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(Circulation Research. 1995;77:317-325.)
© 1995 American Heart Association, Inc.

Articles

Pulmonary Microvascular Permeability

Responses to High Vascular Pressure After Induction of Pacing-Induced Heart Failure in Dogs

Mary I. Townsley, Zhenxing Fu, Odile Mathieu-Costello, John B. West

From the Department of Physiology, University of South Alabama, Mobile, and the Department of Medicine, University of California at San Diego, La Jolla.

Correspondence to Mary I. Townsley, PhD, Department of Physiology, MSB 3024, University of South Alabama, Mobile, AL 36688.

	Abstract

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Abstract The pressure threshold for injury of pulmonary capillaries is 50 to 55 cm H₂O in the canine lung, as measured by changes in the filtration coefficient (K_f,c). Since the pulmonary endothelial basement membrane has been observed to thicken in patients with heart failure and pulmonary venous hypertension, we hypothesized that both baseline permeability and the threshold for high-vascular-pressure injury would be altered as a result. Dogs (n=12) were chronically paced at 245 beats per minute for 4 weeks, then were paced at 225 beats per minute for an additional 3 weeks. Lung lobes from anesthetized paced dogs and additional control dogs (n=14) were then isolated, ventilated, and perfused with blood. Although vascular resistance was increased nearly threefold and vascular compliance reduced by 50% in the paced group, K_f,c referenced to 1 g blood-free dry weight was no different from control. Despite this lack of difference at normal pulmonary vascular pressures, several significant results were obtained. First, in the paced group there was a significant increase in the threshold for high-vascular-pressure injury: K_f,c measured at pulmonary vascular pressures commonly seen in heart failure (20 to 50 cm H₂O) were significantly less in this group compared with control. Model predictions showed that in vivo, this difference in K_f,c would result in a 50% reduction in the amount of water and protein cleared across the pulmonary capillary endothelial barrier in the paced group. Next, challenge with high vascular pressure resulted in less accumulation of residual blood in this group compared with control. Finally, in separate groups of animals, morphometric analysis of the alveolocapillary barrier showed that after either 4 or 7 to 8 weeks of pacing, endothelial, interstitial, and epithelial thicknesses were increased compared with control. Together, these results suggest that vascular remodeling confers a modest but important increase in the resistance of these lungs to high-vascular-pressure injury and the development of alveolar edema.

Key Words: pulmonary circulation • isolated lung • capillary filtration coefficient • osmotic reflection coefficient • stretched pore phenomenon

	Introduction

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In the normal canine lung, the threshold for acute high-vascular-pressure injury, as measured by changes in the K_f,c, is 55 cm H₂O or 40 mm Hg.^{1 2} This threshold cannot be shifted by prior chemical injury of the lung,² suggesting that it does not depend on endothelial integrity per se. In contrast, morphological assessment of the changes in alveolocapillary barrier ultrastructure in response to high pulmonary vascular pressure has suggested that the threshold for mechanical failure is related to the strength of the capillary basement membrane.³ Increases in the thickness of the capillary endothelial basement membrane in lung biopsies from patients with a history of pulmonary venous hypertension secondary to heart failure or mitral stenosis^{4 5} has been interpreted as evidence for decreased pulmonary microvascular permeability.^{6 7}

If microvascular permeability indeed decreases, this adaptation would clearly be beneficial. As a consequence, the movement of fluid and proteins into the pulmonary interstitium would be limited despite the pulmonary venous hypertension. A further benefit, which has not previously been addressed, could be an increase in the resistance of the pulmonary capillaries to injury and mechanical failure during pulmonary venous hypertension. However, there is little information in the literature regarding pulmonary microvascular permeability in congestive heart failure. Kaplan et al⁸ reported that the pulmonary transcapillary escape rate for transferrin was no different in patients with congestive heart failure and normotensive control subjects. These results are difficult to interpret, because the measurement of tracer macromolecule accumulation in lung tissue is influenced by the rate of transcapillary fluid filtration and lymphatic drainage, as well as by microvascular permeability per se.^{9 10} To specifically evaluate microvascular permeability, K_f,c, , or the permeability–surface area product must be measured. We previously evaluated vascular adrenergic reactivity and K_f,c in the canine lung after pacing-induced heart failure.¹¹ Although there was substantial cardiac dysfunction and enhanced pulmonary vascular adrenergic reactivity after 4 weeks of pacing in that study, the pulmonary K_f,c in the paced group was not different from that in control. We are not aware of any other studies in which permeability coefficients were measured in patients or animals in heart failure.

In the present study, we hypothesized that baseline permeability would be decreased and the threshold for high-pressure injury increased after heart failure as a result of vascular remodeling. We measured K_f,c and for total proteins and albumin in isolated lung lobes from control dogs and those in pacing-induced heart failure. Since we had previously found no change in K_f,c after 4 weeks of pacing,¹¹ we continued pacing the dogs for up to 7 or 8 weeks. In addition, the ultrastructure of the alveolocapillary barrier was evaluated in several lobes to determine the extent of pacing-induced changes compared with controls. Portions of this work have been reported in abstract form.^{12 13}

	Materials and Methods

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Pacing Model of Heart Failure
Conditioned, microfilaria-negative mongrel dogs (n=12; weight, 24.6±0.9 kg, mean±SEM) were anesthetized with sodium pentobarbital (30 mg/kg IV). Under sterile operating conditions, a transvenous unipolar pacing lead (model 4011, Medtronic Target Tip) was introduced into the right ventricle via a jugular vein, as described.¹¹ A pacemaker generator (custom SX-8329, Medtronic) was implanted in a subcutaneous pocket anterior to the first rib. After the lead was tunneled subcutaneously to the pocket, it was attached to the generator, and the skin incisions were closed in layers. The dogs were given buprenorphine (0.2 mg IM) during recovery for analgesia and antibiotics (cefazolin sodium, 500 mg PO twice daily) for 5 days after surgery. They were allowed free access to food and water throughout the postoperative period and the subsequent pacing period.

Pacing at 245 bpm^{11 14} was initiated 1 to 2 days after recovery from surgery via an external programmer (model 9710, Medtronic). The progression of heart failure was monitored biweekly by echocardiogram of the left ventricle. LVSF was measured in sinus rhythm while the dogs were lying in a right lateral decubitus position and was calculated as the difference between left ventricular end-diastolic and end-systolic diameters divided by the end-diastolic diameter. Pacing was maintained at 245 bpm until LVSF fell to 18% (3 to 4 weeks). At that time, the pacing rate was decreased to 225 bpm to stabilize cardiac function for the remainder of the pacing period. Total pacing time averaged 46.6±1.0 days (range, 38 to 51 days). A group of microfilaria-negative mongrel dogs (n=14, 22.0±0.9 kg) studied acutely served as controls.

Terminal Experiment
Dogs were fasted overnight. Analgesia was induced with ketamine hydrochloride (200 mg IM). Then heparin (10 000 U IV) was administered, followed by intravenous sodium pentobarbital (<15 mg/kg in the paced animals; up to 30 mg/kg in controls) to induce a surgical plane of anesthesia. Anesthesia was maintained by intravenous administration of -chloralose. The dogs were then intubated and ventilated (15 mL/kg tidal volume, 15 breaths per minute). In the paced group, hemodynamics were evaluated in vivo before lung isolation. The left carotid artery and jugular vein and a femoral artery (with the catheter advanced into the left ventricle) were cannulated for measurement of arterial blood gases and blood withdrawal, central venous pressure, and left ventricular end-diastolic pressure, respectively. All in vivo pressure measurements were measured in sinus rhythm. In some paced animals, these measurements could not be made because of anesthetic-induced cardiovascular collapse. After the in vivo measurements, the lung lobes were isolated.

Isolated Lung Preparation
Surgical details of the lower left lung lobe isolation and cannulation have been described.^{1 2 11 15 16} Briefly, after an intercostal incision, the left upper and middle lobes were isolated and excised for measurement of blood-free extravascular lung water (see below). The lower left or middle right lobe was then isolated for ex vivo perfusion. A mixture of 200 mL blood, removed via the carotid cannula, and 100 mL sterile Earle's buffer solution (heated to 37°C) was used to fill the perfusion system. Lobes were rapidly excised, and wide-bore plastic cannulas were tied into the lobar artery, vein, and bronchus. The lobes were then suspended from a counterbalanced force transducer (Grass model FT-10) calibrated so that a 1.0-cm deflection equaled a 1.0-g weight gain. Lobes were ventilated with 30% O₂/5% CO₂ with the rate set to 6 breaths per minute and end-expiratory and inspiratory pressures set to 2 and 8 to 10 cm H₂O, respectively. Perfusate pH was measured and corrected to 7.35 to 7.40 with the addition of sodium bicarbonate.

Lobar Hemodynamics
Thin catheters were placed near the orifices of the inflow and outflow cannulas to measure Pa and Pv, respectively, both referenced to the level of the lung hilum. Pv was set to 4 to 5 cm H₂O, and |$$ was increased via the perfusion pump to the maximum attainable while the lobe was kept isogravimetric, and then was held constant. |$$ was measured by timed collection of venous effluent in a graduated cylinder and referenced either to 100 g initial lobe wet weight or to 1 g blood-free dry weight, the latter obtained from the lung water analysis detailed below. Pc was measured by the double vascular occlusion technique.¹⁵ Pressures and lung weights were recorded on a Grass polygraph. Total pulmonary vascular resistance (R_t) was calculated as R_t=(Pa-Pv)/|$$. Total vascular compliance (C_t) was measured with the venous occlusion technique¹⁷ and was calculated as C_t=|$$/(linear rate of Pv/t after rapid venous occlusion).

Evaluation of Permeability and Transvascular Fluid Exchange
K_f,c and were used as measures of microvascular permeability.^{1 2} K_f,c was evaluated after a step increase in Pv that was maintained for 15 minutes. Unlike the calculation of K_f,c in our earlier studies, only the resulting steady rate of weight gain (W/t) between minutes 13 and 15 was used to calculate K_f,c according to the equation K_f,c=(W/t)/Pc. Pc was calculated as the difference between Pc before and at the end of the 15-minute period of elevated pressure. If the lobe was not isogravimetric before the K_f,c maneuver, the pre-K_f,c rate of weight gain was subtracted from that measured when pressure was elevated, and the difference was used to calculate K_f,c. K_f,c was expressed in mL · min^-1 · cm H₂O^-1 per 100 g initial lobe wet weight or per 1 g blood-free dry weight, assuming the density of the filtered fluid to be 1 g/mL.

In these isolated lung lobes, was evaluated by measurement of the relative changes in perfusate Hct and protein concentration after a period of fluid filtration.^{2 18 19} Hct and protein concentrations were measured before (i) and after (f) a period of fluid filtration sufficient to increase Hct by 8%. was calculated via the equation

(1)

where Pr is protein concentration and Pr* is the arithmetic mean of Pr_i and Pr_f.¹⁹ Total protein concentration was measured by the Lowry technique.²⁰ Both Hct and the total protein concentration were corrected for loss of red cell mass via hemolysis,²¹ by use of measures of free perfusate hemoglobin.²² Albumin concentrations were measured via rocket immunoelectrophoresis²³ using the IgG fraction of rabbit monospecific antisera to dog albumin (Capell).

Measurement of Blood-Free EVLW
EVLW was measured by a modification²⁴ of the method devised by Pearce et al.²⁵ After homogenization of the lobe with distilled water, aliquots of blood, lung homogenate, and supernatant (obtained by centrifugation of the homogenate at 15 000 rpm for 1 hour) were weighed and then dried to constant weight (65°C). Total hemoglobin was measured²² in both blood and supernatant, allowing correction of lung water for blood water. The total lobar blood-free dry weight was determined and EVLW expressed as milliliters per gram blood-free dry weight.

Experimental Protocol
After the initial hemodynamic measurements, a baseline value of K_f,c was obtained (Pv=15 cm H₂O for 15 minutes). After recovery of an isogravimetric state, both K_f,c and were measured while Pv was set to one pressure between 48 and 86 cm H₂O (initial HiPv). In each case, the Pv for each lung was chosen randomly from pressures within this range, and that Pv was maintained for 15 minutes. When the lobe had regained an isogravimetric state (60 to 90 minutes), measurements of K_f,c and were repeated. For the second HiPv challenge, Pv was set randomly to one pressure between 17 and 52 cm H₂O (second HiPv), and K_f,c was determined during the first 15 minutes. Pv was maintained until the Hct had increased by 8% to allow measurement of . This protocol differs from our earlier reports of high-pressure injury when the HiPv was maintained for only 5 minutes.^{1 2}

Equivalent Pore Analysis
With the data, an equivalent pore analysis of the pulmonary capillary exchange barrier at high pressure was made, as previously described.² Briefly, knowing and the solute radius (37 Å) for albumin, we solved for an equivalent pore radius of the barrier at each time point using the Drake and Davis equation.²⁶

Morphometry
Additional lung lobes from 4 control dogs, 2 dogs paced for 4 weeks, and 3 paced for 7 to 8 weeks were cannulated and perfused with autologous blood, as described above. After lobar hemodynamics had stabilized, Pv was elevated to 23 cm H₂O, and the lobe was flushed with 1.5 L saline at the same flow rate. Saline perfusion was followed by perfusion with 1 to 1.5 L buffered glutaraldehyde (2.5%) to fix the lobe. During fixation, pump flow was gradually decreased to maintain lobar arterial pressure constant. Saline and glutaraldehyde were not recirculated. One 0.5-cm tissue slab was cut from the fixed lobe in a direction parallel to the cranial-caudal axis at approximately two thirds of the distance lateral from this central axis, vertical sections were generated, and tissue blocks were processed for electron microscopy as previously described.^{27 28 29 30} Five blocks randomly chosen from each lobe were cut with an LKB Ultratome III. Sections 1 µm thick were cut from each block, stained with 0.1% toluidine blue aqueous solution, and examined by light microscopy. From the same tissue blocks, ultrathin sections (50 to 70 nm) were prepared, contrasted with uranyl acetate and bismuth subnitrate, and examined with a Zeiss 10 electron microscope. Twelve micrographs were taken by systematic random sampling of one thin section from each block (total, 60 micrographs per lobe). With morphometric techniques, the frequency of endothelial and epithelial breaks and the thickness of endothelium, interstitium, epithelium, and the total barrier were measured at a final magnification of x11 000.^{27 28 29} In addition, systematic random sampling of 55 micrographs from each lobe (10 micrographs from each of three blocks taken at a magnification of x26 352 plus those taken at five different magnifications from x18 099 to x42 012 at each of five randomly chosen sites) was used to examine the appearance of the alveolocapillary barrier basement membrane.

Data are presented as mean±SEM. Statistical differences were evaluated by analysis of variance, with a Student-Newman-Keuls post hoc test to determine specific differences when necessary.³¹

	Results

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Overall, LVSF decreased from 35.6±0.8% to 15.0±0.3% (P<.05 versus baseline) after 7 to 8 weeks of pacing (Fig 1), although it was relatively stable after 4 weeks. In the paced animals, at the time of the terminal study under anesthesia, left ventricular end-diastolic pressure averaged 35.1±4.3 mm Hg (48 cm H₂O), and central venous pressure averaged 10.4±1.1 mm Hg (14 cm H₂O). We previously showed that left ventricular end-diastolic pressure and central venous pressure in control dogs averaged 4 and 2 mm Hg, respectively.¹¹ Systemic PO₂ was 63.2±4.7 mm Hg in the paced group, consistent with the higher EVLW (5.68±0.41 mL/g) compared with control (4.05±0.05 mL/g; P<.05). The incidence of pleural effusion, pericardial effusion, and ascites in the paced dogs was 17%, 83%, and 75%, respectively. Airway foam or frank fluid accumulation was absent in 4 paced dogs, rated as minimal (1+) in another 4 animals, and severe (4+) in the remainder of this group. The lobar hemodynamics in the two groups are shown in Table 1. In the paced group, isogravimetric blood flow decreased by 50% (P<.05), total vascular resistance increased nearly threefold (P<.05), and vascular compliance decreased by 50% (P<.05) compared with controls. These differences in , R_t, and C_t were retained when the measurements were referenced to lobar blood-free dry weight.

View larger version (19K): [in this window] [in a new window]   Figure 1. Graph showing LVSF measured in awake animals in sinus rhythm as a function of pacing time. Note that the pacing rate was decreased after 3 to 4 weeks to stabilize left ventricular function.

View this table: [in this window] [in a new window]   Table 1. Baseline Hemodynamics in Isolated Lung Lobes

At baseline, K_f,c was significantly less in the paced group than in controls (0.042±0.005 versus 0.063±0.005 mL · min^-1 · cm H₂O^-1 · 100 g wet wt^-1, respectively; P<.05). When Pv was increased to an average of 64 to 68 cm H₂O (47 to 50 mm Hg), K_f,c increased significantly in both groups. The distribution of individual responses to HiPv is shown in Fig 2. In the control group at the initial HiPv, K_f,c averaged 0.243±0.038 mL · min^-1 · cm H₂O^-1 · 100 g^-1. Although K_f,c in the paced group tended to be less (0.151±0.032 mL · min^-1 · cm H₂O^-1 · 100 g^-1), statistical significance was not achieved (P=.08). During the second challenge, when Pv averaged 37 to 38 cm H₂O (or 27 to 28 mm Hg), the resultant K_f,c was less than at the initial HiPv (0.121±0.016 versus 0.051±0.006 mL · min^-1 · cm H₂O^-1 · 100 g^-1 in control and paced groups, respectively; P<.05). In addition, this second K_f,c was significantly lower in the paced group than in controls (P<.05). The overall pattern of individual K_f,c responses (Fig 2) suggests that the threshold for HiPv injury was shifted to the right by 20 to 30 cm H₂O in the paced group. Average K_f,c values, referenced per gram blood-free dry weight, are shown in Fig 3. Importantly, there was no difference in the baseline K_f,c between the two groups when the data were normalized in this manner. However, the initial HiPv K_f,c still tended to be lower, and the second HiPv K_f,c was significantly lower in the paced group compared with control (P<.05).

View larger version (24K): [in this window] [in a new window]   Figure 2. Scatterplot showing pulmonary capillary filtration coefficients (Kf,c) referenced to 100 g initial wet weight plotted as a function of Pv during the measurement. Kf,c during baseline (circles) are compared with those during an initial (triangle) and second (inverted triangle) 15-minute exposure to HiPv. Lungs from animals paced for 7 to 8 weeks (solid symbols) were more resistant to high-vascular-pressure injury than control lungs (open symbols) over the middle range of Pv. Even at Pv >55 cm H2O, Kf,c tended to be less. Overall, the threshold for pressure-induced increases in Kf,c was shifted to the right in the paced group.

View larger version (25K): [in this window] [in a new window]   Figure 3. Bar graph showing Kf,c measured during baseline and during the two HiPv periods also referenced to 1 g blood-free dry weight and averaged (±SEM). During the initial HiPv, the average Kf,c rose similarly in both groups (*P<.05 vs baseline). However, during the second HiPv, Kf,c in the paced group was significantly less than that in controls (**P<.05 vs control) and no different from the baseline Kf,c. In contrast, the second HiPv Kf,c in the control group was 86% greater on average than the baseline value (P=.06). #P<.05 vs initial HiPv.

These differences in K_f,c in the two groups, despite the similar pressure history, were echoed by measurements of EVLW. The final values for EVLW in the two groups were similar (9.11±0.45 versus 9.20±0.42 mL/g in controls and paced lobes, respectively). However, the difference between the final EVLW and that observed in the initial lung samples was substantially greater in controls (5.06±0.45 mL/g) than in the paced group (3.52±0.61 mL/g; P<.05). Similarly, the residual blood (intravascular plus extravascular blood) in the perfused control lobes (7.4±0.7 mL/g dry wt) was significantly greater than in the paced group (3.2±0.6 mL/g dry wt; P<.05). However, there was no difference in residual lobar blood between the control and paced groups (1.7±0.1 and 1.7±0.3 mL/g dry wt, respectively) when the left upper and middle lobes were evaluated for lung water.

There was no discernible difference in for total proteins or albumin at any Pv between the two groups (Table 2). The criterion for measurement of (ie, that Hct increase by >8%) was not reached in all lobes. In control lobes, for albumin averaged 0.79±0.04 and 0.73±0.03 at the initial and second HiPv, respectively, while the average total protein were slightly lower. There were no differences in for albumin or total protein between measurements at the two pressures.

View this table: [in this window] [in a new window]   Table 2. Equivalent Pore Analysis

Results of the morphometric analysis on lung tissue from control and paced dogs are shown in Table 3. Tissues were obtained from dogs paced for 4 weeks, in which LVSF averaged 16.5%, and from those paced for 7 to 8 weeks, in which LVSF averaged 16.1%. Ultrastructural analysis provided quantitative evidence for thickening of the entire blood-gas barrier in the paced dogs, as suggested in representative light (Fig 4) and electron (Fig 5) micrographs. The average total barrier thickness increased from 0.65 µm in control to 1.09 µm after 4 weeks of pacing (P<.05). Interstitial and total barrier thicknesses were intermediate at 7 to 8 weeks (0.85 µm) but significantly different from measures in both the 4-week paced group and controls (P<.05). Although the greatest contributor to the increased barrier thickness in both paced groups was the increase in interstitial thickness (P<.05), the endothelial and epithelial thicknesses increased as well (P<.05). Despite the markedly high pulmonary pressures measured in vivo in the paced dogs, no endothelial or epithelial breaks were observed. In the control group, only one break was observed in the barrier among 240 micrographs from the four lungs. A systematic comparison of the ultrastructure of the blood-gas barrier at higher power showed that the basement membrane was more clearly delineated after 4 weeks of pacing-induced pulmonary venous hypertension (Fig 6), although the appearance of the basement membrane after 7 to 8 weeks of pacing could not be distinguished from that in controls.

View this table: [in this window] [in a new window]   Table 3. Morphometry of the Alveolocapillary Barrier

View larger version (60K): [in this window] [in a new window]   Figure 4. Light micrographs of representative portions of the lung parenchyma from control dogs (A) and those paced for 4 weeks (B) or 7 to 8 weeks (C). Tissues were fixed by perfusion at 23 cm H2O (see text for details).

View larger version (115K): [in this window] [in a new window]   Figure 5. Electron micrographs of the same lungs, showing the ultrastructure of the alveolocapillary barrier in control dogs (A) and those paced for 4 (B) or 7 to 8 weeks (C). Note the thickening of the interstitium (i) in the 4-week paced group (B). a indicates alveolar space; c, capillary.

View larger version (69K): [in this window] [in a new window]   Figure 6. Electron micrographs at higher magnification show the qualitative difference in basement membrane thickness in lung from control dogs (A) and those paced for 4 (B) or 7 to 8 weeks (C). The better delineation of the basement membrane after 4 weeks (B, arrow) compared with control was not apparent in the 7- to 8-week paced group. The micrographs are examples of the systematic random sampling of micrographs used to examine the appearance of the alveolocapillary barrier basement membrane (see text).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
In the normal canine lung, acute elevations in Pc to levels >24 cm H₂O (18 mm Hg) result in pulmonary edema.³² In contrast, in congestive heart failure or mitral stenosis, pulmonary venous hypertension of this magnitude is not always accompanied by pulmonary edema.^{6 33 34 35} In a large group of patients with mitral stenosis, pulmonary edema never occurred with left atrial pressures <30 mm Hg, and pressures up to 50 mm Hg could be tolerated for short periods without distress.³⁴ A decrease in the permeability of the capillary endothelial barrier^{6 7} is one possible explanation for these findings, although to date few experimental studies have been designed to directly address this problem.

Pacing-Induced Heart Failure and Pulmonary Microvascular Permeability
In the present study, we used the canine model of pacing-induced heart failure and a protocol that maintained LVSF at 50% of baseline levels for nearly 1 month. Lobar hemodynamics were markedly altered as a result, while EVLW ranged from normal values to values 80% greater than normal. These patterns are similar to those observed in animals paced for only 4 weeks.¹¹ Despite these changes, we did not detect any substantial alteration in baseline pulmonary microvascular permeability in the paced group compared with controls, when permeability was evaluated at low venous pressures.

In contrast, after the development of pacing-induced heart failure, there was a significant increase in the resistance to high-vascular-pressure lung injury. The normal canine lung microvasculature has previously been shown to resist injury after acute, transient increases in pulmonary venous pressure, as long as that pressure remained less than 50 to 55 cm H₂O. At higher Pv, K_f,c increases.^{1 2 36} Our present results suggest that the threshold for the pressure-induced increase in K_f,c in control canine lung may actually be lower. In 6 of 14 control lobes exposed to Pv between 20 and 50 cm H₂O, K_f,c remained at or near baseline levels, with an average K_f,c ratio of 1.15. This contrasts with the response in the remaining 8 lobes in this group, in which the average K_f,c increased approximately threefold compared with baseline. This lower pressure threshold in control lung may be due to the longer duration of the HiPv challenge (15 minutes) compared with the 5-minute exposure used previously^{1 2} or with differences in the K_f,c measurement per se. The K_f,c measurement used in our present study appears to be more sensitive to small changes in lung microvascular permeability³⁷ than the zero-time extrapolation measure used in earlier studies.^{1 2} Nonetheless, compared with the concurrent control group, it is clear that lobes from paced animals were more resistant to high-pressure injury (see Fig 2). This is evidenced by the lower K_f,c during acute exposure to high venous pressures (particularly in the 20 to 50 cm H₂O range) and by the diminished accumulation of residual blood in the paced group. Together, these data provide evidence that barrier strength in the paced group was modestly increased.

However, despite the documented increase in K_f,c at high Pv, there does not appear to be any effect of this mechanical stress on the barrier permeability to macromolecules in the canine lung. The for total proteins has not been shown to vary significantly in control lung during exposure to Pv ranging from 25 to >100 cm H₂O.^{2 36 38 39} Similarly, we found no difference in for total proteins or albumin at any Pv between the control and paced groups (Table 2). For comparison, in lobes from control and paced (4 weeks) animals perfused only at low venous pressures, for total proteins was 0.65^{2 40} and 0.70±0.06 (Reference 40⁴⁰ and unpublished results), values similar to those reported here.

Morphometric Analysis of the Alveolocapillary Barrier
There have been no previous morphometric analyses of stress failure per se after the development of chronic pulmonary venous hypertension, although the basement membrane of pulmonary capillaries in alveolar septa has been shown by electron microscopic analysis to be thickened in patients with chronic pulmonary venous hypertension.^{4 5} A number of pieces of evidence supported the notion that this thickening would alter the pulmonary response to high-pressure challenge. These include the similarity of the threshold for high-pressure injury in the normal canine lung^{1 2 36} to the compliant limit for the pulmonary capillary bed⁴¹ and the invariance of the pressure threshold for mechanically induced injury after chemical damage to the lung alveolocapillary barrier.² Further, although focal endothelial discontinuities have been observed in neurogenic pulmonary edema, the basement membrane and alveolar epithelium were normal in appearance.⁴² Similarly, although the frequency of endothelial and epithelial breaks in lung after acute HiPv challenge has been shown to be related to magnitude of the mechanical stress,^{3 27 28 29} basement membrane rupture was less common. In our study, the basement membrane was better delineated after 4 weeks of pacing (Fig 6). However, this may be secondary to local edema⁴³ rather than to any matrix deposition per se, since the appearance of the basement membrane after 7 to 8 weeks of pacing could not be distinguished from that in controls.

Recently, the thickness of the total barrier has been suggested to be a more important determinant of resistance to high-vascular-pressure injury, since wall stress is inversely related to total barrier thickness.³ Birks et al³⁰ reported that the total thickness of the alveolocapillary barrier was least in rabbits, intermediate in the dog, and greatest in the horse, a ranking that correlated well with the pulmonary vascular pressures required to cause stress failure (50, 90, and 130 cm H₂O, respectively). After 7 to 8 weeks of pacing in the present study, we found that the total thickness of the barrier was increased by 30% on average. Thus, this adaptation may underlie the increased resistance of lungs from these paced animals to high-vascular-pressure injury.

Consequences for Fluid and Protein Exchange In Vivo
One can use the measures of microvascular permeability obtained in this study to predict that the structural alterations associated with pacing and heart failure, albeit modest, do nonetheless confer a measure of protection against pressure-induced increases in transcapillary fluid flux and protein clearance. The Starling equation^{9 44} describes the determinants of transcapillary fluid flux (J_v):

(2)

where the undefined variables are the interstitial hydrostatic pressure (P_t) and colloid osmotic pressure (_t) and the plasma colloid osmotic pressure (_p). Using Equation 2 and the K_f,c and data measured in the present study, we can estimate fluid flux in the normal and hypertensive pulmonary circulations. Similarly, we can calculate clearances for total protein (ie, the transcapillary flux normalized to the plasma protein concentration) using a modification of the Patlak equation^{9 44} under the same conditions:

(3)

where the undefined variables are C, the plasma (C_p)–to–interstitial protein concentration gradient, and the Peclet number, x, which relates convective to diffusive protein exchange across the capillary endothelium.

An important comparison is that between predictions for the normal lung, with a Pc of 10 cm H₂O, and the lung in an animal or patient in heart failure, in which Pc is 40 or 70 cm H₂O (Table 4). We have assumed that the barrier coefficients measured in the canine lung are similar to those in the human lung (other assumptions used in this calculation are detailed in Table 4^{44 45} ). Note that at moderately high vascular pressures in control lung, pulmonary transcapillary fluid flux and protein clearance should be elevated 4.6- and 3-fold above baseline, respectively. When the lung after heart failure was exposed to comparable pressures, fluid flux and protein clearance again increased. However, both measures remained 50% less than that in control lung because of the attenuated pressure-induced change in K_f,c.

View this table: [in this window] [in a new window]   Table 4. Estimation of Fluid and Protein Exchange In Vivo

In patients with decompensated congestive heart failure, Kaplan et al⁸ found that the transcapillary escape rate of transferrin was normal. Transcapillary escape rate measures the accumulation of tracer in lung normalized to blood concentrations of the tracer, ie, tracer clearance. Transferrin has a Stokes-Einstein radius of 43 Å and thus should have a clearance similar to that of net total proteins. That transferrin clearance was normal with heart failure⁸ is somewhat difficult to interpret, since pulmonary vascular pressures were not documented. However, the patients in that study probably had pulmonary venous hypertension, since lung density was increased, indicating pulmonary edema. The differences between these in vivo measurements and our predictions for fluid and protein transfer into the lung can be resolved if lymphatic drainage is taken into account. Ishibashi et al¹⁰ showed that tissue accumulation of tracer protein can markedly underestimate transcapillary protein clearance, particularly when fluid flux is elevated, because of removal of tracer from the lung in lymph. Together, these observations point toward a critical role for lymphatic drainage as a safety factor in heart failure–induced pulmonary venous hypertension.

Conclusions
The results of this study demonstrate that baseline pulmonary microvascular permeability was not altered after pacing-induced heart failure. Such alterations may require longer-term exposure to heart failure–induced pulmonary venous hypertension than that evaluated in this study. Nonetheless, several physiological results obtained from this work are significant and relevant to the in vivo condition. First, K_f,c in lungs from paced animals was significantly less than that in control lung at pulmonary vascular pressures commonly seen in heart failure (20 to 50 cm H₂O); ie, there was an increase in the threshold for high-vascular-pressure injury in the paced group. This was supported by the reduction in residual blood accumulation after high-pressure challenge in the paced group. Although the alveolocapillary basement membrane was more clearly delineated after 4 weeks of pacing, the resolution of this change after 7 to 8 weeks of pacing suggests that increased thickness of other barrier components (ie, the endothelial and epithelial layers) underlies the differences in K_f,c during exposure to HiPv. These alterations appear to constitute a beneficial adaptation to the pulmonary venous hypertension that accompanies heart failure. Indeed, we predict that after heart failure, transcapillary fluid flux and protein clearance would be 50% less than that predicted for normal lung at comparably increased vascular pressures. In conclusion, our findings suggest that remodeling of the alveolocapillary barrier probably plays an important role in the response to heart failure and pulmonary venous hypertension.

	Selected Abbreviations and Acronyms

 

= osmotic reflection coefficient bpm = beats per minute EVLW = extravascular lung water Hct = hematocrit HiPv = high venous pressure Kf,c = capillary filtration coefficient LVSF = left ventricular shortening fraction Pa = arterial pressure Pc = capillary pressure Pv = venous pressure |$$ = blood flow

	Acknowledgments

 
This study was supported by NIH grants HL-39045, HL-17331, and HL-46910. Dr Townsley is an Established Investigator of the American Heart Association. The authors wish to thank Vicki Pitts, Sue Barnes, and Paula Flowers for their excellent technical assistance.

Received February 27, 1995; accepted May 1, 1995.

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