What is the significance of Po2 (partial pressure of oxygen) in an arterial blood gas (ABG) analysis?

PO2 in Arterial Blood Gas Analysis

PO2 (partial pressure of oxygen) in an ABG measures the pressure exerted by dissolved oxygen in arterial blood plasma and is the primary indicator of lung function and oxygen diffusion capacity, with normal values ranging from 80-100 mmHg (or approximately 11-13 kPa) at sea level. 1

Normal Reference Values

• Healthy young adults (18-24 years): 90-111 mmHg 2
• General adult range: 80-110 mmHg, though this varies with age 2
• Adults over 64 years: 68-111 mmHg (PaO2 naturally decreases with age, particularly after age 55) 2
• Normal arterial oxygen saturation (SaO2): 95-98% in healthy adults 1

Clinical Significance and Interpretation

Defining Hypoxemia

Hypoxemia is defined as PaO2 <8 kPa (60 mmHg), which corresponds to an oxygen saturation of approximately 90%. 3 However, clinical thresholds vary:

• Type 1 respiratory failure: PaO2 <8 kPa (60 mmHg) with normal or low PaCO2 3
• Investigation threshold: PaO2 <80 mmHg or alveolar-arterial oxygen gradient (P[A-a]O2) ≥15 mmHg warrants further investigation 1
• Elderly patients (≥65 years): Use P[A-a]O2 ≥20 mmHg as the cut-off 1

Understanding What PO2 Reflects

PO2 specifically measures lung function and oxygen diffusion into blood, representing the result of O2 diffusion within the lungs. 4 It is important to understand that:

• PO2 alone does not determine tissue oxygenation, which also depends on hemoglobin concentration, cardiac output, and tissue oxygen extraction 2
• Diagnostic hierarchy: The clinical significance of oxygen variables increases in the order of PO2 → sO2 → cO2 (oxygen content) 4

Relationship Between PO2 and Oxygen Saturation

Above a PaO2 of approximately 16 kPa (120 mmHg), oxygen saturation will not change from 100%, yet further increases in PaO2 may have important clinical effects in certain conditions such as COPD. 3

The oxygen-hemoglobin dissociation curve explains this relationship:

• Mixed venous blood PO2 rises from approximately 6 kPa (45 mmHg) in the pulmonary artery to about 16 kPa (120 mmHg) by the end of the pulmonary capillary 3
• Actual arterial PO2 is lower at 13 kPa (100 mmHg) due to ventilation-perfusion mismatch 3

When to Obtain Arterial Blood Gas

Critical Indications

For critically ill patients or those with shock or hypotension (systolic BP <90 mmHg), the initial blood gas measurement must be obtained from an arterial sample. 3

Additional indications include: 3

• All critically ill patients
• Unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia
• Any patient requiring increased supplemental oxygen

Clinical Decision Algorithm

1. Initial screening: Use pulse oximetry (SpO2) 1
2. If SpO2 <96%: Proceed to ABG analysis 1
3. Assess PaO2: <80 mmHg indicates hypoxemia 1
4. Calculate alveolar-arterial gradient: ≥15 mmHg (≥20 mmHg in adults ≥65 years) warrants further investigation 1

Target Oxygen Ranges

Standard Patients

For acutely ill patients without risk of hypercapnic respiratory failure, target oxygen saturation should be 94-98%. 3

High-Risk Patients (COPD, Hypercapnia Risk)

For patients at risk of hypercapnic respiratory failure, target oxygen saturation should be 88-92%. 3

• Use 24% Venturi mask at 2-3 L/min or 28% Venturi mask at 4 L/min or nasal cannulae at 1-2 L/min 3
• Adjust target range to 94-98% if PCO2 is normal (unless history of previous ventilatory support) 3
• Recheck blood gases after 30-60 minutes 3

Common Pitfalls and Caveats

Limitations of Pulse Oximetry

A normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen. 3 Pulse oximetry will be normal in patients with:

• Normal PO2 but abnormal blood pH or PCO2 3
• Low blood oxygen content due to anemia 3
• Carbon monoxide poisoning (standard pulse oximetry cannot differentiate carboxyhemoglobin, leading to falsely normal readings) 1

Sampling Considerations

• Local anesthesia should be used for all ABG specimens except in emergencies 3
• Earlobe capillary samples underestimate PO2 by 0.5-1 kPa and should not be relied upon in hypoxemic patients 3, 1
• Capillary PO2 does not adequately reflect arterial PO2 in hypoxemic COPD patients, potentially leading to unnecessary long-term oxygen therapy prescriptions in 20-30% of cases 5

Spurious Results

In patients with very high WBC or platelet counts (typically hematological malignancies), "spurious hypoxemia" or "oxygen steal" can occur due to consumption of oxygen by active cellular elements during ABG analysis. 6 Suspect this when:

• ABG shows low PO2 and oxygen saturation
• Pulse oximetry shows normal oxygen saturation
• No other obvious explanation for hypoxemia exists 6

Hyperoxemia Concerns

A PO2 >155 mmHg constitutes hyperoxemia and can lead to increased oxidative stress and potential organ damage. 2 When encountering hyperoxemia:

• Assess whether patient is receiving supplemental oxygen 2
• Consider titrating oxygen down to achieve normoxemia (94-98% saturation) 2
• Avoid abrupt withdrawal in patients with physiologic dependence 2

Types of Hypoxia and PO2 Relationship

Understanding which types of hypoxia affect PO2 is critical: 3

• Hypoxemic hypoxia: Decreases PO2, sO2, and oxygen content (lung dysfunction) 3, 4
• Anemic hypoxia: Normal PO2 and sO2, but reduced oxygen content due to low hemoglobin 3, 4
• Stagnant hypoxia: Normal PO2, but inadequate blood flow causes tissue hypoxia 3
• Histotoxic hypoxia: Normal PO2, but tissues cannot utilize oxygen (e.g., cyanide poisoning) 3

Oxygen therapy only corrects hypoxia due to hypoxemia; other causes require different interventions. 3