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Arterial line: setting up, levelling and zeroing the transducer, the waveform, sampling and complications

  • Procedure

An arterial line shows the blood pressure beat by beat and allows blood samples without new punctures; the nurse keeps it accurate, clean and safe.

Updated

What it is

An arterial line is a short catheter in an artery, usually the radial, connected through stiff tubing to a transducer and a pressurised flush system. It gives a continuous, beat-by-beat blood pressure and allows repeated arterial blood samples. The doctor or a trained clinician inserts it; the nurse prepares the system, helps with the asepsis, levels and zeroes the transducer, checks the waveform, watches the limb and the site, and takes samples according to protocol. The reading is only as good as the system: a transducer at the wrong height, a missed zero, air or a clot in the line all give a false number. Nothing is ever injected into an arterial line.

Indications

  • Continuous blood pressure monitoring in shock or with vasopressor infusions; the Surviving Sepsis Campaign suggests invasive arterial monitoring as soon as practical in septic shock.
  • Repeated arterial blood gases and other blood samples without new punctures.
  • Patients whose pressure changes fast: after major surgery, on titrated infusions, in severe instability.

Contraindications and precautions

  • The radial, brachial or dorsalis pedis sites are preferred over the femoral or axillary sites in adults; a femoral line carries more infection risk and can cause a pseudoaneurysm.
  • Radial lines are avoided in conditions with a high risk of hand ischaemia, such as scleroderma; check the circulation of the hand before insertion (see the Allen test page).
  • Keep every part of the pressure system sterile, enter it as little as possible, and scrub any access port with an antiseptic before using it.
  • If heparin is used in the flush, watch for heparin-induced thrombocytopenia; many units use plain flush solution, as the protocol says.

Equipment

  • Arterial catheter set and insertion tray, as the protocol lists.
  • Cap, mask, sterile gloves and a small sterile fenestrated drape for a radial line; full sterile barrier for a femoral or axillary line.
  • Chlorhexidine-alcohol skin antiseptic.
  • Pressure transducer set with short stiff tubing, flush device and stopcocks; a pressure bag for the flush solution.
  • Monitor cable and a module for invasive pressure; a transparent dressing and securing device.

Step by step (and why)

  1. Check the order, identify the patient and explain the procedure — consent and cooperation reduce movement during insertion.
  2. Prime the transducer set with the flush solution and remove every air bubble — air damps the waveform and gives false readings.
  3. Inflate the pressure bag as the protocol says (commonly 300 mmHg) — a continuous slow flush keeps the catheter from clotting.
  4. Assist the insertion with the barrier the site needs and chlorhexidine-alcohol skin prep — this lowers the risk of catheter infection.
  5. Place the transducer at the phlebostatic axis and keep it there when the bed moves — a wrong height shifts every reading.
  6. Zero: close the line to the patient, open the transducer to air, press zero, then reopen to the patient — this removes atmospheric pressure and drift.
  7. Run a fast-flush (square-wave) test and look at the trace — it shows over- or under-damping before you trust the number.
  8. Secure the catheter, keep the site visible and identify the line as the institutional protocol says — it helps prevent dislodgement and wrong injections.
  9. Check colour, warmth, capillary refill and sensation of the limb as the protocol says — thrombosis or spasm can cause ischaemia.
  10. For samples, use the closed system and the discard method of the protocol, then flush and check the trace returns — it limits blood loss and clotting.

Complications

  • False readings from a transducer at the wrong height, a missed zero, air bubbles, clots, kinks or a soft bag.
  • Over-damping (no dicrotic notch) under-reads the systolic pressure; under-damping (several notches) over-reads it.
  • Pale, discoloured or cold fingers or toes from arterial thrombosis or spasm.
  • Bleeding or leakage at the site, and a line that no longer gives blood back.
  • Catheter-related bloodstream infection, more likely with femoral lines and frequent handling of the system.

Next step: when to call for help

  • Call at once if the limb becomes pale, cold, painful, discoloured or numb: the line may need to come out.
  • Press firmly on the site and call if it bleeds or if the catheter comes out.
  • Before acting on a sudden change in pressure, look at the patient, the trace and the transducer height; if the patient looks unwell, treat the patient first.
  • Report a damped trace that does not improve after you check for air, clots, kinks and the bag pressure.
  • Report redness, pus or fever: the line is replaced only for a clinical reason and is removed as soon as it is no longer needed.

Test yourself

Think of the answer, then tap the question to see it.

Where is the transducer of an arterial line levelled in a patient lying in bed?

At the phlebostatic axis: the 4th intercostal space at the mid-axillary line, roughly the level of the right atrium.

What does a waveform with no dicrotic notch suggest?

Over-damping, for example from air, a clot, a kink or a soft pressure bag; it under-reads the systolic pressure.

Which value is most reliable when the arterial trace is doubtful?

The mean arterial pressure, because it is less affected by damping and catheter site than the systolic pressure.

What must never be given through an arterial line?

Drugs, and dextrose-containing or parenteral nutrition fluids, which must not run through the pressure monitoring circuit.

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Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.

Sources: Intravascular Catheter-related Infection (BSI) Prevention Guidelines: Summary of Recommendations (peripheral arterial catheters and pressure monitoring devices) (Centers for Disease Control and Prevention (CDC), 2017); Intraoperative Invasive Blood Pressure Monitoring and the Potential Pitfalls of Invasively Measured Systolic Blood Pressure (Lam S et al., Cureus 2021;13(8):e17610) (Cureus, open access in PubMed Central, 2021); INPRESS: INvasive PRESSure Monitoring Knowledge and Practice Survey (Coolens O et al., Nurs Crit Care 2026;31(2):e70443) (Nursing in Critical Care, open access in PubMed Central, 2026); Comparison of invasive blood pressure monitoring vs. non-invasive blood pressure monitoring in critically ill children receiving vasoactive agents (Front Pediatr 2024) (Frontiers in Pediatrics, open access in PubMed Central, 2024).

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