Remember that Recruitment of Alveoli Using an Increased Level of Positive End-Expiratory Pressure can Take 6 to 12 Hours
David N. Hager MD
There are two forms of positive end-expiratory pressure (PEEP). These are extrinsic PEEP and intrinsic-PEEP (aka, auto-PEEP). Extrinsic PEEP is the pressure applied to the airways by the ventilator at end expiration. Intrinsic PEEP is the pressure remaining in the airways in excess of extrinsic PEEP upon occluding the expiratory valve at end expiration. Extrinsic PEEP is typically used for three reasons: (a) to decrease work of breathing in patients with intrinsic PEEP, (b) to reduce or avoid atelectasis, and (c) to improve arterial oxygenation and avoid or limit low-volume lung injury. The purpose of this chapter is to review the use of PEEP in each of these situations.
Work of Breathing and PEEP
The use of extrinsic PEEP in the context of obstructive lung disease may decrease the work of breathing. At end expiration, many of these patients will have measurable intrinsic PEEP. They must therefore drop their pleural pressure far enough to (a) overcome their intrinsic PEEP and (b) trigger the ventilator. For example, if a given patient has an intrinsic PEEP of 5 cm H2O and the ventilator trigger threshold is set at negative 2 cm H2O, the patient has to decrease his or her pleural pressure by 7 cm H2O before the ventilator will deliver the next assisted breath. In this situation, that application of 5 cm H2O of extrinsic PEEP will decrease the patient’s work of breathing. The patient will then need to decrease his or her pleural pressure by only 2 cm H2O. The often cited concern that the use of low levels of extrinsic PEEP (5 cm H2O) will substantially increase the hazards of hyperinflation have been overemphasized.
Atelectasis and PEEP
Ventilation at low lung volumes with deep sedation predisposes patients to the development of atelectasis. Though there are no convincing data to suggest that important outcomes such as duration of mechanical ventilation, morbidity, or mortality are reduced by the
empiric use of low levels of PEEP (5 cm H2O) in healthy patients, many clinicians have adopted this practice based on physiologic measurements that show that such maneuvers reduce shunt.
empiric use of low levels of PEEP (5 cm H2O) in healthy patients, many clinicians have adopted this practice based on physiologic measurements that show that such maneuvers reduce shunt.
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