2.3 Arterial & Capillary Blood-Gas Sampling
Key Takeaways
- The radial artery is a common pediatric arterial-puncture site when anatomy, perfusion, collateral circulation, operator skill, and local policy support its use; assess distal perfusion before and after sampling.
- For heel capillary sampling, use the approved lateral or medial plantar area, an age-appropriate safety lancet, and the institution’s specimen protocol. If warming is used, control temperature and time to prevent burns; do not apply one warming regimen to every patient.
- Capillary pH and PCO2 may approximate arterial values when perfusion and technique are adequate, but capillary PO2 is unreliable for arterial oxygenation; use pulse oximetry or an arterial sample for oxygen decisions.
2.3 Arterial & Capillary Blood-Gas Sampling
Arterial blood gas analysis directly measures pH, PaCO2, and PaO2 and remains an important reference for managing neonatal and pediatric respiratory failure. Selecting the appropriate sampling modality—arterial puncture, capillary blood gas, or indwelling catheterization—requires understanding the technical limitations, vascular anatomy, and clinical risks unique to developing pediatric patients.
Arterial Puncture in Neonatal & Pediatric Patients
Direct arterial puncture provides an undisturbed sample of arterial blood for measuring pH, PaCO2, and PaO2.
Anatomical Site Selection
- Radial Artery (Primary Choice): Located on the thumb side of the volar wrist. It is the preferred site due to its superficial course and collateral perfusion supplied to the hand via the ulnar artery through the deep and superficial palmar arches.
- Dorsalis Pedis & Posterior Tibial: Secondary acceptable sites in neonates and older children. Assess collateral perfusion before radial puncture using the local procedure, recognizing that no bedside test perfectly predicts ischemic risk.
- Femoral artery: Generally avoid routine puncture in neonates and small infants because of ischemic, bleeding, infection, and adjacent-structure risk; use only when the benefit and local expertise justify it. Puncture risks femoral head avascular necrosis, septic arthritis of the hip, injury to the adjacent femoral nerve, and occult retroperitoneal hemorrhage.
- Brachial Artery (Avoid): Lacks adequate collateral circulation; arterial spasm or hematoma can cause ischemic compromise of the forearm and hand (Volkmann ischemic contracture), with high risk of median nerve injury.
Modified Allen Test Protocol
Before radial artery puncture, assess collateral hand perfusion according to local policy:
- Elevate the pediatric patient's hand.
- Occlude both the radial and ulnar arteries simultaneously by applying digital pressure at the wrist.
- Instruct the child to open and close the hand repeatedly (or gently squeeze an infant's hand) until blanching occurs.
- Release pressure only on the ulnar artery, maintaining continuous occlusion on the radial artery.
- Observe the palm and fingers for reperfusion (erythema):
- Interpretation: Look for prompt, symmetric reperfusion according to the local procedure. Timing thresholds and terminology vary, and the test is imperfect. If perfusion is delayed or uncertain, select another site or method and reassess distal perfusion after sampling.
- Neonatal Modification: In infants unable to cooperate, place a pulse oximeter sensor on the right index finger. Occlude both vessels until the photoplethysmographic pulse waveform disappears; release the ulnar artery and verify prompt waveform and SpO2 recovery.
Technical Nuances & Anticoagulation Artifacts
- Needle size: 23 to 25-gauge small butterfly needle or short bevel hypodermic needle.
- Anticoagulation: Dry, lyophilized electrolyte-balanced lithium heparin is preferred. If liquid sodium heparin is used, syringe deadspace volume must be minimized. An excess liquid heparin volume (> 10% of total sample volume) produces a dilutional artifact: falsely lowers PaCO2, falsely decreases calculated HCO3-, and shifts pH toward the acidic buffer pH (~7.0).
Capillary Blood Gas (CBG) Sampling & Clinical Boundaries
Capillary blood gas sampling is widely utilized in neonatal intensive care to assess ventilation and acid-base status while conserving vascular access sites and preventing arterial vessel trauma.
Plantar View of Neonatal Heel
┌──────────────────────────────┐
│ Toes (Avoid) │
│ │
SAFE │ ░░ ░░ │ SAFE
LATERAL│ ░░ ░░ │ MEDIAL
BORDER │ ░░ UNSAFE ZONE ░░ │ BORDER
│ ░░ (Center/Arch) ░░ │
│ ░░ ░░ │
│ │
│ UNSAFE ZONE │
│ (Posterior Calcaneus) │
└──────────────────────────────┘
Pre-Puncture Heel Warming Protocol
Follow the neonatal sampling protocol for site preparation. If warming is used to improve local flow, use a temperature-controlled approved method for the specified time and inspect the skin; improvised hot packs or fixed heating rules can burn fragile tissue.
- Controlled warming may increase local blood flow, but it does not convert the sample into true arterial blood or make capillary PO2 reliable.
- Heating above 42°C is strictly prohibited due to high risk of cutaneous thermal burns.
Anatomical Boundaries & Calcaneal Osteomyelitis Prevention
- Safe Puncture Zones: The lateral or medial plantar border of the heel (outer borders of a line drawn posteriorly from the 4th/5th toe to the heel, or from the great toe to the heel).
- Prohibited Sites:
- Posterior curvature of the heel: The calcaneus bone lies within 1.0 to 2.0 mm of the dermal surface. Puncturing the posterior curve risks penetrating the periosteum, causing calcaneal osteomyelitis and permanent skeletal deformities.
- Central plantar arch: High risk of lacerating the medial plantar artery, deep posterior tibial nerve branches, and plantar fascia.
- Previous puncture sites, bruised tissue, or edematous skin.
- Lancet Depth: Automated, spring-loaded lancet with a penetration depth ≤ 2.0 mm (in ELBW preterms, depth ≤ 0.85 to 1.0 mm).
Diagnostic Limits: The 'PO2 Trap'
| Measured Parameter | Arterial Correlation | Diagnostic Reliability & Clinical Utility |
|---|---|---|
| Capillary pH | Often clinically useful | May approximate arterial pH when perfusion and collection technique are adequate; interpret discrepancies with the clinical picture. |
| Capillary PCO2 | Often clinically useful for trends | May approximate arterial PaCO2, but shock, poor flow, air, delay, and squeezing can produce clinically important error. |
| Capillary PO2 | Unreliable for arterial oxygenation | Venous admixture and tissue extraction make it unsuitable as a substitute for arterial PaO2. |
Clinical Rule
Capillary PO2 correlates poorly with arterial oxygen tension and should not guide FiO2 or exclude hyperoxia. Use pulse oximetry for continuous titration and arterial PaO2 or another approved method when oxygen tension is required. Chasing a capillary PO2 target can expose a preterm infant to unnecessary oxygen and increase hyperoxia-related risk.
Squeezing ('Milking') Artifact
Vigorously squeezing, milking, or massaging the punctured heel releases intracellular fluid and causes mechanical hemolysis. This invalidates the sample by:
- Falsely elevating potassium (K+).
- Diluting blood gases, leading to falsely low PCO2 and altered pH.
A 32-week gestation neonate receiving supplemental oxygen via nasal cannula requires blood gas evaluation for ventilatory assessment. The respiratory therapist plans to perform a capillary blood gas (CBG). Which technique and limitation are most appropriate?