Oxygen dissociation curve

Definition:
The oxygen dissociation curve is a graphical representation that depicts the relationship between the
partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen (oxygen saturation or
%HbO2). It is a fundamental concept in understanding oxygen transport and tissue oxygenation.

1. Sigmoidal Shape: The sigmoidal shape of the oxygen dissociation curve is due to the cooperative binding
of oxygen to hemoglobin. As the first oxygen molecule binds to hemoglobin, it induces conformational
changes that increase the affinity of hemoglobin for subsequent oxygen molecules. This positive
cooperativity enhances oxygen uptake in the lungs and oxygen release in the tissues.

2. Saturation of Hemoglobin: The y-axis of the oxygen dissociation curve represents the saturation of
hemoglobin with oxygen, expressed as a percentage. At the top of the curve, hemoglobin is almost fully
saturated (typically around 97-99%) when all oxygen-binding sites are occupied. At the bottom,
hemoglobin is minimally saturated, such as in venous blood returning to the heart.

3. p50: The p50 value is the pO2 at which hemoglobin is 50% saturated with oxygen. It reflects the
affinity of hemoglobin for oxygen. A lower p50 indicates higher affinity, meaning that hemoglobin binds
oxygen more readily at lower oxygen pressures. For example, fetal hemoglobin has a lower p50
compared to adult hemoglobin, enabling efficient oxygen transfer across the placenta.

4. Factors Shifting the Curve to the Right: Various factors cause the oxygen dissociation curve to shift
to the right, resulting in reduced hemoglobin affinity for oxygen and enhanced oxygen release to tissues.
These factors include increased temperature (e.g., during exercise), decreased pH (acidosis), increased
carbon dioxide (CO2) levels (Bohr effect), increased 2,3-DPG levels (e.g., in chronic hypoxia), and
elevated altitude.

5. Factors Shifting the Curve to the Left: Conversely, factors that shift the oxygen dissociation curve
to the left increase hemoglobin's affinity for oxygen, making it more difficult for oxygen to be released
to tissues. These factors include decreased temperature (e.g., hypothermia), increased pH (alkalosis),
decreased carbon dioxide levels (e.g., hyperventilation), and decreased 2,3-DPG levels (e.g., blood
 Oxygen dissociation curve
transfusion).

6. Clinical Significance: Understanding the oxygen dissociation curve has clinical implications. In
situations where tissues require increased oxygen delivery, such as during exercise or in hypoxic
conditions, the curve shifting to the right promotes oxygen unloading from hemoglobin to meet the
demand. In contrast, a leftward shift of the curve can impede oxygen release, as seen in conditions like
carbon monoxide poisoning or certain hemoglobinopathies.

7. Arterial Blood Gas Analysis: The oxygen dissociation curve helps interpret arterial blood gas (ABG)
results. ABG measurements of pO2 and oxygen saturation can be plotted on the curve to assess the
adequacy of oxygen delivery and tissue oxygenation.

8. Relationship with Bohr Effect and Haldane Effect: The oxygen dissociation curve is closely linked to
the Bohr effect and Haldane effect. The Bohr effect describes how changes in pH and carbon dioxide
levels influence the oxygen affinity of hemoglobin. The Haldane effect explains how the saturation of
hemoglobin with oxygen affects its capacity to carry carbon dioxide.

Anaesthesia concerns:

1. Oxygen Delivery: The oxygen dissociation curve provides insights into the efficiency of oxygen
delivery to tissues. A rightward shift of the curve indicates decreased hemoglobin affinity for oxygen,
facilitating oxygen release in the tissues. This is beneficial in situations where increased oxygen delivery
is required, such as during exercise or in hypoxic conditions.

2. Monitoring Oxygenation: Pulse oximetry is commonly used to monitor arterial blood oxygen
saturation (SpO2) in real-time. However, it's important to note that pulse oximetry only provides an
estimate and cannot capture changes in the shape of the oxygen dissociation curve or detect alterations
in oxygen content or partial pressure.

3. Perfusion-Oxygenation Mismatch: The steep part of the oxygen dissociation curve indicates that
small changes in pO2 result in significant changes in oxygen saturation. This concept is important in
situations where oxygen consumption is compromised, such as sepsis or shock, as even small reductions
in oxygen delivery can lead to tissue hypoxia.

4. Factors Affecting Oxygenation: pH, temperature, carbon dioxide levels, 2,3-DPG levels, and
abnormal hemoglobins can influence the position of the oxygen dissociation curve. Understanding how
these factors affect oxygen affinity helps guide clinical interventions to optimize oxygenation.

5. Oxygen Therapy: The oxygen dissociation curve assists in determining appropriate oxygen therapy
strategies. Patients with chronic obstructive pulmonary disease (COPD), for example, may have a
blunted response to hypoxemia due to a right-shifted curve. In such cases, maintaining higher oxygen
tensions may be necessary to achieve adequate tissue oxygenation.
 Oxygen dissociation curve
6. Hemoglobin Abnormalities: Certain hemoglobinopathies, such as sickle cell disease, affect the oxygen
dissociation curve. Understanding these alterations is crucial for managing patients with such conditions,
as they may have impaired oxygen delivery and increased susceptibility to tissue hypoxia.

7. Ventilation-Perfusion Mismatch: The oxygen dissociation curve helps assess the impact of
ventilation-perfusion (V/Q) mismatch on oxygenation. Areas with low V/Q ratios (e.g., due to lung
pathology or regional lung collapse) may exhibit a right-shifted curve, indicating reduced oxygen affinity
and facilitating oxygen unloading in those areas.

8. Assessment of Oxygen Transport: The oxygen dissociation curve complements other parameters,
such as arterial blood gas analysis, to assess oxygen transport and tissue oxygenation comprehensively.
It aids in interpreting pO2 and oxygen saturation values and evaluating the relationship between them in
different clinical scenarios.

Factors affecting the affinity of oxygen for hemoglobin:

1. pH (Bohr effect): Changes in pH can shift the curve to the right or left. In conditions where there is an
increase in acidity (decrease in pH), such as during exercise or in tissues with high carbon dioxide levels,
the curve shifts to the right. This rightward shift promotes oxygen release from hemoglobin, ensuring
efficient oxygen delivery to the tissues.
2. Temperature: An increase in temperature shifts the curve to the right, while a decrease in temperature
shifts it to the left. This temperature-dependent shift is particularly relevant during exercise or when
there are variations in body temperature. Higher temperatures facilitate oxygen release to meet
increased metabolic demands.
3. Carbon Dioxide (CO2) Levels: Elevated levels of carbon dioxide in the blood, such as during exercise
or in tissues with increased metabolic activity, cause the curve to shift to the right. This phenomenon,
known as the Bohr effect, reflects the role of carbon dioxide in promoting oxygen release from
hemoglobin.
4. 2,3-DPG (2,3-diphosphoglycerate): 2,3-DPG is a byproduct of glycolysis in red blood cells.
Increased 2,3-DPG levels, as seen in conditions like chronic hypoxia or anemia, shift the curve to the
right. This shift enhances oxygen release to the tissues, allowing better adaptation to chronic low
oxygen levels.
5. Fetal Hemoglobin: Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin. This higher
affinity enables the transfer of oxygen from the mother's hemoglobin to the fetal hemoglobin in the
placenta. Fetal hemoglobin has a left-shifted curve compared to adult hemoglobin, facilitating efficient
oxygen uptake by the developing fetus.
6. Abnormal Hemoglobins: Certain abnormal hemoglobins, such as those found in individuals with sickle
cell disease or other hemoglobinopathies, can alter the oxygen dissociation curve. These abnormalities
can result in decreased oxygen affinity, leading to impaired oxygen transport and tissue hypoxia.