How does positive end‑expiratory pressure (PEEP) during mechanical ventilation affect hemodynamics?

Hemodynamic Effects of PEEP During Mechanical Ventilation

PEEP decreases cardiac output primarily by reducing venous return to the right ventricle, while simultaneously decreasing left ventricular afterload—effects that are mediated through increases in pleural pressure and transpulmonary pressure. 1

Primary Mechanisms of PEEP's Hemodynamic Impact

Effects on Venous Return and Right Ventricular Preload

PEEP reduces the pressure gradient for venous return throughout the entire respiratory cycle, which is the dominant mechanism for decreased cardiac output. 1 The normal pressure gradient from the systemic venous reservoir to the heart is only 4-8 mmHg, meaning small increases in right atrial back pressure from PEEP can exert disproportionately large effects on cardiac output. 1

• In normal subjects, approximately 50% of alveolar pressure changes are transmitted to pleural pressure, though diseased lungs transmit less due to increased stiffness. 1
• The entire fall in cardiac output with PEEP is due to reduced left ventricular stroke volume, as heart rate typically remains unchanged. 2
• Transmural right atrial pressure rises with PEEP (approximately 3.1 cmH2O with 10 cmH2O PEEP), increasing right ventricular end-diastolic diameter by about 15%. 3

Effects on Right Ventricular Afterload

Transpulmonary pressure increases with PEEP can substantially elevate right ventricular afterload, particularly in patients with lung disease. 1 When pleural pressure exceeds pulmonary venous pressure, microvascular collapse creates West zone 2 conditions; when it exceeds pulmonary arterial pressure, West zone 1 conditions develop, making alveolar pressure the effective outflow pressure for the right ventricle. 1

• The right ventricle is relatively more sensitive to afterload than preload variations due to its high compliance but limited myocardial thickness and contractile power. 1
• In ARDS patients with reduced lung compliance, inflammation, consolidation, and microthrombosis, pulmonary vascular resistance increases proportionally to mean alveolar pressure. 1
• This effect is particularly important in septic, post-cardiac surgery, and ARDS patients with pre-existing right ventricular dysfunction. 1

Effects on Left Ventricular Function

PEEP decreases left ventricular afterload by increasing pleural pressure, but this potential benefit is negated by the simultaneous reduction in preload. 1 Left ventricular end-diastolic diameter decreases by approximately 21% with 10 cmH2O PEEP, while ejection time increases by 11% and velocity of circumferential fiber shortening falls by 30%. 3

• The reduction in left ventricular filling occurs secondary to decreased right ventricular output over subsequent cardiac cycles. 1
• For a given cardiac output, transmural left ventricular filling pressure is higher with PEEP compared to control states, suggesting impaired ventricular performance. 4
• PEEP may improve left ventricular function in heart failure patients by reducing afterload, provided no deleterious effects occur on the right ventricle. 5

Magnitude and Time Course of Hemodynamic Changes

Cardiac output falls approximately 19% with 10 cmH2O PEEP in normal subjects, with partial compensation occurring over time. 3 During the first 15 minutes of PEEP application, stroke volume decreases substantially, but cardiac output gradually increases over the subsequent hour due to a 7% rise in heart rate and further increases in transmural right atrial pressure. 3

• Coronary blood flow decreases by 5% with 4 cmH2O PEEP and by 25% with 14 cmH2O PEEP. 6
• Low PEEP values (under 10 cmH2O) reduce left ventricular end-diastolic pressure, while higher values cause increases. 6
• Ventricular function remains impaired for the entire duration of PEEP application, returning to baseline only after resuming spontaneous breathing. 3

Clinical Implications and Compensatory Mechanisms

A reflex decrease in vascular capacitance or fluid infusion is necessary to maintain the venous pressure gradient and cardiac output when PEEP is applied. 1 The hemodynamic effects of PEEP vary according to respiratory mechanics and the interaction with other hemodynamic variables such as central venous pressure and mean arterial pressure. 5

• In patients with heart failure, PEEP may provide beneficial afterload reduction for the left ventricle. 5
• In ARDS patients, the combination of pulmonary hypertension, sepsis-induced vascular dysfunction, and mechanical ventilation can produce right ventricular failure through either preload insufficiency or excessive afterload. 1
• High PEEP levels can decrease cardiac output by increasing intrathoracic pressure, reducing venous return, and potentially worsening right ventricular function in patients with pulmonary hypertension or right heart failure. 7

Critical Monitoring Considerations

Intensive care physicians must maintain vigilance toward PEEP's hemodynamic effects on cardiac function and end-organ circulation. 5 The effects are not inherently beneficial or detrimental but depend on the specific clinical context, including underlying cardiac function, volume status, and respiratory mechanics.

• Monitor for sudden hypotension or cardiovascular collapse, which suggests significant auto-PEEP in mechanically ventilated patients with obstructive airway disease. 8
• Plateau pressures should be maintained below 25 cmH2O in patients without ARDS and below 30 cmH2O in ARDS patients to prevent ventilator-induced lung injury while considering hemodynamic effects. 7
• When implementing PEEP in cardiac patients, careful cardiovascular monitoring is essential to detect and address adverse hemodynamic effects. 7