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Lucas Kimmig @Lucas_Kimmig
, 7 tweets, 3 min read Read on Twitter
#Tweetorial on hypoxemic #respiratoryfailure. #FITSurvivalGuide #FOAMed
1️⃣ Shunt
2️⃣ V/Q mismatch
3️⃣ Low FiO2/low O2 tension
4️⃣ Alveolar hypoventilation
5️⃣ Diffusion limitation
6️⃣ Venous admixture
1️⃣Shunt - can be intra or extrapulmonary. No V, only Q. Does not respond to O2 - shunt fraction not exposed to o2. Inflamm alveolar dz (eg PNA) can p/w shunt (imp. vasoconstr.) Think abt PFO w R➡️L shunt in PE, ARDS (elevation in R sided pressures), check bubble study. Aa grad ⬆️
2️⃣V/Q mismatch. More Q than V but regions with low V not entirely excluded from gas exchange. Can improve w increasing FiO2. Probably 75-90% of hypoxemia cases. Wide Aa gradient.
3️⃣Low PAO2 due to low FiO2 or low Patm/PO2. This one is usually apparent from the history. Breathing in gases with low FiO2 or going to Mt. Everest 🏔. Aa gradient is normal (Art pO2 is low b/c Alv pO2 is low).
4️⃣ Alveolar hypoventilation. Similar to 3️⃣: Aa gradient is low and hypoxemia is due to low Alv pO2. Alveolus is filled with CO2, leaving less room for O2. Common post-OP (analgesics, sedation). Check resp rate for clue, EtCO2 or ABG ➡️ calc Aa gradient
5️⃣Diffusion limitation. Rare cause of resting hypoxemia. RBC Hgb saturated early on trip along alveolar capillary➡️even if low diffusivity, usually enough interface time to saturate fully. Exertion ⬆️❤️output,⬆️RBC speed➡️shorter interface time. Wide Aa gradient.
6️⃣Venous admixture. Low venous SO2➡️lungs can’t fully re-oxygenate. Contributes to hypoxemia but healthy, V/Q matched lungs can re-oxygenate even severely desaturated Hgb. Improve by ⬆️O2 delivery or ⬇️O2 consumpt;both ➡️higher venous SO2, less “work” for lungs to resat. Wide Aa
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