Oxygen Delivery and Hemoglobin Dissociation Curve

Introduction to Hemoglobin Dissociation Curve

The American Thoracic Society recommends understanding the hemoglobin dissociation curve to comprehend oxygen transport and release to tissues, with its position directly determining the capacity of hemoglobin to capture oxygen in the lungs and release it in peripheral tissues 1
The relationship between partial oxygen pressure (PaO2) and oxygen saturation (SaO2) is represented by a sigmoidal curve, reflecting the cooperative effect of oxygen binding to hemoglobin 2

A shift to the right facilitates oxygen release to tissues and occurs with increased H+ ions (acidosis/low pH), increased CO2, increased 2,3-DPG, and increased temperature (hyperthermia) 3, 4
A shift to the left increases oxygen capture in the lungs but hinders its tissue release, resulting in "functional anemia" 4

The goal of oxygen therapy is not just to increase SaO2 but to optimize oxygen delivery to tissues (DO2) 1, 2
Target saturation ranges: 94-98% for patients without risk of hypercapnia, and 88-92% for patients with chronic obstructive pulmonary disease (COPD) to reduce mortality 2
Titrated oxygen therapy to 88-92% in COPD patients significantly reduces mortality compared to high-concentration oxygen 2

Normal SpO2 can mask significant tissue hypoxia in patients with a left-shifted curve 4
Standard pulse oximeters cannot differentiate carboxyhemoglobin from oxyhemoglobin, reporting SpO2 >90% even with COHb ≥25% 5
In methemoglobinemia, SpO2 can be normal while tissues suffer severe hypoxia 4
Always confirm with laboratory co-oximetry when suspecting elevated COHb or MetHb 5

CaO2 depends mainly on hemoglobin level, oxygen saturation, and a small amount of dissolved oxygen 1, 4
Hemoglobin targets: 70 g/L (7 g/dL) for critically ill patients without coronary disease, and 100 g/L (10 g/dL) for patients with unstable or symptomatic coronary disease to ensure adequate DO2 1

Stored blood has depleted 2,3-DPG levels, causing a leftward shift of the curve and hindering oxygen release 4
2,3-DPG levels return to ~50% of normal after 6 hours post-transfusion and normalize completely in 48 hours 4

Alkalosis or hypothermia can critically reduce capillary oxygen tension 4
Avoid excessive pH correction in patients with chronic kidney disease and anemia, as decreased oxygen affinity is compensatory 4

Do not rely solely on SpO2 by pulse oximetry in CO poisoning or methemoglobinemia - always confirm with laboratory co-oximetry 5, 4
Do not administer excessive supplemental oxygen in COPD patients - it can cause hypercapnia and increase mortality 2
Do not ignore tissue hypoxia when SpO2 is normal but the curve is left-shifted 4