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Let's have a look

click image to enlarge

The Y axis represents the Cardiac output (CO) or Venous return (VR) in l/min

The X axis represents the right atrial pressure (RAP) in mm Hg

Curves that slope from Y to X represent venous return

Curves that slope from X axis and go up represent the cardiac output

The intersection of each two curves represent the actual status of a given case, for example the central point represent the normal physiological resting state with 5 l/min CO and VR and RAP of 2 mm Hg.

The intersection of VR curves with the X axis represent what's called the

**mean systemic filling pressure MSFP**(the tightness with which the circulatory system is filled with blood) and the greater the MSFP the easier it is to push blood back into the heart, it's the pressure in the circulation when the heart stops and blood redistributes and equilibrate in all the vessels (classic USMLE question, is few seconds to one minute after ventricular fibrillation).

Higher VR curves represent higher volumes and and likewise lower VR curves represent hypovolemia, for CO it's the higher contractility that shift the curve up and heart failure that shift the curve down.

Examples:

**Point H**is a failed heart with increased preload

**Point C**can be seen in excercise where we have higher cardiac output because of sympathetic stimulation and higher venous retrun because of pressurized peripheral veins and dilated muscular arterioles