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Abstract
lIYlPERINFLATION AND DYNAMIC AUTO POSIT MEND EXPIRATORY PRESSURE
the approach to mechanical ventilation of critically in patients may vary according to the pathopbysiologic events underlying the development of acute respiratory failure. In that regard, it is useful to classify acute respiratory failure into two major categories: type I, or hypoxemic respiratory failure, and type 2, or hypercapnic ventdlatory failure 2) The former category, exemplified by tile acute respiratory distress syndrome, is characterized by severe hypoxemia, generally caused by alveolar or interstitial pulmonary edema or alveolar collapse. In contrast, hypercapnic vendlatory failure corresponds to acute ventilatory failure (AVF) and is characterized by the inability of a failing respiratory pump to provide a level of alveolar ventilation sufficient to meet the required metabolic demands 423~ Altbough this may be caused by central depression of respiratory drive, neuromuscular disorders, or chest wall abnorrnalities, perhaps the most common cause in the ICU setting is an exacerbation of severe underlying chronic obstructive pulmonary disease (COPD). Mechanical ventilation therefore provides an appropriate level of alveolar ventilation while allowing for improved pulmonary function and recovery from respiratory muscle fatigue. In this setting, positive end~expiratory pressure (PEEP) has been considered unhelpful and contraindicated, for the following reasons: (1) the level of hypoxemia in patients with COPD generally is mild and responds readily to low level of supplemental oxygen (02), (2) Severe COPD is characterized by augmentation of lung volume, a further increase in lung volume eventually induced by application of PEEP would impair respiratory muscle efficiency and enhance risk of barotraurna and hemodynamic depression.