Lung mechanics and airway reactivity in sheep during development of oxygen toxicity

Acute lung injury and pulmonary vascular permeability: use of transgenic models

Acute lung injury is a general term that describes injurious conditions that can range from mild interstitial edema to massive inflammatory tissue destruction. This review will cover theoretical considerations and quantitative and semi-quantitative methods for assessing edema formation and increased vascular permeability during lung injury. Pulmonary edema can be quantitated directly using gravimetric methods, or indirectly by descriptive microscopy, quantitative morphometric microscopy, altered lung mechanics, high-resolution computed tomography, magnetic resonance imaging, positron emission tomography, or x-ray films. Lung vascular permeability to fluid can be evaluated by measuring the filtration coefficient (Kf) and permeability to solutes evaluated from their blood to lung clearances. Albumin clearances can then be used to calculate specific permeability-surface area products (PS) and reflection coefficients (σ). These methods as applied to a wide variety of transgenic mice subjected to acute lung injury by hyperoxic exposure, sepsis, ischemia-reperfusion, acid aspiration, oleic acid infusion, repeated lung lavage, and bleomycin are reviewed. These commonly used animal models simulate features of the acute respiratory distress syndrome, and the preparation of genetically modified mice and their use for defining specific pathways in these disease models are outlined. Although the initiating events differ widely, many of the subsequent inflammatory processes causing lung injury and increased vascular permeability are surprisingly similar for many etiologies.

The role of endothelin-1 in hyperoxia-induced lung injury in mice

Background

As prolonged hyperoxia induces extensive lung tissue damage, we set out to investigate the involvement of endothelin-1 (ET-1) receptors in these adverse changes.

Methods

Experiments were performed on four groups of mice: control animals kept in room air and a group of mice exposed to hyperoxia for 60 h were not subjected to ET-1 receptor blockade, whereas the dual ETA/ETB-receptor blocker tezosantan (TEZ) was administered via an intraperitoneal pump (10 mg/kg/day for 6 days) to other groups of normal and hyperoxic mice. The respiratory system impedance (Zrs) was measured by means of forced oscillations in the anesthetized, paralyzed and mechanically ventilated mice before and after the iv injection of ET-1 (2 μg). Changes in the airway resistance (Raw) and in the tissue damping (G) and elastance (H) of a constant-phase tissue compartment were identified from Zrs by model fitting.

Results

The plasma ET-1 level increased in the mice exposed to hyperoxia (3.3 ± 1.6 pg/ml) relative to those exposed to room air (1.6 ± 0.3 pg/ml, p < 0.05). TEZ administration prevented the hyperoxia-induced increases in G (13.1 ± 1.7 vs. 9.6 ± 0.3 cmH₂O/l, p < 0.05) and H (59 ± 9 vs. 41 ± 5 cmH₂O/l, p < 0.05) and inhibited the lung responses to ET-1. Hyperoxia decreased the reactivity of the airways to ET-1, whereas it elevated the reactivity of the tissues.

Conclusion

These findings substantiate the involvement of the ET-1 receptors in the physiopathogenesis of hyperoxia-induced lung damage. Dual ET-1 receptor antagonism may well be of value in the prevention of hyperoxia-induced parenchymal damage.

Hyperoxia-induced changes in mouse lung mechanics: forced oscillations vs. barometric plethysmography

Hyperoxia-induced lung damage was investigated via airway and respiratory tissue mechanics measurements with low-frequency forced oscillations (LFOT) and analysis of spontaneous breathing indexes by barometric whole body plethysmography (WBP). WBP was performed in the unrestrained awake mice kept in room air ( n = 12) or in 100% oxygen for 24 ( n = 9), 48 ( n = 8), or 60 ( n = 9) h, and the indexes, including enhanced pause (Penh) and peak inspiratory and expiratory flows, were determined. The mice were then anesthetized, paralyzed, and mechanically ventilated. Airway resistance, respiratory system resistance at breathing frequency, and tissue damping and elastance were identified from the LFOT impedance data by model fitting. The monotonous decrease in airway resistance during hyperoxia correlated best with the increasing peak expiratory flow. Respiratory system resistance and tissue damping and elastance were unchanged up to 48 h of exposure but were markedly elevated at 60 h, with associated decreases in peak inspiratory flow. Penh was increased at 24 h and sharply elevated at 60 h. These results indicate no adverse effect of hyperoxia on the airway mechanics in mice, whereas marked parenchymal damage develops by 60 h. The inconsistent relationships between LFOT parameters and WBP indexes suggest that the changes in the latter reflect alterations in the breathing pattern rather than in the mechanical properties. It is concluded that, in the presence of diffuse lung disease, Penh is inadequate for characterization of the mechanical status of the respiratory system.

Clearance of filtered fluid from the lung during exercise: role of hyperpnea

During strenuous exercise in sheep, lung lymph flow increases within seconds and rises to levels 7- to 10-fold over baseline. Concomitant with the flow increase, the lymph protein content rapidly decreases to levels consistent with severe capillary hypertension. This pattern of clearance of filtered fluid is quite different than is seen with the passive capillary hypertension that results from mechanical obstruction of the mitral valve. In passive capillary hypertension, the increase in lymph flow and reduction in lymph protein content develop over several hours. The purpose of this study was to discover if these observed differences in edema clearance are related to the hyperpnea that accompanies exercise. Sheep were instrumented for continuous measurement of pulmonary arterial, left atrial, and systemic pressures, cardiac output by ultrasound, lung lymph flow, and ventilation. First, hemodynamics, ventilatory, and lymph clearance variables were measured during moderate exercise at 2.8 mph on a treadmill. Second, on a separate occasion, sheep were induced to hyperventilate to the same minute ventilation as during exercise, using modest CO2 stimulation. Lymph flow and hemodynamics were unaffected by this hyperpnea. The third arm of the experiment was to raise pulmonary microvascular pressure at rest to the level seen with exercise by means of a balloon catheter placed in the mitral valve. Lymph flow rose and protein content decreased slowly and to a lower degree than seen with exercise despite a comparable microvascular pressure. Finally, left atrial hypertension and induced hyperpnea were combined in sheep at rest, and the resulting lymph flow and protein content were the same as seen with exercise at similar pressures and ventilation. We conclude that hyperpnea is a major mechanism of interstitial liquid clearance during exercise, and may be largely responsible for preventing pulmonary edema that might occur at the high microvascular pressures of strenuous exercise.

Pre mortem analysis of lung injury and lung function in oxygen toxic rabbits

OBJECTIVES

To determine whether respiratory system mechanics measurements could detect lung injury in oxygen toxic rabbits before clinical deterioration. To determine whether respiratory system mechanics measurements, using a power analysis, have the statistical power to detect significant reductions in hyperoxic lung injury due to an intervention when compared with traditional post mortem measurements of lung injury, extravascular lung water, and bronchoalveolar lavage protein concentration.

DESIGN

Prospective, controlled study.

SETTING

Institutional animal laboratories.

SUBJECTS

Adult New Zealand white rabbits.

INTERVENTIONS

Spontaneously breathing adult New Zealand white rabbits were exposed continuously to either > 95% oxygen or room air.

MEASUREMENTS AND MAIN RESULTS

We measured arterial pH, blood gas tensions, and respiratory system mechanics in rabbits twice, both before exposure to > 95% oxygen, and after the rabbits developed symptoms of mild lung dysfunction. After the second set of respiratory system mechanics measurements, we measured extravascular lung water and bronchoalveolar lavage protein concentration in the hyperoxia-exposed rabbits and compared the values with those values obtained in animals that breathed room air only. Our hyperoxia-exposed rabbits developed symptoms of mild respiratory impairment at 69 +/- 2 hrs. In these hyperoxia-exposed rabbits, measurements of static compliance, quasi-static compliance and resistance all changed significantly (p < .05) when compared with baseline measurements. Functional residual capacity and arterial blood gas values did not change significantly. Furthermore, assuming that an intervention reduced hyperoxic lung injury by a given amount, we performed a power analysis and found that the measurement of static compliance had at least equivalent power to detect a reduction in lung injury from an intervention when compared with measurement of extravascular lung water and bronchoalveolar lavage protein concentration.

CONCLUSIONS

Measurements of respiratory system mechanics can detect lung injury in hyperoxic rabbits before the onset of severe clinical deterioration or death. Furthermore, measurement of static compliance of the respiratory system is likely to be a powerful tool to detect a reduction in lung injury produced by an intervention.

Persistent increased lung response to methacholine after normobaric hyperoxia in rabbits

This study was performed to determine the occurrence and time course of airway hyperreactivity following exposure to normobaric hyperoxia. Twenty-six rabbits were studied. Twelve served as control (group 1), and 14 were exposed to normobaric hyperoxia (FiO2 > or = 95%) for 48 h: 9 rabbits (group 2) were studied after 1 day of recovery in room air and 5 (group 3) after 7 days. The rabbits were anesthetized, curarized and artificially ventilated. Respiratory resistance (Rrs) and elastance (Ers) and their respective changes induced by cumulative doses of aerosolized methacholine were assessed by the multiple linear regression analysis of airway pressure, tidal flow and volume. Weight-specific Rrs and Ers were significantly higher in group 2 (respectively, 87.7 +/- 6.5 cmH2O.L-1.sec.kg and 1100.2 +/- 87.1 cmH2O.L-1.kg, mean +/- SEM) than in group 1 (respectively, 65.2 +/- 3.2 cmH2O.L-1.sec.kg and 904.4 +/- 49.7 cmH2O.L-1.kg (P < 0.05)), but were not different from group 3 (79.4 +/- 7.9 cmH2O.L-1.sec.kg and 952.3 +/- 125.0 cmH2O.L-1.kg, respectively). The dose of methacholine required to increase Rrs by 50% (PDRrs50) was significantly lower in both treated groups: 0.37 +/- 0.11 mg in group 2 and 0.51 +/- 0.19 mg in group 3 vs 2.07 +/- 0.51 mg in group 1 (P < 0.05)). PDErs50 was significantly lower in group 2 (0.45 +/- 0.15 mg) and 3 (0.75 +/- 0.43 mg) compared with controls (1.11 +/- 0.26 mg (P < 0.05)). These results show that hyperoxia induces an increase in Rrs and Ers, and airway hyperreactivity in the rabbit. The latter is prolonged beyond the immediate post-exposure period.

Increases in lung tissue expression of intercellular adhesion molecule-1 are associated with hyperoxic lung injury and inflammation in mice

Lung injury caused by breathing enriched oxygen continues to be a major problem in clinical medicine. Experimentally, hyperoxic lung injury is characterized by pulmonary edema and associated neutrophil accumulation. Although extensively investigated, the mechanisms for neutrophil accumulation and the role of this accumulation in hyperoxic lung injury remain controversial. Intercellular adhesion molecule-1 (ICAM-1) is an adhesion molecule that when increased on endothelium by inflammatory cytokines leads to increased adhesion of neutrophils to the inflamed endothelium and transendothelial migration. The purpose of this study was to examine the role of inflammation in hyperoxia-induced lung injury by investigating ICAM-1 expression in the lungs of mice exposed to > 95% oxygen continuously. Lung tissue from mice exposed to > 95% oxygen was analyzed for ICAM-1 mRNA by slot blot analysis and for ICAM-1 protein expression. We also examined lungs from mice exposed to hyperoxia for up to 96 h by light microscopy to correlate pulmonary inflammation with ICAM-1 expression. We found that mRNA for ICAM-1 increased 56% over baseline after 48 h of exposure to hyperoxia, that ICAM-1 protein increased by more than 5-fold over baseline after 96 h of exposure to hyperoxia, and that lung inflammation and injury were not evident until 96 h of exposure. Our data demonstrate that exposure to hyperoxia causes an increase in ICAM-1 gene transcription and/or mRNA stability in mouse lungs, and that this increase is followed by an increase in ICAM-1 protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hyperoxia on bronchial wall dimensions and lung mechanics in rats

The effects of exposure to hyperoxic conditions (> 95 kPa at normobaric pressure) on bronchial wall dimensions and lung mechanics were examined in adult rats. Measurements of baseline pulmonary resistance and changes in pulmonary resistance following acetylcholine aerosol inhalation were made in rats exposed to hyperoxia for 48 and 60 h and in control rats exposed to air. Exposures for 48 h were carried out in humid (80% relative humidity) or dry (35-40% relative humidity) conditions. Morphometric measurements of airway wall thickness in lobar bronchi were made in separate groups of similarly exposed rats. Exposure to hyperoxia was associated with an increase in baseline pulmonary resistance (control rats 0.043 (0.016) cmH2O ml-1 s-1, 60 h exposed rats 0.125 (0.042) cmH2O ml-1 s-1) but hyper-responsiveness to acetylcholine inhalation did not occur. Thickness of the airway wall and its subdivisions, epithelium, lamina propria and muscularis, was not altered by hyperoxic exposure in humid conditions. However, epithelial thickening in the lobar bronchi was observed in rats exposed for 48 h to hyperoxia in dry conditions compared to rats exposed in humid conditions (mean (SD) thickness 13.2 (3.3) microns for controls, 14.5 (1.5) microns for humid exposed rats and 16.5 (3.3) microns for dry exposed rats). The increase in pulmonary resistance caused by hyperoxic exposure is unlikely to be due to airway damage as airway hyper-responsiveness did not occur, and is more likely to be associated with the development of alveolar oedema. Environmental humidity may modulate lung damage induced by hyperoxia, as exposure in dry conditions was associated with significant epithelial thickening.

Hyperoxic lung injury in Fischer-344 and Sprague-Dawley rats in vivo

Supplemental oxygen remains an important therapy for pulmonary insufficiency, despite the potential adverse effects of hyperoxic exposures. Recently, He et al. reported that hyperoxic ventilation more readily damaged isolated perfused lungs from Fischer-344 rats than from Sprague-Dawley rats (Am. J. Physiol. 259:L451), which correlates with the previously reported strain differences in hepatic responses to diquat-induced oxidant stress in vivo (J. Pharmacol. Exp. Ther. 235:172). We therefore examined the differences in hyperoxic lung injury in Fischer-344 and Sprague-Dawley rats in vivo. Adult male rats were exposed to > 95% O2 and were sacrificed after 24, 48, or 60 h. Control animals were maintained in room air. Dramatically greater increases in pleural effusions and bronchoalveolar lavage protein concentrations in response to hyperoxia were observed in the Fischer-344 rats than in the Sprague-Dawley rats (p < .05 at both 48 and 60 h for both measurements). Additionally, the glutathione concentrations in alveolar lining fluid decreased from 800 microM to 115 microM in Fischer-344 rats after 60 h of > 95% O2, but did not change in Sprague-Dawley rats. We conclude that the greater susceptibility of Fischer-344 than of Sprague-Dawley rats to hyperoxic lung injury in vitro reported previously also is observed in vivo and that this strain difference offers unique opportunities to study mechanisms of hyperoxic lung injury.

Hyperoxia-induced airway remodeling in immature rats. Correlation with airway responsiveness

We recently found that exposure of 21-day-old rats to hyperoxia (> 95% O2 for 8 days) significantly increased in vivo airway cholinergic responsiveness and that O2 exposure also increased airway epithelial and smooth muscle layer thicknesses in a separate cohort of animals. There was substantial variation in the magnitude of both the functional and structural responses to hyperoxia. The present study was designed to test whether the magnitude of O2-induced airway remodeling could account for individual differences in airway responsiveness after O2 exposure, as well as for the difference in responsiveness between air- and O2-exposed animals. We assessed in vivo airway responsiveness to aerosolized acetylcholine (ACh) and airway architecture in 14 O2- and 5 air-exposed, immature rats. Total respiratory system resistance was determined using a plethysmographic method. The mean thicknesses and fractional areas of the airway epithelial and smooth muscle layers were determined by contour tracing using a digitizing pad and microcomputer. Both the small (circumference < 1,000 microns) and central (circumference 1,000 to 4,000 microns) airways were studied. For O2-exposed rats, individual values of EC200 ACh correlated negatively with small airway smooth muscle layer thickness (r = -0.59, p < 0.05; ANOVA), small airway smooth muscle layer fractional area (r = -0.75, p < 0.01), small airway epithelial thickness (r = -0.54, p < 0.05), small airway epithelial fractional area (r = -0.69, p < 0.01), and central airway smooth muscle layer thickness (r = -0.53, p < 0.05). When both air- and O2-exposed animals were considered, EC200 ACh correlated negatively with all eight parameters of airway layer thickness and fractional area.(ABSTRACT TRUNCATED AT 250 WORDS)

Differing effects of nitrogen mustard and hydroxyurea on lung O2 toxicity in adult sheep

We measured the effects of leukocyte depletion on the pulmonary response to breathing 100% O2 in adult sheep, using two dissimilar agents. A group of eight sheep received 4.0 g/day of hydroxyurea for 4 to 6 days, and six sheep received two to four doses of 0.4 mg/kg of nitrogen mustard before exposure to 100% O2. A group of seven sheep breathed 100% O2 with no drug treatment. A group of five control sheep breathed compressed air for 96 h. Hydroxyurea selectively reduced circulating neutrophils (602 +/- 245 neutrophils, 2,537 +/- 394 lymphocytes per mm3), and nitrogen mustard decreased both circulating neutrophils (633 +/- 326) and lymphocytes (521 +/- 129). In untreated sheep breathing 100% O2, survival time was 92.6 +/- 3.4 h and postmortem blood-free lung water to dry lung ratio was 4.4 +/- 0.2. Hydroxyurea significantly delayed the onset of oxygen toxicity as measured by changes in lung lymph flow, PaO2, PaCO2, and time to respiratory failure (111.8 +/- 7.7 h), although the degree of final lung injury was unchanged and postmortem lung water was elevated (5.3 +/- 0.3). Nitrogen mustard shortened the time to hypoxemia, and CO2 retention and decreased time to respiratory failure (84.0 +/- 4.3 h). Neutrophils were markedly reduced but not absent in the lungs of both groups of leukocyte-depleted sheep. We conclude that neutrophils are not essential for the full expression of lung O2 toxicity in adult sheep. Secondary effects of hydroxyurea and nitrogen mustard appear to influence the rate of development, but not the outcome of oxygen toxicity, by effects independent of leukocyte depletion.

Simultaneous exposure of sheep to endotoxin and 100% oxygen

The purpose of this study was to determine the effect of endotoxin on the development of vascular and airway dysfunction during O2 toxicity. Sheep were prepared for chronic measurement of vascular pressures, cardiac output, gas exchange, and collection of lung lymph. Tracheostomies were made for accurate delivery of gas mixtures. Sheep were placed in one of three experimental groups: those receiving endotoxin (n = 9), those breathing 100% O2 and receiving endotoxin (n = 7), and those exposed to 100% O2 alone (n = 6). Sheep had daily measurements of hypoxic vasoconstriction (FIO2 = 0.12), gas exchange, circulating white blood cell counts, lymph flow, and lymph and plasma protein concentrations. Lung neutrophils were counted, and copper-zinc superoxide dismutase and manganous superoxide dismutase were measured in lung samples from some sheep biopsies taken at baseline surgery and postmortem. Endotoxin markedly prolonged survival time and partially protected against the increased lung vascular permeability in sheep breathing 100% oxygen, but impairment of gas exchange, loss of hypoxic pulmonary vasoconstriction, and ultimate progression of respiratory failure were not prevented. Induction of MnSOD occurred in sheep breathing 100% O2, in sheep receiving endotoxin alone, and in those exposed to 100% O2 plus endotoxin. We conclude that endotoxin markedly increases tolerance to O2 toxicity but that some of the pathophysiology of O2 toxicity is unaltered. The role of superoxide dismutase in the observed protection is unclear.