Lungs normally empty by passive recoil and the pressure in the alveoli at end-expiration is the intrinsic positive end-expiratory pressure (PEEP). During spontaneous respirations and with normal lungs, thePEEP is equal to atmospheric pressure. On a ventilator, auto-PEEP occurs when gas is trapped in the alveoli at end-expiration. Auto-PEEP can be measured by measuring airflow at end-exhalation in controlled ventilation. In the absence of auto-PEEP, there should be no airflow at end-exhalation. Practically, the practitioner performs an end-expiratory hold and measures the airway pressure, the total PEEP. This measured pressure with an end-expiratory hold minus the extrinsic PEEP is the auto-PEEP (Total PEEP – PEEP(external) = auto-PEEP. Auto-PEEP happens in up to 35% of mechanically ventilated patients and can be higher in those with severe obstructive lung disease.[i]
There are scenarios during mechanical ventilation for acute respiratory failure when auto-PEEP may develop. If this happens, Auto-PEEP can cause respiratory and hemodynamic compromise. Auto-PEEP can develop when the ventilator provides a machine breath prior to full exhalation (breath stacking). Risk factors for auto-PEEP include medical conditions with severe bronchospasm (status asthmaticus or COPD exacerbation), mucous plugging, and mechanical ventilation at high respiratory rates and with long inspiratory times (short expiratory times). It is important to recognize auto-PEEP and clues to its presence are: a breath begins before the full exhalation and failure of the peak airway pressure to change when external PEEP is applied.
The main problem with auto-PEEP is that gas trapping in the alveoli leads to dynamic hyperinflation of the lungs and increased positive pressure in the alveoli. This makes it harder for gas to escape the alveoli until airway pressures exceed alveolar pressures. In order for the body to overcome the positive pressure in the alveoli, the patient must take a stronger negative inspiratory force during inspiration to exceed the auto-PEEP pressures thereby transmitting negative pressures to the central airways and generating airflow.
The consequences of auto-PEEP are myriad. Auto-PEEP increases the work of breathing, worsens gas exchange, and increases intra-thoracic pressures which can cause decreased cardiac output and hypotension. Auto-PEEP can also falsely elevate a measured central venous pressure (CVP) or pulmonary capillary wedge pressure (PCWP) which can lead to mistaken interpretations during central hemodynamic monitoring.
There are multiple ventilator adjustments you can do to treat auto-PEEP. First, you can apply extrinsic PEEP at a level of about 75%-85% auto-PEEP level to increase airway pressures and allow gas release from the alveoli (the waterfall effect). Second, you can increase expiratory time, decrease respiratory rates and if possible decrease tidal volumes. Third, you can increase sedation to prevent hyperventilation by the patient. Fourth, increase the frequency of suctioning and bronchodilators to address both mucous plugging and bronchospasm.
[i] Cleveland Clin J of Med. 2005;72(9):801-808