What is the effect of positive-pressure ventilation on venous return and LV preload?

• A. Increases venous return and LV preload.
• B. Decreases venous return, reducing LV preload.
• C. Venous return increases briefly but LV preload remains the same.
• D. LV preload increases while venous return decreases.

Answer: (B)

Positive-pressure ventilation raises intrathoracic pressure during inspiration. That higher thoracic pressure compresses the great veins and the right atrium, reducing the pressure gradient that normally drives venous return to the heart. With less blood returning to the heart, right ventricular filling drops, and LV filling (preload) falls as well because LV preload depends on the amount of blood delivered to the left ventricle at end-diastole. So venous return decreases and LV preload decreases. This effect can be more pronounced with higher PEEP, which keeps thoracic pressure elevated.