What are PO2 (partial pressure of oxygen) and SO2 (oxygen saturation) levels?

Understanding PO2 and SO2: Oxygen Measurement Parameters

PO2 (partial pressure of oxygen) is the pressure exerted by oxygen dissolved in blood, while SO2 (oxygen saturation) is the percentage of hemoglobin binding sites occupied by oxygen molecules. These two parameters are fundamental measurements used to assess oxygenation status in clinical settings.

PO2 (Partial Pressure of Oxygen)

• Definition: PO2 (also written as PaO2 when measured in arterial blood) represents the pressure exerted by oxygen dissolved in blood plasma, measured in kilopascals (kPa) or millimeters of mercury (mmHg) 1.

• Normal values:

  • Ages 18-24: Mean 13.4 kPa (11.98-14.82 kPa range)
  • Ages 25-34: Mean 13.4 kPa (12.08-14.72 kPa range)
  • Ages 35-44: Mean 13.18 kPa (11.14-15.22 kPa range)
  • Ages 45-54: Mean 13.0 kPa (10.86-15.14 kPa range)
  • Ages 55-64: Mean 12.09 kPa (10.89-13.29 kPa range)
  • Ages >64: Mean 11.89 kPa (9.02-14.76 kPa range) 1

• Clinical significance: PO2 reflects lung function and oxygen diffusion capacity. Low PO2 (hypoxemia) indicates impaired oxygen uptake in the lungs 2.

SO2 (Oxygen Saturation)

• Definition: SO2 (or SaO2 when measured in arterial blood) represents the percentage of hemoglobin binding sites occupied by oxygen molecules 1.

• Normal values:

  • Ages 18-24: Mean 96.9% (96.1-97.7% range)
  • Ages 25-34: Mean 96.7% (95.3-98.1% range)
  • Ages 35-44: Mean 96.7% (95.5-97.9% range)
  • Ages 45-54: Mean 96.5% (94.4-98.5% range)
  • Ages 55-64: Mean 95.1% (94.5-97.3% range)
  • Ages >64: Mean 95.5% (92.7-98.3% range) 1

• Clinical significance: SO2 indicates how much oxygen is bound to hemoglobin and available for tissue delivery. Values below 90% are generally considered concerning for tissue hypoxia 1.

Relationship Between PO2 and SO2

• The relationship between PO2 and SO2 is represented by the oxyhemoglobin dissociation curve, which has a sigmoid (S-shaped) form 3.

• Key features of this relationship:

  • Upper flat portion: SO2 remains >90% despite significant reductions in PO2
  • Lower steep portion: SO2 drops rapidly when PO2 falls below approximately 8 kPa (60 mmHg)
  • This shape optimizes oxygen transport by maintaining near-complete saturation in lungs while facilitating oxygen release to tissues 3

Factors Affecting the Oxyhemoglobin Dissociation Curve

• Rightward shifts (facilitate oxygen release to tissues):

  • Increased temperature
  • Increased 2,3-DPG levels
  • Acidosis (increased H+)
  • Increased PaCO2
  • Exercise 3

• Leftward shifts (enhance oxygen uptake but may limit tissue delivery):

  • Decreased temperature
  • Decreased 2,3-DPG levels
  • Alkalosis
  • Decreased PaCO2
  • Carboxyhemoglobin 3

Clinical Applications and Considerations

• Measurement methods:

  • PO2: Directly measured via arterial blood gas analysis
  • SO2: Can be measured directly via arterial blood sampling (SaO2) or estimated non-invasively via pulse oximetry (SpO2) 4

• Limitations of measurement:

  • Pulse oximetry may be inaccurate in certain conditions (e.g., carbon monoxide poisoning, methemoglobinemia)
  • Blood gas calculations assume normal oxygen affinity and no dyshemoglobins 4

• Time considerations:

  • When changing FiO2 (fraction of inspired oxygen), PO2 equilibration requires approximately 10 minutes following an increase and 16 minutes following a decrease 5

• Clinical interpretation:

  • Hypoxemia is often defined as PaO2 <8 kPa (60 mmHg) or SaO2 <90%
  • Significant arterial oxygen desaturation is defined as SpO2 <90% or a fall of 4% or more that is prolonged (>1 min) 1
  • In palliative care, oxygen therapy should be restricted to patients with SpO2 consistently <90% 1

Types of Hypoxemia Based on PO2 and SO2

• Hypoxic hypoxemia: Decreased PO2, SO2, and oxygen content due to lung function disturbances 2
• Toxic hypoxemia: Normal PO2 but decreased SO2 and oxygen content (e.g., carbon monoxide poisoning) 2
• Anemic hypoxemia: Normal PO2 and SO2 but decreased oxygen content due to reduced hemoglobin 2

Understanding the relationship between PO2 and SO2 is crucial for proper assessment of oxygenation status and appropriate clinical decision-making in respiratory care.