When MAP drops, the peripheral compartment closes off. Drug pools in a smaller central volume — and a larger fraction of that flow goes to the brain. Same dose, much higher effect.
FOAMfrat Education
Volume of Distribution Trainer
Two-Compartment Model
PeripheralCentralDrug
Patient Inputs
Patient Weight
kg
Ketamine Dose2.0mg/kg
0.5 mg/kginduction range3.0 mg/kg
Mean Arterial Pressure70mmHg
35 (arrest)65 (threshold)90 (normal)
Pharmacokinetic Output
Total Dose Given
160mg
2.0 mg/kg × 80 kg
Volume of Distribution
3.0L/kg
normal Vd
Plasma Concentration
0.67mg/L
therapeutic
Brain Exposure
1.0×
baseline
Clinical Pearl
At a normal MAP, the dose distributes across roughly 3.0 L/kg. Plasma and brain levels sit in the therapeutic window. Now drop the MAP and watch the math change.