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VENTILATORY STRATEGIES in ARDS Low-Tidal-Volume Ventilation The cornerstone of ARDS management is a strategy of low-tidal-volume ventilation pioneered by the ARDSnet trial in 2000. This involves the following:  Tidal volume 6 ml/kg based on ideal body weight  Plateau pressure , 30 cm H2O  Permissive hypercapnoea tolerated as long as pH . 7.2 The lower tidal volumes were strongly associated with a decrease in moality and an increase in ventilator-free days. This was thought to be as a result of reducing excessive distension or stretch of the aerated lung, which could be associated with the release of proinflammatory cytokines that could contribute to lung (volutrauma) and other organ injury. Positive End-Expiratory Pressure PEEP prevents alveolar derecruitment, minimising atelectrauma. It increases functional residual capacity, displaces fluid from the alveoli into the interstitium, and moves the lung to the steeper pa of the compliance curve. However, this has to be balanced against potential hyperinflation and decreased venous return, and thus cardiac output.PEEP is patient-specific and determining the ''optimal'' PEEP for each patient is challenging. A number of different strategies (none of which have proven superiority) have been suggested to set PEEP:  Use of PEEP/FiO2 tables  Titrating based on plateau pressure  Stepwise increase until maximal compliance  Recruitment manoeuvre followed by decremental titration  Based upon the inflection point of the pressure/volume curve Guidelines recommend use of high PEEP (defined variably, but on average 15 cm H2O) on patients with moderate and severe ARDS, in whom a moality benefit was demonstrated in clinical trials. Prone-Position VentilationAlthough this has long been known to improve oxygenation, evidence of moality benefit has been lacking until recently. There are a number of postulated physiological benefits: improved recruitment of atelectatic dorsal lung tissue leading to its increased compliance of lungs (total compliance may actually decrease), more even distribution of tidal volume, and improved V/Q (V 1/4 pulmonary ventilation; Q 1/4 pulmonary perfusion) matching. Prone ventilation also decreases alveolar hyperinflation, reduces cardiac compression of the underlying lung, and improves drainage of secretions. Suggested Combinations of PEEP and FiO2 for ARDS/ALI GOAL Pa02 Fi02 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7 0.7 0.8 0.9 0.9 0.9 1.0 1.0 1.0 1.0 PEEP (cm H2O) 5-14 5-14 8-16 8-16 10-20 10-20 10-20 12-20 14-20 14-22 14-22 14-22 16-22 18-22 20-22 22 24 Goals of mechanical ventilation in ARDS/ALI Pa02: 55-80 mm Hg Plateau Pressure : <30 cm H20 Tidal volume: 6ml/kg predicted body weight Ph: > 7.15 Titrating the ventilator in ARDS/ALI Sta with AC with Tidal volume of 8 mL/kg of predicted body weight and decrease by 1ml/kg over tour four until tidal volume is 6 ml/kg goal is reached If plateau pressure is >30 cm H20, decrease tidal volume by 1 ml/kg until tidal volume is 4mL/kg or aerial ph reaches 7.15 Initiate PEEP at 5 cm H20 and titrate up in increments of 2cm H20 according to table above Options PEEP setting are between 8-15 cm H20