The approach to mechanical ventilation of critically ill patients may vary according to the pathophysiologic events underlying the development of acute respiratory failure. In that regard, it is useful to classify acute respiratory failure into two major categories—type 1, or hypoxemic respiratory failure, and type 2, or hypercapnic ventilatory failure.<sup>22 Kimball W.R., Leith D.E., Robins A.G. Dynamic hyperinflation and ventilatory dependence in chronic obstructive pulmonary disease Am Rev Respir Dis 1982 ;  126 : 991-995

Haga clic aquí para ir a la sección de Referencias, 44 Roussos C. The failing ventilatory pump Lung 1982 ;  160 : 59-84  [cross-ref]

Haga clic aquí para ir a la sección de Referencias</sup> The former category, exemplified by the acute respiratory distress syndrome, is characterized by severe hypoxemia, generally caused by alveolar or interstitial pulmonary edema or alveolar collapse. Its physiologic and clinical aspects are discussed in the article by Marini.

In contrast, hypercapnic ventilatory 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.<sup>24 Macklem P.T. Hyperinflation Am Rev Respir Dis 1984 ;  129 : 1-2

Haga clic aquí para ir a la sección de Referencias, 44 Roussos C. The failing ventilatory pump Lung 1982 ;  160 : 59-84  [cross-ref]

Haga clic aquí para ir a la sección de Referencias</sup> Although this may be caused by central depression of respiratory drive, neuromuscular disorders, or chest wall abnormalities, 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 levels of supplemental oxygen (O₂). (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 barotrauma and hemodynamic depression.<sup>2 Ashbaugh D.G., Petty T.L. Positive end-expiratory pressure. Clinical indications and contraindications J Thorac Cardiovasc Surg 1973 ;  65 : 165-170

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Deviation of end-expiratory lung volume (EELV) from the elastic equilibrium volume or relaxation volume (Vr) of the respiratory system is recognized as a cardinal feature in mechanically ventilated patients with severe COPD and AVF.<sup>22 Kimball W.R., Leith D.E., Robins A.G. Dynamic hyperinflation and ventilatory dependence in chronic obstructive pulmonary disease Am Rev Respir Dis 1982 ;  126 : 991-995

Haga clic aquí para ir a la sección de Referencias</sup> The presence of dynamic hyperinflation implies that alveolar pressure remains positive throughout expiration. At the end of expiration, this positive pressure is termed auto<sup>33 Pepe P.E., Marini J.J. Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction. The auto-PEEP effect Am Rev Respir Dis 1982 ;  126 : 166-170

Haga clic aquí para ir a la sección de Referencias</sup> or intrinsic PEEP.<sup>43 Rossi A., Gottfried S.B., Higgs B.D. , y al. Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation Am Rev Respir Dis 1985 ;  131 : 672-677

Haga clic aquí para ir a la sección de Referencias</sup> Auto-PEEP and dynamic hyperinflation have been described in mechanically ventilated COPD patients in whom expiratory flow limitation occurred as a consequence of dynamic airway compression.<sup>5 Broseghini C., Brandolese R., Poggi R. , y al. Respiratory mechanics during the first day of mechanical ventilation in patients with pulmonary edema and chronic airway obstruction Am Rev Respir Dis 1988 ;  138 : 355-361

Haga clic aquí para ir a la sección de Referencias, 11 Fleury B., Murciano D., Talamo C. , y al. Work of breathing in patients with obstructive pulmonary disease in acute respiratory failure Am Rev Respir Dis 1985 ;  131 : 822-827

Haga clic aquí para ir a la sección de Referencias, 12 Gay C.G., Rodarte J.R., Hubmayer R.D. The effects of positive expiratory pressures on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation Am Rev Respir Dis 1989 ;  139 : 621-626

Haga clic aquí para ir a la sección de Referencias, 15 Gottfried S.B., Rossi A., Higgs B.D. , y al. Noninvasive determination of respiratory mechanics during mechanical ventilation for acute respiratory failure Am Rev Respir Dis 1985 ;  131 : 414-420

Haga clic aquí para ir a la sección de Referencias, 33 Pepe P.E., Marini J.J. Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction. The auto-PEEP effect Am Rev Respir Dis 1982 ;  126 : 166-170

Haga clic aquí para ir a la sección de Referencias, 43 Rossi A., Gottfried S.B., Higgs B.D. , y al. Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation Am Rev Respir Dis 1985 ;  131 : 672-677

Haga clic aquí para ir a la sección de Referencias</sup> Recent work has suggested that, in COPD patients with expiratory flow limitation, the application of external PEEP during assisted mechanical ventilation<sup>46 Smith T.C., Marini J.J. Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction J Appl Physiol 1988 ;  65 : 1488-1499

Haga clic aquí para ir a la sección de Referencias</sup> or the use of continuous positive airway pressure (CPAP) in spontaneously breathing patients<sup>35 Petrof B.J., Legare´ M., Goldberg P. , y al. Continuous positive airway pressure reduces work of breathing and dyspnea during weaning from mechanical ventilation in severe chronic obstructive pulmonary disease Am Rev Respir Dis 1990 ;  141 : 281-289

Haga clic aquí para ir a la sección de Referencias</sup> can counterbalance and reduce the inspiratory threshold load imposed by auto-PEEP without causing further hyperinflation. Under those circumstances, application of PEEP may facilitate weaning from mechanical ventilation by reducing the work of breathing and dyspnea without causing further hyperinflation.<sup>13 Gottfried S.B. The role of PEEP in the mechanically ventilated COPD patient Ventilatory Failure New York: Springer-Verlag (1991).  392-418

Haga clic aquí para ir a la sección de Referencias, 35 Petrof B.J., Legare´ M., Goldberg P. , y al. Continuous positive airway pressure reduces work of breathing and dyspnea during weaning from mechanical ventilation in severe chronic obstructive pulmonary disease Am Rev Respir Dis 1990 ;  141 : 281-289

Haga clic aquí para ir a la sección de Referencias, 46 Smith T.C., Marini J.J. Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction J Appl Physiol 1988 ;  65 : 1488-1499

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The aims of this article are to (1) review the physiologic mechanisms of auto-PEEP and the use of PEEP in counterbalancing auto-PEEP and (2) examine the clinical criteria for application of PEEP in patients with COPD.