Resp Care
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Traditional ventilator management of acute respiratory distress syndrome (ARDS), emphasizing normalization of blood gases, promoted high rates of conventional barotrauma. Research revealed a broader range of ventilator-induced lung injury, physiologically and histopathologically indistinguishable from ARDS itself. It is now known that overdistention and cyclic inflation of injured lung can exacerbate lung injury and probably promote systemic inflammation, effects minimized by low tidal volumes/plateau pressures and by application of positive end-expiratory pressure. ⋯ There may thus be disparate goals in ARDS management: rapid institution of a restrictive ventilatory strategy, and avoidance of significant acidosis. We review data pertaining to ARDS physiology, ventilator-induced lung injury, lung-protective ventilatory strategies, and the physiology of respiratory acidosis. Tracheal gas insufflation is considered as a means to reconcile the clinical goals of ventilatory reduction and control of acidosis.
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Tracheal gas insufflation (TGI) is the continuous or phasic insufflation of fresh gas into the central airways for the purpose of improving the efficiency of alveolar ventilation and/or minimizing the ventilatory pressure requirements. Fresh gas is insufflated near the main carina, usually at flow rates of 2-15 L/min. During expiration, TGI clears the anatomic and apparatus dead space proximal to the catheter tip, thus improving carbon dioxide (CO2) clearance. ⋯ At that point, increasing catheter flow rate decreases PaCO2 much less, probably because of turbulent mixing in the airways distal to the catheter tip. In clinical practice, TGI can be applied either to decrease PaCO2 while maintaining tidal volume constant or to decrease tidal volume while keeping PaCO2 constant. In the former strategy, TGI is used to protect pH, whereas in the latter it is used to minimize the stretch forces acting on the lung parenchyma, to minimize ventilator-associated lung injury.