You are in the intensive care unit (ICU) and you are called to see Mrs. Smith, a woman with chronic obstructive pulmonary disease (COPD) who had been admitted for respiratory failure secondary to pneumonia. You quickly go to her bed to evaluate. You are told that she was doing well with the planned weaning from mechanical ventilation, but she became acutely unstable. Upon your arrival she is found to be tachypneic, diaphoretic, and hypotensive. Of course, today is your first day in the ICU and you have not received sign-out yet. All of a sudden the ICU fellow arrives and quickly disconnects Mrs. Smith from the ventilator and her blood pressure improves and she starts looking better. What happened?

Patients with COPD have regions of the lung with flow limitation, needing longer time constants for their alveoli to deflate (i.e., they take longer to complete expiration). In these patients, especially during mechanical ventilation, these areas that take longer to deflate may still be in expiration once the lung is making the next inspiratory effort, therefore trapping air in the alveoli. This phenomenon is called auto positive end-expiratory pressure (PEEP) or intrinsic PEEP (iPEEP). This basically means that the patient’s expiratory flow gets interrupted before the alveoli are completely empty, thus creating a pressure difference between the alveoli and the proximal airway.

How do you measure if a patient has autoPEEP? The easiest method in a ventilated patient is to review the expiratory flow in the ventilator graphics. If the expiratory flow does not reach zero and continues until the onset of the following inspiratory cycle, your patient has autoPEEP (Fig. 120.1). There are also more sophisticated and more complicated ways to measure. Using an esophageal balloon, the intrapleural pressure can be measured and related to the onset of inspiratory flow in patients who take spontaneous breaths. If the patient is not triggering the ventilator, the static autoPEEP can be measured by
occluding the expiratory port of the ventilator, allowing the pressure of the alveoli to equilibrate with the pressure of that port (not all ventilators allow you to do this fancy measurement). Another way to measure autoPEEP can be done by applying an end-expiratory pause for 1 to 2 seconds and measuring the pressure in excess of the PEEP set on the ventilator.