TCI · TIVA Pathway  /  Bolus-Speed Lab
Subbu Insights · Interactive Lab Guide

How fast you push → how hard it hits

A two-minute guide to the propofol bolus-speed simulator — how to read it, what each control does, and five bedside scenarios that make the pharmacokinetics click.

Launch the simulator → Eleveld model · plasma vs effect-site
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The same induction dose, pushed slowly or fast, reaches almost the same depth of hypnosis — but a fast push produces a far higher plasma peak, and the plasma peak is what drops the blood pressure and stops the breathing.

Speed buys plasma risk, not depth.
01

Two curves, two meanings

Everything in this lab hangs on telling the two line styles apart. Get this, and the rest reads itself.

Solid line

Plasma concentration · Cp

The drug arriving in the blood. This is the side-effect hit — it drives hypotension and apnoea. A fast push spikes it; a slow push flattens it.

Dashed line

Effect-site · Ce

The concentration at the brain — what actually produces hypnosis. It is buffered by ke₀, so it barely cares how fast you pushed.

Watch the gap between them. The solid plasma curves fan out wildly with push speed; the dashed effect-site curves all converge to nearly the same peak. That gap is the whole lesson.

What changes when you slow the push
2 mg/kg · 70 kg · 40 yr · male. Bars = plasma peak (Cp); dots = effect-site peak (Ce).
10 s
21.2 µg/mL
30 s
18.7
60 s
15.9
120 s
11.9
Plasma peak nearly halves: 21.2 → 11.9 µg/mL Effect-site peak barely moves: 3.11 → 3.04 µg/mL
02

The controls

Six inputs. The first four set the patient and the dose; the last two are the experiment.

Bolus dose
mg/kg. Sets how much drug — and therefore scales both the plasma and the effect-site peaks. This is your depth lever.
Weight
kg. Total dose = mg/kg × weight, and the Eleveld model uses it to size the compartments.
Age
years. Shifts the model's clearances and ke₀ — older patients reach a given effect-site level at lower doses.
Sex
Male / female. A covariate in the Eleveld pharmacokinetics.
Your push speed
seconds. The star control. Slide it and watch your custom curve move against the fixed 10 / 30 / 60 / 120 s references. This is your side-effect lever.
▶ Replay  ·  show Ce
Replay re-runs the injection sweep so you can watch the peaks build in real time. show Ce toggles the dashed effect-site curves on and off.
03

How to drive it

A clean first run that surfaces the core idea in under a minute.

  1. Leave the defaults (2 mg/kg · 70 kg · 40 yr · male) and make sure show Ce is on.
  2. Look only at the solid curves first. Note how the 10 s line peaks roughly twice as high as the 120 s line.
  3. Now follow the dashed curves to their peaks. They land almost on top of each other — speed hardly touched the depth.
  4. Drag Your push speed from fast to slow and watch your custom plasma curve collapse downward while its effect-site twin stays put.
  5. Hit ▶ Replay to see the spike-then-fall in motion, and read the Where the bolus goes diagram alongside it.
  6. Change dose and notice the difference: now both curves rise together. Dose moves depth; speed moves only the plasma hit.
04

Where the bolus goes

Why the plasma curve spikes and then falls — the three-compartment story, made visible.

V1 central · small V2 muscle fast V3 fat slow · large Ce redistribute

The whole dose lands in V1 — a deliberately small central volume. Briefly, it has nowhere to go, so the plasma concentration spikes.

Within seconds it begins to redistribute into muscle (V2) and fat (V3), and the plasma level falls just as quickly. A faster push crowds more drug into that small space at once — a taller spike for the same dose.

This is also why a single bolus wears off: the patient lightens because the drug redistributes, not because it has been cleared.

05

Five scenarios at the bedside

Each one is a 30-second experiment. Set the inputs, run it, and read the lesson off the curves.

1

The fragile heart — slow the push, keep the depth

78 yr · 55 kg · aortic stenosis / poor LV reserve
Set
dose 1.5 mg/kg · weight 55 · age 78 · then push at 15 s vs 90 s
You'll see
The 15-second plasma peak towers; the 90-second peak is dramatically lower — yet the two dashed effect-site curves reach almost the same height.
Lesson
In the haemodynamically fragile, the plasma spike is the enemy. Slowing the injection blunts the pressure drop without under-dosing the brain. Slow is smooth.
2

RSI — counting the cost of speed

Emergency airway · shocked / hypovolaemic
Set
dose 2 mg/kg · compare push at 10 s vs 30 s
You'll see
Plasma peak ~21 µg/mL at 10 s versus ~19 at 30 s — and the effect-site peak is essentially identical (~3.1).
Lesson
A 20–30 s push secures the same depth at a lower plasma peak. Reserve the true slam for when the airway risk genuinely outweighs the cardiovascular one — make it a decision, not a reflex.
3

"I gave it slowly and it still worked"

The trainee's worry about under-dosing
Set
defaults · show Ce on · compare 30 s vs 120 s
You'll see
The dashed effect-site curves converge to nearly the same peak (3.08 vs 3.04); only the time-to-peak shifts slightly later.
Lesson
Depth is set by ke₀ and dose, not push speed. A slow push is not an inadequate induction — the effect site is buffered against how fast the syringe moved.
4

Dose vs speed — two independent levers

Building the mental model
Set
first change push speed only · then change dose 1.5 → 2.5 mg/kg
You'll see
Speed moves the plasma curve but barely the effect-site one. Dose moves both curves together.
Lesson
To deepen induction, raise the dose; to soften the haemodynamic hit, slow the push. Two separate dials — don't confuse one for the other.
5

Why the patient wakes — redistribution, not metabolism

Single bolus, no maintenance
Set
any speed · hit ▶ Replay · watch the curve fall + the V1→V2/V3 diagram
You'll see
Plasma spikes, then drops steeply as drug leaves the small central compartment into muscle and fat.
Lesson
A single induction bolus is terminated by redistribution, not clearance. The three-compartment model, seen directly in the shape of one curve.

The five things to carry to the bedside

  • 01
    Solid = plasma (side-effects), dashed = effect-site (depth). Read the gap between them.
  • 02
    Push speed scales the plasma peak almost inversely with time — fast push, tall spike.
  • 03
    Effect-site peak is governed by ke₀ and dose, not speed — depth is preserved when you slow down.
  • 04
    Dose is the depth lever; push speed is the side-effect lever. Independent controls.
  • 05
    Slowing the push lowers the haemodynamic hit essentially for free. That is the clinical gift.

Now go and break it on purpose

Push an elderly patient fast. Push a young one slow. Watch the gap between the curves move.

Open the Bolus-Speed Lab →
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