Theory 01/Anesthesiology & Critical Care/Neuropharmacology
Depth Is a Hysteresis Loop
Two patients at the same concentration can be awake and asleep. The difference is which way they got there.
The depth of anaesthesia is set by hysteresis in the neural circuit rather than by the effect-site concentration, so emergence happens at a lower concentration than induction and concentration-based monitoring mispredicts awareness.
01The evidence
Neural inertia has been described across species, with the transition into and out of anaesthesia occurring at different concentrations for the same drug, and the gap varying between agents.
02The tension
Depth monitors are calibrated against concentration and against processed EEG, both of which assume a single state for a given level. If the state depends on history, the same reading means different things on the way in and the way out.
03The mechanism
Anaesthetic state is maintained by a network of mutually excited and inhibited populations. Such a network has two stable states over a range of drive, and which one it occupies depends on where it came from. That is hysteresis, and it is a property of the circuit rather than of the drug.
The width of the loop sets the ambiguity. A wide loop means a large concentration range in which either state is possible, so a monitor reading in that band cannot tell you which state the patient is in. The clinical consequence is that awareness risk is highest inside the loop, not at a particular number.
04What this is not
Concentration models assume one state per level. This assumes two states over a band, which predicts that the risky zone is defined by the loop width rather than by a threshold.
05Predictions
- P1Awareness should occur inside the hysteresis band, not at a fixed concentration.
- P2Agents with wider loops should show more awareness events at matched monitoring.
- P3Interventions that narrow the loop should reduce awareness without changing dose.
06Refutation
Emergence occurring at the same concentration as induction, in a controlled comparison, would remove the loop.
07What it changes
It would make loop width a drug-selection criterion and reframe depth monitoring around history.