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Microvascular membranes are heteroporous, so the mean osmotic reflection coefficient for a microvascular membrane (sigma d) is a function of the reflection coefficient for each pore. Investigators have derived equations for sigma d based on the assumption that the protein osmotic pressure gradient across the membrane (delta II) does not vary from pore to pore. However, for most microvascular membranes, delta II probably does vary from pore to pore. In this study, we derived a new equation for sigma d. According to our equation, pore-to-pore differences in delta II increase the effect of small pores and decrease the effect of large pores on the overall membrane osmotic reflection coefficient. Thus sigma d for a heteroporous membrane may be much higher than previously derived equations indicate. Furthermore, pore-to-pore delta II differences increase the effect of plasma protein osmotic pressure to oppose microvascular fluid filtration.

Many investigators have used the lymphatic protein washdown technique to estimate the pulmonary microvascular membrane reflection coefficient to protein (sigma d). With that technique, the investigator causes a high microvascular filtration rate then estimates sigma d from the lymph and plasma protein concentrations. However the lymph may contain protein washed from the lung tissue, and the tissue protein may cause investigators to underestimate sigma d. Plasma protein osmotic pressure (IIc) may cause investigators to underestimate sigma d because IIc opposes fluid filtration. To minimize the effect of IIc, we decreased IIc to 5.6 +/- 1.1 mmHg in five anesthetized sheep. We increased the microvascular filtration rate by increasing pulmonary microvascular pressure to 22 +/- 3 mmHg. Then we tagged plasma protein with Evans blue dye and estimated sigma d from the lymph and plasma dye concentrations. Because tissue protein was not tagged, it did not interfere with our sigma d estimate. Our sigma d estimate (0.79 +/- 0.08) was much higher than previous estimates in anesthetized animals.

Lymphatic vessels normally remove excess fluid from the lung tissue and thus prevent pulmonary edema. However, when lungs do become edematous, the edema fluid may be cleared via several pathways. Each clearance pathway has unique properties, and recent studies have helped to define those properties.

We used a new technique to estimate the pulmonary microvascular membrane reflection coefficient to plasma protein (sigma d) in anesthetized dogs. In five animals we continuously weighed the lower left lung lobe and used a left atrial balloon to increase the pulmonary microvascular pressure (Pc). We determined the relationship between the rate of edema formation (S) and Pc and estimated the fluid filtration coefficient (Kf) as delta S/delta Pc. From the S vs. Pc relationship and Kf, we estimated the Pc at which S/Kf = 10 mmHg for each dog. This pressure (P10) was 38.0 +/- 5.8 (SD) mmHg, and the plasma protein osmotic pressure (pi c) was 14.9 +/- 3.7 mmHg. In five additional dogs in which we decreased pi c to 2.9 +/- 1.7 mmHg, P10 = 27.2 +/- 2.6 mmHg. The P10 vs. pi c regression line fit to the data from all 10 dogs was P10 = 0.92 pi c +/- 24.4 mmHg (r = 0.88). We estimated sigma d from the slope of the regression line as sigma d = square root of delta P10/delta pi c. With this technique, we estimated that, with 95% probability, sigma d lies between 0.72 and unity. This is higher than most previous sigma d estimates.

We used lymph flow rate (QL) to lymphatic vessel outflow pressure (Po) relationships to analyze lymphatic flow in five unanesthetized sheep with liver lymphatic cannulas. The olecranon was the zero reference level for pressures. Increases in Po did not change QL until Po exceeded 19 +/- 4 (SD) cmH2O. However, for Po greater than 19 +/- 4 cmH2O, QL decreased linearly with increases in Po. We fit regression lines to the QL vs. Po data for Po greater than 19 cmH2O and estimated the effective lymphatic resistance (RL) as -delta Po/delta QL. The effective pressure driving lymph (PL) was the Po at which QL = 0. At baseline, RL = 0.18 +/- 0.10 cmH2O.min.microliter-1 and PL = 29.6 +/- 3.4 cmH2O. When we increased hepatic vein pressure by 5.7 +/- 1.7 cmH2O, QL increased to 6.2 +/- 3.2 times baseline, RL decreased to 0.050 +/- 0.015 cmH2O.min.microliter-1, and PL increased to 37.1 +/- 3.5 cmH2O (P less than 0.05). Thus 1) liver lymph flow is very sensitive to increases in hepatic vein pressure, 2) there is a substantial QL vs. Po plateau for liver lymphatics, and 3) after hepatic venous pressure elevations, liver lymph flow increases as if it were driven by a higher pressure through a lower resistance.

Postnodal intestinal lymphatic pressure increases during rapid intravenous infusions with Ringer solution in sheep. Part of the lymphatic pressure increase is due to increased venous pressure at the lymphatic outflow (the neck veins). We tested the hypothesis that other factors besides increased neck vein pressure may cause increased lymphatic pressure during intravenous infusions. We placed cannulas into postnodal lymphatic vessels in eight sheep. After the sheep recovered from the surgery, we infused Ringer solution [46 +/- 21 (SD) ml/kg body wt in 30 min] intravenously into the sheep and inflated a balloon in the inferior vena cava. We adjusted the balloon inflation to prevent any increase in neck vein pressure during the infusions. At baseline, the intestinal lymphatic pressure was 15.5 +/- 2.5 cmH2O. During the infusions, lymphatic pressure increased significantly, and for the last 10 min of the infusion period, intestinal lymphatic pressure was 24.0 +/- 6.1 cmH2O. These results are consistent with the hypothesis that factors in addition to increased neck vein pressure may cause increased intestinal lymphatic pressure during rapid intravenous infusions.

Intravenous fluid infusions cause increased venous pressure and increased lymph flow throughout the body. Together the increased lymph flow and increased venous pressure (the outflow pressure to the lymphatic system) should increase the pressure within the postnodal intestinal lymphatics. To test this, we measured the pressure in postnodal intestinal lymphatics and the neck vein pressure in five awake sheep. At baseline, the neck vein pressure was 1.2 +/- 1.5 (SD) cmH2O and the lymphatic pressure was 12.5 +/- 1.7 cmH2O. When we infused Ringer solution intravenously (10% body weight in approximately 50 min), the neck vein pressure increased to 17.3 +/- 0.9 cmH2O and the lymphatic pressure increased to 24.6 +/- 3.8 cmH2O (both P < 0.05). In two additional sheep, the thoracic duct lymph flow rate increased from 0.8 +/- 0.4 ml/min at baseline to 5.5 +/- 2.0 ml/min during the infusions. Our results show that postnodal intestinal lymphatic pressure may increase substantially during intravenous fluid infusions. This is important because increases in postnodal lymphatic pressure may slow lymph flow from the intestine.

The lymph from most organs drains through the thoracic duct and into veins in the neck. We hypothesized that increases in neck vein pressure (Pnv) are reflected through the thoracic duct to the lung lymphatic-thoracic duct junction. To test this, we cannulated the lung lymphatics in the direction of flow in four sheep. We advanced each cannula until it entered the thoracic duct. Thus the pressure at the tip of the lymphatic cannula (Px) was the pressure at the outflow of the lung lymphatics. We also placed a balloon into the superior vena cava. One to two days later, we measured Px in the awake sheep as we inflated the balloon and increased Pnv in steps to 25–45 cmH2O. We found no significant differences in Px and Pnv. Furthermore, Px closely followed Pnv after each step increase in Pnv. These results support our hypothesis that increases in Pnv cause increases in the outflow pressure to lung lymphatics.

Blood plasma contains many factors that influence cellular proliferation. However, cells within the tissue spaces of most organs are exposed to interstitial fluid, and the composition of interstitial fluid may be much different from plasma. Accordingly, the effect of interstitial fluid on cellular proliferation may be much different from the effect of plasma. The aim of this study was to test the effect of blood plasma and interstitial fluid (lymph) from the lung and intestine on the proliferation of fibroblasts. Plasma and lung and intestinal lymph were collected from anesthetized sheep and added to standard culture medium (final concentration = 10%). Cells (fibroblasts) were cultured from the lungs of the sheep and grown to confluence. Then the cells were subcultured at low density and incubated with the medium containing plasma or lymph. Control dishes contained only cells and medium. The cells were counted over a 6-day period. Cells incubated with medium alone grew very little over the 6-day period, but cells exposed to plasma increased approximately 100-fold, and cells exposed to lymph increased approximately 10-fold. We found no significant difference in cell growth for cells incubated with lung versus intestinal lymph. Our results show lung and intestinal lymph are less effective than plasma in stimulating cell growth.

Increases in systemic venous pressure (Pv) associated with heart failure cause an increase in microvascular fluid filtration into the tissue spaces. By removing this excess filtrate from the tissues, lymphatic vessels help to prevent edema. However, the lymphatics drain into systemic veins and an increase in Pv may interfere with lymphatic flow. To test this, we cannulated caudal mediastinal node efferent lymphatics in sheep. We used rapid cardiac ventricular pacing (240-275 beats/min) to cause heart failure for 4-7 days. Each day we determined the lymph flow rate two ways. First, we adjusted the lymph cannula height so that the pressure at the outflow end of the lymphatic was zero. After we determined the lymph flow with zero outflow pressure, we raised the cannula so that outflow pressure was equal to the actual venous pressure. We quantitated the effect of venous pressure on lymph flow rate by comparing the flow rate with outflow pressure = Pv to the flow rate with zero out low pressure. At baseline, Pv = 5.0 +/- 2.5 (SD) cmH2O and we found no difference in the two lymph flow rates. Pacing caused Pv and both lymph flow rates to increase significantly. However for Pv < 15 cmH2O, we found little difference in the two lymph flow rates. Thus increases in Pv to 15 cmH2O at the outflow to the lymphatics had little effect on lymph flow. By comparison, Pv > 15 cmH2O slowed lymph flow by 55 +/- 29% relative to the lymph flow rate with zero outflow pressure. Thus Pv values > 15 cmH2O interfere with lymph flow from the sheep caudal mediastinal lymph node.

We determined the relationship between flow rate and inflow pressure for intestinal lymphatic vessels in six sheep. First we anesthetized the sheep and cannulated both ends of a 6- to 10-cm-long segment of intestinal lymphatic. We allowed the sheep to recover from the anesthesia for 2-24 h. To determine the flow rate-inflow pressure relationship, we recorded the inflow pressure and infused Ringer solution into the lymphatic at rates from 34 to 510 microliters/min. The flow rate-pressure relationship was not linear and it had two regions. For flow rates less than approximately 150 microliters/min, inflow pressure was greater than outflow pressure. Thus the lymphatic pumped fluid against a pressure gradient. For flow rates > 150 microliters/min, inflow pressure was greater than outflow pressure, and we attributed most of the flow to the favorable inflow-outflow pressure gradient (passive flow). When we used verapamil to inhibit lymphatic pumping, we found no flow for inflow pressure less than outflow pressure, and flow increased linearly for inflow pressure greater than outflow pressure. Our data for actively pumping lymphatic vessels are consistent with the flow vs. pressure relationships derived from mathematical models of the lymphatic pump. Furthermore, our data with verapamil confirm that active lymphatic pumping was responsible for the nonlinear flow vs. pressure relationship for the lymphatic vessels.

After an increase in microvascular filtration rate, lung lymph may contain protein washed from the tissue spaces plus protein from the filtrate. If so, then the lymphatic protein concentration may be significantly higher than the filtrate protein concentration (Cf). To test this hypothesis, we decreased the plasma protein concentration from 5.1 +/- 0.6 to 0.54 +/- 0.15 g/dl and increased the pulmonary microvascular filtration rate in four dogs. We estimated Cf to be 0.16 +/- 0.05 g/dl after we reduced the plasma protein concentration, and the lymphatic protein concentration (0.43 +/- 0.04 g/dl) was significantly greater than Cf. Our results indicate that lung microvascular membrane reflection coefficients estimated from lung lymph data may be too low. However, the amount of error caused by tissue protein washout is probably small. To account for the protein washout error, we estimated the lung microvascular membrane reflection coefficient to be approximately 0.74–0.76 instead of the approximately 0.70 previously reported for dogs (J. C. Gabel et al. J. Appl. Physiol. 55: 866–869, 1983).

Active (intrinsic) lymphatic pumping may be an important factor determining lymph flow from the lungs. Unfortunately, in most experiments, it is very difficult to determine the influence of active pumping vs. passive factors on lymph flow. However, 1) the pumping activity (stroke volume and frequency) of isolated lymphatic segments varies nonlinearly with transmural pressure, and 2) the lung lymph flow from awake sheep varies nonlinearly with lymphatic outflow pressure. Accordingly, if lymphatic pumping significantly influences lung lymph flow, then it should be possible to describe the sheep lung lymph flow vs. outflow pressure data with the pumping activity data. To test this, we used published lymphatic pumping activity data to develop a mathematical model of the lymphatic pump for a segment of lymphatic vessel. Flow vs. outflow pressure relationships obtained from simulations with this model were very similar to the data from sheep. Our results indicate that both passive factors and active lymphatic pumping contribute to lymph flow, and our model may allow investigators to distinguish the effects of active pumping vs. passive factors in the regulation of lymph flow.

Positive end-expiratory pressure (PEEP) increases central venous pressure, which in turn impedes return of systemic and pulmonary lymph, thereby favoring formation of pulmonary edema with increased microvascular pressure. In these experiments we examined the effect of thoracic duct drainage on pulmonary edema and hydrothorax associated with PEEP and increased left atrial pressure in unanesthetized sheep. The sheep were connected via a tracheostomy to a ventilator that supplied 20 Torr PEEP. By inflation of a previously inserted intracardiac balloon, left atrial pressure was increased to 35 mmHg for 3 h. Pulmonary arterial, systemic arterial, and central venous pressure as well as thoracic duct lymph flow rate were continuously monitored, and the findings were compared with those in sheep without thoracic duct cannulation (controls). At the end of the experiment we determined the severity of pulmonary edema and the volume of pleural effusion. With PEEP and left atrial balloon insufflation, central venous and pulmonary arterial pressure were increased approximately threefold (P less than 0.05). In sheep with a thoracic duct fistula, pulmonary edema was less (extra-vascular fluid-to-blood-free dry weight ratio 4.8 +/- 1.0 vs. 6.1 +/- 1.0; P less than 0.05), and the volume of pleural effusion was reduced (2.0 +/- 2.9 vs. 11.3 +/- 9.6 ml; P less than 0.05). Our data signify that, in the presence of increased pulmonary microvascular pressure and PEEP, thoracic duct drainage reduces pulmonary edema and hydrothorax.

Lymphatic vessels are important in removing excess fluid from the intestines and preventing intestinal edema. In this study we used the relationship between intestinal lymph flow rate (QL) and lymphatic outflow pressure (PO) to analyze the flow from intestinal lymphatics in unanesthetized sheep. We cannulated intestinal lymphatic vessels in six anesthetized sheep. One to 3 days after the surgery, we measured QL as we increased PO in steps. We found no QL decrease until PO greater than 15 cmH2O, but QL decreased significantly for PO greater than 15 cmH2O and was decreased to zero at PO = 34 +/- 13 (SD) cmH2O. In three experiments, we used the pressure pulses generated by the active lymphatic pump to estimate the pump stroke volume and frequency. These data indicate that increases in lymphatic pumping prevented a QL decreased for PO less than 15 cmH2O and the QL decrease for PO greater than 15 cmH2O was associated with lymphatic pump failure. When we increased portal venous pressure from baseline (10.1 +/- 4.1 cmH2O) to 24.3 +/- 6.2 cmH2O, lymph flow increased, but it was much more sensitive to outflow pressure. These results are important because they indicate that the ability of the lymphatics to remove fluid from edematous intestines may be compromised by small increases in lymphatic outflow pressure.

Elevation of left atrial pressure to 25–40 mmHg causes continuous pulmonary edema formation in dog lungs. However, after 5–120 min, the rate of edema formation often increases (acceleration of edema). Acceleration of edema could be associated with an increase in microvascular membrane permeability because an increase in permeability would cause fluid to filter through the microvascular membrane more rapidly. To test the hypothesis that acceleration is associated with increased permeability, we used the continuous weight-gain technique to estimate the pulmonary microvascular membrane filtration coefficient (Kf) before and after acceleration of edema in 10 dogs. Acceleration occurred 36 +/- 38 (SD) min after elevation of left atrial pressure to 35.2 +/- 5.4 mmHg. Rate of weight gain increased from 0.47 +/- 0.17 g/min before acceleration to 0.88 +/- 0.26 g/min (P less than 0.05) after acceleration of pulmonary edema. Kf was increased from initial values of 0.058 +/- 0.027 to 0.075 +/- 0.029 ml.min-1.mmHg-1 (P less than 0.05) after acceleration. In five additional dogs we cannulated lung lymphatics and determined the lymph to plasma protein concentration ratio (CL/CP) before and after acceleration. CL/CP increased from base-line values of 0.37 +/- 0.07 to 0.44 +/- 0.06 (P less than 0.05) after acceleration. Both the increase in Kf and CL/CP data support the hypothesis that acceleration of edema is due, in part, to a slight increase in microvascular membrane permeability. However, the findings could also have been caused by an increase in interstitial conductance, washout of interstitial proteins, or alveolar flooding.

Estimates of the pulmonary microvascular membrane reflection coefficient (sigma) and permeability-surface area product (PS) are frequently made with the assumption that a percent change in transmicrovascular fluid flux (Jv) will be represented by an equal percent change in the lymph flow rate (QL) from a single cannulated lung lymph vessel. To test this, we measured QL in seven anesthetized dogs with the outflow end of the lymph cannula set at several heights (H) above and below the lung hilus. The left atrial pressure was then elevated to increase Jv, and QL was again measured at several H's. The percent increase in QL at elevated left atrial pressure depended on H. We used the QL data and lymph and plasma protein concentrations to estimate sigma and PS with a modified form of the Kedem and Katchalsky equations. The calculated values varied considerably with H. Our results indicate that changes in Jv are not represented by equal changes in QL. Therefore, techniques for estimating permeability that depend upon QL as an estimate of Jv may lead to erroneous estimates of sigma and PS.

We used a gravimetric technique to test for increased pulmonary capillary permeability after Escherichia coli endotoxin infusion in unanesthetized sheep. The sheep were chronically prepared with cannulas placed into the left atrium and pulmonary artery 1–2 wk before the experiments. We estimated pulmonary capillary pressure (Pc) as the average of pulmonary arterial and left atrial pressures, and used the modified method of Pierce to estimate the ratio of extravascular fluid weight (EVF) to blood-free dry weight. In 15 sheep we inflated a left atrial balloon to raise Pc to -10.7, 5, 10, or 15 mmHg above plasma oncotic pressure (IIc) for 3 h, then measured EVF. EVF averaged 4.0 +/- 0.2 (base line), 4.3 +/- 0.1, 4.5 +/- 0.1, and 5.1 +/- 0.5 (SD), respectively, for the four levels of Pc - IIc. We gave seven additional sheep 1 microgram/kg of E. coli endotoxin (0127:B8) and measured EVF after 3 h of stable Pc. Endotoxin increased Pc in each sheep. EVF was higher than control for the endotoxin sheep with Pc - IIc greater than -1. This finding is consistent with an increase in pulmonary capillary permeability caused by endotoxin. However, EVF was not elevated in the endotoxin sheep with Pc - IIc less than 1 mmHg. This shows that the increased permeability was insufficient to cause edema unless Pc was elevated. Thus endotoxin may cause edema by two mechanisms, 1) an increase in capillary permeability, and 2) an increase in Pc.

Lung weight is a useful indicator of increases in lung extravascular volume. In addition, the lung lymph flow rate (QL) is an important factor in lung fluid balance. We have studied the weight and QL responses to elevations in capillary pressure (Pc) in intact dog lung lower left lobes. We measured lobe weight continuously. We also measured QL from small lymph vessels from the same lobes. The base-line QL was 1.7 +/- 1.5 microliter/min, and the weight was constant. After we increased Pc by 8-20 cmH2O, both weight and QL increased transiently. In most lungs the weight reached a new steady state. When we increased Pc further, weight increased continuously; however, QL reached a plateau. The continuous weight gain was due to edema. These results show that weight and QL respond similarly in nonedematous lungs; however, the weight and QL responses in edematous lungs may be different.

The preparation for collecting lung lymph from sheep caudal mediastinal lymph node (CMN) efferent vessels is widely used to study the effects of endotoxin on lung microvascular permeability. However, there are nonpulmonary lymph vessels that drain into the CMN along with the afferent lymph vessels from the lung. Thus CMN lymph is a mixture of lymph from the lung and diaphragm lymph vessels as well as from other nonpulmonary lymph vessels. We studied the effect of 0.5–1.0 microgram/kg Escherichia coli endotoxin on the flow rates in diaphragm and CMN efferent lymph vessels (Qdi and QCMN, respectively) in unanesthetized sheep. For the time period between 2 and 5.5 h after endotoxin QCMN was increased from its base line of 7.2 +/- 4.4 (SD) to 17.3 +/- 10.6 ml/h and the lymph-to-plasma protein concentration ratio (L/PCMN) had increased from 0.68 +/- 0.11 to 0.81 +/- 0.06. During the same time period, Qdi was 4.5 +/- 3.1 ml/h compared with 1.0 +/- 0.8 ml/h at base line and the diaphragm lymph-to-plasma protein concentration ratio (L/Pdi) was 0.92 +/- 0.07 (base line = 0.74 +/- 0.15). The increases in flow rate and protein concentration were significant for each type of vessel (P less than 0.05). We conclude that the period of increased QCMN and L/PCMN after endotoxin is associated with an increase in Qdi and L/Pdi. Thus, it is difficult to determine how much of the CMN lymph response comes from the lungs and how much comes from diaphragm lymph vessels.

We used a gravimetric method to determine the ratio (gamma) of pulmonary venous to total pulmonary vascular resistance in intact dog lungs. From this ratio, pulmonary capillary pressure (Pc) can be calculated. The average value of gamma was 0.50 +/- 0.06 (mean +/- SD) in 10 dogs. We found no correlation between gamma and PO2, PCO2, pH, or hematocrit in the narrow ranges of these experiments. Over the capillary pressure range of 22.4--35.2 mmHg we found no correlation between gamma and Pc. The value of gamma found in this study is not significantly different from the value found in isolated perfused lungs.

Many investigators have used the chronic sheep lung lymph preparation to collect caudal mediastinal node (CMN) efferent lymph. These investigators have assumed that the lymph collected with the preparation is almost pure lung lymph. We examined 17 sheep for possible systemic contamination to the lymph, and in each sheep we found one to five lymph vessels that ran from the diaphragm to the CMN. Contamination from these vessels would not be eliminated in the chronic sheep preparation. We estimated the flow rate from these vessels to be 3.0 +/- 2.6 (SD) ml/h in anesthetized sheep. This represents 25-60% of the lymph flow rate in the chronic lymph preparation. In five sheep, we also located 1-4 esophageal lymph vessels that entered th CMN. These results show that lymph collected with the chronic sheep lung lymph preparation contains a significant nonpulmonary contamination.

The liver lymphatic system plays an important role in removing excess fluid from the hepatic tissue. A complete analysis of the liver lymphatic system would be difficult. However, we used a simple circuit-analysis technique to represent the intrahepatic portion of the lymph system as a single pressure source (PL) pushing lymph through a single resistance (RL). Liver lymphatic vessels were cannulated in nine halothane-anesthetized dogs. The lymphatic vessel outflow pressure (PO) was varied by raising the outflow end of the cannula. Lymph flow from the cannula (QL) decreased linearly with PO, and we calculated RL as -delta PO/delta QL and PL as the extrapolated PO at which QL = 0. At base line, PL = 8.5 +/- 2.9 cmH2O, and RL = 0.05 +/- 0.03 cmH2O.min/microliter. After we increased inferior vena caval pressure from 5.8 +/- 2.7 to 15.2 +/- 2.5 cmH2O, PL increased significantly to 13.7 +/- 3.4 cmH2O, and RL decreased to 0.02 +/- 0.02 cmH2O.min/microliter (P less than 0.05). The results indicate that increases in QL occur because the effective pressure pushing lymph from the liver (PL) increases, and the effective resistance of the intrahepatic lymph vessels (RL) decreases.

We estimated the pulmonary capillary membrane filtration coefficient (Kf) and the maximum capillary pressure (PCcritical) at which the lung could maintain a constant weight in 1) 5 control experiments in anesthetized open-chested dogs, 2) 7 experiments in which the dogs were given 3.6-8.3 microgram . kg-1 . min-1 of histamine phosphate, and 3) in 6 experiments after 75-100 mg/kg of alloxan. In additional experiments, pulmonary lymph flow (QL) and protein concentration (CL) were measured during the infusion of histamine and alloxan. After histamine, Kf averaged 0.045 +/- 0,008 ml . min-1mmHg-1 (SE) and PCcritical was 22.1 +/- 1.1 mmHg. These values were not significantly different from the control Kf and PCcritical (0.036 +/- 0.006 and 22.5 +/- 2.3, respectively). After alloxan, Kf (1.43 +/- 0.69) was larger and PCcritical (12.4 +/- 1.3) was significantly less than control (P less than 0.05). Histamine caused no significant change in QL or CL; however, both were increased after alloxan. These results show that Kf, PCcritical, QL, and CL are all changed by an increase in capillary membrane permeability caused by alloxan. Because none of these factors as significantly affected by histamine, dog lung capillary membrane permeability may not be affected by histamine.

Fluid accumulation within the lungs is a potentially fatal complication in critically ill patients. Sepsis and increased microvascular permeability are often implicated as the cause. This article shows that edema can be prevented by lowering systemic venous pressure (to permit pulmonary lymph to drain), by lowering pulmonary arterial pressure, and by maintaining plasma colloid osmotic pressure. It points out the importance of understanding the basic physiology behind pulmonary edema and therapeutic measures.

Pulmonary lymph drains into the thoracic duct and then into the systemic venous circulation. Since systemic venous pressure (SVP) must be overcome before pulmonary lymph can flow, variations in SVP may affect lymph flow rate and therefore the rate of fluid accumulation within the lung. The importance of this issue is evident when one considers the variety of clinical interventions that increase SVP and promote pulmonary edema formation, such as volume infusion, positive-pressure ventilation, and various vasoactive drug therapies. We recorded pulmonary arterial pressure (PAP), left atrial pressure (LAP), and SVP in chronic unanesthetized sheep. Occlusion balloons were placed in the left atrium and superior vena cava to control their respective pressures. The superior vena caval occluder was placed above the azygos vein so that bronchial venous pressure would not be elevated when the balloon was inflated. Three-hour experiments were carried out at various LAP levels with and without SVP being elevated to 20 mmHg. The amount of fluid present in the lung was determined by the wet-to-dry weight ratio method. At control LAP levels, no significant difference in lung fluid accumulation could be shown between animals with control and elevated SVP levels. When LAP was elevated above control a significantly greater amount of pulmonary fluid accumulated in animals with elevated SVP levels compared with those with control SVP levels. We conclude that significant excess pulmonary edema formation will occur when SVP is elevated at pulmonary microvascular pressures not normally associated with rapid fluid accumulation.

The pulmonary lymph flow rate (QL) should be a function of the lymph vessels' resistance and the pressure gradient along the vessels. We attempted to study how these factors affect lymph flow. We assumed that the lymph system could be represented by a single pressure generated within the lung (PL) and a single resistance (RL). Thus, QL should be a function of the lymph vessel outflow pressure (Po): QL = (PL - Po)/RL. We cannulated tracheobronchial lymph vessels in eight anesthetized dogs and varied Po by raising the outflow end of the cannula. QL decreased linearly when we increased Po. We estimated RL as - delta Po/ delta QL and PL as the extrapolated Po at which QL = 0. At baseline PL = 7.7 +/- 2.7 (SD) cm H2O and RL = 0.36 +/- 0.25 cm H2O. min/microliters. After we increased capillary pressure to produce edema, PL and RL averaged 22.8 +/- 8.8 and 0.14 +/- 0.12, respectively. After we reduced the capillary pressure to baseline in the edematous lungs, PL and RL averaged 11.6 +/- 2.8 and 0.08 +/- 0.09, respectively. All changes in PL and RL were significant (P less than 0.05). These results show that (1) lymph flow rate depends upon lymph vessel outflow pressure, and (2) the QL vs. Po relationship is changed by edema. PL may be equal to the pressure causing lymph to flow and RL may equal the lymph vessel resistance.

We studied the effect of left atrial pressure (LAP) elevation on the formation of pleural effusion in unanesthetized sheep. We prepared the animals by placing catheters in the left atrium, pulmonary artery, femoral artery, and vein. We also placed a balloon catheter in the left atrium. After a recovery period of at least 1 wk, we measured LAP, pulmonary artery pressure (PAP), systemic arterial pressure, systemic venous pressure, cardiac output, plasma protein concentration, and plasma colloid osmotic pressure (pi c). We calculated capillary pressure (Pc) as 0.5(PAP - LAP). We then elevated LAP such that Pc-pi c was between -10 and 19.5 mmHg for 6-24 h. At the end of the experiment, we killed the sheep and measured the volume and protein concentration of the right pleural effusion. We also determined the extravascular fluid to blood free dry weight of the right lung. We found that pleural effusions and pulmonary edema formed when Pc-pi c greater than 5 mmHg. We also found that the pleural effusion volume correlated with the amount of pulmonary edema. Our data show that elevated LAP may cause pleural effusions, but only after pulmonary edema has developed.

Several equations to calculate plasma oncotic pressure (pi) from the total protein concentration (C) have been previously described. These equations were derived empirically from samples with a wide range of C obtained by diluting or concentrating normal plasma samples. To test these equations over a range of naturally occurring C, we measured C and pi of plasma samples from 40 dogs. C ranged from 5.3 to 8.7 g/dl and averaged 6.5 +/- 0.1 (mean +/- SE) and pi averaged 17.9 +/- 0.3 mmHg. The regression equation was pi = 78.14 + 1.67 C (r = 0.74). pi increased with C much less than predicted with the commonly used equations. The albumin-to-globulin concentration ratios (A/G), determined in 27 of the dogs, decreased with increasing C (A/G = 1.56-0.128 C, r = 0.62). The lower A/G at the higher C's could cause the lower than predicted increase in pi with C, because the equations were developed from data in which A/G was constant.

Recent studies have indicated that pulmonary microvascular permeability may be increased after coronary arterial ligation. We tested this by estimating the pulmonary microvascular reflection coefficient (delta) in six anesthetized control dogs and in six dogs in which we ligated the left anterior descending coronary artery. We cannulated lung lymph vessels and measured the lymph and plasma protein concentrations (CL and CP, respectively). The left atrial pressure was increased to decrease CL to a minimum. We used the minimum CL to estimate delta as 1 -CL/CP. delta for the control [0.70 +/- 0.03 (SD)] and coronary artery ligated groups (0.70 +/- 0.04) were almost identical. This data indicates that coronary artery ligation does not increase pulmonary microvascular permeability.

The effect of superior vena caval pressure (SVCP) elevation on the formation of pleural effusions (PE) was studied in sheep. Through a right thoracotomy, a Silastic cuff was placed around the superior vena cava. Catheters for monitoring SVCP and pulmonary artery pressure (PAP) were also placed. After a 1- to 3-wk recovery period, we measured the SVCP, PAP, cardiac output, and plasma protein concentration (Cp). We then elevated the SVCP to various levels from base line [5.3 +/- 2.6 (SD) mmHg] to 33 mmHg. The cardiac output, PAP, and Cp were remeasured 1–2 h and 24 h after SVCP elevation. At the end of the 24-h period, the animals were killed. The PE volume and pleural fluid protein concentration (Cpl) were measured, and the Cpl/Cp was calculated. PE generally did not occur until the SVCP was elevated above 15 mmHg. To study the effect of the thoracotomy on the subsequent pleural effusion, we studied six additional sheep in which we did not perform a thoracotomy. In these animals, the SVCP was elevated to between 5 and 28 mmHg for 24 h by use of a 16-Fr balloon catheter placed via a left external jugular vein and a right carotid-external jugular shunt. We found that the PE volume, for a given SVCP elevation, was similar to that present in sheep that received a thoracotomy. For all sheep the volume of PE was related to SVCP by the equation PE (ml) = 0.24e0.26SVCP, r = 0.85. In the sheep without a thoracotomy, Cpl/Cp rose with increasing volume of PE. Our data demonstrate that elevation of SVCP greater than 15 mmHg for 24 h results in the formation of PE. The rise in Cpl/Cp with PE volume suggests that filtration through the pleural vessels is not the major contributor to PE formation.

We measured the flow rate (QLV) from cannulated lung lymph vessels in anesthetized dogs. Low-resistance lymph cannulas were used and the vessels were cannulated at the lung hilus. When we increased left atrial pressure to 42.9 +/- 5.7 (SD) cmH2O (base line = 6.6 +/- 4.6 cmH2O), the lungs became edematous and QLV increased from a base line of 20.4 +/- 21.5 microliters/min to 388 +/- 185 microliters/min. QLV plateaued at the higher level. We also measured the relationship between lymph flow rate and the height of the outflow end of the lymph cannula. From this relationship, determined at the end of the period of elevated left atrial pressure, we calculated the effective resistance and pressure driving lymph from the lungs. We also cannulated lymph vessels in the downstream direction and estimated the effective resistance and pressure opposing flow into the part of the lymphatic system between the lung hilus and the veins (extrapulmonary lymph vessels). We found that the effective resistance of the extrapulmonary part of the lymph system (0.042 +/- 0.030 (SD) cmH2O X min X microliter-1) was large compared with the resistance of the lymph vessels from the lungs (0.026 +/- 0.027). These data indicate that the resistance of the extrapulmonary part of the lung lymph system limits the maximum flow of lymph from edematous lungs.

We have previously reported that lymph from the chronic sheep lung lymph preparation contains 25-60% lymph from nonpulmonary sources. In subsequent studies we found that the lymph flow rate from cannulated lymph vessels depends on the resistance and position of the lymph cannula. Because we did not account for these factors in our estimate of the amount of nonpulmonary lymph in the sheep lung lymph preparation, our data were not accurate. We probably overestimated the amount of nonpulmonary lymph. However, the presence of nonpulmonary lymph remains a potentially serious problem with the sheep lung lymph preparation.

In many sheep Escherichia coli endotoxin results in pulmonary hypertension, increased microvascular permeability, pulmonary edema, and increased central venous pressure. Since lung lymph drains into the systemic veins, increases in venous pressure may impair lymph flow sufficiently to enhance the accumulation of extravascular fluid. We tested the hypothesis that, following endotoxin, elevating the venous pressure would increase extravascular fluid. Thirteen sheep were chronically instrumented with catheters to monitor left atrial pressure (LAP), pulmonary arterial pressure (PAP), and superior vena caval pressure (SVCP) as well as balloons to elevate LAP and SVCP. These sheep received 4 micrograms/kg endotoxin, and following the pulmonary hypertensive spike the left atrial balloon was inflated so that (PAP + LAP)/2 = colloid osmotic pressure. It was necessary to control PAP + LAP in this way to minimize the sheep-to-sheep differences in the pulmonary hypertension. We elevated the SVCP to 10 or 17 mmHg or allowed it to stay low (3.2 mmHg). After a 3-h period, we killed the sheep and removed the right lungs for determination of the extravascular fluid-to-blood-free dry weight ratio (EVF). Sheep with SVCP elevated to 10 or 17 mmHg had significant increases in EVF (5.2 +/- 0.1 and 5.6 +/- 1.2) compared with the sheep in which we did not elevate SVCP (EVF = 4.5 +/- 0.4). These results indicate that sustained elevation in central venous pressure in patients contributes to the amount of pulmonary edema associated with endotoxemia.

Endotoxin may cause an increase in pulmonary capillary permeability and thus promote edema formation. We used a gravimetric technique to estimate the pulmonary capillary filtration coefficient (KF) and the maximum capillary pressure at which the lung could maintain a constant weight (Pccritical) in dogs after intravenous administration of Escherichia coli (E. coli) endotoxin. KF should be increased and Pccritical should be decreased by an increase in permeability. Four groups of three to four dogs were given 1, 10, 1,000, or 3,000 micrograms/kg of endotoxin. A fifth group of five dogs, which served as controls, was given no endotoxin. KF was significantly (P less than 0.05) greater than control [0.049 +/- 0.031 (SD) ml . min-1 . mmHg-1] in only the 1-micrograms/kg group (0.100 +/- 0.027), indicating a possible increase in permeability. However, changes in capillary surface area may have affected KF. Pccritical was not significantly different from control (20.7 +/- 2.4 mmHg) in any of the E. coli groups. We conclude from these results that E. coli endotoxin may have caused a slight increase in permeability; however, the lung retained its ability to resist edema formation.

We have developed an electromechanical feedback system to control blood flow to the lower left lung lobe of dogs. Blood flow is measured with an electromagnetic flowmeter. The feedback system compares the blood flow signal to an adjustable reference voltage and causes a motor to turn. The direction of motor rotation depends on the relative magnitude of the flow signal and the reference. The motor pushes the plunger of a syringe that is attached to a balloon in the right pulmonary artery. Inflation of the balloon causes increased blood flow to the lower left lobe. We have used the system to control lobe blood flow in three dogs.

Lymph vessels branch and interconnect in a manner similar to a complex electronic circuit. Accordingly, we have applied circuit analysis techniques to the analysis of lymphatic systems. A lymph vessel is cannulated and the "equivalent circuits" are determined for the parts of the vessel upstream and downstream of the site of cannulation. Each equivalent circuit consists of a single resistor in series with a single pressure source. A diode is included to represent the lymphatic valves. The lymph flow rate may be determined by calculating the flow in the circuits when they are connected to each other. This technique can be applied to larger lymph trunks that receive lymph from many tissues.

Escherichia coli endotoxin causes increased capillary membrane permeability and increased pulmonary arterial pressure (PAP) in sheep. If the pulmonary hypertension extends to the level of the microvasculature, then the increased microvascular pressure may contribute to the pulmonary edema caused by endotoxin. We tested the hypothesis that reducing the pulmonary hypertension would reduce the amount of edema caused by endotoxin. Twelve sheep were chronically instrumented with catheters to measure PAP, left atrial pressure, and central venous pressure. The sheep were divided into two groups. One group (E) of six sheep received an intravenous infusion of 4 micrograms/kg of E. coli endotoxin. The second group (E + SNP) received the same dose of endotoxin as well as a continuous infusion of sodium nitroprusside (SNP) to reduce PAP. Three hours after the endotoxin infusions, the sheep were terminated and the extravascular fluid-to-blood-free dry weight ratios of the lungs were determined (EVF). The base-line PAP was 17.5 +/- 2.7 mmHg. A two-way analysis of variance demonstrated a significant difference (P less than 0.01) in PAP between the E and E + SNP groups. Although PAP in each group varied as a function of time, the difference between the two groups did not. The mean PAP for the E + SNP group (20.9 +/- 1.5 mmHg) was lower than the E group PAP of 27.3 +/- 2.1 mmHg after the endotoxin spike. Furthermore, the E + SNP group EVF (3.9 +/- 0.2) was significantly less than the EVF of the E group (4.7 +/- 0.5).(ABSTRACT TRUNCATED AT 250 WORDS)

Studies in anesthetized animals have shown that the flow rate from lung lymphatics (QL) depends on the pressure at the outflow end of the vessels (Po). We tested this in unanesthetized sheep prepared with chronic lung lymph cannula. We measured QL with the lymph cannula held at various heights above the olecranon and calculated Po as the height + QL X cannula resistance. QL decreased with increases in Po (delta QL/delta Po = -8.2 +/- 6.4 microliter X min-1 X cmH2O-1, mean +/- SD). We increased QL by raising left atrial pressure or infusing Ringer solution or Escherichia coli endotoxin and found that QL was even more sensitive to Po (delta QL/delta Po = -32 +/- 22). Cannula resistance caused a 9–70% reduction in QL. Changes in QL caused by increasing Po were not associated with changes in lymph protein concentration for up to 330 min. This indicates that increases in Po shunt lymph away from cannulated vessels but do not substantially effect microvascular filtration rate. The shunted lymph may flow into other vessels or collect in the lung. We conclude that QL does not accurately represent microvascular filtration rate because it depends on the cannula resistance and position at which the investigator chooses to place the cannula.

To determine whether the microvascular membrane of isolated perfused dog lungs is damaged by the isolation procedure, we compared the filtration coefficient (Kf,c) and minimum capillary pressure (Pcc) required to cause continuous edema formation in 10 intact and 8 isolated dog lower left lobes. We used the same gravimetric technique to measure Kf,c and Pcc in both preparations. The Kf,c averaged 0.09 +/- 0.03 (SD) ml . min-1 . Torr-1 . 100 g-1 in the intact and 0.30 +/- 0.16 in the isolated lobes; Pcc averaged 20.1 +/- 4.7 and 1.7 +/- 3.6 Torr in the intact and isolated lobes, respectively. Both Pcc and Kf,c were significantly different in the two preparations (P less than 0.01). These data indicate that the increased tendency of isolated lungs to become edematous at lower pressures may be caused by damage to the microvascular membranes.