From the Guidelines
Positive end-expiratory pressure (PEEP) therapy will result in a decrease in extravascular lung water (option A). PEEP works by maintaining positive pressure in the airways at the end of expiration, which prevents alveolar collapse and improves oxygenation 1. This positive pressure helps to push fluid from the alveolar spaces back into the pulmonary circulation, thereby reducing extravascular lung water. PEEP actually decreases cardiac preload by increasing intrathoracic pressure, which reduces venous return to the heart, as explained by the hemodynamic effects of mechanical ventilation 1. It increases functional residual capacity (not decreases it) by keeping alveoli open throughout the respiratory cycle. PEEP also typically increases atrial natriuretic peptide release due to the stretching of atrial walls from the altered hemodynamics. The reduction in extravascular lung water is particularly beneficial in conditions like pulmonary edema, acute respiratory distress syndrome (ARDS), and other conditions characterized by increased lung water content, as it improves gas exchange and lung compliance, as supported by the clinical practice guideline for mechanical ventilation in adult patients with ARDS 1. Key points to consider when using PEEP include:
- The level of PEEP should be tailored to the individual patient's needs, taking into account factors such as lung recruitability and end-expiratory transpulmonary pressure 1.
- Higher PEEP levels may be beneficial in patients with moderate or severe ARDS, as they can improve oxygenation and reduce mortality 1.
- The use of recruitment maneuvers (RMs) in combination with PEEP may also be beneficial in improving oxygenation and reducing mortality, although the evidence for this is not as strong 1.
From the Research
Effects of Positive End-Expiratory Pressure Therapy
- Positive end-expiratory pressure (PEEP) therapy is used to improve gas exchange, increase functional residual capacity, recruit air spaces, and decrease pulmonary shunt in patients suffering from respiratory failure 2.
- The application of PEEP is expected to increase PaO2, and four mechanisms have been proposed to explain the improved pulmonary function and gas exchange with PEEP:
- increased functional residual capacity
- alveolar recruitment
- redistribution of extravascular lung water
- improved ventilation-perfusion matching 3.
- PEEP may decrease expiratory shunting by maintaining alveolar patency, thereby increasing functional residual capacity 4.
- However, PEEP may not prevent and may actually favor accumulation of interstitial lung water 4, and the increase in PEEP resulted in a decrease in lung lymph flow and an increase in extravascular lung water 2.
Cardiac Effects of PEEP
- PEEP may have a variable effect on cardiac output based in part on the level of end-expiratory pressure, the state of intravascular volume, and the pathophysiology of the underlying pulmonary abnormality 4.
- The institution of PEEP does not affect cardiac output, but it produces a reversible increase in extravascular lung water that is linked to a decrease in lung lymph flow 2.
Answer to the Question
- Based on the studies, the correct answer is A. Decrease in extravascular lung water is not supported by the evidence, as PEEP may actually favor accumulation of interstitial lung water 4 and increase extravascular lung water 2.
- The correct answer is not B, as the effect of PEEP on cardiac preload is not clearly stated in the studies.
- The correct answer is not C, as the effect of PEEP on atrial natriuretic peptide is not mentioned in the studies.
- The correct answer is not D, as PEEP may actually increase functional residual capacity 4, 2.
- Therefore, none of the options are supported by the evidence, but option A is the least incorrect as PEEP may increase extravascular lung water, which is the opposite of what option A states.