A nurse has inadvertently contaminated an arterial blood sample with venous blood. What will be the effect on arterial blood gas analysis?

Appeared in: PGICH Noida NO-2025

Explanation

Venous blood has lower partial pressure of oxygen (PaO₂) because it has already delivered oxygen to the body's tissues.
Venous blood has a higher partial pressure of carbon dioxide (PaCO₂) as it carries this metabolic waste product away from the tissues.
The higher concentration of CO₂ in venous blood makes it more acidic, resulting in a lower pH compared to arterial blood.
Contaminating an arterial sample with venous blood will cause the results to shift towards venous values: PaO₂ will decrease, PaCO₂ will increase, and pH will decrease.

Option A: This pattern (increased PaO₂, decreased PaCO₂, increased pH) suggests hyperventilation or an error like an air bubble in the sample, not venous contamination.
Option B: This option incorrectly suggests that PaO₂ would increase and PaCO₂ would decrease. Venous blood is low in oxygen and high in carbon dioxide.
Option C: This option incorrectly states that PaO₂ would increase. Venous blood is deoxygenated, so mixing it with arterial blood will always lower the PaO₂.

Clinical Relevance

Nursing practice connection: Knowing Interpretation of erroneous Arterial Blood Gas (ABG) results due to sample contamination helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Accurate ABG results are crucial for diagnosing and managing life-threatening conditions like respiratory failure and metabolic acidosis. Incorrect results from a contaminated sample can lead to inappropriate and harmful treatments, such as incorrect ventilator settings or bicarbonate administration.
A nurse who suspects a sample may be contaminated (e.g., if the blood return was sluggish or dark) must discard the sample and redraw it to ensure patient safety.
What if? If the sample was contaminated with an air bubble instead of venous blood, the effect would be different: PaO₂ would falsely increase (towards room air O₂), and PaCO₂ would falsely decrease (as it diffuses into the air bubble). The pH would consequently increase.

How to Approach the Question

First, identify the core of the question, which is to compare the composition of arterial and venous blood.
Recall the primary function of the circulatory system: arterial blood delivers oxygen, and venous blood removes carbon dioxide.
Based on function, deduce the relative gas levels: Arterial blood has high oxygen (PaO₂) and low carbon dioxide (PaCO₂). Venous blood has low oxygen and high carbon dioxide.
Relate the level of carbon dioxide to pH. CO₂ acts as an acid in the blood, so higher CO₂ means a lower (more acidic) pH.
Determine how mixing venous blood (low O₂, high CO₂, low pH) into an arterial sample would alter the results. It will 'pull' the arterial values towards the venous values.
Conclude that the mixture will result in decreased PaO₂, increased PaCO₂, and decreased pH. Finally, select the option that matches this conclusion.

Concept Tested & Keywords

Concept Tested: Interpretation of erroneous Arterial Blood Gas (ABG) results due to sample contamination.
Stem keywords: arterial blood sample, venous blood, contaminated, arterial blood gas analysis
Lead-in keywords: What will be the effect

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E6 Nelson Textbook of Pediatrics(2024) — Volume 1 p. 662-664

E6 Nursing Fundamentals Taylor p. 785-787

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