One beat as a loop
Everything so far becomes a single picture in the pressure–volume loop. We plot ventricular pressure (up) against ventricular volume (right) and let one beat draw a closed loop. Read it counter-clockwise and it walks through the four phases of the cycle — filling, building pressure, ejecting, and relaxing — in order.
- Filling (diastole). Mitral valve open; volume rises along the bottom from end-systolic to end-diastolic volume, pressure barely changes.
- Isovolumetric contraction. Both valves shut; the squeeze raises pressure with no change in volume — a vertical line up. This is what makes the “lub.”
- Ejection (systole). Aortic valve opens; volume falls as blood leaves — the stroke volume is the width of the loop.
- Isovolumetric relaxation. Both valves shut again; pressure falls with no volume change — a vertical line down. Then the loop closes and the next beat begins.
Two ways a pump can fail
A pump can break in two distinct ways, and the cardiac cycle has two phases to match. A systolic problem is a weak squeeze: the ventricle can't generate enough force, ejection fraction falls, and stroke volume drops. On the loop, the left edge shifts right — the heart can't empty as far. This is heart failure with reduced EF.
A diastolic problem is the opposite — the squeeze is fine, but the relaxation is not. A stiff ventricle (impaired diastolic function) resists filling, so to pack in enough blood the filling pressure must climb steeply. The EF can look perfectly normal, yet the heart struggles, backing pressure up into the lungs. This is heart failure with preserved EF — and it explains why a “normal EF” never rules heart failure out.