Episode notes
Fick's Law of Diffusion: The Equation Behind Every Inhaled Anesthetic
Stuff spreads from where it is crowded to where it is not. Fick's law is that intuition written as arithmetic — and it governs every membrane in the body, from the alveolar wall between air and blood to the cell membrane between an anesthetic in your bloodstream and the neuron it needs to reach.
This episode unpacks each lever, then follows them to the bedside. Preoxygenation and apneic oxygenation. Why a volatile agent crosses more slowly in both directions when the membrane is compromised, so induction and emergence both drag. Why a recruitment maneuver or a little PEEP speeds a sluggish case back up. And why a widening alveolar-to-arterial oxygen gradient is the fingerprint of a membrane that has lost area or gained thickness.
Key Takeaways
- Diffusion runs down a gradient and spends no energy. The working anesthesia form: rate ∝ (A × ΔP × solubility) / (T × √MW).
- Five levers — surface area, gradient, thickness, solubility, molecular weight. Area, gradient, and thickness move the rate one-for-one; molecular weight moves it only by its square root.
- "Crosses faster" is not "faster onset."
- Preoxygenation is Fick's law in action
- In a healthy lung, oxygen transfer is perfusion-limited, not diffusion-limited. Disease flips it
Timestamps
00:00 — The pool and the fence
02:57 — Five levers, one equation
04:07 — Why CO₂ crosses faster than O₂
05:05 — The word that trips everyone: solubility
06:00 — Fick, Fourier, and a borrowed equation
07:07 — From Theory to Practice
07:20 — Apneic oxygenation and THRIVE
08:30 — When disease flips the levers
09:12 — Atelectasis, PEEP, and the sigh
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