Effect of volume status on systolic pressure variation (SPV)

The physiological basis of SPV reflects the effect of VR on the Frank-Starling curve. Mechanical ventilation moves the Frank-Starling from its expiratory position (dashed red curve) to its inspiratory position (solid blue curve). The three points on the VR (cardiac output) curve represent normovolemia, mild hypovolemia, and severe hypovolemia during expiration and inspiration. In normovolemia, there is no significant reduction in VR or cardiac output with mechanical ventilation; this would translate to minimal volume responsiveness and minimal SPV. In contrast, in severe hypovolemia, there is a large reduction in VR and cardiac output with mechanical ventilation, reflected as significant SPV.