Regional Lung Mechanics and Influence of an Active Diaphragm in Experimental Lung Injury

Sammanfattning: Despite being an essential life-support strategy in severe respiratory failure, mechanical ventilation can, if not optimally set and monitored, lead to injury of the lung parenchyma and diaphragm. These conditions are called ventilator-induced lung injury and ventilator-induced diaphragmatic dysfunction (VIDD), respectively. Although substantial progress has been made in the ventilator management of severely lung-injured patients, we are still far from a fully protective mechanical ventilation. In consideration of this gap of knowledge, this doctoral thesis aimed at investigating regional lung mechanics during both inspiration and expiration, in both controlled and assisted ventilation. Particular emphasis was placed on the expiratory phase, which is involved in expiratory flow limitation, airway closure and atelectasis formation, although commonly considered non-harmful.A novel methodological approach has been the fundamental basis for this research project. The combination of respiratory mechanics, diaphragmatic electromyographic activity and lung imaging enabled a breath-by-breath analysis at high temporal and spatial resolution.In Study I, the gravitational field affected the distribution of gas and transpulmonary pressures, as previously shown. This effect differed between healthy and injured lungs. Moreover, lung injury induced a heterogeneous distribution of gas within the lungs, as well as an increased gravitational gradient in transpulmonary pressure. Study I was mainly aimed at testing the new methodological approach centred on the investigation of regional lung mechanics.In Study II, the focus was on assisted ventilation and the phenomenon of gas redistribution within the lungs. Large pendelluft events had been demonstrated in disproportionate inspiratory efforts. In Study II, we showed that large pendelluft resulting from pathological respiratory drive could be attenuated by high positive end expiratory pressure (PEEP). Moreover, we showed that transient and widespread small gas redistribution events occur at all times during inspiration. Assisted ventilation and high PEEP reduced the size of gas redistribution as compared with controlled ventilation and low PEEP.In Study III, we demonstrated a diaphragmatic expiratory contraction in lungs prone to collapse, serving to brake the expiratory flow. It preserved end expiratory lung volume (EELV) and counteracted tidal atelectasis. However, the expiratory brake induced by diaphragmatic contraction is a known cause of VIDD.In Study IV, we tested the effects of external expiratory resistances (ExpR). We showed that, by applying ExpR, an expiratory brake was induced. The beneficial effects on EELV were retained, while the diaphragm could quickly relax during the expiration, thus reducing the risk of VIDD.