Arterial & Capillary Blood Gas Sampling

Objective 1.1.1 — Understand the indications, contraindications, hazards, sample sites, and equipment used for ABG sampling; perform radial, brachial, and femoral arterial puncture; describe care and troubleshooting of indwelling arterial lines (A-lines); and perform a capillary blood gas in infants and pediatrics.

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Arterial Blood Gases (ABGs)

Definition

Arterial puncture — puncture of a peripheral artery to obtain an arterial blood sample for direct measurement of:

• pH
• PaCO₂
• PaO₂

Indications

1. Evaluate the adequacy of the patient's:

   • Ventilation → PaCO₂
   • Oxygenation → PaO₂
   • Acid–base status → pH

2. Assess the need for therapeutic intervention:

   • Oxygen therapy
   • Mechanical ventilation
   • Diagnostic evaluation (e.g., exercise desaturation)

3. Monitor the severity and progression of a documented disease process.

Contraindications

Complications & Hazards

• Thrombosis
• Hematoma
• Arteriospasm — transient constriction of the artery
• Pain
• Air emboli
• Infection
• Peripheral nerve damage
• Vasovagal reaction — can result in loss of consciousness

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Sample Sites by Age Group

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Equipment & Supplies

Syringe

• 1-, 3-, or 5-mL self-filling, disposable plastic syringe
• Pre-filled with dry lithium heparin anticoagulant

Needle

• 20- to 25-gauge, short-beveled for adult arterial sampling
• Needle length: ⅝ to 1½ in (longest needles → brachial or femoral)
• Children/neonates: 25-gauge syringe

Glass vs. Plastic

Glass syringes are no longer the standard for ABG collection. Plastic syringes are now used.

• Plastic syringes are more permeable and susceptible to substantial diffusion of room air
• PaO₂ can be skewed if bubbles are present in the sample
• Use quality plastic syringes with or without pre-filled heparin

Anticoagulant

• Lithium heparin — anticoagulant of choice for ABG sampling
• Dry anticoagulant is preferred; liquid heparin may be added if dry is unavailable

Other Supplies

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Radial Arterial Blood Gas Procedure

Why the Radial Artery?

• Vessel of choice for arterial puncture
• Use the non-dominant hand
• Avoid surgical scar sites
• Pulsations are readily palpable approximately 1 inch from the wrist where the artery passes above the radius bone

Adequate Collateral Circulation

Before puncturing the radial artery, verify there is collateral circulation. If absent, radial artery puncture is not recommended.

Modified Allen Test

The Modified Allen test determines the adequacy of ulnar circulation:

1. Patient clenches the fist, forcing blood from the hand
2. Clinician applies external pressure to both the radial and ulnar arteries, obstructing blood flow
3. Patient relaxes the hand (not full extension) — the palm and fingers will blanch
4. Release the ulnar artery — the hand should flush within 5–10 seconds if ulnar flow is adequate

Alternatively, ulnar circulation can be assessed with a Doppler ultrasonic flow indicator.

Procedure

1. Patient seated or lying down; wrist extended ~30° with towel support
2. Wash hands, don gloves
3. Palpate a definite pulse — do not puncture if a palpable pulse cannot be distinguished
4. Prep site with 70% isopropyl alcohol
5. Palpate again with one hand; hold the syringe like a pencil or dart in the other hand
6. With pulse maintained, insert the needle at a 30–45° angle (or less) with the bevel up
7. Advance slowly

Redirection & Avoiding Venous Contamination

• If redirection is needed: withdraw the needle almost to the skin surface before redirecting
• Be very cautious to avoid contamination with even a small amount of venous blood

Artery vs. Vein: Arterial blood flashes/pulsates as it enters the syringe.

After the Sample

1. Once 1–2 cc is obtained, withdraw the needle quickly and place sterile gauze over the puncture site
2. Apply pressure for 3–5 minutes (or 10–15 minutes if on anticoagulants or has a bleeding disorder)
3. Inspect for bleeding before bandaging
4. Remove air bubbles, cap the syringe
5. Gently mix the specimen by inversion and rotation to prevent clot formation

Specimen Labeling

Label with:

• Patient name
• Date and time
• ID #
• Puncture site
• Allen's test result (positive or negative)
• Oxygen device or invasive/noninvasive settings
• Patient temperature

Then analyze the specimen, assess results, enter into the chart, and report critical results to the attending physician.

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Brachial Artery Blood Gas Procedure

Indication

• Second choice in adults — when the radial arteries are unsuitable

Anatomy

• The brachial artery is the major artery of the upper arm
• It bifurcates into the radial and ulnar arteries just below the elbow
• Located a short distance above the bend of the elbow, on the internal surface of the arm where it passes over the humerus

Hazards (in addition to the general ABG hazards)

• Puncture of the median nerve
• Permanent median nerve damage from trauma
• Use the non-dominant side

Procedure

1. Extend the patient's arm
2. Rotate the wrist until the strongest pulse is obtained just above the skin crease
3. Trace the artery 2–3 cm up the arm with another finger
4. Insert at a 45° angle
5. After puncture → follow the same post-puncture procedure as a radial ABG

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Femoral Artery Blood Gas Procedure

Indication

Reserved for emergencies! Used in hypotensive patients with poor peripheral pulses.

Anatomy

• Located along the inguinal ligament
• Patient lies flat with legs extended
• Large diameter makes it an easy target
• The vessel lies deep below the skin, adjacent to the femoral nerve and vein

Procedure

• Insert the needle at an angle nearly perpendicular to the skin surface

Hazards

• Large quantities of blood may seep from this vessel and may go unnoticed because of its deep, inconspicuous location
• Increased risk of infection
• Collateral circulation is almost nonexistent — limb ischemia is a serious concern

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Indwelling Catheters — Arterial Lines (A-Lines)

An A-line is an indwelling arterial catheter used for continuous monitoring and frequent sampling.

Indications

• Frequent arterial blood gas measurements
• Continuous blood pressure monitoring

Contraindications

• Negative Modified Allen test
• Coagulopathy / bleeding disorders
• Poor peripheral / distal circulation

Hazards

Troubleshooting

Dampening

A dampened waveform appears flattened with poor systolic/diastolic definition. Causes:

• Clots in the catheter
• Tip of the catheter against the wall of the vessel
• Air bubbles in the line, or a clot in the transducer

Abnormally High or Low Readings

Causes:

• Improper calibration
• Improper transducer position

Transducer Position Rule

The transducer should be leveled at the phlebostatic axis (level of the heart) for accurate readings.

Drawing Blood from an A-Line

Access for sampling from most intravascular lines is provided by a three-way stopcock.

See Box 19.4 in Egan's for the full step-by-step three-way stopcock sampling procedure.

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Capillary Blood Gas Procedure

When It's Used

• Alternative to direct arterial access in infants and small children
• Estimates arterial pH and PCO₂
• Capillary PO₂ is of little value in estimating arterial oxygenation

Clinicians must be very cautious when using capillary blood gases to guide clinical decisions.

Contraindications

Capillary punctures should not be performed at the following sites or in these situations:

• Fingers of neonates
• Previous puncture sites
• Swollen or edematous tissue
• Cyanotic, poorly perfused tissue
• Areas of infection
• Patients less than 24 hours old (insufficient peripheral perfusion)
• When accurate oxygenation analysis is needed

Hazards

• Infection
• Contamination
• Burns (from warming pads)
• Hematoma
• Nerve damage
• Pain
• Bleeding
• Scarring

Equipment

Procedure

• The most common site is the heel — specifically the lateral aspect of the plantar surface
• Warm the site to ~42 °C to arterialize the capillary bed before puncture

Problem Solving & Troubleshooting

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Quick Reference

Allen Test Cheat Sheet

Positive = good ulnar flow (flushes 5–10 s) → puncture OK. Negative = poor ulnar flow → do not puncture the radial.

Hold Pressure After ABG

Arterial Site Choices in Order

1. Radial (vessel of choice — adults)
2. Brachial (second choice in adults)
3. Femoral (emergencies / hypotensive patients only)
4. Dorsalis pedis (alternate adult site)

A-Line Pressure Errors

• Transducer below heart → falsely HIGH
• Transducer above heart → falsely LOW

Capillary Sampling Pearl

• Heel (lateral plantar surface) is the standard site
• Warm to 42 °C; never squeeze the site
• Capillary PO₂ is unreliable — use ABG when oxygenation accuracy matters