Auto-PEEP, or intrinsic positive end-expiratory pressure, occurs when air remains trapped in the lungs at the end of exhalation. This trapped volume creates a positive pressure within the alveoli that exceeds the pressure set by the mechanical ventilator. Auto-PEEP is a serious complication because it significantly increases the effort required to take the next breath. If untreated, this air trapping can lead to high pressures within the chest, potentially reducing blood return to the heart and causing a drop in blood pressure. Treatment aims to eliminate this excess pressure and restore efficient breathing mechanics.
The Mechanism of Air Trapping
Auto-PEEP occurs when the time allowed for exhalation is too short to fully empty the lungs before the next mechanical breath is delivered. This insufficient duration is known as a short expiratory time (T-E). The issue is often worsened by high airway resistance, such as narrowed breathing passages seen in conditions like asthma or chronic obstructive pulmonary disease.
When the patient cannot fully exhale, residual gas accumulates over several breaths, leading to dynamic hyperinflation. This “breath stacking” increases lung stiffness, making it harder for the patient to overcome the trapped pressure and trigger the ventilator for the next breath.
Identifying and Measuring Auto-PEEP
Since Auto-PEEP is an internal pressure not explicitly set on the ventilator, clinicians use specific diagnostic techniques to confirm its presence. A primary clue is visible on the ventilator’s flow-time graphic: the expiratory flow curve fails to return to the zero baseline before the next inhalation begins. This visual cue indicates that air is still exiting the lungs when the next breath starts.
The definitive way to quantify the pressure is through an end-expiratory hold maneuver. During this brief maneuver, the exhalation valve is temporarily closed, allowing the pressure in the airways and alveoli to equalize. The resulting pressure reading is the total pressure remaining in the lungs. The difference between this total pressure and the external pressure set on the ventilator represents the actual level of Auto-PEEP. Patients may also show clinical signs like agitation, increased work of breathing, or sudden hypotension, prompting the medical team to check for air trapping.
Ventilator Strategies for Reduction
The most effective treatment involves adjusting mechanical ventilator settings to maximize the time available for exhalation. The strategy is to either reduce the amount of air the patient needs to exhale or provide more time for air to escape. This is achieved by manipulating three primary ventilator parameters: respiratory rate, tidal volume, and inspiratory flow rate.
Adjusting Respiratory Rate
Reducing the respiratory rate (RR) is often the most impactful change, as it directly lengthens the total time between breaths, increasing the expiratory time (T-E). Clinicians may reduce the rate, sometimes aiming for 10 to 14 breaths per minute, to force a slower breathing pattern. This adjustment may lead to a temporary rise in carbon dioxide levels, known as permissive hypercapnia, which is accepted to prioritize lung safety.
Decreasing Tidal Volume
Decreasing the tidal volume (the volume of air delivered with each breath) reduces the amount of air that must be exhaled during the limited T-E. Clinicians often set the tidal volume to a lower range, such as 6 to 8 milliliters per kilogram of predicted body weight. Less air entering the lungs simplifies the exhalation phase.
Increasing Inspiratory Flow Rate
Increasing the inspiratory flow rate is a third adjustment that indirectly increases T-E. A higher flow rate delivers the breath into the lungs more quickly, shortening the inspiratory time. This automatically lengthens the remaining time available for exhalation. To favor the exhalation phase, the inspiratory-to-expiratory ratio (I:E ratio) is often widened, sometimes set to 1:3 or 1:4.
Applying Extrinsic PEEP
For patients with severe airway obstruction, a counterintuitive strategy involves applying a small amount of extrinsic positive end-expiratory pressure (PEEPe). This external pressure is often titrated to 80 to 85 percent of the measured Auto-PEEP level. This small amount of external pressure acts as a splint, holding the airways open. This facilitates the escape of trapped air and reduces the patient’s work of breathing, especially in patients whose airways tend to collapse during forced exhalation.
Medications and Supportive Care
While adjusting ventilator settings is the main focus, pharmacological and supportive measures play an important adjunctive role. Medications address the underlying physiological issues contributing to airway resistance and air trapping.
Bronchodilators, such as inhaled beta-agonists, relax the smooth muscles surrounding the airways. This widens the breathing passages, decreasing airway resistance and allowing air to exit the lungs faster during exhalation. Corticosteroids may also be used to reduce underlying airway inflammation, particularly in patients with severe lung disease exacerbations.
Reducing the patient’s own respiratory drive is beneficial if rapid breathing is worsening air trapping. Sedation can suppress spontaneous, rapid breathing efforts, bringing the patient’s rate closer to the lower rate set on the ventilator. This synchronization ensures that the prolonged expiratory time programmed into the ventilator is utilized.
Supportive care includes positioning the patient to optimize lung mechanics, such as raising the head of the bed to reduce pressure on the diaphragm. Managing the primary medical condition, such as treating an infection or administering diuretics for fluid overload, is also necessary to address the root cause of the respiratory failure.