Indications
- Continuous blood pressure monitoring — haemodynamic instability, vasopressor/inotrope infusion, major resuscitation.
- Frequent arterial blood sampling — repeated blood gases in the critically unwell or ventilated patient.
- Beat-to-beat monitoring where non-invasive cuff readings are unreliable (arrhythmia, obesity, shock).
Site selection & the Allen test
Site choice
- Radial artery — first choice: superficial, generally good collateral flow via the ulnar artery and palmar arch, and easy to secure.
- Femoral artery — alternative when radial access fails or in profound shock with poor peripheral pulses; larger vessel, easier in low-flow states.
- Brachial and dorsalis pedis are less commonly used second-line options.
The modified Allen test
- Traditionally taught to assess collateral ulnar flow before radial cannulation.
- Modern evidence has not shown that an abnormal Allen test reliably predicts hand ischaemia, and cannulation is commonly performed safely without it — permanent ischaemic complications after radial cannulation are rare.
- Many departments still perform it as part of a standard pre-procedure check; follow local policy, but do not treat an abnormal result as an absolute contraindication in isolation.
Equipment
Wrist position
Step-by-step technique
Position
Extend the wrist over a support or rolled towel to bring the artery closer to skin and reduce its mobility.
Locate the artery
Palpate the pulse just proximal to the wrist crease, or use ultrasound in short-axis to directly visualise the artery — ultrasound guidance improves first-pass success, particularly in weak, impalpable, or previously punctured pulses.
Clean and anaesthetise
Skin antisepsis; local anaesthetic infiltration in the conscious patient (a small subcutaneous bleb, avoiding the artery itself).
LAST rescue — AAGBI algorithm ↗Cannulate
Catheter-over-needle: advance at a shallow angle (~30–45°) until flashback of pulsatile blood, then either advance the whole unit slightly and thread the catheter off the needle, or use a transfixion technique (through-and-through, then withdraw slowly until flow returns and thread). Seldinger kits use a smaller entry needle, wire, then catheter over the wire — often easier in a weak-pulse or previously punctured artery.
Confirm and connect
Confirm pulsatile return, connect to the pre-flushed, air-free pressure line and transducer.
Zero the transducer
Position the transducer at the phlebostatic axis (approximately heart level, 4th intercostal space, mid-axillary line) and zero it to atmospheric pressure with the stopcock open to air before relying on the reading.
Secure
Suture or adhesive securement device, clear dressing, splint if needed to protect against movement.
Trace interpretation basics
- Normal trace — a sharp systolic upstroke, a dicrotic notch on the downstroke (aortic valve closure), and a smooth diastolic decay.
- Overdamped trace — rounded, slow upstroke with loss of the dicrotic notch and a narrowed pulse pressure; causes include air bubbles, kinked tubing, clot at the tip, or a loose connection. Check the fast-flush ("square wave") test — an overdamped system shows a slow return to baseline with minimal or no oscillation.
- Underdamped trace — exaggerated systolic overshoot with artefactual widening of pulse pressure; often from long or narrow tubing, or excessive stopcocks in the circuit.
- Always correlate the numeric reading with the trace shape and the patient's clinical state — a normal-looking number on a damped trace can be misleading.
Complications
Early
- Haematoma — apply firm pressure after any failed attempt or on removal.
- Distal ischaemia — rare with radial access; monitor hand colour, warmth, and capillary refill after insertion.
- Arterial spasm making cannulation more difficult with repeated attempts.
Late & critical
- Accidental intra-arterial drug injection — a serious, avoidable error causing severe distal ischaemia. Label arterial lines clearly and never inject drugs intended for IV use through them.
- Line infection with prolonged dwell time.
- Thrombosis of the artery — usually asymptomatic given collateral supply, but monitor distal perfusion.
- Pseudoaneurysm (rare).
Pitfalls & pearls
- An abnormal Allen test alone should not automatically stop a radial attempt — weigh it against the clinical need and local policy.
- Zero the transducer at heart level before trusting the number — a mis-zeroed line gives a confidently wrong reading.
- A dampened trace with a "normal" mean pressure can still be clinically misleading — always check the fast-flush test if the trace looks unusual.
- Ultrasound guidance helps most in weak, impalpable, or previously attempted pulses — use it early rather than after several failed blind attempts.
DOPS pointers
RCEM curriculum: Core Procedural Skill. What assessors look for at each entrustment level:
| 1 | Observed only — can describe indications, site choice, and basic trace interpretation. |
| 2a | Performs with supervisor present: correct wrist positioning, cannulation technique, transducer zeroing. |
| 2b | Performs with supervisor in the department: full sequence including securing and recognising a damped trace. |
| 3 | Indirect supervision: troubleshoots damping, manages failed radial attempts and selects an alternative site. |
| 4 | Independent; can teach and supervise others, including trace interpretation and complication management. |
References
- Faculty of Intensive Care Medicine / GPICS. Guidelines for the Provision of Intensive Care Services — arterial line insertion and monitoring standards.
- Nuttall G, et al. Surgical and patient risk factors for severe arterial line complications in adults. Anesthesiology 2016;124(3):590–597.
- RCEM Curriculum 2021 — Emergency Medicine Specialty Learning Outcomes: core procedural skills (DOPS).