Positive End-Expiratory Pressure (PEEP) is a fundamental setting used in mechanical ventilation to support patients with acute respiratory failure. PEEP is the air pressure maintained in the lungs at the end of the breathing cycle, specifically after a patient exhales. While lung pressure normally returns to zero upon full exhalation, PEEP keeps the pressure positive. Applying PEEP prevents the tiny air sacs in the lungs from completely collapsing. This continuous pressure is maintained by the ventilator to ensure the lungs remain open and ready for the next breath.
The Mechanics of PEEP
The primary function of PEEP is to counteract atelectasis, the collapse of the small air sacs (alveoli) at the end of expiration. When a patient’s lungs are diseased or injured, the natural substance that prevents collapse, called surfactant, may be depleted. Without PEEP, these delicate alveoli deflate fully, requiring significant effort and pressure to reopen with every breath, which can cause further injury.
By maintaining a constant positive pressure, PEEP physically splints these airways and alveoli open. This process, called alveolar recruitment, increases the number of functional air sacs available for gas exchange. Keeping the alveoli open increases the total surface area, directly enhancing the lung’s ability to oxygenate the blood.
PEEP is a baseline pressure that is always present, unlike the pressure delivered during an inhale, which is called pressure support or peak inspiratory pressure. PEEP provides the floor of pressure, while the inspiratory pressure provides the ceiling to deliver the full breath volume. This combination protects the lung tissue from the damaging effects of repeated collapse and re-expansion.
A separate concept is intrinsic PEEP, also called auto-PEEP, which is not a setting applied by the ventilator but an undesirable consequence of air trapping. Auto-PEEP occurs when a patient’s exhalation time is too short or their airways are obstructed, causing air to be unable to fully exit the lungs before the next breath begins. This buildup of trapped air creates an uncontrolled positive pressure within the lung. Applied PEEP is a controlled therapeutic tool, while auto-PEEP is a problem that clinicians must identify and correct.
Clinical Goals and Applications of PEEP
PEEP is a central component of ventilation strategies designed to maximize oxygen delivery while minimizing damage to the lungs. By recruiting collapsed alveoli, PEEP improves the ratio of ventilation to perfusion, ensuring blood flowing through the lungs is more effectively exposed to oxygen. This efficiency allows clinicians to reduce the Fraction of Inspired Oxygen (\(\text{FiO}_2\)), the concentration of oxygen delivered by the ventilator.
Reducing the \(\text{FiO}_2\) is a significant clinical goal because high concentrations of oxygen over long periods can be toxic to lung tissue. PEEP allows the medical team to achieve acceptable blood oxygen levels with safer, lower oxygen concentrations, thereby reducing the risk of oxygen-induced lung injury. This balancing act is important in managing severe respiratory failure.
PEEP is frequently utilized in the management of conditions such as Acute Respiratory Distress Syndrome (ARDS), severe pneumonia, and pulmonary edema (fluid buildup in the lungs often associated with heart failure). In these conditions, lung tissue is heavy, stiff, and prone to collapse, making PEEP necessary to keep the alveoli stable. The use of PEEP is part of lung-protective ventilation, which aims to prevent injury caused by the mechanical breathing process itself.
The pressure prevents atelectrauma, which is trauma caused by the repeated opening and closing of airways with every breath cycle. Keeping the lung stable and open reduces the physical stress on lung tissue and limits the release of inflammatory mediators.
Risks and Monitoring High PEEP Levels
While PEEP is a powerful tool for improving oxygenation, high levels can introduce significant systemic side effects that require careful monitoring. The positive pressure applied inside the lungs is transmitted to the chest cavity, compressing the major blood vessels returning blood to the heart. This compression impedes the flow of blood from the veins back into the heart, reducing venous return.
A decrease in venous return directly reduces the amount of blood the heart can pump out, leading to hypotension and decreased cardiac output. Clinicians must constantly monitor the patient’s blood pressure and cardiac function against the benefit of improved oxygenation when PEEP levels are increased. This balance is the deciding factor in determining the optimal PEEP setting.
Excessive pressure can also cause lung injury known as barotrauma or volutrauma, which is physical damage from too much pressure or volume. This can rupture alveoli, potentially causing an air leak into the chest cavity, a life-threatening condition called a pneumothorax. Ventilator settings, including PEEP, are carefully managed to keep the peak inspiratory pressure and volume within safe limits to avoid this complication.
PEEP levels are not fixed but are constantly titrated, meaning they are adjusted based on a patient’s specific needs and response. The medical team uses blood gas analysis (which measures oxygen and carbon dioxide levels) and blood pressure readings to fine-tune the PEEP. When a patient’s lung function improves, the PEEP is carefully weaned, or lowered gradually, to ensure the patient can maintain adequate oxygenation before the ventilator is eventually removed.