What is the significance of elevated oxygen levels on a venous blood draw from an ultrasound-guided Intravenous (IV) line in a patient not receiving oxygen or Intravenous (IV) fluids?

From the Guidelines

The elevated oxygen level in a venous blood sample drawn from an ultrasound-guided IV likely represents an arterial puncture rather than a true venous sample, and therefore, the blood draw should be repeated from a different site to obtain a true venous sample for accurate results. This is a common occurrence when attempting venous access, as arteries can be inadvertently accessed instead of veins. According to the British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings 1, blood gases should be checked in specific situations, but an elevated oxygen level in a venous sample is not a recommended indication for blood gas analysis. To differentiate between arterial and venous blood during collection, observe the color (arterial blood is typically brighter red), pulsatility (arterial blood often pulsates into the collection tube), and pressure (arterial blood enters the syringe with greater force). This mix-up occurs because arteries and veins often run parallel to each other, and ultrasound guidance, while helpful, doesn't always guarantee venous access. Some key points to consider when collecting blood samples include:

• Observing the color of the blood to differentiate between arterial and venous samples
• Noting the pulsatility of the blood flow into the collection tube
• Assessing the pressure of the blood entering the syringe
• Being aware of the anatomical proximity of vessels, particularly in areas like the antecubital fossa, which increases the likelihood of arterial puncture during attempted venous access. If the patient needs venous access, reposition the IV in a confirmed venous location to ensure accurate laboratory results and proper patient management.

From the Research

Elevated Oxygen on Venous Blood Draw

• The patient's elevated oxygen levels on a venous blood draw from an ultrasound-guided IV, despite not being on oxygen or fluids, may be attributed to various factors.
• According to 2, pulse oximetry may not always be a reliable predictor of arterial oxygen saturation (SaO2), especially in hypoxemic patients.
• The study 3 suggests that SpO2 levels may not accurately reflect severe resting hypoxemia in patients with chronic obstructive pulmonary disease (COPD), highlighting the importance of arterial blood gas (ABG) analysis in such cases.

Factors Affecting Oxygen Saturation Readings

• Several factors can affect oxygen saturation readings, including:
  • Anaemia 4
  • Peripheral vasoconstriction 4
  • Dark skin tone 4
  • Skin discolouration 4
• These factors may contribute to discrepancies between SpO2 and SaO2 readings, as observed in studies 2 and 5.

Comparison of Oxygen Saturation Measurement Methods

• Studies 2 and 5 compared the accuracy of pulse oximetry and ABG analysis in measuring oxygen saturation.
• The results showed that pulse oximetry can be a reliable method for estimating oxygen saturation, especially when SpO2 levels are above 90% 5.
• However, ABG analysis remains the gold standard for assessing oxygenation and acid-base status, particularly in critically ill patients or those with suspected alveolar hypoventilation 2.

Clinical Implications

• The findings of these studies have important implications for clinical practice, highlighting the need for careful interpretation of oxygen saturation readings and consideration of potential factors that may affect their accuracy.
• In patients with elevated oxygen levels on venous blood draw, further evaluation using ABG analysis may be necessary to determine the underlying cause and guide appropriate management 2, 3.