes in osmotic pressure and density as they are transported by the blood along the pulmonary vasculature. Because the model has taken into account the effect of dispersion and pulmonary blood flow, the equations developed here provide the basis to calculate from the density change in pulmonary venous blood the characteristics of osmotic extraction intrinsic to the lung.

Porous phase model of the lung interstitial fluid motion

A mathematical model of nonsteady-state fluid movement in the lung interstitium during hydrodynamic loading is evaluated. The model is based on experimental results obtained in isolated perfused canine lungs. The interstitium is treated as a porous phase with an elastic framework, able to relax; all resistance to flux is in the interstitial tissue and not in the endothelium. A numerical iteration procedure was applied for the calculation of fluid transfer coefficients. The model describes space distribution of driving pressures in the interstitium.

Fibronectin decreases pulmonary vascular permeability under baseline conditions and after administration of arachidonic acid in rabbit lungs

In blood- and plasma-free perfused isolated rabbit lungs, the influence of albumin and soluble fibronectin on vascular permeability was investigated. The lungs were perfused with Krebs Henseleit buffer containing no protein (KHB), containing 1 g/100 ml bovine albumin (KHAB) or containing albumin together with 100 micrograms/ml soluble fibronectin. The absence or presence of albumin had no influence on the perfusion pressure, the vascular compliance and the capillary filtration coefficient (CFC), determined by zero time extrapolation of the slope of weight gain which was induced by a sudden venous pressure elevation. Fibronectin caused a slight increase in pulmonary artery pressure, a slight decrease in vascular compliance and an approximately 50% reduction of CFC. In a second set of experiments, KHAB-perfused lungs were stimulated by the administration of 100 microM arachidonic acid (AA) during the second hydrostatic challenge within a sequence of three venous pressure elevations. In the presence of indomethacin, which blocks any significant increase in pulmonary vascular pressure after AA application, this procedure caused an immediate gain in lung weight due to increased pulmonary vascular permeability, with greater than 10-fold increased CFC values subsequent to the AA application. In the presence of 100 micrograms/ml fibronectin, this AA-induced increase in CFC was mitigated to less than 20% of the controls without the glycoprotein, with correspondingly severalfold reduced lung weight gain. In conclusion, the present study provides evidence for a direct influence of circulating soluble fibronectin on lung microvascular integrity and fluid balance under baseline conditions and after stimulation of the pulmonary AA cascade.

Pulmonary microvascular injury induced by Pseudomonas aeruginosa cytotoxin in isolated rabbit lungs

The effects of Pseudomonas aeruginosa cytotoxin on the pulmonary microvasculature were studied in blood-free, perfused, isolated rabbit lungs. Cytotoxin was administered to the recirculating Krebs Henseleit albumin (1%) buffer during two consecutive 30-min-perfusion phases (phases 1 and 2) at a concentration of 13 micrograms/ml, followed by a third perfusion phase (phase 3) without toxin. After perfusion phases 2 and 3, the capillary filtration coefficient (Kf,c) and vascular compliance were determined gravimetrically from two-step microvascular pressure increments under zero-flow conditions. Cytotoxin caused a continuous release of K+ and lactate dehydrogenase, which started within the first 5 min and amounted to about 50% of the total lung cellular K+ and 5 to 7% of the total lactate dehydrogenase by the end of the experiment. The toxin caused the continuous generation of prostaglandin I2, which was detectable in the perfusates of all perfusion phases at maximum values five times above the control values and which was measured in the bronchoalveolar lavage fluid at the end of the experiment. Thromboxane generation in toxin-treated lungs did not significantly exceed that of control lungs or of lungs with mechanically induced edema. Cytotoxin caused a gradual increase in pulmonary vascular resistance, to maximum values 2.5 times above the control, starting within 1 min; the increase was partially reversible after washout of the toxin. After a lag period of 20 to 30 min, the lungs gained weight, amounting to a mean gain of 9.1 g at the end of the experiments. After perfusion phases 2 and 3, an almost fourfold increase in Kf,c, which was not reversible after washout of the toxin, was measured, whereas the values of vascular compliance were not altered. We conclude that pseudomonal cytotoxin may be an important factor in the pathogenesis of prolonged microvascular injury, encountered in states of P. aeruginosa sepsis or acute lung failure with secondarily acquired P. aeruginosa pneumonia.

Effects of 100% oxygen breathing on the capillary filtration coefficient in rabbit lungs

Prolonged exposure to 100% O2 at 1 atm is known to result in a progressive increase of the alveolar epithelial permeability to lipid-insoluble molecules. To investigate whether the damage to the capillary endothelium precedes or follows this event, conscious, unanesthetized rabbits were exposed to 100% O2 from 24 to 66 hr, and (a) the filtration coefficient (Kf) of the pulmonary capillary endothelium in isolated, perfused lungs and (b) the arterial and carbon dioxide gas tensions and right and left heart vascular pressures were measured in intact animals. The mean value of the filtration coefficient (+/- SEM) in air-breathing animals was 0.036 +/- 0.002 ml/(min x Torr x g dry lung). After 48 and 66 hr in 100% O2, it increased by 58 and 114% from its baseline value, respectively. At the later period the lung wet/dry weight of the isolated, but not the intact lungs, increased also from 5.42 +/- .2 to 7.3 +/- .3 (means +/- 1 SEM) due to the combination of a higher capillary conductance and the lack of lymph flow in this preparation. All other variables remained normal throughout the exposure. Thus, in contrast to previous morphological findings, these results indicate that the oxygen damage to the capillary endothelium is progressive and occurs concurrently with the increase of the alveolar permeability to solute but before the appearance of pulmonary edema and the compromise of gas exchange.

Alveolar microvessels in isolated perfused dog lungs: structural and functional studies after production of moderate and severe hydrodynamic edema

We have reported earlier that increased endothelial vesiculation follows the development of septal edema and alveolar flooding in isolated dog lung preparations. In this report, established ultrastructural morphometric analyses are coupled with data from physiologic and indicator-dilution studies to evaluate the stage of edema development at which de novo formation of alveolar microvessel plasmalemmal vesicles occurs. The interpretation that alveolar microvessel plasmalemmal vesicles increase prior to alveolar flooding, the final stage of edema formation, is consistent with the results reported here. Moderate pulmonary edema, characterized by substantial fluid cuffing around extra-alveolar arteries and veins and by fluid accumulation restricted to the thick sides of the alveolar septa, is associated with increased vesiculation in alveolar vessel endothelium. Further, a larger percentage of the vesicle population seen is directly attached to the endothelial luminal or abluminal surfaces. The functional significance of an increased population and an altered intracellular distribution of vesicles remains undetermined. The vesicles may provide a minor defense against excessive septal interstitial fluid accumulation, and subsequent alveolar flooding, by contributing to retrograde transport to the blood. Increased vesiculation, on the other hand, may represent an adaptive cellular response to interstitial fluid accumulation.