One beat, drawn as pressure against volume
A left ventricular (LV) pressure–volume loop traces one heartbeat. Move right for more blood in the ventricle; move up for higher pressure. Follow the arrows anticlockwise. These are explanatory drawings, not live measurements.
End-diastolic volume (EDV) is the volume just before contraction. End-systolic volume (ESV) is the blood remaining after ejection. Their difference is stroke volume (SV) in an ordinary beat with normally functioning valves. Left ventricular end-diastolic pressure (LVEDP) is the filling pressure at the end of diastole.
Illustrative loop. Aortic-valve closure pressure is not the peak systolic pressure. During ejection, pressure can rise and then fall.
| Phase | What happens? | Valves |
|---|
| 1 → 2 · Isovolumetric contraction | Pressure rises while blood volume stays almost constant. | Both closed |
| 2 → 3 · Ejection | Blood leaves for the aorta; ventricular volume falls. | Aortic open; mitral closed |
| 3 → 4 · Isovolumetric relaxation | Pressure falls while blood volume stays almost constant. | Both closed |
| 4 → 1 · Filling | Blood enters from the left atrium. Atrial contraction adds a late filling contribution. | Mitral open; aortic closed |
“Isovolumetric” means the volume stays constant. Pressure opens a valve when upstream pressure exceeds downstream pressure; the valve is not actively pulling blood through.
Read the width, height and position. Width shows ejected volume; height reflects pressure. The area enclosed represents external mechanical stroke work, not the heart’s total oxygen or energy use. Ejection fraction (EF) is SV ÷ EDV × 100% for an ordinary beat.
Frank–Starling: filling changes the next beat
At the same contractility and afterload, greater filling usually lets the ventricle eject more blood. The response becomes smaller as the curve flattens. More filling is therefore not a guarantee of a useful increase in output.
Schematic curves at a fixed afterload. A → B is more filling on the same curve; a change in contractility gives a different curve.
Preload describes the muscle’s loading before contraction; this app represents it using end-diastolic volume or filling pressure. Contractility describes active pumping strength. Afterload is the load against which the ventricle ejects; arterial pressure, resistance and compliance all contribute.
Volume and filling pressure are related but are not interchangeable: a stiff ventricle can have high filling pressure without a large volume. Watch the loop and filling pressure together.
Try a small experiment
- Increase circulating volume slightly and watch filling, stroke volume and atrial pressure.
- Reduce LV contractility and see how the loop and Frank–Starling curve change.
- Increase systemic resistance. Compare ejection pressure, blood remaining after ejection and the next beats as the whole circulation adjusts.
What shifts the loop?
| Isolated change | Typical initial effect | Interpretation |
|---|
| More preload | Right edge moves right; the loop usually widens. | More filling can recruit more ejection at a fixed contractility and afterload. |
| More afterload | Higher ejection pressure, less ejection and more blood remaining. | The following beats can change preload as blood redistributes. |
| More contractility | More ejection and a smaller end-systolic volume at comparable loading. | This is a change in ventricular pressure-generating capacity. |
| Lower diastolic compliance | More filling pressure at a given volume. | The passive filling relationship becomes steeper; pressure alone is not a volume measurement. |
These are conditional trends, not guaranteed final changes. In this closed loop, afterload, filling and output interact. In a susceptible overloaded ventricle, more filling can reduce pressure-generating capacity.
The two pressure–volume relationships
The end-diastolic pressure–volume relationship (EDPVR) describes passive filling stiffness and is curved. The gold curve on this app’s ventricular plots shows its passive pressure law. Active atrial contraction and residual activation can make a live end-diastolic point differ from that passive curve.
The end-systolic pressure–volume relationship (ESPVR) is obtained from multiple beats with altered loading. A local straight-line approximation is written as:
Pes ≈ Ees × (Ves − V₀)
End-systolic elastance (Ees) is the slope; pressure is in mmHg and volume in mL. V₀ is the extrapolated volume-axis intercept. It is not the blood left in the ventricle or an actual “empty-heart” volume. Ignoring V₀ changes the slope inferred from a single pressure–volume point.
Arterial load and coupling
Effective arterial elastance (Ea) summarises arterial load as end-systolic pressure divided by stroke volume. It is not the same as systemic vascular resistance (SVR): resistance, arterial compliance and beat timing all influence the load faced by the ventricle.
Ea = Pes / SV
Coupling ratio = Ea / Ees
This app uses interpolated aortic-valve closure pressure for Ea and integrated forward stroke volume. For estimated Ees it makes five conserved preload changes in copied circulations, freezes effective parameters, and iteratively fits the maximum P/(V − V₀) points. Those points need not occur at aortic closure. Five independent resistance changes provide an afterload comparison.
The labelled Preload ESPVR slope (Estimated Ees) is therefore a fitted model estimate, not the ventricular model’s active-pressure coefficient. Shortening losses and the other model relationships can make the slope depend on loading. A good fit or agreement between preload and afterload slopes does not establish clinical validity. Uninterpretable beats suppress the estimate and ratio.
Why the live point can miss the Frank–Starling curve
Each reference curve holds aortic pressure fixed for isolated test beats. The connected circulation has pulsatile, changing afterload and redistribution between compartments. Its live operating point can lie away from those reference curves.
The optional overload susceptibility control adds a downturn beyond a chosen end-diastolic-volume scale of 140 mL. This is a simulation assumption, not a universal physiological threshold. Normal reference curves have no imposed overload downturn.
Where the simple loop needs care
Ideal one-way valves, simplified activation and lumped pressure–volume laws shape these loops. Active relaxation, valve inertance, septal interaction and pericardial restraint are not independently modelled. Extreme loading can permit passive through-flow or late outflow, making integrated forward volume differ from EDV − ESV; the heart page flags these cases. Ejection fraction uses ventricular volume change.
Pressure–volume loop area represents external stroke work, not total myocardial energy expenditure or oxygen demand. The guide’s diagrams are illustrative and are not patient reference targets.
Further reading
