Cerebral blood flow (CBF) stays roughly constant across a wide range of mean arterial pressures (MAP) because resistance-vessel smooth muscle constricts or dilates within seconds of a pressure change — the myogenic (Bayliss) response. Dynamic cerebral autoregulation describes how fast this correction happens, and is clinically tested with a thigh-cuff release: bilateral thigh cuffs inflated above systolic pressure are suddenly deflated, producing a rapid, brief step-down in MAP. CBF velocity (CBFV) initially falls with the pressure, then — if autoregulation is intact — recovers toward baseline within a few seconds even though pressure stays low.
This simulator drives a second-order damped model (in the style of Tiecks et al., 1995) of that recovery. Let u = ΔMAP/MAP₀ be the normalized pressure step and y the resistance-vessel compensation state:
T² · y'' + 2·D·T · y' + y = K · u(t)
ΔCBFV/CBFV₀ = u(t) − y(t)
T (time constant), D (damping) and K (gain) are set from the Autoregulation Index (ARI, 0–9), the standard clinical scale for dynamic autoregulation: ARI 0 means y never moves (CBFV simply tracks pressure — pressure-passive flow, as seen after severe traumatic brain injury or stroke); higher ARI drives y toward u faster and with less oscillation, restoring flow while pressure stays low.
- Run Thigh-Cuff Test — steps MAP down by ~22% and holds it there, exactly as cuff release does; watch CBFV dip then recover (or fail to) depending on ARI.
- ARI slider — sets T, D and K from the standard 0–9 table used in clinical dynamic-autoregulation testing (Tiecks-style parameters).
- PaCO₂ slider — hypercapnia (elevated CO₂) is a potent cerebral vasodilator that competes with and blunts the myogenic response, so the effective gain is scaled down above ~45 mmHg; hypocapnia sharpens it slightly.
- Baseline MAP — sets the resting operating pressure the step is taken from.
Clinical relevance: a low measured ARI after stroke, subarachnoid hemorrhage or traumatic brain injury identifies pressure-passive cerebral flow, where blood pressure management directly controls brain perfusion — a key input to neuro-ICU treatment targets.