Arterial Blood Gas Sampling

Overview

An arterial blood gas (ABG) requires arterial blood — usually drawn from the radial artery. The radial site is preferred because it is superficial, easily compressed against the underlying radius, and receives collateral perfusion from the ulnar artery through the palmar arch. That collateral supply matters: if the radial artery goes into spasm or sustains injury after puncture, the hand is not left ischemic.

Alternative sites include the brachial artery (less collateral protection, used when radial access fails) and the femoral artery (last resort — high hematoma risk, no collateral compression). An indwelling arterial line, when present, is the cleanest option because it avoids repeated punctures entirely.

Before any radial puncture, confirm collateral circulation with the modified Allen test. A failed Allen test is an absolute contraindication to that radial site.

Key Concepts

• Arterial vs. venous blood. Arterial blood self-fills the syringe under arterial pressure and appears bright red. A dark, non-pulsatile sample that must be aspirated suggests venous contamination.
• Dry/lyophilized heparin syringes.Pre-heparinized syringes prevent clotting. Dry heparin is strongly preferred — excess liquid heparin dilutes the sample and falsely lowers PCO₂ and HCO₃⁻.
• Collateral circulation. The dual blood supply to the hand (radial + ulnar via the palmar arch) is what makes the radial site safe. The Allen test verifies it is intact before puncture.
• Pre-analytic errors. Most ABG errors occur before the sample reaches the analyzer: air contamination, heparin dilution, venous admixture, and metabolic drift in a delayed sample are the four classic sources.
• Patient preparation.The patient should be at rest and breathing at their baseline (or documented FiO₂) for at least 20–30 minutes before sampling. Note the FiO₂ and patient temperature on the requisition — temperature corrects PO₂ and PCO₂ values.

Modified Allen Test

The modified Allen test confirms adequate ulnar collateral perfusion before radial puncture. Perform it on every patient before the first radial draw.

1. Have the patient make a tight fist to exsanguinate the palm.
2. Apply firm pressure over both the radial and ulnar arteries simultaneously with your fingertips.
3. Ask the patient to open the hand (the palm will appear pale/blanched).
4. Release ulnar pressure only while maintaining radial occlusion.
5. Observe for color return. Normal (positive): pink color returns within 5–15 seconds — proceed with radial puncture. Abnormal (negative):delayed or absent return beyond 15 seconds — choose an alternative site.

Sampling Technique

After confirming a positive Allen test, gather equipment: a pre-heparinized syringe (dry heparin preferred), a 22–25 gauge needle, antiseptic prep, gloves, gauze, and tape or a bandage.

1. Position the wrist. Extend and slightly dorsiflex the wrist (a towel roll helps) to bring the radial artery closer to the surface.
2. Prep the site. Cleanse with antiseptic and allow it to dry fully.
3. Palpate the pulse. Locate the strongest pulse point with two fingertips; the needle enters just distal to your proximal finger.
4. Insert bevel-up at 30–45°.Advance the needle at a 30–45° angle, bevel facing up, against the direction of blood flow (toward the heart). Advance slowly until pulsatile blood fills the syringe.
5. Let arterial pressure fill the syringe.Do not aspirate — the arterial pressure should drive 1–3 mL into the barrel spontaneously.
6. Withdraw and cap immediately. Remove the needle, engage the safety, and cap the syringe to prevent air exposure.
7. Apply firm pressure. Hold direct pressure over the puncture site for at least 5 minutes (10+ minutes if the patient is anticoagulated). Do not peek early.
8. Expel air, mix, label, and transport.Immediately remove any visible air bubbles, gently roll the syringe to mix, label with patient ID and FiO₂, and send to the analyzer. Place on ice if analysis will be delayed beyond 15–30 minutes.

Assessment & Findings

Confirm you have an arterial sample before sending it to the analyzer:

• Pulsatile flashback.Blood should enter the syringe in synchrony with the patient’s pulse, driven by arterial pressure — not by aspiration.
• Bright red color. Well-oxygenated arterial blood is bright red. Dark, maroon-colored blood that does not self-fill suggests a venous draw.
• Self-filling barrel. The syringe plunger should rise on its own. If you had to pull back the plunger to collect the sample, suspect venous blood.
• Post-draw site assessment. After releasing pressure, inspect for hematoma, check distal capillary refill and sensation, and confirm radial pulse is intact.

RT Priorities & Interventions

• Prevent air contamination. Expel all visible air bubbles immediately after withdrawal. Even a small bubble can measurably alter PO₂ and PCO₂ if the sample equilibrates with room air.
• Use dry heparin syringes.Liquid heparin left in excess of the dead-space volume dilutes the sample. The resulting drop in PCO₂ and HCO₃⁻ can mimic a metabolic or respiratory disturbance that isn’t real.
• Avoid venous contamination. If the sample is dark and did not self-fill, do not send it. Discard, apply pressure, and attempt again at a new site or after the artery recovers.
• Ice delayed samples. Cellular metabolism (especially in patients with leukocytosis or polycythemia) will progressively lower PO₂ and pH if the sample sits at room temperature. An ice slurry slows this drift significantly.
• Mix to prevent clotting.Gently roll the capped syringe between your palms to distribute the heparin. Do not shake vigorously — excessive agitation can lyse red cells and alter potassium values.
• Document clinical context.Note the patient’s FiO₂ (or O₂ delivery device and flow rate), temperature, and time of draw on the requisition. These values are required for accurate interpretation and temperature correction.

Common Pitfalls

• Air bubble in the syringe. Trapped room air equilibrates with the sample, falsely elevating PO₂ and lowering PCO₂. Expel immediately after withdrawal.
• Excess liquid heparin. Liquid heparin dilutes the sample, causing a dilutional drop in PCO₂ and HCO₃⁻. Use a dry/lyophilized heparin syringe whenever possible.
• Venous contamination. A dark, non-pulsatile sample that does not self-fill likely represents venous blood — it will show falsely low PO₂ and falsely high PCO₂.
• Delayed analysis without icing. Ongoing cellular metabolism lowers PO₂ and pH over time, particularly with leukocytosis. Ice the sample if analysis will take more than 15–30 minutes.
• Inadequate post-puncture pressure. Releasing pressure too soon — especially in anticoagulated patients — can cause a hematoma large enough to compress the artery. Hold firm pressure for at least 5 minutes, longer if the patient is on anticoagulants.

Board Exam Pearls

FAQ

Related Resources

Sources

1. Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC Clinical Practice Guideline: Blood gas analysis and hemoximetry: 2013. Respir Care. 2013;58(10):1694-1703.
2. Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Analysis and monitoring of gas exchange.