Positive expiratory pressure plateau: improved gas exchange during mechanical ventilation.

IT IS "WELL KNOWN that the pattern of mechanical ventilation can affect the efficiency of pulmonary gas exchange. Despite this, the optimum ventilatory wave form has yet to be defined. In the past, most research has pertained to the pattern of inspiration T M and relatively little attention has been directed to the expiratory phase of ventilation. The concept of a maintained expiratory pressure as a means of improving gas exchange during mechanical ventilation is not new. Enthusiasm for the clinical use of maintained pressure during expiration has been tempered somewhat by Cournand's work, 5 which predicts a fall in cardiac output because of the inevitable associated rise in mean inflating pressure. There are relevant reports from animal studies ",7 and from studies using anaesthetized human subjects. 8-1~ In these studies, methodology varies and the results are conflicting. More recently, however, several workers have ventilated critically ill patients using a maintained expiratory pressure and have shown improved oxygenation, n-13 It was the object of the present study to examine the effects of a Positive Expiratory Pressure Plateau (PEEP) on gas exchange, cardiac output, and functional residual capacity, and to define its place in the clinical management of patients receiving artificial ventilation. Pm, P refers to a particular feature of the ventilator), wave form. Figure 1 is a tracing of airway pressure and shows the characteristic feature. A constant pressure is maintained throughout the expiratory phase, and airway pressure at no time returns to ambient pressure. This should be distinguished from an expiratory flow retard resulting from a restriction in the expiratory line.