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

Effect of ringers acetate in different doses on plasma volume in rat models of hypovolemia

Background

Even though crystalloids are the first choice for fluid resuscitation in hemodynamically unstable patients, their potency as plasma volume expanders in hypovolemia of different etiologies is largely unknown. The objective of the study was to investigate dose–response curves of a crystalloid in hypovolemia induced by either sepsis or hemorrhagic shock.

Results

Rats were randomized to resuscitation with Ringers acetate at a dose 10, 30, 50, 75, or 100 ml/kg at 4 h after induction of sepsis by cecal ligation and puncture (CLP) or 2.5 h after a 30 ml/kg hemorrhage. Plasma volume (¹²⁵I–albumin) was the primary outcome. Plasma volume decreased by about 11.8 (IQR 9.9–14.5) ml/kg relative baseline after CLP and increased dose-dependently by at most 5.8 (IQR 3.3–7.0) ml/kg in the 100 ml/kg group at 15 min after resuscitation. In the hemorrhage group, the plasma volume increased by at most 13.8 (IQR 7.1–15.0) ml/kg in 100 ml/kg group. Blood volumes at baseline, calculated using hematocrit and plasma volumes, were 72.4 (IQR 68.2–79.5) ml/kg in sepsis group and 71.1 (IQR 69.1–74.7) ml/kg in hemorrhage group. At 15 min after resuscitation with a dose of 100 ml/kg blood volumes increased to 54.8 (IQR 52.5–57.7) ml/kg and ; 49.6 (IQR 45.3–56.4) ml/kg, in the sepsis and hemorrhage groups, respectively. Plasma volume expansion as the percentage of dose at 15 min was 5.9 (IQR 2.5–8.8)% and 14.5 (IQR 12.1–20.0)% in the sepsis and hemorrhage groups, respectively. At 60 min, average plasma volume as the percentage of dose had decreased to 2.9 (IQR ([−2.9] − 8.3)% (P = 0.006) in the sepsis group whereas no change was detected in the hemorrhage group. A dose-dependent decrease in the plasma oncotic pressure, which was more marked in sepsis, was detected at 60 min after resuscitation.

Conclusions

We conclude that the efficacy of Ringers acetate as a plasma volume expander is context dependent and that plasma volume expansion is lower than previously realized across a wide range of doses. Ringers acetate decreases plasma oncotic pressure in sepsis, in part, by mechanisms other than dilution.

Effect of charge on microvascular permeability in early experimental sepsis in the rat

A key feature of sepsis is hypovolemia due to increased microvascular permeability. It has been suggested that the negative charge of albumin and of the endothelial glycocalyx is important for maintenance of the normally low permeability for albumin. Here we tested the hypothesis that charge effects contribute to the increased permeability in sepsis. Transcapillary escape rate (TER) and initial distribution volume for (125)I-labeled bovine serum albumin (BSA, isoelectric point pH 4.6) and for (131)I-labeled charge modified BSA (cBSA, average isoelectric point, pH 7.1) was measured 3h after sepsis was induced by cecal ligation and incision (CLI) (n=11) and in control animals (n=12). The importance of charge for permeability in sepsis was estimated by comparing the ratio between TER for cBSA and TER for BSA during control conditions to that after CLI. Plasma concentration of the glycocalyx component glycosaminoglycans (GAGs) was measured in separate control and CLI animals. The initial distribution volume for BSA and cBSA in control animals was 38 ± 3 ml/kg and 47 ± 4 mL/kg and decreased by 17% and 19%, respectively, following CLI. TER for BSA increased from 16.7 ± 4.1% in the controls to 20.1 ± 1.9% following CLI. Corresponding values for cBSA were 26.7 ± 5.6% and 29.8 ± 3.5%, respectively. The ratio between TER for cBSA and TER for BSA was 1.62 ± 0.1 in the control group and 1.49 ± 0.1 following CLI (p<0.05). Plasma GAG concentrations were higher in CLI animals than in the control group. We conclude that CLI induce hypovolemia secondary to increased microvascular permeability. Negative charge contributes to the normally low permeability of albumin and the importance of charge is decreased in early experimental sepsis. The observed charge effects are associated with CLI-induced breakdown of the glycocalyx.

Assessment of vascular permeability in an ovine model of acute lung injury and pneumonia-induced Pseudomonas aeruginosa sepsis

OBJECTIVE

To assess the time changes and mechanism of pulmonary and peripheral vascular permeability in sheep with acute lung injury and sepsis.

DESIGN

Prospective, controlled, randomized trial.

SETTING

University research laboratory.

SUBJECTS

A total of 21 chronically instrumented, adult female sheep.

INTERVENTIONS

Sheep were instrumented with lung and prefemoral lymph fistulas and allocated to either an uninjured control group (n = 5) or sepsis group (n = 5). The sheep in the sepsis group received cotton smoke inhalation injury followed by instillation of Pseudomonas aeruginosa into the lungs. All sheep were mechanically ventilated and fluid resuscitated for the entire duration of the 24-hr experiment. Additional sheep (n = 11) received injury and were killed at different time points for the measurement of vascular endothelial growth factor in lung tissue.

MEASUREMENTS AND MAIN RESULTS

The injury induced a hypotensive-hyperdynamic circulation; increases in pulmonary capillary pressure, net fluid balance, lung and prefemoral lymph flow and protein content, lung water content, abdominal and thoracic fluid and protein content, neutrophil accumulation in the lung, and vascular endothelial growth factor expression in lung tissue; and decreases in PaO2/FiO2 ratio, plasma protein concentration, plasma oncotic pressure, and myocardial contractility.

CONCLUSIONS

Lung edema formation in this model was the result of marked increases in both pulmonary microvascular permeability and pressure. Pulmonary vascular hyperpermeability peaked 12 hrs postinjury and was related to vascular endothelial growth factor overexpression. Early myocardial failure was a potential contributor to the constant increase in pulmonary capillary pressure. The sepsis-induced increase in peripheral microvascular permeability was associated with significant accumulation of fluid and protein in the third space.

New method for evaluation of lung lymph flow rate with intact lymphatics in anaesthetized sheep

AIM

Lung lymph has commonly been studied using a lymph fistula created by tube cannulation into the efferent duct of the caudal mediastinal node in sheep. In this method, the tail region of the caudal mediastinal node is resected and the diaphragm is cauterized to exclude systemic lymph contamination, and cannulation is performed into one of the multiple efferent ducts originating from the caudal mediastinal node. Moreover, the pumping activity of lymphatics might be diminished by cannulation. Therefore, the purpose of the study was to evaluate the flow rate of lung lymph with maintenance of intact lymphatic networks around the caudal mediastinal node to the thoracic duct in sheep.

METHODS

An ultrasound transit-time flow meter was used to measure lung lymph flow. The thoracic duct was clamped just above the diaphragm and the flow probe was attached to the thoracic duct just after the last junction with an efferent duct from the caudal mediastinal node. The lung lymph flow rate was measured at baseline and under conditions of lung-oedema formation.

RESULTS

The baseline lung lymph flow rate in our model was three- to sixfold greater than values obtained with the cannulation method. With oedema-formation, the lung lymph flow rate was the same as that measured using cannulation.

CONCLUSION

The lung lymph flow was unexpectedly large under the conditions of the study, and our data suggest that the drainage effect of lymphatics is significant as a safety factor against pulmonary oedema formation.

Effects of adenosine on extravascular lung water content in endotoxemic pigs

OBJECTIVE

To investigate whether adenosine protects against endotoxin-induced increments in extravascular lung water content.

DESIGN

Prospective, randomized, animal study.

SETTING

University research laboratory.

SUBJECTS

Twenty-one anesthetized juvenile pigs.

INTERVENTIONS

The animals were divided into two groups subjected to endotoxin infusion: Endotoxin alone (n = 7), or endotoxin combined with adenosine infusion (n = 7) administered during the whole experimental period. Two other groups were exposed to anesthesia alone (n = 4) or adenosine infusion alone (n = 3), respectively.

MEASUREMENTS AND MAIN RESULTS

Central hemodynamic variables and extravascular lung water, as assessed by the thermal dye dilution double indicator technique, were monitored. Plasma endothelin-1 concentrations were measured hourly. Extravascular lung water increased significantly in response to endotoxemia (p <.001) along with an increase in pulmonary microvascular pressure (P(mv) [p <.01]). Although the Pmv increased less in endotoxemic animals exposed to adenosine infusion, no intergroup difference was found. From 4 through 6 hrs, adenosine-treated pigs displayed only half of the extravascular lung water content of nontreated animals (p <.01). The latter did not differ from that of anesthetized controls receiving anesthesia or adenosine alone. Adenosine administered alone had no effect on P(mv). In pigs receiving adenosine alone, extravascular lung water content reached nadir after 3 hrs. In both endotoxin groups, plasma endothelin-1 concentration increased two-fold, peaking 4-6 hrs after the start of endotoxin infusion (p <.001).

CONCLUSIONS

The endotoxin-induced increase in lung extravascular water was hampered by intravenously infused adenosine in the presence of a nonsignificantly reduced microvascular pressure. This leaves reduced microvascular permeability the most likely reason for the beneficial effect of adenosine.

Methylene blue reduces lung fluid filtration during the early phase of endotoxemia in awake sheep

OBJECTIVE

To determine whether methylene blue (MB), an inhibitor of soluble guanylate cyclase and nitric oxide synthase, alters lung hemodynamics and fluid filtration after endotoxin in sheep.

DESIGN

Prospective, randomized, controlled experimental study with repeated measurements.

SETTING

University animal laboratory.

SUBJECTS

Eight yearling, awake sheep.

INTERVENTIONS

Sheep were instrumented for a chronic study with vascular and lung lymph catheters. In two experiments, separated by 1 wk of recovery, the animals received intravenously either an injection of MB 10 mg/kg or a corresponding volume of 0.9% sodium chloride as pretreatment. Thirty minutes later, sheep received a bolus injection of Escherichia coli endotoxin 1 microg/kg, followed by either an infusion of MB 2.5 mg/kg/hr or a corresponding volume of 0.9% sodium chloride for 5 hrs.

MEASUREMENTS AND MAIN RESULTS

MB decreased the early phase endotoxin-induced rises in pulmonary capillary pressure and pulmonary vascular resistance. MB also reduced the increments in lung lymph flow (QL) and protein clearance (CL) as well as the rightward shift of the permeability-surface area product (PS). In addition, MB diminished the decrease in cardiac output, stabilized mean arterial pressure, and precluded the rise in plasma and lung lymph cyclic guanosine 3'-5' monophosphate. However, during the late phase, MB-treated sheep presented with a faster rise in QL with no difference in CL and PS from the endotoxemic controls.

CONCLUSIONS

During the early phase of endotoxemia in sheep, MB attenuates lung injury by decreasing the enhanced lung fluid filtration as a result of reduced pulmonary capillary pressure and permeability. However, MB does not counteract the late phase increase in lung fluid filtration.

Inhaled nitric oxide reduces lung fluid filtration after endotoxin in awake sheep

We studied the effect on lung fluid filtration of 37.6 ppm inhaled nitric oxide (NO) imposed for 1 h 2.5 h after endotoxin in seven awake sheep, with seven control subjects. The effects of NO on the longitudinal distribution of pulmonary vascular resistance (PVR) before and after endotoxin were specifically addressed in six sheep. Following endotoxin, sheep developed respiratory distress; PaO2, the alveolar-arterial oxygen tension difference (AaPO2) and venous admixture (Q S/Q T) changed significantly, as did the pulmonary artery pressure (Ppa), PVR, and lung lymph flow (Q L). Inhaled NO reduced Ppa and PVR by 50%; Q L decreased from 7.8 +/- 0.34 ml/15 min to 4.7 +/- 0.80 ml/15 min (mean +/- SEM), and lymph protein clearance from 4.9 +/- 0.18 ml/15 min to 3.6 +/- 0.75 ml/15 min. Lymph/plasma protein concentration ratio (L/P) increased from 0.63 +/- 0.016 to 0.72 +/- 0.006, concomitant with the decrease in Q L. The L/P - Q L relationships shifted from left, at baseline, to the right during endotoxemia, as did the permeability surface product (PS) isolines. The rightward shift was significantly less in the NO group. Inhaled NO significantly improved PaO2, AaPO2, and Q S/Q T, reduced the increase in pulmonary microwedge pressure back to baseline and decreased upstream and downstream PVR at 3.0 through 4. 0 h. We conclude that, in sheep, inhaled NO reduces lung fluid filtration by decreasing microvascular pressure and apparently also by declining the enhanced microvascular permeability during the late phase of endotoxemia.

Neutrophils pretreated with granulocyte colony-stimulating factor (G-CSF) are not related to the severity of endotoxin-induced lung injury

Neutrophils play an important role in mediating acute lung injury that is characteristic of adult respiratory distress syndrome. Granulocyte colony-stimulating factor (G-CSF) has been shown to increase neutrophil counts and to enhance their biological functions. This study investigated the effects of neutrophils pretreated with G-CSF on endotoxin-induced lung injury in conscious sheep. Nineteen sheep were chronically instrumented with a lung lymph fistula and vascular catheters for monitoring. Sheep were randomly allocated into three groups: group 1-sheep were infused only with endotoxin; group 2-G-CSF (250 micrograms/day) was administered intravenously for 5 days prior to endotoxin; and group 3-G-CSF (125 micrograms) was administered just before endotoxin. In each group, sheep received E. coli endotoxin (1 microgram/kg) for 30 min and observations were made for 5 h after endotoxin administration. Circulating leukocyte counts before endotoxin markedly increased in group 2 and significantly decreased in group 3, when compared with the level in group 1 (9700 +/- 900 (SEM) in group 1, 49,900 +/- 10,000 in group 2, and 3600 +/- 600/microL in group 3). In each group, circulating leukocyte counts significantly decreased 1 h after endotoxin administration and then returned to baseline values. However, there were no significant differences in either pulmonary hemodynamic or lung lymph responses to endotoxin among the groups. The results indicate that G-CSF does not adversely affect physiologic responses of the lung to endotoxin in sheep.