Mechanical ventilation induces cyclic changes in vena cava blood flow, pulmonary artery blood flow and aortic blood flow. At the bedside, changes in the aortic blood flow are reflected by swings in the blood pressure curve (and thus variations in stroke volume and blood pressure). The magnitude of these variations is highly dependent on the volume responsiveness of the patient.
With controlled ventilation, the rise in intrathoracic pressure during early inspiration leads to a squeezing of the pulmonary blood into the left ventricle. This process in turn increases the left ventricular preload. With a volume responsive patient, this results in an increased stroke volume or pulse pressure.
An increase in intrathoracic pressure also results in reduced right ventricular filling. With a volume responsive right heart, this will reduce the volume being ejected. Thus, during late inspiration a couple of heartbeats later, the left ventricular preload will decrease as will the stroke volume or pulse pressure. The variations in stroke volume and pulse pressure are analysed over a 30 second time frame with the following formula: