Effects of hypoproteinemia on lung microvascular protein sieving and lung lymph flow

Pathophysiology of the Lymphatic System in Patients With Heart Failure: JACC State-of-the-Art Review

The removal of interstitial fluid from the tissues is performed exclusively by the lymphatic system. Tissue edema in congestive heart failure occurs only when the lymphatic system fails or is overrun by fluid leaving the vascular space across the wall of the capillaries into the interstitial space. This process is driven by Starling forces determined by hydrostatic and osmotic pressures and organ-specific capillary permeabilities to proteins of different sizes. In this review, we summarize current knowledge of the generation of lymph in different organs, the mechanics by which lymph is returned to the circulation, and the consequences of the inadequacy of lymph flow. We review recent advances in imaging techniques that have allowed for new research, diagnostic, and therapeutic approaches to the lymphatic system. Finally, we review how efforts to increase lymph flow have demonstrated potential as a viable therapeutic approach for refractory heart failure.

Isolation of rat trachea interstitial fluid and demonstration of local cytokine production in lipopolysaccharide-induced systemic inflammation

Access to interstitial fluid from trachea is important for understanding tracheal microcirculation and pathophysiology. We tested whether a centrifugation method could be applied to isolate this fluid in rats by exposing excised trachea to G forces up to 609 g. The ratio between the concentration of the equilibrated extracellular tracer 51Cr-labeled EDTA in fluid isolated at 239 g and plasma averaged 0.94 +/- 0.03 (n = 14), suggesting that contamination from the intracellular fluid phase was negligible. The protein pattern of the isolated fluid resembled plasma closely and had a protein concentration 83% of that in plasma. The colloid osmotic pressure in the centrifugate in controls (n = 5) was 18.8 +/- 0.6 mmHg with a corresponding pressure in plasma of 22 +/- 1.5 mmHg, whereas after overhydration (n = 5) these pressures fell to 9.8 +/- 0.4 and 11.9 +/- 0.4 mmHg, respectively. We measured inflammatory cytokine concentration in serum, interstitial fluid, and bronchoalveolar lavage fluid in LPS-induced inflammation. In control animals, low levels of IL-1 beta, IL-6, and TNF-alpha in serum, trachea interstitial fluid, and bronchoalveolar lavage fluid were detected. LPS resulted in a significantly higher concentration in IL-1 beta and IL-6 in interstitial fluid than in serum, showing a local production. To conclude, we have shown that interstitial fluid can be isolated from trachea by centrifugation and that trachea interstitial fluid has a high protein concentration and colloid osmotic pressure relative to plasma. Trachea interstitial fluid may also reflect lower airways and thus be of importance for understanding, e.g., inflammatory-induced airway obstruction.

Use of multiple fluorophores for evaluating microvascular permeability in control rats and rats with sepsis

Capillary leak accompanying systemic inflammatory response conditions is a significant clinical problem. In the present study, we describe and verify a method for studying capillary leak that is based on the injection of proteins that differ significantly in size and have spectrally distinguishable fluorophores. Control (n=11) and post-CLP (caecal ligation and puncture; n=14) Sprague–Dawley rats were injected with tracer amounts of albumin and PEG–Alb [albumin covalently linked to methoxy-poly(ethylene glycol)] labelled with fluorescein and Texas Red. Blood samples were withdrawn between 5 min and 144 h, and the fluorescence of the labelled proteins was determined. The relative retention of the PEG–Alb and albumin was assessed via measurement of the TER (transcapillary escape rate; in %/h) and the t50% estimate, defined as the time when the actual concentration reached 50% of its baseline. The concentration–time trends for both albumin and PEG–Alb tracers exhibited two-compartmental behaviour and were analysed using bi-exponential modelling. Retention times were significantly greater for PEG–Alb in both control and CLP rats. TERPEG-Alb was significantly lower than TERalbumin for both control (8.1±5.6 compared with 14.8±7.1 %/h respectively; P<0.01) and CLP (14.8±6.6 compared with 22.5±7.3 %/h respectively; P<0.001) rats. The t50%[PEG–Alb] was substantially greater than the corresponding t50%[albumin] for both control (29.8±9.8 compared with 7.2±2.0 h respectively; P<0.001) and CLP (12.9±5.6 compared with 5.1±1.6 h respectively; P<0.001) rats. The result was similar irrespective of the fluorophore–protein combination, validating the multifluorophore technique. In conclusion, the double-fluorophore approach described in the present study may provide the future basis for a method to quantify capillary leak in disease.

Effect of concentration and hyaluronidase on restriction of hetastarch flux through lung interstitial segments

The transport properties of lung interstitium were studied by measuring the flow of hetastarch solution (2 and 6%) through 1-cm perivascular interstitial segments of rabbit lungs. Hetastarch (10(4)-10(7) Da) solution has a colloid osmotic pressure similar to that of albumin solution. Driving pressure was 5 cm H(2)O and mean interstitial pressure was 0 cm H(2)O. The flows of 2 and 6% hetastarch solutions were measured before (Q(1)) and after (Q(2)) the addition of 0.02% hyaluronidase. Hetastarch molecular distributions in effluent samples were measured by high-performance size-exclusion chromatography (HPSEC) to determine sieving ratio (C(out)/C(in), downstream-to-upstream concentration ratio). Hyaluronidase significantly (P < 0.0004) increased flow sixfold, but the increase in flow (Q(2)/Q(1)) was reduced through the interstitium around smaller vessels. A similar behavior was observed with the flow of albumin solution without and with hyaluronidase. C(out)/C(in) decreased monotonically with molecular weight, was greater with 6% than with 2% (low colloid osmotic pressure) hetastarch, and increased with hyaluronidase. Modeling the transport through uniform pores, equivalent pore radius was 10 and 15 nm with 2 and 6% hetastarch, respectively, and doubled with hyaluronidase. In conclusion, interstitial pores expand in response to an increase in colloid osmotic pressure both before and after tissue degradation by hyaluronidase.

Isolation of pulmonary interstitial fluid in rabbits by a modified wick technique

Interstitial fluid protein concentration (C(protein)) values in perivascular and peribronchial lung tissues were never simultaneously measured in mammals; in this study, perivascular and peribronchial interstitial fluids were collected from rabbits under control conditions and rabbits with hydraulic edema or lesional edema. Postmortem dry wicks were implanted in the perivascular and peribronchial tissues; after 20 min, the wicks were withdrawn and the interstitial fluid was collected to measure C(protein) and colloid osmotic pressure. Plasma, perivascular, and peribronchial C(protein) values averaged 6.4 +/- 0.7 (SD), 3.7 +/- 0.5, and 2.4 +/- 0.7 g/dl, respectively, in control rabbits; 4.8 +/- 0.7, 2.5 +/- 0.6, and 2.4 +/- 0.4 g/dl, respectively, in rabbits with hydraulic edema; and 5.1 +/- 0.3, 4.3 +/- 0.4 and 3.3 +/- 0.6 g/dl, respectively, in rabbits with lesional edema. Contamination of plasma proteins from microvascular lesions during wick insertion was 14% of plasma C(protein). In control animals, pulmonary interstitial C(protein) was lower than previous estimates from pre- and postnodal pulmonary lymph; furthermore, although the interstitium constitutes a continuum within the lung parenchyma, regional differences in tissue content seem to exist in the rabbit lung.

Pulmonary capillary sieving of hetastarch is not altered by LPS-induced sepsis

BACKGROUND

Gram-negative lipopolysaccharide (LPS) has been demonstrated to increase pulmonary capillary permeability as judged by the increased flow of protein-rich lymph from the lungs of sheep infused with LPS. This finding suggests that LPS-injured pulmonary capillaries might be less restrictive than uninjured capillaries to the filtration of large hetastarch molecules. Hetastarch has a broad molecular mass spectrum (35-1,500 kilodaltons (kDa)), and one way to test the restrictiveness of pulmonary capillaries is to measure the size of the largest hetastarch molecules that cross the microvascular barrier and enter the lymph. To evaluate the effects of LPS, we compared hetastarch molecular distributions in the lung lymph of normal and LPS-injured sheep.

METHODS

Adult sheep (38.2 +/- 0.8 kg) were surgically prepared for the collection of lung lymph, with study initiation after a 5- to 7-day recovery period. Hetastarch (6%) was infused (10 mL/kg) 24 hours before study to allow for stabilization of the hetastarch molecular distribution. On the day of study, LPS (Escherichia coli lipopolysaccharide, 2 microg/kg; n = 6) was infused, and plasma and lymph samples were collected for 12 hours. An additional group of animals not infused with LPS (n = 6) served as controls. Hetastarch molecular distributions in plasma and lymph were measured by using high performance size exclusion chromatography.

RESULTS

In control sheep, the largest hetastarch molecules in lymph averaged 861 +/- 18 kDa (mean +/- SEM) (plasma, 1,065 +/- 18 kDa). In LPS-treated sheep, the largest hetastarch molecules in lymph averaged 845 +/- 19 kDa (not significant vs. normal) (plasma, 1,025 +/- 14 kDa). Hetastarch concentrations in plasma and lung lymph of normal sheep, respectively, were 0.61 +/- 0.05% and 0.34 +/- 0.07%. In LPS-treated sheep, hetastarch concentrations in plasma and lymph were 0.56 +/- 0.08 (not significant vs. normal) and 0.29 +/- 0.07, respectively (p < or = 0.05). Lymph concentrations were lower after LPS because of increased lymph flows (19.9 +/- 5.4 mL/30 min, compared with 3.6 +/- 0.8 mL/30 min in normal sheep).

CONCLUSION

Our results suggest that LPS does not alter the diameter of the largest pores perforating the walls of pulmonary capillaries. Rather, the number of these pores in the capillary wall appears to be increased. This increase would explain why lymph flows rise after LPS with little change in the lymph protein concentration. Our results are also consistent with a filtration model in which capillaries are assumed to be perforated by small pores (protein reflection coefficient = 1) as well as large pores (protein reflection coefficient = 0).

[Role of albumin in pulmonary edema and septic shock, plasma volume expansion excluded]

Patients with septic shock deserve a global approach. Intravascular volume loading is part of the treatment. However several questions remain open: what are the respective contributions of volume expansion and vaso-active drugs in the restoration of blood pressure and increase of cardiac output, which volumes and type of solutions should be used, which pulmonary capillary wedge pressure should be targetted, and which evaluation criteria are the most appropriate? Few experimental and clinical studies provide evidence of a superiority of colloids over crystalloids, although none of them has documented a reduction of mortality, length of stay in ICU or duration of mechanical ventilation. There are no data support-ing a superiority of albumin over artificial colloids, which are also much cheaper. Moreover, hydroxyethylstarch could have promising properties in case of increased capillary permeability. In summary and in agreement with the North American consensus conference, albumin should not be recommended for the treatment of septic shock, whether associated with non cardiogenic pulmonary oedema or not.

Organ-specific support in multiple organ failure: pulmonary support

The catastrophic pulmonary failure that complicates management of patients with multiple trauma or sepsis syndrome with shock is recognizable to nearly all experienced surgeons. However, the spectrum of injury is broad, the distribution of lung injury may be heterogeneous within a single patient, and many patients will not develop acute respiratory distress syndrome (ARDS) even after a major predisposing insult. The lung responds stereotypically to many disparate insults, so a better conceptual construct of ARDS may be to consider it as one component of the multiple organ dysfunction syndrome. Support of oxygen transport with positive pressure ventilation and high levels of positive end-expiratory pressure, long the mainstay of pulmonary support, has been criticized for its predilection to cause barotrauma. Newer modes of ventilation, such as pressure-controlled, inverse-ratio ventilation and permissive hypercapnia, are under investigation but have not yet been reported with scientific rigor. However, pulmonary support extends beyond the support of gas exchange. Fluid management requires close attention so that the circulation is supported but lung water accumulation is minimized. Nosocomial pneumonia greatly increases the mortality rate in ARDS, but is difficult to diagnose and must be sought aggressively. Until recently, pharmacologic therapy has held little promise, but inhalation of very low concentrations of nitric oxide appear to decrease pulmonary vascular pressures and intrapulmonary shunt. It remains unknown whether nitric oxide is effective therapy for the underlying injury, or is simply treatment for certain manifestations.

Transvascular albumin and IgG flux in skin and skeletal muscle following plasmapheresis

The extravascular uptake for labeled albumin and IgG and the extravascular masses for endogenous albumin and IgG were measured in skin and skeletal muscle from anesthetized rabbits following 24 hr of intermittent plasmapheresis. An amount of protein equivalent to the total intravascular protein mass was removed. There was a significant reduction in the extravascular mass for albumin in both tissues and for IgG in skin. The shift of albumin out of the extravascular space of skin and skeletal muscle could account for 95% of the vascular replacement of albumin. The extravascular uptake for the labeled proteins was measured as the 1-hr extravascular distribution space at plasma concentration divided by time and expressed as a plasma clearance. The plasma volume in the tissue samples was estimated from the 3-min distribution space for labeled transferrin. Following plasmapheresis the rate of extravascular uptake for both labeled proteins was greater than that for control or sham-operated animals, suggesting an increase in transvascular protein permeability. The transvascular fluxes for native albumin and IgG were not increased due to the decrease in the plasma concentrations. The results were consistent with the major mechanism for a shift of plasma proteins being due to a decrease in plasma protein concentration and a subsequent increase in lymph flow instead of a decrease in transvascular protein permeability.

A simplified two-pore filtration model explains the effects of hypoproteinemia on lung and soft tissue lymph flux in awake sheep

We used a simplified two-pore filtration model to examine the effects of hypoproteinemia on lung and soft tissue lymph flux in awake sheep (n = 7). To induce hypoproteinemia, we subjected each animal to 3 days of batch plasmapheresis (6 units per day). Data were collected in near steady-state conditions, 15-18 hr following completion of the last plasmapheresis episode. At this time, plasma protein concentration had fallen by 34%, while lung and soft tissue lymph protein concentrations had fallen by 55 and 62%, respectively. Lung and soft tissue lymph flows increased 52 and 87%, respectively. The plasma-to-lymph osmotic pressure gradients for lung and soft tissue lymph were unchanged by protein depletion (soft tissue, 7.7 mm Hg; lung, 4.8 mm Hg). We applied these results to a heteropore model of the microvascular barrier that consisted of two types of pores: those which plasma proteins could not cross (sigma = 1) and those which proteins could cross without restriction (sigma = 0). We varied the proportion of small pores to large pores until the measured data fit a model in which the calculated microvascular hydrostatic pressures in normal and hypoproteinemic conditions were equal. This was based on the assumption that microvascular hydrostatic pressure did not change with plasma protein depletion. These conditions could be satisfied when the small pores accounted for 90% of total barrier porosity. According to the model, lymph flow increased in hypoproteinemia because of an increase in protein-free liquid flux through the large percentage of small pores; protein flux through the small percentage of large pores remained unchanged. The net result was an increase in lymph flow and a decrease in the lymph protein concentration. The model reproduced these changes even though the plasma-to-lymph osmotic pressure gradients were unchanged. We conclude that a simplified heteropore model can explain the effects of hypoproteinemia on lung and soft tissue lymph flux.

A simple, dual tracer method for the measurement of transvascular flux of albumin into the lung

A simple, first-order model of albumin kinetics in the rat lung is presented and validated with a more sophisticated model. The simple model assumes that intravascular concentration of tracer albumin is constant over 30 min after injection and transvascular flux of tracer albumin is unidirectional and proportional to the permeability-surface area product (PS). 125I-albumin is injected initially and 131I-albumin at 20 min. At 30 min the rat is sacrificed and plasma and tissue samples are obtained for gamma counting. Simultaneous equations are set up for the two tracers and solved for PS and plasma volume. The accuracy of this approach is examined with data generated from a more complete model. This model uses the concepts of hydraulic conductivity, solvent drag, reflection coefficients, hydrostatic and osmotic pressures, exclusion volumes, and lymph flow, as well as PS. Based on known PS and clearance rates from the complex model, the simple model estimates tracer albumin leakage rate with less than 5% error over the range of PS encountered in rat studies.

Pulmonary transvascular fluid filtration response to hypoproteinemia and Hespan infusion

Management of major blood loss utilizing protein-free fluids for volume replacement frequently results in plasma protein depletion and plasma volume expansion. These factors can increase pulmonary transvascular fluid filtration which may lead to life-threatening pulmonary edema. We studied the combined effects of plasma protein depletion and plasma volume expansion on lung lymph flow (QL) in awake sheep prepared with chronic lung lymph fistulae. Animals were first chronically protein-depleted by batch plasmapheresis and then infused for 2 hr with either lactated Ringer's (Hypo/LR; n = 7) or 6% hydroxyethyl starch (Hespan) (Hypo/HES; n = 6). Control normoproteinemic animals (Norm/LR; n = 13) only received lactated Ringer's. Hypoproteinemia alone resulted in an average 2-fold increase in QL over normoproteinemic baseline levels (P less than or equal to 0.05). Infusion of LR into hypoproteinemic animals caused a 7.9-fold increase in QL (P less than or equal to 0.05). By comparison, HES infusion under similar hypoproteinemic conditions limited the increase in QL to 3.2-fold over baseline. We attributed this reduced rise in QL to Hespan's high oncotic pressure, which dramatically widened (by 4-5 mm Hg) the pulmonary-to-lymph oncotic pressure gradient. We did not observe this with LR infusion, or in previous studies employing intravenous infusion of plasma protein. Thus, the oncotic pressure of Hespan appears to significantly limit pulmonary fluid filtration during hypoproteinemia compared to LR. We do not believe that these effects are the results of any changes in microvascular porosity.

Human recombinant interleukin 2-activated sheep lymphocytes lyse sheep pulmonary microvascular endothelial cells

Administration of lymphokine-activated killer (LAK) cells in combination with interleukin 2 (IL-2) has been effective in reducing tumor mass in humans, but has been accompanied by significant toxicity. We used a chronic awake sheep model to investigate the cause of the vascular leak syndrome associated with IL-2 administration. Sheep repeatedly infused with human recombinant IL-2 (hrIL-2) developed mild pulmonary hypertension, systemic hypotension, acidemia, hypoxemia, and increased flow of protein rich lung lymph. We hypothesized that LAK cells may damage lung endothelium in vivo and cause increased lung vascular permeability. Sheep peripheral blood and lung lymph lymphocytes incubated in vitro with hrIL-2 generated cytotoxic activity for human K-562 cells and sheep pulmonary microvascular endothelial cells. In addition, cytotoxic effector cells were isolated from the peripheral blood of a sheep which had received hrIL-2. These observations suggest that LAK cells possess the ability to damage endothelial cells and may contribute to an increased pulmonary vascular permeability observed following hrIL-2 infusion in sheep.