Rightward Shift of the Oxyhemoglobin Dissociation Curve
A decrease in pH (acidosis) causes a rightward shift of the oxyhemoglobin dissociation curve. 1
Physiological Basis of the Oxyhemoglobin Dissociation Curve
The oxyhemoglobin dissociation curve represents the relationship between partial pressure of oxygen (PaO2) and hemoglobin saturation (SaO2). This curve has two important features that protect against tissue hypoxia:
- The upper portion is flat - allowing hemoglobin to remain highly saturated despite significant drops in PaO2
- The steep lower portion - allowing oxygen to be released to tissues while maintaining relatively stable PO2 levels
Factors That Cause Rightward Shift
The rightward shift of the oxyhemoglobin dissociation curve enhances oxygen release to tissues and improves oxygen availability. This phenomenon is known as the Bohr effect 1. The following factors cause a rightward shift:
- Increased hydrogen ion concentration (decreased pH/acidosis) - The primary factor in option B
- Increased temperature
- Increased PaCO2
- Increased 2,3-diphosphoglycerate (2,3-DPG)
A rightward shift means that at any given PaO2, hemoglobin releases more oxygen to tissues, which is particularly beneficial during periods of increased metabolic demand or tissue hypoxia.
Analysis of Answer Options
Let's analyze each option:
Option A: Hypothermia - Incorrect. Hypothermia (decreased temperature) causes a leftward shift of the curve, increasing hemoglobin's affinity for oxygen and making it more difficult for oxygen to be released to tissues 1.
Option B: Decrease in pH (acidosis) - Correct. Acidosis shifts the curve to the right, decreasing hemoglobin's affinity for oxygen and facilitating oxygen release to tissues 1.
Option C: Hypocapnia - Incorrect. Hypocapnia (decreased PaCO2) causes a leftward shift of the curve. Increased PaCO2 (hypercapnia) would cause a rightward shift 1.
Option D: Methemoglobinemia - Incorrect. Methemoglobinemia causes a leftward shift of the oxyhemoglobin dissociation curve due to the altered structure of hemoglobin, which increases its affinity for oxygen but reduces its ability to release oxygen to tissues.
Clinical Significance
The rightward shift seen in acidosis is a compensatory mechanism that helps deliver more oxygen to tissues during periods of increased metabolic demand. This is particularly important in:
- Exercise, where lactic acid production decreases pH
- Severe infections with metabolic acidosis
- Respiratory failure with CO2 retention
- Chronic hypoxemic conditions where 2,3-DPG increases
However, it's important to note that in severe acidosis (pH < 6.4), the beneficial effects of the rightward shift diminish 2. Additionally, at very low PaO2 levels, a rightward shift may actually be detrimental as it reduces oxygen uptake in the lungs 3.
In chronic hypoxemia, increased 2,3-DPG in erythrocytes shifts the curve to the right, enhancing oxygen delivery to tissues 1. This adaptation is seen in conditions like chronic anemia, high-altitude living, and certain hemoglobinopathies.