Lucio A. Pavone

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

IntroductionOne 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.MethodsIn 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.ResultsHigh 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.ConclusionA 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.

Alveolar instability caused by mechanical ventilation initially damages the nondependent normal lung

BackgroundSeptic shock is often associated with acute respiratory distress syndrome, a serious clinical problem exacerbated by improper mechanical ventilation. Ventilator-induced lung injury (VILI) can exacerbate the lung injury caused by acute respiratory distress syndrome, significantly increasing the morbidity and mortality. In this study, we asked the following questions: what is the effect of the lung position (dependent lung versus nondependent lung) on the rate at which VILI occurs in the normal lung? Will positive end-expiratory pressure (PEEP) slow the progression of lung injury in either the dependent lung or the nondependent lung?Materials and methodsSprague–Dawley rats (n = 19) were placed on mechanical ventilation, and the subpleural alveolar mechanics were measured with an in vivo microscope. Animals were placed in the lateral decubitus position, left lung up to measure nondependent alveolar mechanics and left lung down to film dependent alveolar mechanics. Animals were ventilated with a high peak inspiratory pressure of 45 cmH2O and either a low PEEP of 3 cmH2O or a high PEEP of 10 cmH2O for 90 minutes. Animals were separated into four groups based on the lung position and the amount of PEEP: Group I, dependent + low PEEP (n = 5); Group II, nondependent + low PEEP (n = 4);Group III, dependent + high PEEP (n = 5); and Group IV, nondependent + high PEEP (n = 5). Hemodynamic and lung function parameters were recorded concomitant with the filming of alveolar mechanics. Histological assessment was performed at necropsy to determine the presence of lung edema.ResultsVILI occurred earliest (60 min) in Group II. Alveolar instability eventually developed in Groups I and II at 75 minutes. Alveoli in both the high PEEP groups were stable for the entire experiment. There were no significant differences in arterial PO2 or in the degree of edema measured histologically among experimental groups.ConclusionThis open-chest animal model demonstrates that the position of the normal lung (dependent or nondependent) plays a role on the rate of VILI.

Absence of Alveolar Tears in Rat Lungs with Significant Alveolar Instability

Background: Lung injury associated with the acute respiratory distress syndrome can be exacerbated by improper mechanical ventilation creating a secondary injury known as ventilator-induced lung injury (VILI). We hypothesized that VILI could be caused in part by alveolar recruitment/derecruitment resulting in gross tearing of the alveolus. Objectives: The exact mechanism of VILI has yet to be elucidated though multiple hypotheses have been proposed. In this study we tested the hypothesis that gross alveolar tearing plays a key role in the pathogenesis of VILI. Methods: Anesthetized rats were ventilated and instrumented for hemodynamic and blood gas measurements. Following baseline readings, rats were exposed to 90 min of either normal ventilation (control group: respiratory rate 35 min–1, positive end-expiratory pressure 3 cm H2O, peak inflation pressure 14 cm H2O) or injurious ventilation (VILI group: respiratory rate 20 min–1, positive end-expiratory pressure 0 cm H2O, peak inflation pressure 45 cm H2O). Parameters studied included hemodynamics, pulmonary variables, in vivovideo microscopy of alveolar mechanics (i.e. dynamic alveolar recruitment/derecruitment) and scanning electron microscopy to detect gross tears on the alveolar surface. Results: Injurious ventilation significantly increased alveolar instability after 45 min and alveoli remained unstable until the end of the study (electron microscopy after 90 min revealed that injurious ventilation did not cause gross tears in the alveolar surface). Conclusions: We demonstrated that alveolar instability induced by injurous ventilation does not cause gross alveolar tears, suggesting that the tissue injury in this animal VILI model is due to a mechanism other than gross rupture of the alveolus.

Chemically Modified Tetracycline Improves Contractility in Porcine Coronary Ischemia/Reperfusion Injury

Abstract  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/m2; p < 0.05) and stroke volume index (22 ± 2 vs. 17 ± 1 L/m2/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.