Which of the following increases pulmonary vascular resistance (PVR)?

Excessive PEEP (Positive End-Expiratory Pressure) increases pulmonary vascular resistance (PVR) primarily because it elevates the pressure in the thoracic cavity, particularly in the lungs' vascular structures. When PEEP is applied, it maintains lung inflation at the end of expiration, which can lead to increased intrathoracic pressure. This increase can compress blood vessels within the lungs, particularly the pulmonary arteries and veins, thereby increasing resistance to blood flow.

The elevation in PVR due to excessive PEEP can lead to reduced pulmonary blood flow and potentially cause right heart strain or failure, as the right ventricle must work harder to pump blood through the increased resistance.

In contrast, decreased cardiac output typically leads to reduced blood flow to the lungs, which might lower PVR rather than increase it. Increased fluid intake could potentially decrease blood viscosity or expand blood volume, which does not inherently raise PVR. Finally, improved oxygenation generally indicates better perfusion and can actually promote vasodilation in the pulmonary vessels, leading to decreased PVR.