Systolic pressure variation: a dynamic measure of the adequacy of intravascular volume

Central venous pressure and pulmonary artery occlusion pressure measurements are complicated by ventilatory changes in intrathoracic pressure and have a poor record as a preload parameter. The hemodynamic changes of mechanical ventilation have the effect of withholding a volume of blood and then giving it back during each respiratory cycle. The inspiratory phase of mechanical ventilation reduces preload and increases afterload of the right heart, while the preload is increased and afterload is reduced. These changes are reversed during expiration. The systolic pressure variation (SPV) by mechanical ventilation is closely related to the intravascular volume according to Starling’s law. The effect of mechanical ventilation on the left ventricular stroke volume has given birth to dynamic monitors of preload obtained from the arterial side in the form of SPV and pulse pressure variation. Animal studies and numerous clinical studies in surgical and critically ill patients have shown the superiority of dynamic monitors over the venous and pulmonary artery occlusion pressures.