Positive End-Expiratory Pressure, or PEEP, refers to the positive pressure maintained in the lungs and airways at the end of each breath. This pressure helps keep the small air sacs, called alveoli, from fully collapsing after exhalation, improving oxygen transfer into the bloodstream. In healthy breathing, a small amount of natural PEEP occurs, but it is also a common setting used in mechanical ventilation to support lung function.
Auto PEEP, also known as intrinsic PEEP, develops unintentionally when air becomes trapped in the lungs at the end of exhalation. Unlike the PEEP intentionally set on a ventilator, this occurs spontaneously.
How Auto PEEP Happens
Auto PEEP develops when the lungs do not have enough time to fully exhale before the next breath begins, leading to air trapping. This incomplete emptying of the lungs causes a progressive buildup of pressure within the air sacs. Several factors can contribute to this phenomenon, including rapid breathing rates, which shorten the available time for exhalation.
Airflow obstruction is another common reason for auto PEEP. Conditions like asthma or chronic obstructive pulmonary disease (COPD) can narrow the airways, making it harder for air to leave the lungs efficiently. Certain ventilator settings, such as a high respiratory rate or a short expiratory time, can inadvertently create conditions favorable for air trapping.
When air is trapped, the lung volume increases with each subsequent breath, leading to lung hyperinflation. This state means the lungs are unable to return to a relaxed, baseline state before the next inhalation, making the act of breathing more challenging.
Impact on the Body
The presence of auto PEEP can have effects on the body, particularly impacting the respiratory and cardiovascular systems. When air becomes trapped in the lungs, it increases the pressure inside the chest cavity. This elevated pressure can make it more difficult for the heart to fill with blood, as it impedes the return of blood from the body to the heart, a process known as venous return.
A reduction in venous return can lead to a decrease in the amount of blood the heart pumps out with each beat, potentially affecting overall blood circulation. In the lungs, the increased pressure from auto PEEP can cause the air sacs to overstretch, a condition called lung overdistension. This overstretching can compromise the effectiveness of gas exchange, making it harder for oxygen to enter the bloodstream and carbon dioxide to be removed.
Auto PEEP increases the effort required to breathe. Because the lungs are already partially inflated and under pressure, the muscles involved in breathing must work harder to draw in the next breath. Patients may experience this as shortness of breath or increased discomfort during breathing.
Addressing Auto PEEP
Recognizing auto PEEP involves observing specific indicators, especially in individuals receiving mechanical ventilation. One common method is to monitor the expiratory flow tracing on a ventilator display. If the flow does not return to baseline before the next breath is delivered, it suggests air trapping and the presence of auto PEEP. Another assessment involves performing an end-expiratory hold maneuver, where the exhalation valve is briefly closed; a pressure reading higher than the set PEEP indicates auto PEEP.
Strategies to reduce auto PEEP often focus on allowing more time for complete exhalation. This can involve adjusting ventilator settings, such as decreasing the respiratory rate or the volume of air delivered with each breath.
For patients with underlying airway narrowing, bronchodilator medications can be administered to open the airways and improve airflow. In some situations, carefully applying a small amount of external PEEP can paradoxically help reduce the work of breathing associated with auto PEEP, improving patient comfort without worsening lung hyperinflation.