Effect of positive end-expiratory pressure and tidal volume on lung injury induced by alveolar instability.

INTRODUCTION: One potential mechanism of ventilator-induced lung injury (VILI) is due to shear stresses associated with alveolar instability (recruitment/derecruitment). It has been postulated that the optimal combination of tidal volume (Vt) and positive end-expiratory pressure (PEEP) stabilizes alveoli, thus diminishing recruitment/derecruitment and reducing VILI. In this study we directly visualized the effect of Vt and PEEP on alveolar mechanics and correlated alveolar stability with lung injury. METHODS: In vivo microscopy was utilized in a surfactant deactivation porcine ARDS model to observe the effects of Vt and PEEP on alveolar mechanics. In phase I (n = 3), nine combinations of Vt and PEEP were evaluated to determine which combination resulted in the most and least alveolar instability. In phase II (n = 6), data from phase I were utilized to separate animals into two groups based on the combination of Vt and PEEP that caused the most alveolar stability (high Vt [15 cc/kg] plus low PEEP [5 cmH2O]) and least alveolar stability (low Vt [6 cc/kg] and plus PEEP [20 cmH2O]). The animals were ventilated for three hours following lung injury, with in vivo alveolar stability measured and VILI assessed by lung function, blood gases, morphometrically, and by changes in inflammatory mediators. RESULTS: High Vt/low PEEP resulted in the most alveolar instability and lung injury, as indicated by lung function and morphometric analysis of lung tissue. Low Vt/high PEEP stabilized alveoli, improved oxygenation, and reduced lung injury. There were no significant differences between groups in plasma or bronchoalveolar lavage cytokines or proteases. CONCLUSION: A ventilatory strategy employing high Vt and low PEEP causes alveolar instability, and to our knowledge this is the first study to confirm this finding by direct visualization. These studies demonstrate that low Vt and high PEEP work synergistically to stabilize alveoli, although increased PEEP is more effective at stabilizing alveoli than reduced Vt. In this animal model of ARDS, alveolar instability results in lung injury (VILI) with minimal changes in plasma and bronchoalveolar lavage cytokines and proteases. This suggests that the mechanism of lung injury in the high Vt/low PEEP group was mechanical, not inflammatory in nature.

Chemically modified tetracycline (COL-3) improves survival if given 12 but not 24 hours after cecal ligation and puncture.

Sepsis can result in excessive and maladaptive inflammation that is responsible for more than 215,00 deaths per year in the United State alone. Current strategies for reducing the morbidity and mortality associated with sepsis rely on treatment of the syndrome rather than prophylaxis. We have been investigating a modified tetracycline, COL-3, which can be given prophylactically to patients at high risk for developing sepsis. Our group has shown that COL-3 is very effect at preventing the sequelae of sepsis if given before or immediately after injury in both rat and porcine sepsis models. In this study, we wanted to determine the "treatment window" for COL-3 after injury at which it remains protective. Sepsis was induced by cecal ligation and puncture (CLP). Rats were anesthetized and placed into five groups: CLP (n = 20) = CLP without COL-3, sham (n = 5) = surgery without CLP or COL-3, COL3@6h (n = 10) = COL-3 given by gavage 6 h after CLP, COL3@12h (n = 10) = COL-3 given by gavage 12 h after CLP, and COL3@24h (n = 20) = COL-3 given by gavage 24 h after CLP. COL-3 that was given at 6 and 12 h after CLP significantly improved survival as compared with the CLP and the CLP@24h groups. Improved survival was associated with a significant improvement in lung pathology assessed morphologically. These data suggest that COL-3 can be given up to 12 h after trauma and remain effective.

Chemically modified tetracycline improves contractility in porcine coronary ischemia/reperfusion injury.

BACKGROUND: Reperfusion of ischemic myocardium has been implicated in extension of infarct size and deleterious clinical outcomes. Anti-inflammatory agents reduce this reperfusion injury. Chemically modified tetracycline-3 (CMT-3) (Collagenex Pharmaceuticals, Newtown, PA, USA) lacks antimicrobial properties yet retains anti-inflammatory activity. We examined infarct size and myocardial function in a porcine coronary artery occlusion/reperfusion model in CMT-3-treated and control animals. METHODS: Yorkshire pigs (n = 8) underwent median sternotomy, pretreatment with heparin (300 U/kg and 67 U/kg/hr IV) and lidocaine (1 mg/kg IV) and were divided into two groups. Group one (n = 4) had the left anterior descending artery (LAD) occluded for 1 hour, after which it was reperfused for 2 hours. Group two (n = 4) had an identical protocol to group one except CMT-3 (2 mg/kg IV) was administered prior to occlusion of the LAD. RESULTS: Animals receiving CMT-3 had significantly decreased infarct size in relation to the ventricular area-at-risk (AAR) (28 +/- 9% vs. 64 +/- 8%; p < 0.05). Myocardial contractile function was superior in the CMT-3 treatment, indicated by a higher cardiac index (2.9 +/- 0.3 vs. 2.0 +/- 0.3 L/min/m(2); p < 0.05) and stroke volume index (22 +/- 2 vs. 17 +/- 1 L/m(2)/beat; p < 0.05). CONCLUSIONS: CMT-3 decreased infarct size in relation to the AAR resulting in relative preservation of contractility, suggesting CMT-3 may improve outcomes during myocardial ischemia reperfusion.

Wood smoke inhalation causes alveolar instability in a dose-dependent fashion.

BACKGROUND: Wood smoke inhalation causes severe ventilation and oxygenation abnormalities. We hypothesized that smoke inhalation would cause lung injury by 2 mechanisms: (1) direct tissue injury by the toxic chemicals in the smoke and (2) a mechanical shear-stress injury caused by alveolar instability (ie, alveolar recruitment/derecruitment). We further postulated that alveolar instability would increase with the size of the cumulative smoke dose. METHODS: Anesthetized pigs were ventilated and instrumented for hemodynamic and blood-gas measurements. After baseline readings, the pigs were exposed to 5 separate doses of wood smoke, each dose lasting 1 min. Factors studied included hemodynamics, pulmonary variables, and in vivo photomicroscopy of alveolar mechanics (ie, the dynamic change in alveolar size with ventilation). RESULTS: Smoke inhalation significantly increased alveolar instability with 4 min and 5 min of smoke exposure. Significant rises in carboxyhemoglobin levels and in pulmonary shunt were also observed at 4 min and 5 min of smoke exposure. Lung histology demonstrated severe damage characteristic of acute lung injury. CONCLUSIONS: We demonstrated that wood smoke inhalation causes alveolar instability and that instability increases with each dose of smoke. These data suggest that smoke inhalation may cause a "2-hit" insult: the "first hit" being a direct toxic injury and the "second hit" being a shear-stress injury secondary to alveolar instability.

Alveolar instability causes early ventilator-induced lung injury independent of neutrophils.

Intratracheal instillation of Tween causes a heterogeneous surfactant deactivation in the lung, with areas of unstable alveoli directly adjacent to normal stable alveoli. We employed in vivo video microscopy to directly assess alveolar stability in normal and surfactant-deactivated lung and tested our hypothesis that alveolar instability causes a mechanical injury, initiating an inflammatory response that results in a secondary neutrophil-mediated proteolytic injury. Pigs were mechanically ventilated (VT 10 cc/kg, positive end-expiratory pressure [PEEP] 3 cm H2O), randomized to into three groups, and followed for 4 hours: Control group (n = 3) surgery only; Tween group (n = 4) subjected to intratracheal Tween (surfactant deactivator causing alveolar instability); and Tween + PEEP group (n = 4) subjected to Tween with increased PEEP (15 cm H2O) to stabilize alveoli. The magnitude of alveolar instability was quantified by computer image analysis. Surfactant-deactivated lungs developed significant histopathology only in lung areas with unstable alveoli without an increase in neutrophil-derived proteases. PEEP stabilized alveoli and significantly reduced histologic evidence of lung injury. Thus, in this model, alveolar instability can independently cause ventilator-induced lung injury. To our knowledge, this is the first study to directly confirm that unstable alveoli are subjected to ventilator-induced lung injury whereas stable alveoli are not.

Positive end-expiratory pressure after a recruitment maneuver prevents both alveolar collapse and recruitment/derecruitment.

We tested the hypothesis that collapsed alveoli opened by a recruitment maneuver would be unstable or recollapse without adequate positive end-expiratory pressure (PEEP) after recruitment. Surfactant deactivation was induced in pigs by Tween instillation. An in vivo microscope was placed on a lung area with significant atelectasis and the following parameters measured: (1) the number of alveoli per field and (2) alveolar stability (i.e., the change in alveolar size from peak inspiration to end expiration). We previously demonstrated that unstable alveoli cause lung injury. A recruitment maneuver (peak pressure = 45 cm H2O, PEEP = 35 cm H2O for 1 minute) was applied and alveolar number and stability were measured. Pigs were then separated into two groups with standard ventilation plus (1) 5 PEEP or (2) 10 PEEP and alveolar number and stability were again measured. The recruitment maneuver opened a significant number of alveoli, which were stable during the recruitment maneuver. Although both 5 PEEP and 10 PEEP after recruitment demonstrated improved oxygenation, alveoli ventilated with 10 PEEP were stable, whereas alveoli ventilated with 5 PEEP showed significant instability. This suggests recruitment followed by inadequate PEEP permits unstable alveoli and may result in ventilator-induced lung injury despite improved oxygenation.

Alveolar mechanics alter hypoxic pulmonary vasoconstriction.

OBJECTIVES: Hypoxic pulmonary vasoconstriction is the primary physiologic mechanism that maintains a proper ventilation/perfusion match, but it fails in diffuse lung injuries such as acute respiratory distress syndrome. Acute respiratory distress syndrome is associated with pulmonary surfactant loss that alters alveolar mechanics (i.e., dynamic change in alveolar size and shape during ventilation), converting normal stable alveoli into unstable alveoli. We hypothesized that alveolar instability stents open pulmonary microvessels and is the mechanism of hypoxic pulmonary vasoconstriction failure associated with acute respiratory distress syndrome. DESIGN: Prospective, randomized, controlled study. SETTING: University research laboratory. SUBJECTS: Ten adult pigs. INTERVENTIONS: Anesthetized ventilated pigs were prepared surgically for hemodynamic monitoring and were subjected to a right thoracotomy. An in vivo microscope was attached to the right lung, and the microvascular response to hypoxia (F(IO(2)), 15%) was measured in a lung with normal stable alveoli and in a lung with unstable alveoli caused by surfactant deactivation (Tween lavage). MEASUREMENTS AND MAIN RESULTS: Alveolar instability, defined as the difference between alveolar area at peak inspiration and end expiration and assessed as a percentage change (I-E Delta%), was significantly increased after Tween (23.9 +/- 3.0, I-E Delta%) compared with baseline (2.4 +/- 1.0, I-E Delta%). Alveolar instability was associated with the following microvascular changes: a) increased vasoconstriction (Tween, 14.9 +/- 1.0%) in response to hypoxia compared with baseline (10.8 +/- 1.2%, p <.05); and b) increased mean vascular diameter (Tween, 41.2 +/- 1.5 microm) compared with the mean diameter at baseline (24.6 +/- 1.0 microm, p <.05). CONCLUSION: Unstable alveoli stent open pulmonary vessels, which may explain the failure of hypoxic pulmonary vasoconstriction in acute respiratory distress syndrome.