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The Shunt Equation: Why More Oxygen Won't Fix True Shunt

Every arterial blood gas is a mixture of two streams — blood that saw ventilated alveoli and picked up oxygen, and blood that skipped them entirely. The shunt equation solves for what fraction came from the second stream.

This episode covers: the mass-balance algebra of Qs/Qt = (CcO₂ − CaO₂) / (CcO₂ − CvO₂); PAO₂ versus PaO₂; why normal shunt sits near 5% and anything above 10% is pathologic; true shunt versus V̇/Q̇ mismatch; the FiO₂ test; mixed venous sampling and why cardiac output changes the answer; the Riley–Cournand history; the perioperative causes — atelectasis, pneumonia, hemothorax, ARDS — and the anatomic ones: PFO, ASD, VSD, pulmonary AVM; and why PEEP — not more oxygen — is the response to true shunt.

Against true shunt, extra FiO₂ is shouting at a room no one is in.

Key Takeaways

• Qs/Qt = (CcO₂ − CaO₂) / (CcO₂ − CvO₂) — the shunt fraction.

• PAO₂ is calculated. PaO₂ is measured.

• Normal shunt ≈ 5% — thebesian and bronchial veins. Above 10% is pathologic.

• True shunt ignores FiO₂. V̇/Q̇ mismatch doesn't.

• FiO₂ test: PaO₂ above 200 on 100% oxygen suggests mismatch; below 200 suggests true shunt.

• Anatomic shunts — PFO, ASD, VSD, pulmonary AVM — ignore FiO₂ too.

• A falling cardiac output deepens hypoxemia from an unchanged shunt.

• PEEP reopens the lung. More oxygen doesn't.

Timestamps

00:00 — Two pipes, one mixing chamber

01:53 — The equation, term by term

03:14 — PAO₂ versus PaO₂

04:06 — Normal shunt and the 10% line

04:19 — True shunt versus V̇/Q̇ mismatch

05:08 — The FiO₂ test

05:24 — Why cardiac output changes the answer

05:56 — Riley, Cournand, and Swan-Ganz

06:48— From Theory to Practice

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0.9% Sodium Chloride: The Drug We Give Without Thinking (Free Preview) | Anesthesia Drug Compendium Series

Normal" saline is not normal. Plasma runs Na⁺ ≈ 140 and Cl⁻ ≈ 100; the bag holds 154 of each at 308 mOsm/L. The supraphysiological chloride — half again above normal — defines everything modern medicine worries about in the bag.

This episode covers: the 1832 cholera outbreak that invented IV fluid resuscitation, Hamburger's 1896 red-cell experiments that gave us the "0.9%" (and overshot the true isotonic value), hyperchloremic metabolic acidosis via the Stewart strong-ion-difference mechanism, the 2018 SMART and SALT-ED trials, and the three jobs no balanced crystalloid does better than normal saline.

The full episode, available on Supercast >>> https://diaryofaloststudent.supercast.com/

Timestamps

00:42 — "Normal" is not normal

01:16 — The modern formulation

01:40 — The full episode continues on Supercast...

Full Episode

The complete saline episode — is on Supercast https://diaryofaloststudent.supercast.com/

If this made something click, leave a 5-star rating and follow the show. That's how other anesthesia providers — residents, SRNAs, SAAs, CRNAs, and anesthesiologists — find it. New episodes every week.

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Massive Transfusion Protocol: The 1:1:1 Ratio, and the Diamond of Death | Anesthesia Clinical Concepts

Ten units of red cells in 24 hours. That was the old definition — arbitrary, borrowed because ten units approximates one adult blood volume. It never accounted for rate. A patient can lose half their volume in an hour and never reach ten units before they arrest. Massive transfusion is not a number. It is a physiology problem with a clock attached.

This episode covers the protocol from activation to deactivation — the ABC score, the 1:1:1 ratio, and what the PROPPR trial actually showed. Then the mechanism underneath: the lethal triad of hypothermia, acidosis, and coagulopathy, plus the fourth variable that turns a triad into a death spiral — ionized hypocalcemia from citrate. Viscoelastic testing, tranexamic acid, calcium, warming, and the team roles that decide outcomes.

Key Takeaways

• Massive transfusion is traditionally defined as 10 or more units of red cells in 24 hours. Modern practice favors dynamic criteria

• The ABC score uses four bedside variables — penetrating mechanism, positive FAST, systolic BP ≤90, heart rate ≥120. Two or more activates the protocol.

• Balanced resuscitation with a 1:1:1 ratio of plasma to platelets to red cells improves early hemorrhage control. The PROPPR trial showed no significant 24-hour mortality difference versus 1:1:2, but more patients in the 1:1:1 group achieved hemostasis and fewer died of exsanguination.

• The lethal triad — hypothermia, acidosis, and coagulopathy. Citrate is the fourth variable that creates the diamond. Every unit of blood carries citrate that binds ionized calcium. Hypothermia and hypoperfusion slow citrate metabolism, prolonging hypocalcemia and worsening coagulopathy.

• Transexamic acid works by blocking plasmin activation, preventing clot degradation rather than promoting clot formation.

• Rapid infusers warm and deliver 200–1,200 mL/min. Warming is not comfort — it is coagulation management.

Timestamps

00:00 — The falling wall

02:00 — 1:1:1 ratio

02:36 — The lethal triad and the diamond of death

03:33 — Calcium, Transexamic Acid, Cryoprecipitate

04:10 — The rediscovery of balance

05:05 — PROPPR Trial

05:31 — From Theory to Practice

If this made something click, leave a 5-star rating and follow the show. That's how other anesthesia providers — residents, SRNAs, SAAs, CRNAs, and anesthesiologists — find it. New episodes Tuesday and Friday.