Is time the missing component in protective ventilation strategies?

Purinergic signalling links mechanical breath profile and alveolar mechanics with the pro-inflammatory innate immune response causing ventilation-induced lung injury

Severe pulmonary infection or vigorous cyclic deformation of the alveolar epithelial type I (AT I) cells by mechanical ventilation leads to massive extracellular ATP release. High levels of extracellular ATP saturate the ATP hydrolysis enzymes CD39 and CD73 resulting in persistent high ATP levels despite the conversion to adenosine. Above a certain level, extracellular ATP molecules act as danger-associated molecular patterns (DAMPs) and activate the pro-inflammatory response of the innate immunity through purinergic receptors on the surface of the immune cells. This results in lung tissue inflammation, capillary leakage, interstitial and alveolar oedema and lung injury reducing the production of surfactant by the damaged AT II cells and deactivating the surfactant function by the concomitant extravasated serum proteins through capillary leakage followed by a substantial increase in alveolar surface tension and alveolar collapse. The resulting inhomogeneous ventilation of the lungs is an important mechanism in the development of ventilation-induced lung injury. The high levels of extracellular ATP and the upregulation of ecto-enzymes and soluble enzymes that hydrolyse ATP to adenosine (CD39 and CD73) increase the extracellular adenosine levels that inhibit the innate and adaptive immune responses rendering the host susceptible to infection by invading microorganisms. Moreover, high levels of extracellular adenosine increase the expression, the production and the activation of pro-fibrotic proteins (such as TGF-β, α-SMA, etc.) followed by the establishment of lung fibrosis.

Multicenter randomized controlled trial of inhaled nitric oxide for pediatric acute respiratory distress syndrome

OBJECTIVES

To test the hypothesis that inhaled nitric oxide (iNO) would lead to improved oxygenation and a decrease in duration of mechanical ventilation in pediatric patients with acute respiratory distress syndrome.

STUDY DESIGN

A total of 55 children with acute respiratory distress syndrome were enrolled from 9 centers. Patients were randomized to iNO or placebo and remained on the study drug until death, they were free of ventilator support, or day 28 after the initiation of therapy.

RESULTS

Mean baseline oxygenation indexes (OIs) were 22.0 ± 18.4 and 25.6 ± 14.9 (iNO and placebo groups, respectively, P = .27). There was a trend toward an improved OI in the iNO group compared with the placebo group at 4 hours that became significant at 12 hours. There was no difference in the OI between groups at 24 hours. Days alive and ventilator free at 28 days was greater in the iNO group, 14.2 ± 8.1 and 9.1 ± 9.5 days (iNO and placebo groups, respectively, P = .05). Although overall survival at 28 days failed to reach statistical significance, 92% (22 of 24) in the iNO group and 72% (21 of 29) in the placebo group (P = .07), the rate of extracorporeal membrane oxygenation-free survival was significantly greater in those randomized to iNO 92% (22 of 24) vs 52% (15 of 29) for those receiving placebo (P < .01).

CONCLUSION

The use of iNO was associated with a significantly reduced duration of mechanical ventilation and significantly greater rate of extracorporeal membrane oxygenation-free survival.

Effect of PEEP on phosgene-induced lung edema: pilot study on dogs using protective ventilation strategies

Various therapeutic regimes have been proposed for treatment of phosgene-induced acute lung injury (P-ALI). Most of these treatments rely on late-stage supportive measures to maintain the oxygenation of the lung. This exploratory proof-of-concept study on Beagle dogs focused on protective positive end-expiratory pressure (PEEP) ventilation, initiated early at the yet asymptomatic stage after phosgene exposure. Conscious, spontaneously breathing dogs were head-only exposed to a potentially lethal inhalation dose of phosgene (870 ppm × min). Shortly after exposure, the dogs were anesthetized, intubated and then subjected to mechanical ventilation (PEEP; tidal volume (VT)=10-12 mL/kg body weight, 40 breaths/min) at 0, 4, or 12 cm H2O over a post-exposure period of 8h (one dog per setting). For reference, one additional dog received the same dose of phosgene without anesthesia and mechanical ventilation. Time-course changes of hematocrit, leukocytes, and thrombocytes were determined in peripheral blood. At necropsy, changes lung weights, bronchoalveolar lavage, and histology were used to assess the efficacy of treatment. The most salient outcome in the non-ventilated dog was a time-related hemoconcentration and leukocytosis and autopsy findings suggestive of pulmonary congestion and edema. The pulmonary epithelium of the major airways was generally intact; however, in their lumen inflammatory cells, cellular debris and mucus were present. Relative to the dog receiving no intervention, the lung edema was markedly alleviated by PEEP at both 4 and 12 cm H2O but not at 0 cm H2O PEEP. In summary, the time-dependent progression into a life-threatening pulmonary edema can effectively be suppressed by protective, low-pressure PEEP when implemented early enough after exposure to phosgene. However, due to the exploratory nature of this study, the findings may suggest an association between PEEP and protection from pulmonary edema. However, definite conclusions and recommendations cannot be made yet based upon the small sample size and the limited variables examined.