Oxyhemoglobin Curve Shifts: Loading, Unloading, Stored Blood

S-shaped relationship: arterial O₂ saturation (y-axis) vs PaO₂ (x-axis). Normal P50 (the PaO₂ at which Hgb is 50% saturated) = ~26-27 mmHg. The shape matters: above PaO₂ 60 mmHg, the curve is flat — small drops in PaO₂ = small drops in SaO₂. Below PaO₂ 60 mmHg, the curve is steep — small drops in PaO₂ = big drops in SaO₂. This is why SpO₂ stays ~90% for a long time as PaO₂ falls, then collapses.

P50 increases. Same PaO₂, lower SaO₂ — Hgb releases O₂ more readily to tissues. Causes: ↑ temperature (fever, exercise), ↑ CO₂ (Bohr effect), ↑ H+ (acidosis), ↑ 2,3-DPG, certain hemoglobinopathies. Clinical: a febrile septic patient with metabolic acidosis has a right-shifted curve — better tissue O₂ delivery, partly compensating for poor cardiac output. Don't 'fix' the fever or acidosis without thinking about this trade-off.

P50 decreases. Same PaO₂, higher SaO₂ — Hgb holds onto O₂. Causes: ↓ temperature (hypothermia, post-CPB rewarming), ↓ CO₂ (alkalosis from hyperventilation), ↓ H+ (alkalosis), ↓ 2,3-DPG. CRITICAL: stored PRBC have severely depleted 2,3-DPG (drops to 1/3 normal by day 14, near zero by day 21). Massive transfusion of stored blood = profound left shift. The patient has 'normal' SpO₂ but shockingly poor tissue O₂ delivery. Clinical: persistent lactic acidosis after MTP can be a 2,3-DPG issue, not just perfusion. Newer / younger blood preserves 2,3-DPG better; activity restored within 24 hours of transfusion as red cells regenerate.

CO has 240× the Hgb affinity of O₂. Each CO molecule blocks an O₂ binding site AND left-shifts the remaining Hgb (cooperativity poisoning). SpO₂ reads falsely normal (CO-Hgb absorbs the same wavelengths). Diagnosis requires co-oximetry. Treatment: 100% O₂ (half-life of CO drops from 5 hr → 90 min) ± hyperbaric for severe / pregnant / cardiac. Methemoglobin (Fe³⁺ form) cannot bind O₂ + left-shifts neighboring sites. SpO₂ saturates at ~85% regardless of true PaO₂. Treatment: methylene blue 1-2 mg/kg IV (caution G6PD deficiency).

(1) Two patients with same SpO₂ may have very different tissue oxygen delivery (DO₂). DO₂ = CO × CaO₂ (which depends on Hgb, SaO₂, and the curve position). (2) Stored blood + hypothermic post-CPB patient = double left-shift = consider giving fresher blood when possible + warming the patient. (3) Permissive acidosis in ARDS may help tissue O₂ delivery — don't reflexively correct unless severe. (4) Pulse oximetry is wavelength-based and can lie: CO toxicity (falsely high), methemoglobinemia (saturates at 85%), methylene blue infusion (falsely low). Suspect → co-oximetry.