Effects of hypobaria on lung fluid balance in awake sheep

Effects of hypobaria on lung fluid balance were studied in five awake sheep with chronic lung lymph fistulas using a decompression chamber. Each sheep was exposed to three conditions of 6,600-m-simulated high altitude in random order as follows: 1) 6,600-m-simulated hypoxic hypobaria (barometric pressure 326 Torr, 21% inspired O2 fraction), 2) 6,600-m-simulated normoxic hypobaria (barometric pressure 326 Torr, 65% inspired O2 fraction), and 3) 6,600-m-simulated normoxic hypobaria (barometric pressure 326 Torr, 65% inspired O2 fraction) after pretreatment with a 2-h pure O2 inhalation (i.e., denitrogenation) to allow elimination of dissolved gases, especially N2, from the blood and tissues. We observed that under both hypoxic hypobaria and normoxic hypobaria, lung lymph flow (Qlym) significantly increased from the base-line values of 6.4 +/- 0.3 to 13.0 +/- 1.0 ml/h and 6.0 +/- 0.2 to 9.4 +/- 0.3 ml/h, respectively (P less than 0.05) and that the lymph-to-plasma protein concentration ratio remained unchanged. Moreover, pretreatment with a 2-h denitrogenation inhibited the increase in Qlym. These results suggest that rapid exposure to hypobaria causes an increase in pulmonary vascular permeability and that intravascular air bubble formation may account for this permeability change.     

Effects of coronary ischemia on lung fluid balance in conscious sheep

It has been suggested that coronary ischemia increases extravascular lung water. To determine whether pulmonary microvascular permeability is increased by coronary ischemia, we measured pulmonary hemodynamics, lung lymph flow (QL), and lymph-to-plasma protein concentration ratio (L/P) in 12 sheep with chronic lung lymph fistulas. Studies were done in 3 groups: in group 1 (n = 7) a marginal branch of the left circumflex artery (Lcx) was occluded, in group 2 (n = 5) left atrial pressure (Pla) was mechanically raised by 10 mmHg, and in group 3 (n = 5) Lcx was occluded and Pla was raised by 10 mmHg. In group 1, coronary occlusion increased QL (4.6 +/- 0.4 to 8.3 +/- 2.6 ml/h) without changes in L/P. In group 2, elevated Pla increased QL (5.1 +/- 1.2 to 10.1 +/- 3.0 ml/h) with decreases in L/P (0.71 +/- 0.02 to 0.61 +/- 0.02). In group 3, coronary occlusion with elevated Pla caused a further increase in QL (5.0 +/- 1.5 to 16.9 +/- 4.6 ml/h) without significant decreases in L/P (0.71 +/- 0.01 to 0.65 +/- 0.06). Lung lymph concentrations of 6-keto-prostaglandin F1 alpha (a degradation product of prostacyclin) increased transiently after coronary occlusion. These results indicate that coronary occlusion can increase transcapillary protein transport in lungs of conscious sheep and simultaneously increase prostacyclin production in the lung.     

Effects of inspiratory resistance loading on lung fluid balance in awake sheep

Because pulmonary edema has been associated clinically with airway obstruction, we sought to determine whether decreased intrathoracic pressure, created by selective inspiratory obstruction, would affect lung fluid balance. We reasoned that if decreased intrathoracic pressure caused an increase in the transvascular hydrostatic pressure gradient, then lung lymph flow would increase and the lymph-to-plasma protein concentration ratio (L/P) would decrease. We performed experiments in six awake sheep with chronic lung lymph cannulas. After a base-line period, we added an inspiratory load (20 cmH2O) and allowed normal expiration at atmospheric pressure. Inspiratory loading was associated with a 12-cmH2O decrease in mean central airway pressure. Mean left atrial pressure fell 11 cmH2O, and mean pulmonary arterial pressure was unchanged; calculated microvascular pressure decreased 8 cmH2O. The changes that occurred in lung lymph were characteristic of those seen after other causes of increased transvascular hydrostatic gradient, such as increased intravascular pressure. Lung lymph flow increased twice base line, and L/P decreased. We conclude that inspiratory loading is associated with an increase in the pulmonary transvascular hydrostatic gradient, possibly by causing a greater fall in interstitial perimicrovascular pressure than in microvascular pressure.     

Effect of progressive exercise on lung fluid balance in sheep

The purpose of this study is to determine the roles of cardiac output and microvascular pressure on changes in lung fluid balance during exercise in awake sheep. We studied seven sheep during progressive treadmill exercise to exhaustion (10% grade), six sheep during prolonged constant-rate exercise for 45-60 min, and five sheep during hypoxia (fraction of inspired O2 = 0.12) and hypoxic exercise. We made continuous measurements of pulmonary arterial, left atrial, and systemic arterial pressures, lung lymph flow, and cardiac output. Exercise more than doubled cardiac output and increased pulmonary arterial pressures from 19.2 +/- 1 to 34.8 +/- 3.5 (SE) cmH2O. Lung lymph flow increased rapidly fivefold during progressive exercise and returned immediately to base-line levels when exercise was stopped. Lymph-to-plasma protein concentration ratios decreased slightly but steadily. Lymph flows correlated closely with changes in cardiac output and with calculated microvascular pressures. The drop in lymph-to-plasma protein ratio during exercise suggests that microvascular pressure rises during exercise, perhaps due to increased pulmonary venous pressure. Lymph flow and protein content were unaffected by hypoxia, and hypoxia did not alter the lymph changes seen during normoxic exercise. Lung lymph flow did not immediately return to base line after prolonged exercise, suggesting hydration of the lung interstitium.     

Thrombin-induced alterations in lung fluid balance in awake sheep

We examined the effect of fibrinolysis depression on thrombin-induced pulmonary microembolism in awake sheep prepared with chronic lung lymph fistulas. Fibrinolysis was depressed by an intravenous infusion (100 mg) of tranexamic acid [trans-4-(Aminomethyl)cyclohexanecarboxylic acid]. Pulmonary microembolism was induced by an intravenous infusion of alpha-thrombin (80 NIH U/kg) in normal (n = 7) and in tranexamic acid-treated (n = 6) sheep. Thrombin immediately increased pulmonary lymph flow (Qlym) in both groups. The increased Qlym was not associated with a change in the lymph-to-plasma protein concentration (L/P) ratio in the control group and with a small decrease in the tranexamic acid-treated group. The increases in Qlym and pulmonary transvascular protein clearance (Qlym X L/P ratio) in the tranexamic acid-treated group were greater and sustained at four- to fivefold above base line for 10 h after the thrombin and remained elevated at twofold above base line even at 24 h. In contrast, Qlym and protein clearance were transiently increased in the control group. The mean pulmonary arterial pressure (Ppa) and pulmonary vascular resistance (PVR) increased after thrombin in tranexamic acid-treated group; the increases in Ppa and PVR in the control group were transient. Protein reflection coefficient as determined by the filtration independent method decreased after thrombin in tranexamic acid-treated sheep (n = 5), indicating an increased vascular permeability to proteins. We conclude that prolongation of microthrombi retention in the pulmonary circulation results in an increased vascular permeability to proteins. Both increased vascular permeability and vascular hydrostatic pressure are important determinants of the increases in Qlym and transvascular protein clearance after thrombin-induced pulmonary microembolism.     

Comparison of alpha- and gamma-thrombin on lung fluid balance in anesthetized sheep

We examined the effects of varying dosages of thrombin on lung fluid balance in halothane-anesthetized sheep prepared with lung lymph fistulas. A 15-min iv infusion of sublethal doses of alpha-thrombin (2.5 clotting units/micrograms), the native enzyme, at 0.6 or 1.1 nmol active enzyme/kg body wt increased the mean pulmonary arterial pressure (Ppa) and pulmonary vascular resistance (PVR) two- to threefold. Neither parameter increased in a dose-dependent manner. Platelet counts decreased 50% with both dosages. Leukocyte counts decreased 35 and 75% from base line in the low- and high-dosage groups, respectively, and reached comparable levels of 50% below base line at 60-min postinfusion in both groups. Plasma fibrinogen concentrations decreased in a dose-dependent manner preceding dose-dependent increases in pulmonary lymph flow (Qlym) and lymph protein clearance (Clym). Fibrin deposition in pulmonary vessels was greater at 30 than at 180 min postinfusion. In contrast, a 15-min iv infusion of gamma-thrombin (0.002 clotting units/micrograms), which lacks the fibrinogen recognition site, at 1.2 nmol active enzyme/kg produced no significant increases in PVR, Ppa, Qlym, or Clym. The fibrinogen concentration did not change significantly, whereas platelet and leukocyte counts decreased 25% within 15 min. Fibrin microthrombi were less prominent in pulmonary vessels. Fibrin deposition associated with intravascular coagulation may be an important factor mediating thrombin-induced increases in pulmonary transvascular fluid and protein exchange.     

Aerosolized Pseudomonas elastase and lung fluid balance in anesthetized sheep.

The role of the lung epithelium in lung fluid balance was studied by ventilating anesthetized sheep with an aerosol of 20 mg of elastase from Pseudomonas aeruginosa (Ps. elastase) to increase lung epithelial permeability without affecting lung endothelial permeability or lung vascular pressures. Ps. elastase had no effect on the lung vascular pressures, the alveolar-arterial PO2 gradient (A-aPO2), the flow or protein concentration of the lung lymph, or the postmortem water volume of the lungs. The morphological alveolar flooding score in these sheep was 2.5 times the control level, but this was only marginally significant. Elevation of the left atrial pressure by 20 cmH2O alone increased the postmortem lung water volume but had no effect on A-aPO2, the alveolar flooding score, or the lung epithelial permeability assessed by the clearance of 99mTc-labeled human serum albumin. Addition of aerosolized Ps. elastase to these sheep had no effect on the total lung water volume, but it caused a redistribution of water into the air spaces, as evidenced by significant increases in the alveolar flooding score and A-aPO2 (P less than 0.01). Elevation of the left atrial pressure by 40 cmH2O without elastase caused the same response as elevation of the left atrial pressure by 20 cmH2O with elastase, except the higher pressure caused a greater increase in the total lung water volume. We conclude that alteration of the integrity of the lung epithelium with aerosolized Ps. elastase causes a redistribution of lung water into the alveoli without affecting the total lung water volume.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of imidazole and indomethacin on fluid balance in isolated sheep lungs

We determined the effects of extracorporeal perfusion with a constant flow (75 ml . min-1 . kg-1) of autologous blood on hemodynamics and fluid balance in sheep lungs isolated in situ. After 5 min, perfusate leukocyte and platelet counts fell by two-thirds. Pulmonary arterial pressure (Ppa) increased to a maximum of 32.0 +/- 3.4 Torr at 30 min and thereafter fell. Lung lymph flow (QL), measured from the superior thoracic duct, and perfusate thromboxane B2 (TXB2) concentrations followed similar time courses but lagged behind Ppa, reaching maxima of 4.1 +/- 1.2 ml/h and 2.22 +/- 0.02 ng/ml at 60 min. Lung weight gain, measured as the opposite of the weight change of the extracorporeal reservoir, and perfusate 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) concentration increased rapidly during the first 60 min and then more gradually. After 210 min, weight gain was 224 +/- 40 g and 6-keto-PGF1 alpha concentration, 4.99 +/- 0.01 ng/ml. The ratio of lymph to plasma oncotic pressure (pi L/pi P) at 30 min was 0.61 +/- 0.06 and did not change significantly. Imidazole (5 mM) reduced the changes in TXB2, Ppa, QL, and weight and platelet count but did not alter 6-keto-PGF1 alpha, pi L/pi P, or leukocyte count. Indomethacin (0.056 mM) reduced TXB2, 6-keto-PGF1 alpha, and the early increases in weight, Ppa, and QL but did not alter the time courses of leukocyte or platelet counts. Late in perfusion, however, Ppa and QL were greater than in either untreated or imidazole-treated lungs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory failure after endotoxin infusion in sheep: lung mechanics and lung fluid balance

Infusion of Escherichia coli endotoxin (0.12-1.5 micrograms/kg) into unanesthetized sheep causes transient pulmonary hypertension and several hours of increased lung vascular permeability, after which sheep recover. To produce enough lung injury to result in pulmonary edema with respiratory failure, we infused larger doses of E. coli endotoxin (2.0-5.0 micrograms/kg) into 11 chronically instrumented unanesthetized sheep and continuously measured pulmonary arterial, left atrial and aortic pressures, dynamic lung compliance, lung resistance, and lung lymph flow. We intermittently measured arterial blood gas tensions and pH, made interval chest radiographs, and calculated postmortem extravascular bloodless lung water-to-dry lung weight ratio (EVLW/DLW). Of 11 sheep 8 developed respiratory failure; 7 died spontaneously 6.3 +/- 1.1 h, and one was killed 10 h after endotoxin infusion. All sheep that had a premortem room air alveolar-arterial gradient in partial pressure of O2 (PAo2-Pao2) greater than 42 Torr (58 +/- 5 (SE) Torr) died. Of eight sheep that had radiographs made, six developed radiographically evident interstitial or interstitial and alveolar edema. Pulmonary artery pressure rose from base line 22 +/- 2 to 73 +/- 3 cmH2O and remained elevated above baseline levels until death. There was an initial fourfold decrease in dynamic compliance and sixfold increase in pulmonary resistance; both variables remained abnormal until death. EVLW/DLW increased with increasing survival time after endotoxin infusion, suggesting that pulmonary edema accumulated at the same rate in all fatally injured sheep, regardless of other variables. The best predictor of death was a high PAo2-Pao2. The marked increase in pulmonary resistance and decrease in dynamic compliance occurred too early after endotoxin infusion (15-30 min) to be due to pulmonary edema. The response to high-dose endotoxin in sheep closely resembles acute respiratory failure in humans following gram-negative septicemia. Respiratory failure and death in this model were not due to pulmonary edema alone.     

Effects of endotoxemia on the sheep lung microvascular membrane: a two-pore theory

We analyzed the effects of Escherichia coli endotoxin infusion on pulmonary microvessels in sheep by using a two-pore mathematical model of the microvascular barrier. Five sheep were prepared with lung lymph fistulas and instrumented to measure pulmonary arterial and left atrial pressures. Multiple indicator-dilution curves (with 125I-labeled albumin, 51Cr-labeled erythrocytes, [14C]urea, and 3H2O) were measured at base line and during phases 1 and 2 of the endotoxin response. Alterations in the membrane integrity in response to endotoxin infusion were quantified by using a two-pore theory of the microvascular barrier that incorporated lymph, protein, pressure, and multiple indicator measurements. The modeling results showed a slight change in the size of the pores during phase 1 but a 56% decrease in the number of small pores and a twofold increase in the number of large pores with respect to base-line values. During phase 2 the large pore size increased by 40%, and the total number of pores returned to base-line values. The analysis showed that endotoxin effects on fluid and protein exchange in the lung cannot be explained by hemodynamic and surface area changes alone. An apparent increase in lung microvascular permeability occurs during phases 1 and 2 of the endotoxin reaction, with a substantial decrease in perfused microvascular surface area during phase 1.     

Assessment of fluid balance in isolated sheep lungs

In this study we demonstrate the validity and utility of an isolated lung preparation developed for the study of pulmonary fluid balance. Lungs of 2- to 3-mo-old sheep were perfused in situ with autologous blood treated with indomethacin (20 micrograms/ml). Lung lymph flow (QL), uncontaminated by systemic lymph, was measured from either the efferent duct of the caudomediastinal lymph node or the thoracic duct in the superior mediastinum. Lung weight change (delta W) was measured as the opposite of the change in weight of the extracorporeal blood reservoir. A unique feature of this experimental model is the ability to assess lung fluid balance from simultaneous measurements of delta W and QL. In addition, hemodynamic and blood gas variables can be tightly controlled. Our results show that changes in QL and the lymph-to-plasma oncotic pressure ratio caused by an increase in microvascular pressure were comparable with those seen previously in intact sheep. When microvascular pressure was returned to control levels, QL fell despite a sustained increase in the amount of extravascular lung water, suggesting compartmentalization of the filtrate and/or effects of intravascular volume on lymph-driving pressure or resistance. Lymph flow was directly proportional to respiratory frequency over the range of 0-30 min-1 when the change in frequency was maintained for periods as long as 30 min. This preparation should prove useful in the study of lung fluid balance, particularly when it is desired to use interventions which are precluded or difficult in intact animals.