Mechanical ventilation and volutrauma : study in vivo of a healthy pig model

Mechanical ventilation is essential in intensive care units. However, it may itself induce lung injury. Current studies are based on rodents, using exceptionally large tidal volumes for very short periods, often after a "priming" pulmonary insult. Our study deepens a clinically relevant large animal model, closely resembling human physiology and the ventilator setting used in clinic settings. Our aim was to evaluate the pathophysiological mechanisms involved in alveolo/capillary barrier damage due to mechanical stress in healthy subjects. We randomly divided 18 pigs (sedated with medetomidine/tiletamine-zolazepam and anesthetised with thiopental sodium) into three groups (n=6): two were mechanically ventilated (tidal volume of 8 or 20 ml/kg), the third breathed spontaneously for 4 hours, then animals were sacrificed (thiopental overdose). We analyzed every 30' hemogasanalysis and the main circulatory and respiratory parameters. Matrix gelatinase expression was evaluated on bronchoalveolar lavage fluid after surgery and before euthanasia. On autoptic samples we performed zymographic analysis of lung, kidney and liver tissues and histological examination of lung. Results evidenced that high Vt evoked profound alterations of lung mechanics and structure, although low Vt strategy was not devoid of side effects, too. Unexpectedly, also animals that were spontaneously breathing showed a worsening of the respiratory functions.