An apnoea simulator

What this is. A computational model of gas exchange during apnoea, run forward in real time on two patients side by side. Everything on screen is computed from the physiology — the oxygen stores, the dissociation curves, the shunt, the cardiac output and the mechanics of a sealed lung — not replayed from a recording. Nothing is animated for effect.

What you are looking at. Two identical patients, both obstructed from induction. The only difference between them is what sits in the pharynx. In the lungs, bright is oxygen and dull is nitrogen and CO₂; the dark gap above is the vacuum the obstruction creates as gas is absorbed and none replaces it, and the bases go violet as they collapse. The head turns blue on deoxygenated haemoglobin, not on saturation — which is why an anaemic patient never looks as bad as they are.

How to use it. Press Play. Move any slider and the whole simulation re-computes from the new patient; nothing is interpolated. Collapsibility starts at the calibrated median — the patients who desaturate despite good tracheal oxygen sit near the top of its range, and that is worth seeing.

What it is not. Not a medical device and not validated for patient care. It disagrees with the one human measurement of arterial oxygen under complete obstruction by a wide margin, and that disagreement is recorded rather than hidden. Treat it as a way to reason about mechanism, not as a predictor for an individual.

Modelled, not measured. Saturation carries a pulse oximeter delay. There is no end-tidal CO₂ because there is no ventilation; the CO₂ and pH shown are arterial model values you would not have at the bedside. Each arm stops where the model's fixed cardiac output stops being defensible. Closing capacity and the FRC–BMI relation are parameterised, not fitted to source data.