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

Got something to say to this podcast? Send a voicemail, and you might hear yourself on the next episode! 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 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.