Positive End-Expiratory Pressure (PEEP) is a medical strategy used to support lung function by maintaining a positive pressure within the airways at the end of the breathing cycle. This technique is frequently encountered in the care of patients with respiratory failure. PEEP plays a significant role in mechanical ventilation and other forms of respiratory support in hospital settings.
Defining Positive End-Expiratory Pressure
Positive End-Expiratory Pressure is precisely the pressure remaining in the patient’s lungs above the surrounding atmospheric pressure when they finish exhaling. This pressure is not a natural occurrence in a healthy person breathing normally, as airway pressure typically returns to zero (atmospheric pressure) at the end of expiration. The primary objective of applying this pressure is to keep the small air sacs, known as alveoli, partially inflated.
In mechanical ventilation, PEEP is a specific setting on the ventilator, often starting at a low level such as 5 cmH₂O (centimeters of water). This minimum level is often used to mimic the natural pressure provided by the body’s own physiological mechanisms and to mitigate end-expiratory alveolar collapse.
The Physiological Mechanism
The air sacs, or alveoli, are naturally coated with surfactant, a substance that lowers surface tension to keep them from collapsing completely. However, in conditions like acute lung injury, this natural defense can be overwhelmed, leading to widespread collapse, known as atelectasis. PEEP works by exerting a continuous, outward pressure that physically counteracts the inward pull of surface tension and the tendency of the lungs to deflate. This action effectively “splints” the airways and alveoli open at the end of each breath.
Keeping the alveoli open prevents the need for them to be re-opened with every subsequent inhalation, a process that requires significant energy and can cause microscopic damage to the lung tissue. By preventing this collapse and re-opening cycle, PEEP contributes to what is known as alveolar recruitment, which means more air sacs are available for gas exchange. This recruitment leads to an increase in the Functional Residual Capacity (FRC), the volume of air remaining in the lungs after a normal exhalation. An increased FRC improves the surface area available for oxygen to diffuse into the bloodstream, thereby improving overall oxygenation. The maintenance of open alveoli also leads to better ventilation-perfusion matching, ensuring that blood flow is directed to well-aerated lung regions.
Clinical Use and Application
PEEP’s primary role is within the setting of mechanical ventilation, where it is known as extrinsic PEEP because it is applied externally by the machine. This applied pressure is a standard component of ventilator settings for nearly all intubated patients to ensure alveolar stability.
Higher levels of PEEP are often necessary to manage severe forms of respiratory failure, such as Acute Respiratory Distress Syndrome (ARDS). In ARDS, the lungs are stiff and filled with fluid, causing massive alveolar collapse and severely impaired gas exchange.
PEEP principles are also applied in non-invasive respiratory support devices, such as Continuous Positive Airway Pressure (CPAP) machines. The continuous positive pressure delivered by a CPAP device is functionally analogous to PEEP, as it maintains an open airway throughout the entire breathing cycle. CPAP is frequently used for conditions like obstructive sleep apnea and certain types of heart failure to keep the upper airway and alveoli open without the need for an invasive breathing tube.
Associated Risks and Patient Monitoring
One of the most significant risks associated with PEEP is barotrauma, which refers to physical injury to the lung tissue caused by excessive pressure. This can lead to air escaping from the ruptured alveoli into the surrounding chest cavity, potentially causing a pneumothorax, a serious condition where a collapsed lung can impair breathing.
Another major concern is the impact of PEEP on the cardiovascular system. The positive pressure applied to the lungs is transmitted to the chest cavity, increasing the pressure surrounding the heart and major blood vessels. This increased intrathoracic pressure can impede the return of blood to the heart from the body, a process called venous return.
When venous return is reduced, the amount of blood the heart can pump out with each beat, or cardiac output, decreases, potentially leading to low blood pressure and insufficient oxygen delivery to other organs. Due to these risks, medical professionals must constantly monitor and carefully adjust PEEP settings. The titration of PEEP involves finding the optimal balance between maximizing alveolar recruitment and oxygenation while minimizing the risk of lung injury and hemodynamic compromise. Monitoring includes frequent checks of the patient’s blood pressure, heart rate, oxygen saturation levels, and specific ventilator measurements like plateau pressure, which helps assess the pressure within the small airways and alveoli.