Mechanical ventilation of isolated rat lungs changes the structure and biophysical properties of surfactant.

Mechanical ventilation is an essential but potentially harmful therapeutic intervention for patients with acute lung injury. The objective of this study was to investigate the effects of mechanical ventilation on large-aggregate surfactant (LA) structure and function. Isolated rat lungs were randomized to either a nonventilated control group, a relatively noninjuriously ventilated group [1 h, 10 ml/kg tidal volume, 3 cmH(2)O positive end-expiratory pressure (PEEP)], or an injuriously ventilated group (1 h, 20 ml/kg tidal volume, 0 cmH(2)O PEEP). Injurious ventilation resulted in significantly decreased lung compliance compared with the other two groups. LA structure, as determined by electron microscopy, revealed that LA from the injurious group had significantly lower amounts of organized lipid-protein structures compared with LA obtained from the other groups. Analysis of the biophysical properties by using a captive bubble surfactometer demonstrated that adsorption and surface tension reduction were significantly impaired with LA from the injuriously ventilated lungs. We conclude that the injurious mechanical ventilation impairs LA function and that this impairment is associated with significant morphological alterations.     

Dependence of lung injury on surface tension during low-volume ventilation in normal open-chest rabbits.

To evaluate the role of pulmonary surfactant in the prevention of lung injury caused by mechanical ventilation (MV) at low end-expiratory volumes, lung mechanics and morphometry were assessed in three groups of eight normal, open-chest rabbits ventilated for 3-4 h at zero end-expiratory pressure (ZEEP) with physiological tidal volumes (Vt = 10 ml/kg). One group was left untreated (group A); the other two received surfactant intratracheally (group B) or aerosolized dioctylsodiumsulfosuccinate (group C) before MV on ZEEP. Relative to initial MV on positive end-expiratory pressure (PEEP; 2.3 cmH(2)O), quasi-static elastance (Est) and airway (Rint) and viscoelastic resistance (Rvisc) increased on ZEEP in all groups. After restoration of PEEP, only Rint (124%) remained elevated in group A, only Est (36%) was significantly increased in group B, whereas in group C, Est, Rint, and Rvisc were all markedly augmented (274, 253, and 343%). In contrast, prolonged MV on PEEP had no effect on lung mechanics of eight open-chest rabbits (group D). Lung edema developed in group C (wet-to-dry ratio = 7.1), but not in the other groups. Relative to group D, both groups A and C, but not B, showed histological indexes of bronchiolar injury, whereas all groups exhibited an increased number of polymorphonuclear leukocytes in alveolar septa, which was significantly greater in group C. In conclusion, administration of exogenous surfactant largely prevents the histological and functional damage of prolonged MV at low lung volumes, whereas surfactant dysfunction worsens the functional alterations, also because of edema formation and, possibly, increased inflammatory response.     

Cytokine release, small airway injury, and parenchymal damage during mechanical ventilation in normal open-chest rats.

Lung morpho-functional alterations and inflammatory response to various types of mechanical ventilation (MV) have been assessed in normal, anesthetized, open-chest rats. Measurements were taken during protective MV [tidal volume (Vt) = 8 ml/kg; positive end-expiratory pressure (PEEP) = 2.6 cmH(2)O] before and after a 2- to 2.5-h period of ventilation on PEEP (control group), zero EEP without (ZEEP group) or with administration of dioctylsodiumsulfosuccinate (ZEEP-DOSS group), on negative EEP (NEEP group), or with large Vt (26 ml/kg) on PEEP (Hi-Vt group). No change in lung mechanics occurred in the Control group. Relative to the initial period of MV on PEEP, airway resistance increased by 33 +/- 4, 49 +/- 9, 573 +/- 84, and 13 +/- 4%, and quasi-static elastance by 19 +/- 3, 35 +/- 7, 248 +/- 12, and 20 +/- 3% in the ZEEP, NEEP, ZEEP-DOSS, and Hi-Vt groups. Relative to Control, all groups ventilated from low lung volumes exhibited histologic signs of bronchiolar injury, more marked in the NEEP and ZEEP-DOSS groups. Parenchymal and vascular injury occurred in the ZEEP-DOSS and Hi-Vt groups. Pro-inflammatory cytokine concentration in the bronchoalveolar lavage fluid (BALF) was similar in the Control and ZEEP group, but increased in all other groups, and higher in the ZEEP-DOSS and Hi-Vt groups. Interrupter resistance was correlated with indexes of bronchiolar damage, and cytokine levels with vascular-alveolar damage, as indexed by lung wet-to-dry ratio. Hence, protective MV from resting lung volume causes mechanical alterations and small airway injury, but no cytokine release, which seems mainly related to stress-related damage of endothelial-alveolar cells. Enhanced small airway epithelial damage with induced surfactant dysfunction or MV on NEEP can, however, contribute to cytokine production.     

Different ventilation strategies alter surfactant responses in preterm rabbits.

The effect of ventilation strategy on in vivo function of different surfactants was evaluated in preterm rabbits delivered at 27 days gestational age and ventilated with either 0 cmH2O positive end-expiratory pressure (PEEP) at tidal volumes of 10-11 ml/kg or 3 cmH2O PEEP at tidal volumes of 7-8 ml/kg after treatment with one of four different surfactants: sheep surfactant, the lipids of sheep surfactant stripped of protein (LH-20 lipid), Exosurf, and Survanta. The use of 3 cmH2O PEEP decreased pneumothoraces in all groups except for the sheep surfactant group where pneumothoraces increased (P < 0.01). Ventilatory pressures (peak pressures - PEEP) decreased more with the 3 cmH2O PEEP, low-tidal-volume ventilation strategy for Exosurf-, Survanta-, and sheep surfactant-treated rabbits (P < 0.05), whereas ventilation efficiency indexes (VEI) improved only for Survanta- and sheep surfactant-treated rabbits with 3 cmH2O PEEP (P < 0.01). Pressure-volume curves for sheep surfactant-treated rabbits were better than for all other treated groups (P < 0.01), although Exosurf and Survanta increased lung volumes above those in control rabbits (P < 0.05). The recovery of intravascular radiolabeled albumin in the lungs and alveolar washes was used as an indicator of pulmonary edema. Only Survanta and sheep surfactant decreased protein leaks in the absence of PEEP, whereas all treatments decreased labeled albumin recoveries when 3 cmH2O PEEP was used (P < 0.05). These experiments demonstrate that ventilation style will alter a number of measurements of surfactant function, and the effects differ for different surfactants.     

Choosing the frequency of deep inflation in mice: balancing recruitment against ventilator-induced lung injury

Low tidal volume (Vt) ventilation is protective against ventilator-induced lung injury but can promote development of atelectasis. Periodic deep inflation (DI) can open the lung, but if delivered too frequently may cause damage via repeated overdistention. We therefore examined the effects of varying DI frequency on lung mechanics, gas exchange, and biomarkers of injury in mice. C57BL/6 males were mechanically ventilated with positive end-expiratory pressure (PEEP) of 2 cmH2O for 2 h. One high Vt group received a DI with each breath (HV). Low Vt groups received 2 DIs after each hour of ventilation (LV) or 2 DIs every minute (LVDI). Control groups included a nonventilated surgical sham and a group receiving high Vt with zero PEEP (HVZP). Respiratory impedance was measured every 4 min, from which tissue elastance (H) and damping (G) were derived. G and H rose progressively during LV and HVZP, but returned to baseline after hourly DI during LV. During LVDI and HV, G and H remained low and gas exchange was superior to that of LV. Bronchoalveolar lavage fluid protein was elevated in HV and HVZP but was not different between LV and LVDI. Lung tissue IL-6 and IL-1beta levels were elevated in HVZP and lower in LVDI compared with LV. We conclude that frequent DI can safely improve gas exchange and lung mechanics and may confer protection from biotrauma. Differences between LVDI and HV suggest that an optimal frequency range of DI exists, within which the benefits of maintaining an open lung outweigh injury incurred from overdistention.     

Increased lung expansion alters lung growth but not alveolar epithelial cell differentiation in newborn lambs

Although increased lung expansion markedly alters lung growth and epithelial cell differentiation during fetal life, the effect of increasing lung expansion after birth is unknown. We hypothesized that increased basal lung expansion, caused by ventilating newborn lambs with a positive end-expiratory pressure (PEEP), would stimulate lung growth and alter alveolar epithelial cell (AEC) proportions and decrease surfactant protein mRNA levels. Two groups of lambs were sedated and ventilated with either 0 cmH(2)O PEEP (controls, n = 5) or 10 cmH(2)O PEEP (n = 5) for 48 h beginning at 15 +/- 1 days after normal term birth. A further group of nonventilated 2-wk-old lambs was used for comparison. We determined wet and dry lung weights, DNA and protein content, a labeling index for proliferating cells, surfactant protein mRNA expression, and proportions of AECs using electron microscopy. Although ventilating lambs for 48 h with 10 cmH(2)O PEEP did not affect total lung DNA or protein, it significantly increased the proportion of proliferating cells in the lung when compared with nonventilated 2-wk-old controls and lambs ventilated with 0 cmH(2)O PEEP (control: 2.6 +/- 0.5%; 0 PEEP: 1.9 +/- 0.3%; 10 PEEP: 3.5 +/- 0.3%). In contrast, no differences were observed in AEC proportions or surfactant protein mRNA levels between either of the ventilated groups. This study demonstrates that increases in end-expiratory lung volumes, induced by the application of PEEP, lead to increased lung growth in mechanically ventilated 2-wk-old lambs but do not alter the proportions of AECs.     

Effects of ventilation on the surfactant system in sepsis-induced lung injury.

The present study examined the effects of mechanical ventilation, with or without positive end-expiratory pressure (PEEP), on the alveolar surfactant system in an animal model of sepsis-induced lung injury. Septic animals ventilated without PEEP had a significant deterioration in oxygenation compared with preventilated values (arterial PO(2)/inspired O(2) fraction 316 +/- 16 vs. 151 +/- 14 Torr; P < 0.05). This was associated with a significantly lower percentage of the functional large aggregates (59 +/- 3 vs. 72 +/- 4%) along with a significantly reduced function (minimum surface tension 17.7 +/- 1.8 vs. 11.8 +/- 3.8 mN/m) compared with nonventilated septic animals (P < 0.05). Sham animals similarly ventilated without PEEP maintained oxygenation, percent large aggregates and surfactant function. With the addition of PEEP, the deterioration in oxygenation was not observed in the septic animals and was associated with no alterations in the surfactant system. We conclude that animals with sepsis-induced lung injury are more susceptible to the harmful effects of mechanical ventilation, specifically lung collapse and reopening, and that alterations in alveolar surfactant may contribute to the development of lung dysfunction.     

A synthetic surfactant based on a poly-Leu SP-C analog and phospholipids: effects on tidal volumes and lung gas volumes in ventilated immature newborn rabbits.

Available surfactants for treatment of respiratory distress syndrome in newborn infants are derived from animal lungs, which limits supply and poses a danger of propagating infectious material. Poly-Val-->poly-Leu analogs of surfactant protein (SP)-C can be synthesized in large quantities and exhibit surface activity similar to SP-C. Here, activity of synthetic surfactants containing a poly-Leu SP-C analog (SP-C33) was evaluated in ventilated premature newborn rabbits. Treatment with 2.5 ml/kg body wt of 2% (wt/wt) SP-C33 in 1,2-dipalmitoyl-sn-3-glycero phosphoryl choline (DPPC)-1-palmitoyl-2-oleoyl-sn-3-glycero phosphoryl choline (POPC)-1-palmitoyl-2-oleoyl-sn-3-glycero phosphoryl glycerol (POPG), 68:0:31, 68:11:20, or 68:16:15 (wt/wt/wt) suspended at 80 mg/ml gave tidal volumes (Vt) of 20-25 ml/kg body wt, with an insufflation pressure of 25 cmH2O and no positive end-expiratory pressure (PEEP), comparable to the Vt for animals treated with the porcine surfactant Curosurf. Nontreated littermates had a Vt of approximately 2 ml/kg body wt. The Vt for SP-C33 in DPPC-egg phosphatidylglycerol-palmitic acid [68:22:9 (wt/wt/wt)], DPPC-POPG-palmitic acid [68:22:9 (wt/wt/wt)], and DPPC-POPC-POPG [6:2:2 (wt/wt/wt)] was 15-20 ml/kg body wt. Histological examination of lungs from animals treated with SP-C33-based surfactants showed incomplete, usually patchy air expansion of alveolar spaces associated with only mild airway epithelial damage. Lung gas volume after 30 min of mechanical ventilation were more than threefold larger in animals treated with Curosurf than in those receiving SP-C33 in DPPC-POPC-POPG, 68:11:20. This difference could be largely counterbalanced by ventilation with PEEP (3-4 cmH2O). An artificial surfactant based on SP-C33 improves Vt in immature newborn animals ventilated with standardized peak pressure but requires PEEP to build up adequate lung gas volumes.     

Positive end-expiratory pressure and surfactant decrease lung injury during initiation of ventilation in fetal sheep

The initiation of ventilation in preterm, surfactant-deficient sheep without positive end-expiratory pressure (PEEP) causes airway injury and lung inflammation. We hypothesized that PEEP and surfactant treatment would decrease the lung injury from initiation of ventilation with high tidal volumes. Fetal sheep at 128-day gestational age were randomized to ventilation with: 1) no PEEP, no surfactant; 2) 8-cmH(2)O PEEP, no surfactant; 3) no PEEP + surfactant; 4) 8-cmH(2)O PEEP + surfactant; or 5) control (2-cmH(2)O continuous positive airway pressure) (n = 6-7/group). After maternal anesthesia and hysterotomy, the head and chest were exteriorized, and the fetus was intubated. While maintaining placental circulation, the fetus was ventilated for 15 min with a tidal volume escalating to 15 ml/kg using heated, humidified, 100% nitrogen. The fetus then was returned to the uterus, and tissue was collected after 30 min for evaluation of early markers of lung injury. Lambs receiving both surfactant and PEEP had increased dynamic compliance, increased static lung volumes, and decreased total protein and heat shock proteins 70 and 60 in bronchoalveolar lavage fluid compared with other groups. Ventilation, independent of PEEP or surfactant, increased mRNA expression of acute phase response genes and proinflammatory cytokine mRNA in the lung tissue compared with controls. PEEP decreased mRNA for cytokines (2-fold) compared with groups receiving no PEEP. Surfactant administration further decreased some cytokine mRNAs and changed the distribution of early growth response protein-1 expression. The use of PEEP during initiation of ventilation at birth decreased early mediators of lung injury. Surfactant administration changed the distribution of injury and had a moderate additive protective effect.     

Effects of melatonin in an experimental model of ventilator-induced lung injury

Melatonin is a free radical scavenger and a broad-spectrum antioxidant and has well-documented immunomodulatory effects. We studied the effects of this hormone on lung damage, oxidative stress, and inflammation in a model of ventilator-induced lung injury (VILI), using 8- to 12-wk-old Swiss mice (n = 48). Animals were randomized into three experimental groups: control (not ventilated); low-pressure ventilation [peak inspiratory pressure 15 cmH(2)O, positive end-expiratory pressure (PEEP) 2 cmH(2)O], and high-pressure ventilation (peak inspiratory pressure 25 cmH(2)O, PEEP 0 cmH(2)O). Each group was divided into two subgroups: eight animals were treated with melatonin (10 mg/kg ip, 30 min before the onset of ventilation) and the remaining eight with vehicle. After 2 h of ventilation, lung injury was evaluated by gas exchange, wet-to-dry weight ratio, and histological analysis. Levels of malondialdehyde, glutathione peroxidase, interleukins IL-1beta, IL-6, TNF-alpha, and IL-10, and matrix metalloproteinases 2 and 9 in lung tissue were measured as indicators of oxidation status, pro-/anti-inflammatory cytokines, and matrix turnover, respectively. Ventilation with high pressures induced severe lung damage and release of TNF-alpha, IL-6, and matrix metalloproteinase-9. Treatment with melatonin improved oxygenation and decreased histological lung injury but significantly increased oxidative stress quantified by malondialdehyde levels. There were no differences in TNF-alpha, IL-1beta, IL-6, or matrix metalloproteinases caused by melatonin treatment, but IL-10 levels were significantly higher in treated animals. These results suggest that melatonin decreases VILI by increasing the anti-inflammatory response despite an unexpected increase in oxidative stress.     

Inflammation and lung maturation from stretch injury in preterm fetal sheep

Mechanical ventilation is a risk factor for the development of bronchopulmonary dysplasia in premature infants. Fifteen minutes of high tidal volume (V(T)) ventilation induces inflammatory cytokine expression in small airways and lung parenchyma within 3 h. Our objective was to describe the temporal progression of cytokine and maturation responses to lung injury in fetal sheep exposed to a defined 15-min stretch injury. After maternal anesthesia and hysterotomy, 129-day gestation fetal lambs (n = 7-8/group) had the head and chest exteriorized. Each fetus was intubated, and airway fluid was gently removed. While placental support was maintained, the fetus received ventilation with an escalating V(T) to 15 ml/kg without positive end-expiratory pressure (PEEP) for 15 min using heated, humidified 100% nitrogen. The fetus was then returned to the uterus for 1, 6, or 24 h. Control lambs received a PEEP of 2 cmH(2)O for 15 min. Tissue samples from the lung and systemic organs were evaluated. Stretch injury increased the early response gene Egr-1 and increased expression of pro- and anti-inflammatory cytokines within 1 h. The injury induced granulocyte/macrophage colony-stimulating factor mRNA and matured monocytes to alveolar macrophages by 24 h. The mRNA for the surfactant proteins A, B, and C increased in the lungs by 24 h. The airway epithelium demonstrated dynamic changes in heat shock protein 70 (HSP70) over time. Serum cortisol levels did not increase, and induction of systemic inflammation was minimal. We conclude that a brief period of high V(T) ventilation causes a proinflammatory cascade, a maturation of lung monocytic cells, and an induction of surfactant protein mRNA.     

Variable ventilation induces endogenous surfactant release in normal guinea pigs

Variable or noisy ventilation, which includes random breath-to-breath variations in tidal volume (Vt) and frequency, has been shown to consistently improve blood oxygenation during mechanical ventilation in various models of acute lung injury. To further understand the effects of variable ventilation on lung physiology and biology, we mechanically ventilated 11 normal guinea pigs for 3 h using constant-Vt ventilation (n = 6) or variable ventilation (n = 5). After 3 h of ventilation, each animal underwent whole lung lavage for determination of alveolar surfactant content and composition, while protein content was assayed as a possible marker of injury. Another group of animals underwent whole lung lavage in the absence of mechanical ventilation to serve as an unventilated control group (n = 5). Although lung mechanics did not vary significantly between groups, we found that variable ventilation improved oxygenation, increased surfactant levels nearly twofold, and attenuated alveolar protein content compared with animals ventilated with constant Vt. These data demonstrate that random variations in Vt promote endogenous release of biochemically intact surfactant, which improves alveolar stability, apparently reducing lung injury.     

Comparison of lung protection strategies using conventional and high-frequency oscillatory ventilation.

This study compared pathophysiological and biochemical indexes of acute lung injury in a saline-lavaged rabbit model with different ventilatory strategies: a control group consisting of moderate tidal volume (V(T)) (10-12 ml/kg) and low positive end-expiratory pressure (PEEP) (4-5 cmH(2)O); and three protective groups: 1) low V(T) (5-6 ml/kg) high PEEP, 2-3 cmH(2)O greater than the lower inflection point; 2) low V(T) (5-6 ml/kg), high PEEP (8-10 cmH(2)O); and 3) high-frequency oscillatory ventilation (HFOV). The strategy using PEEP > inflection point resulted in hypotension and barotrauma. HFOV attenuated the decrease in pulmonary compliance, the lung inflammation assessed by polymorphonuclear leukocyte infiltration and tumor necrosis factor-alpha concentration in the alveolar space, and pathological changes of the small airways and alveoli. Conventional mechanical ventilation using lung protection strategies (low V(T) high PEEP) only attenuated the decrease in oxygenation and pulmonary compliance. Therefore, HFOV may be a preferable option as a lung protection strategy.     

Effect of lung-protective ventilation on severe Pseudomonas aeruginosa pneumonia and sepsis in rats

Pneumonia caused by Pseudomonas aeruginosa carries a high rate of morbidity and mortality. A lung-protective strategy using low tidal volume (V(T)) ventilation for acute lung injury improves patient outcomes. The goal of this study was to determine whether low V(T) ventilation has similar utility in severe P. aeruginosa infection. A cytotoxic P. aeruginosa strain, PA103, was instilled into the left lung of rats anesthetized with pentobarbital. The lung-protective effect of low V(T) (6 ml/kg) with or without high positive end-expiratory pressure (PEEP, 10 or 3 cmH(2)O) was then compared with high V(T) with low PEEP ventilation (V(T) 12 ml/kg, PEEP 3 cmH(2)O). Severe lung injury and septic shock was induced. Although ventilatory mode had little effect on the involved lung or septic physiology, injury to noninvolved regions was attenuated by low V(T) ventilation as indicated by the wet-to-dry weight ratio (W/D; 6.13 +/- 0.78 vs. 3.78 +/- 0.26, respectively) and confirmed by histopathological examinations. High PEEP did not yield a significant protective effect (W/D, 4.03 +/- 0.32) but, rather, caused overdistension of noninvolved lungs. Bronchoalveolar lavage revealed higher concentrations of TNF-alpha in the fluid of noninvolved lung undergoing high V(T) ventilation compared with those animals receiving low V(T). We conclude that low V(T) ventilation is protective in noninvolved regions and that the application of high PEEP attenuated the beneficial effects of low V(T) ventilation, at least short term. Furthermore, low V(T) ventilation cannot protect the involved lung, and high PEEP did not significantly alter lung injury over a short time course.     

Lung epithelial injury markers are not influenced by use of lower tidal volumes during elective surgery in patients without preexisting lung injury

Clara cell protein levels are elevated in plasma of individuals with mild or subclinical lung injury. We studied the influence of two mechanical ventilation strategies on local and systemic levels of Clara cell protein (CC16) and compared them with levels of soluble receptor for advanced glycation end products (sRAGE) and surfactant proteins (SP)-A and -D in patients undergoing elective surgery. Saved samples from a previously reported investigation were used for the study. Forty patients planned for elective surgery were randomized to mechanical ventilation with either a conventional tidal volume (V(T)) of 12 ml/kg without positive end-expiratory pressure (PEEP) or low V(T) of 6 ml/kg and 10 cmH(2)O PEEP. Plasma and bronchoalveolar lavage fluid (BALF) was collected directly after intubation and after 5 h of mechanical ventilation. While systemic levels of SP-A and SP-D remained unchanged, systemic levels of CC16 and sRAGE increased significantly in both groups after 5 h (P < 0.001 for both). BALF levels of SP-A, SP-D, CC16, and sRAGE remained unaffected. No differences were found between the two mechanical ventilation strategies regarding any of the measured biological markers. In conclusion, systemic levels of CC16 and sRAGE rise after 5 h in patients receiving mechanical ventilation for elective surgery. Mechanical ventilation with lower tidal volumes and PEEP did not have a different effect on levels of biomarkers of lung epithelial injury compared with conventional mechanical ventilation.     

Changes in proteoglycans and lung tissue mechanics during excessive mechanical ventilation in rats

Excessive mechanical ventilation results in changes in lung tissue mechanics. We hypothesized that changes in tissue properties might be related to changes in the extracellular matrix component proteoglycans (PGs). The effect of different ventilation regimens on lung tissue mechanics and PGs was examined in an in vivo rat model. Animals were anesthetized, tracheostomized, and ventilated at a tidal volume of 8 (VT(8)), 20, or 30 (VT(30)) ml/kg, positive end-expiratory pressure of 0 (PEEP(0)) or 1.5 (PEEP(1.5)) cmH(2)O, and frequency of 1.5 Hz for 2 h. The constant-phase model was used to derive airway resistance, tissue elastance, and tissue damping. After physiological measurements, one lung was frozen for immunohistochemistry and the other was reserved for PG extraction and Western blotting. After 2 h of mechanical ventilation, tissue elastance and damping were significantly increased in rats ventilated at VT(30)PEEP(0) compared with control rats (ventilated at VT(8)PEEP(1.5)). Versican, basement membrane heparan sulfate PG, and biglycan were all increased in rat lungs ventilated at VT(30)PEEP(0) compared with control rats. At VT(30)PEEP(0), heparan sulfate PG and versican staining became prominent in the alveolar wall and airspace; biglycan was mostly localized in the airway wall. These data demonstrate that alterations in lung tissue mechanics with excessive mechanical ventilation are accompanied by changes in all classes of extracellular matrix PG.     

Relationship of end-expiratory pressure, lung volume, and 99mTc-DTPA clearance

We investigated the dose-response effect of positive end-expiratory pressure (PEEP) and increased lung volume on the pulmonary clearance rate of aerosolized technetium-99m-labeled diethylenetriaminepentaacetic acid (99mTc-DTPA). Clearance of lung radioactivity was expressed as percent decrease per minute. Base-line clearance was measured while anesthetized sheep (n = 20) were ventilated with 0 cmH2O end-expiratory pressure. Clearance was remeasured during ventilation at 2.5, 5, 10, 15, or 20 cmH2O PEEP. Further studies showed stepwise increases in functional residual capacity (FRC) (P less than 0.05) measured at 0, 2.5, 5, 10, 15, and 20 cmH2O PEEP. At 2.5 cmH2O PEEP, the clearance rate was not different from that at base line (P less than 0.05), although FRC was increased from base line. Clearance rate increased progressively with increasing PEEP at 5, 10, and 15 cmH2O (P less than 0.05). Between 15 and 20 cmH2O PEEP, clearance rate was again unchanged, despite an increase in FRC. The pulmonary clearance of aerosolized 99mTc-DTPA shows a sigmoidal response to increasing FRC and PEEP, having both threshold and maximal effects. This relationship is most consistent with the hypothesis that alveolar epithelial permeability is increased by lung inflation.     

大潮气量致急性肺损伤犬呼吸机相关性肺损伤模型的构建

目的建立大潮气量致急性肺损伤（ALI）犬呼吸机相关性肺损伤（VILI）模型。方法健康雄性杂种犬12只用油酸静脉注射法制备犬ALI模型,造模成功后进行支持通气15min过渡,然后随机分为VILI组及对照组行机械通气6 h,每组6只。VILI组潮气量（Vt）=20 mL/kg,对照组Vt=6 mL/kg,两组呼气末正压（PEEP）均为10 cmH2O。动态观察各组血气交换指标变化。通气6 h后取支气管肺泡灌洗液（BALF）作白蛋白浓度检查,取肺组织作病理切片肺损伤评分。结果各组在油酸静脉注射后（2.50±0.80）h达到ALI标准。VILI组在犬机械通气6 h后PaO2、SaO2及氧合指数（OI）较对照组略下降（P〈0.05）,而PaCO2波动不大,且心率、血压波动也较对照组小（P〈0.05）。VILI组BALF中蛋白浓度和肺组织损伤评分均较对照组显著升高（分别P〈0.05,P〈0.01）。结论本实验成功建立了大潮气量致ALI犬VILI模型。     

Influence of lung volume and alveolar pressure on reverse pulmonary venous blood flow.

We have reported that left atrial blood refluxes through the pulmonary veins to gas-exchanging tissue after pulmonary artery ligation. This reverse pulmonary venous flow (Qrpv) was observed only when lung volume was changed by ventilation. This was believed to drive Qrpv by alternately distending and compressing the alveolar and extra-alveolar vessels. Because lung and pulmonary vascular compliances change with lung volume, we studied the effect of positive end-expiratory pressure (PEEP) on the magnitude of Qrpv during constant-volume ventilation. In prone anesthetized goats (n = 8), using the right lung to maintain normal blood gases, we ligated the pulmonary and bronchial arterial inflow to the left lung and ventilated each lung separately. A solution of SF6, an inert gas, was infused into the left atrium. SF6 clearance from the left lung was determined by the Fick principle at 0, 5, 10, and 15 and again at 0 cmH2O PEEP and was used to measure Qrpv. Left atrial pressure remained nearly constant at 20 cmH2O because the increasing levels of PEEP were applied to the left lung only. Qrpv was three- to fourfold greater at 10 and 15 than at 0 cmH2O PEEP. At these higher levels of PEEP, there were greater excursions in alveolar pressure for the same ventilatory volume. We believe that larger excursions in transpulmonary pressure during tidal ventilation at higher levels of PEEP, which compressed alveolar vessels, resulted in the reflux of greater volumes of left atrial blood, through relatively noncompliant extra-alveolar veins into alveolar corner vessels, and more compliant extra-alveolar arteries.     

PEEP is necessary for exogenous surfactant to reduce pulmonary edema in canine aspiration pneumonitis.

Alveolar edema inactivates surfactant, and surfactant depletion causes edema by reducing lung interstitial pressure (Pis). We reasoned that surfactant repletion might reduce edema by raising Pis after acute lung injury and that positive end-expiratory pressure (PEEP) might facilitate this effect. One hour after tracheal administration of hydrochloric acid in 18 anesthetized dogs with transmural pulmonary capillary wedge pressure of 8 Torr, the animals were randomized into three groups: in the SURF + PEEP group, 50 mg/kg of calf lung surfactant extract (CLSE) was instilled into each main stem bronchus with 8 cmH2O of PEEP; in the SAL + PEEP group, PEEP was followed by an equal volume of saline (SAL); in the SURF group, CLSE was given without PEEP. After 5 h, edema in excised lungs (wet-to-dry weight ratios) was significantly less in the SURF + PEEP group (9.1 +/- 1.0) than in the other groups (11.3 +/- 1.8 and 11.3 +/- 1.8, respectively). In the SURF + PEEP group, pulmonary venous admixture fell by 6%; this change was different from the 7% increase in the SAL + PEEP group and 40% increase in the SURF group (P less than 0.05). Airway secretions obtained in the SURF + PEEP group had normal minimum surface tensions of 4 +/- 2 mN/m, a value much lower than in SAL + PEEP and SURF groups (32 +/- 4 and 22 +/- 7 mN/m, respectively). We conclude that surfactant normalizes surface tension and decreases transcapillary hydrostatic forces in this lung injury model, thereby reducing edema formation and improving gas exchange. These benefits occur only if surfactant is given with PEEP, allowing surfactant access to the alveoli and/or minimizing its inhibition by edema proteins.