Question
How does positive pressure ventilation affect preload and cardiac output?
Answer
In normal spontaneous breathing, the negative pressure generated during inspiration assists venous return, alleviates pressure on the pulmonary capillaries, and promotes effective blood flow. Positive pressure ventilation (PPV), however, reverses this pattern. During inspiration under PPV, intrathoracic pressure increases, which compresses the great veins and heart, leading to decreased venous return. This reduction in venous return lowers preload—the volume of blood in the ventricles at the end of diastole—ultimately decreasing right ventricular output and restricting pulmonary blood flow.
Preload is a key determinant of cardiac output, as the heart can only pump the volume of blood it receives. Excessive mean airway pressures during PPV can effectively “squeeze” the heart, especially the right side, preventing adequate blood from returning. This diminished preload results in reduced cardiac output and compromised systemic perfusion. Conversely, maintaining or increasing preload supports cardiac output by ensuring sufficient blood volume is available for each contraction.
Because preload is so sensitive to changes in intrathoracic pressure, careful management of ventilatory settings—especially mean airway pressure—is essential in mechanically ventilated patients. Protecting venous return helps preserve cardiac output, supporting both oxygen delivery and overall hemodynamic stability.
This Ask the Expert is an edited excerpt from the course, MAP vs. NAP: The Impact of Mechanical Ventilation on Hemodynamics, presented by Evan Richards, Advanced Practice Clinical Consultant, BSc, RT.
