Positive-end expiratory pressure (PEEP) is a common mode of mechanical ventilation used to improve oxygenation in patients with respiratory failure. It works by maintaining a positive pressure in the airways at the end of exhalation. Understanding the physiological effects of PEEP is crucial for its safe and effective application.

Positive-End Expiratory Pressure (PEEP) Explained

PEEP is a ventilatory maneuver that applies positive pressure to the airways at the end of expiration. This prevents the airway pressure from returning to atmospheric pressure, keeping the alveoli partially inflated. The primary goal of PEEP is to prevent alveolar collapse (atelectasis) and improve oxygenation.

PEEP Effects on Respiratory System and Circulation

Let's analyze each statement regarding the effects of positive-end expiratory pressure (PEEP):

• PEEP Reduces Venous Return: This statement is true. When PEEP is applied, it increases the intrathoracic pressure. This increased pressure inside the chest cavity can compress the major veins (like the vena cava) that return blood to the heart. This compression makes it harder for blood to flow back into the right atrium, thereby reducing venous return and potentially cardiac output.
• PEEP Improves Alveolar Gas Exchange: This statement is true. By maintaining positive pressure at the end of expiration, PEEP helps to keep more alveoli open and prevents them from collapsing. This increases the functional residual capacity (FRC) and the surface area available for gas exchange. More open alveoli mean better diffusion of oxygen into the blood and carbon dioxide out of the blood, thus improving alveolar gas exchange and overall oxygenation.
• PEEP Prevents Alveolar Collapse: This statement is true. This is one of the main therapeutic benefits of PEEP. Many lung conditions, such as Acute Respiratory Distress Syndrome (ARDS), cause alveoli to collapse at the end of expiration. PEEP provides a continuous positive pressure that splints these tiny air sacs open, preventing their collapse and maintaining lung volume.
• PEEP Decreases Mean Airway Pressure: This statement is false. PEEP, by definition, adds a positive pressure to the airways throughout the respiratory cycle, particularly at the end of expiration. This sustained positive pressure significantly *increases* the overall mean airway pressure. Mean airway pressure is the average pressure applied to the airway over the entire respiratory cycle. Since PEEP adds pressure, it directly contributes to a higher mean airway pressure, not a decrease. A higher mean airway pressure helps keep the alveoli open but can also have cardiovascular side effects.

Therefore, among the given options, the statement "It decrease mean airway pressure" is the one that is not true about positive-end expiratory pressure (PEEP).