During the inspiratory phase of spontaneous ventilation, negative intrathoracic pressure leads to augmentation of right ventricular (RV) filling. This leads to a leftward shift in the interventricular septum, decreasing the size and compliance of the left ventricle. In addition, the augmented right ventricular stroke volume pools in the pulmonary circulation, causing a further decrease in left ventricular (LV) filling volume. Decreased LV volume is associated with a decrease in LV stroke volume and systemic blood pressure. With exhalation, blood pooled in the right ventricle and pulmonary circulation returns to the left ventricle; stroke volume and systemic blood pressure increase. Under normal conditions, the inspiratory decrease in arterial blood pressure does not exceed 5 to 10 mm Hg.

A reversed pulsus paradoxus, or systolic pressure variation, has been described during mechanical ventilation. The positive intrathoracic pressure of mechanical inspiration decreases venous return to the right heart and increases RV afterload. In addition, increased alveolar pressure “squeezes” blood from the pulmonary circulation into the left ventricle. The result is an inspiratory decrease in RV stroke volume and an inspiratory increase in LV stroke volume and systemic arterial blood pressure. Given static arterial compliance, systemic arterial pulse pressure (the difference between systolic and diastolic blood pressure) is directly proportional to stroke volume. Thus, any changes in venous return or pulmonary blood flow that lead to an increase in LV stroke volume will also lead to an increase in pulse pressure.