Cardiac Mechanics, Start to Finish - Pressure-Volume Loops and Haemodynamics

Preload, afterload and contractility are three of the most confidently used and least clearly understood words in cardiology. They resist explanation because they are not independent in a real patient: change one and the others move. A model is the only place they can be separated.

One time-varying elastance engine drives all thirty-five modules, so every view is the same heart seen differently. Change contractility in an early module and the pressure-volume loop, the filling curve and the coupling ratio all respond together.

Using it in a teaching session

The pressure-volume loop is where this earns its place. Most trainees can label one and few can predict how it moves. Change afterload alone and ask what happens to stroke volume before showing the answer.

The shock states module is the other high-value one. Seeing cardiogenic, hypovolaemic, distributive and obstructive shock as four distinct loop shapes, rather than four lists of numbers, is the version that survives contact with a sick patient at 3am.

Valve lesions work well immediately afterwards, since each deforms the loop in a way that follows from the physiology rather than needing separate memorisation.

For those who want it, ventriculo-arterial coupling and the work-efficiency modules go considerably deeper than most textbooks bother to.

What the model simplifies

A single-chamber elastance model with a lumped arterial load cannot capture regional wall motion, ventricular interaction, or the time course of neurohormonal adaptation. It is a teaching model of the mechanics, not a patient simulator.