What is the effect of Positive End-Expiratory Pressure (PEEP) therapy on extravascular lung water, cardiac preload, atrial natriuretic peptide, and functional residual capacity?

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 pushes fluid from the alveolar spaces back into the pulmonary circulation, reducing extravascular lung water and improving lung compliance. 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 preventing alveolar collapse and recruiting previously collapsed alveoli. PEEP typically increases atrial natriuretic peptide release due to increased right atrial pressure. The primary therapeutic benefit of PEEP is improving oxygenation by increasing functional residual capacity and reducing pulmonary edema in conditions like acute respiratory distress syndrome (ARDS) 1. Some key points to consider when using PEEP include:

• Higher PEEP levels can improve gas exchange and reduce the need for high FiO2 1
• Appropriate levels of PEEP can limit ventilator-induced lung injury (VILI) by maintaining lung recruitment and improving lung homogeneity 1
• PEEP selection criteria may include lung recruitability, end-expiratory transpulmonary pressure, and other factors 1
• The use of higher PEEP levels has been associated with lower mortality in patients with moderate or severe ARDS, as shown in a recent meta-analysis 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.