A spontaneously breathing patient
A mechanically ventilated patient
In a mechanically ventilated patient, it is normal to observe a decrease in arterial pressure baseline, systolic blood pressure and mean arterial pressure.
This phenomenon is exaggerated in patients who are hypovolemic, as it is the influence of the tug-of-war between positive pressure of the ventilator and the diastolic pressure of the venous system.
It's a fight: PEEP vs central venous pressure. If the PEEP wins, the right heart doesn't fill enough, consequently the left heart doesn't fill enough, and the stroke volume is decreased as per the Frank-Starling mechanism.
If the patient has low central venous pressure (eg. dehydrated or hemorrhaging), PEEP wins more often.
Of course all of this goes out the window if the ventricles are abnormal, eg. in LV dysfunction- then, the positive pressure might actually INCREASE the stroke volume, because it augments the transmural pressure of the barely coping ventricle, and assist in ejecting the blood from the thorax.
The delta-up and delta-down
The delta is the interval depicted here: the difference between the highest and the lowest systolic pressures during the respiratory cycle of a mechanically ventilated patient.
There is a delta-up and a delta-down. The total delta is the sum of the two,
Delta-up is the increase in left ventricular pressure when it is augmented by inspiration
Delta-down is the decrease in cardiac output when the venous return decreases (also due to inspiration)
The total delta in well-filled mechanically ventilated patients should be no more than 10mmHg.
A large delta-up demonstrates that the left ventricle was "just waiting" for something to decrease its afterload; this suggests that it is a failing ventricle. A large delta-down demonstrates that the patient is hypovolemic and the central venous return is already barely adequate.