Diagnostics & Procedures · Topic 12 of 16

Arterial Lines

Procedures · invasive pressure monitoring

Arterial Lines

Beat-to-beat pressure for unstable patients — trustworthy only when the transducer is leveled, zeroed and correctly damped.

Key numbers

Zero at the phlebostatic axis10 cm off = 7.5 mmHg

Hand ischemia is rare (under 0.1%)ischemia < 0.1%

01 · The ruleAllen test does not predict ischemia

The modified Allen test has poor diagnostic accuracy and does not reliably predict hand ischemia after radial cannulation, and adding pulse oximetry or Doppler does not substantially improve it.

02 · DoZero at the phlebostatic axis

Level the transducer at the phlebostatic axis (fourth intercostal space, mid-axillary line), open it to air and zero it; every 10 cm of height error changes the reading by about 7.5 mmHg.

03 · Look forOverdamped: systolic falsely low

An overdamped trace (air bubbles, clot or a kinked catheter) looks flattened and blunted: systolic falsely low, diastolic falsely high, pulse pressure narrowed.

Presentation

What you'll see

Underdamped: systolic falsely high must knowAn underdamped (resonant) trace shows a narrow overshooting systolic peak and extra oscillations: systolic falsely high, diastolic falsely low, pulse pressure widened; long compliant tubing, extra stopcocks and catheter movement promote it.

Diagnosis & monitoring

Work-up

Indications: shock, vasopressors, frequent blood gases must knowPlace an arterial line in unstable patients who need minute-to-minute blood pressure (refractory shock, vasopressor titration, major surgery with fluid shifts) or repeated arterial blood gases and blood sampling, as in respiratory failure.

Fast-flush test checks dampingCheck damping with a fast-flush (square-wave) test from the 300-mmHg flush system: prolonged ringing means underdamping, a sluggish return without oscillation means overdamping. Repeat it whenever the waveform changes.

Management

Orders

Overdamped? Clear air, clot and kinksFix overdamping by clearing air bubbles or clot, straightening kinks, repositioning the wrist and keeping the flush bag at 300 mmHg; fix underdamping with short rigid tubing, fewer stopcocks and a stabilized catheter.

Red flags & don'ts

Avoid harm

Hand ischemia is rare (under 0.1%) complicationComplications include hematoma, temporary radial occlusion (1.5% to 35%), infection, pseudoaneurysm and nerve injury; permanent ischemic injury is rare (< 0.1%) because collateral flow compensates, and occluded arteries usually recanalize.

Avoid infected, fistula or ischemic limbs avoidDo not cannulate through local infection or full-thickness burns, in a limb with a dialysis fistula, thrombosis or ischemia, or an artery neither palpable nor seen on ultrasound; with coagulopathy, prefer radial over femoral.

The rules

Know these

Radial first; femoral nearly doubles infectionThe radial artery is the usual first site (easy, rarely serious complications); femoral and axillary are the main alternatives, but femoral lines carry about twice the bloodstream-infection risk of radial lines (RR 1.93).

Never inject drugs into an arterial lineNever inject medications into an arterial line, and secure every connection: accidental intra-arterial injection and disconnection with rapid blood loss are its preventable disasters.

Remove when unneeded; no routine replacementRemove arterial catheters as soon as they are no longer needed, leave the occlusive dressing undisturbed, and do not replace arterial catheters on a routine schedule.

Overdamped or underdamped?

Commonly confused

Systolic pressure	Falsely low
Diastolic pressure	Falsely high
Pulse pressure	Narrowed
Waveform	Flattened, blunted trace
Usual causes	Air bubbles, clot, kinks
Fast-flush test	Sluggish return, no oscillation

Commonly confused

Tell apart

Titrate to MAP, not peripheral systolicSystolic and pulse pressure normally read higher at the radial artery than in the aorta (peripheral amplification), while mean arterial pressure changes little — so titrate vasopressors to MAP.

More detail

Reference

Show 7 reference facts

Flush the system with plain saline in a pressure bag at 300 mmHg; heparinized flush is not recommended because heparin exposure can trigger heparin-induced thrombocytopenia antibodies.

A 20-gauge catheter is generally recommended for radial cannulation: it is less prone to underdamping and causes fewer complications than an 18-gauge catheter.

Ultrasound guidance raises radial cannulation success and helps with nonpalpable arteries (hypotension, obesity, small vessels) or after failed attempts; it is an option for arterial lines, whereas it is standard care for internal jugular CVCs.

If the radial pulse is lost after a failed attempt, do not try again at that radial artery or the ipsilateral ulnar artery; alternatives are brachial or dorsalis pedis peripherally and femoral or axillary centrally.

Radial artery thrombosis is more likely with small arteries (hence more often in women) and longer catheter duration; thrombosis and distal ischemia are much less frequent with femoral catheters.

Puncture the femoral artery below the inguinal ligament to reduce the risk of retroperitoneal or pelvic hemorrhage.

Arterial-line pressure often reads somewhat higher than cuff pressure; calcification, atherosclerosis, proximal occlusion and limb position can all distort arterial-catheter readings.

Pearl

Overdamping hides the peak; underdamping exaggerates it — run a fast-flush before you treat the number.

Sources · Merck Manual Professional — How To Insert a Radial Artery Catheter (Feb 2026) · Saugel B et al. How to measure blood pressure using an arterial catheter: a systematic 5-step approach. Crit Care 2020;24:172 · Merck Manual Professional — Vascular Access (arterial catheterization) · Bertrand M et al. Applied physiology at the bedside: using invasive blood pressure as a true monitoring tool. Ann Intensive Care 2025 · Merck Manual Professional — How To Do Ultrasound-Guided Radial Artery Catheter Insertion · Buetti N et al. Strategies to prevent central line-associated bloodstream infections in acute-care hospitals: 2022 Update (SHEA/IDSA/APIC). Infect Control Hosp Epidemiol 2022;43:553–569 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).