Oxygen tension, or the partial pressure of oxygen (PO2), is measured using different methods depending on whether you're looking at arterial blood, venous blood, or alveolar gas. Direct measurement is possible for arterial and venous blood, while alveolar oxygen tension is usually calculated.
Measuring Arterial and Venous Oxygen Tension
The most common method for directly measuring oxygen tension in blood is with a blood gas analyzer.
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Arterial Blood Gas (ABG) Test: An ABG provides a direct measurement of the partial pressure of oxygen in arterial blood (PaO2). This is considered the gold standard for assessing oxygenation.
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Venous Blood Gas (VBG) Test: A VBG measures the partial pressure of oxygen in venous blood. While it doesn't directly reflect arterial oxygenation, it can be a useful estimate, especially when an ABG is not feasible.
How Blood Gas Analyzers Work:
These analyzers use electrochemical sensors to directly measure the partial pressure of oxygen (PO2), partial pressure of carbon dioxide (PCO2), and pH in a blood sample. A sensor called a Clark electrode is typically used for PO2 measurement. The electrode consists of a platinum cathode and a silver anode immersed in an electrolyte solution and separated from the blood sample by an oxygen-permeable membrane. Oxygen diffuses across the membrane, is reduced at the cathode, and the resulting current is proportional to the PO2 in the sample.
Estimating Alveolar Oxygen Tension
The partial pressure of oxygen in the alveoli (PAO2) cannot be directly measured easily. Instead, it's calculated using the alveolar gas equation:
PAO2 = (Patm – PH2O) FiO2 – PaCO2/RQ
Where:
- PAO2 is the alveolar partial pressure of oxygen.
- Patm is the atmospheric pressure (approximately 760 mmHg at sea level).
- PH2O is the partial pressure of water vapor in the alveoli (approximately 47 mmHg at body temperature).
- FiO2 is the fraction of inspired oxygen (e.g., 0.21 for room air).
- PaCO2 is the arterial partial pressure of carbon dioxide, obtained from an ABG.
- RQ is the respiratory quotient, which is the ratio of carbon dioxide produced to oxygen consumed (typically assumed to be 0.8).
Example Calculation:
Let's say you're breathing room air (FiO2 = 0.21), your PaCO2 is 40 mmHg, and we're at sea level.
PAO2 = (760 mmHg – 47 mmHg) 0.21 – 40 mmHg / 0.8
PAO2 = (713 mmHg) 0.21 – 50 mmHg
PAO2 = 149.73 mmHg - 50 mmHg
PAO2 ≈ 99.73 mmHg
Summary
Measuring oxygen tension involves direct measurement via blood gas analysis (ABG or VBG) or estimation using the alveolar gas equation for alveolar oxygen tension. ABGs are the gold standard for arterial oxygen measurement.
