Set up, zero and level an arterial line, read the waveform, and use the fast-flush (square-wave) test to detect over- and under-damping.
At the bedside
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Indications
Shock on vasopressors, frequent ABGs, labile BP (e.g. hypertensive emergency, dissection), raised ICP, major surgery. Usually radial (check ulnar collateral by ultrasound/clinically), then femoral or brachial.
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Level and zero
Transducer at the phlebostatic axis (4th intercostal space, mid-axillary line) with patient supine; for neuro patients who need cerebral perfusion pressure, some units level at the tragus (be consistent). Open to air and zero at set-up, after moving, and at least once per shift.
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Check the system
Pressure bag inflated to 300 mmHg (flush ~3 mL/h), no air bubbles or clots, short non-compliant tubing, no kinks or three-way taps with stagnant blood.
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Do the fast-flush (square-wave) test
Pull the flush tab for 1 s and release. Optimal: 1-2 oscillations before returning to baseline. Over-damped: < 1 oscillation, slurred upstroke → falsely low SBP, high DBP. Under-damped: > 2 oscillations, spiky trace → falsely high SBP, low DBP.
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Read the waveform
Sharp upstroke (contractility), dicrotic notch (aortic valve closure), area under curve (stroke volume). Narrow pulse pressure suggests low SV or tamponade; wide pulse pressure suggests vasodilation or AR; respiratory variation suggests preload responsiveness.
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Troubleshoot
Flat/dampened trace: check patient first (pulse, BP cuff), then flush, aspirate, reposition wrist, check bubbles, re-zero. Compare with NIBP if in doubt.
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Safety
Label the line clearly as arterial (never inject drugs), keep it visible, check distal perfusion hourly, remove when no longer needed.
Square-wave test interpretation
| Pattern | Oscillations | Effect on readings | Causes |
|---|---|---|---|
| Optimal | 1-2 | Accurate | - |
| Over-damped | < 1, slow return | SBP falsely low, DBP falsely high, MAP OK | Air bubbles, clot, kink, low bag pressure, compliant tubing |
| Under-damped | > 2, ringing | SBP falsely high, DBP falsely low, MAP OK | Long tubing, too many connectors, tachycardia, stiff arteries |
Calculators for this topic
References
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Is the arterial trace accurate?
Step 1 of ~3
Decision: Patient has a pulse and NIBP similar?
Based on Gardner RM. Educational use; follow local protocol.
Pathophysiology
The catheter-tubing-transducer system has a natural frequency and damping coefficient. If the natural frequency is too low relative to the harmonics in the arterial pulse, components are amplified (under-damping, overshoot). If damping is excessive (bubbles, clots, compliant tubing), the high-frequency components are lost and the trace becomes rounded.
Evidence
- Gardner (1981) described the dynamic response requirements and the fast-flush test still used today.
- ESICM 2014 recommends an arterial catheter in patients with shock not responding to initial therapy and/or requiring vasopressors [Cecconi 2014].
Controversies
- Leveling for CPP calculation (tragus vs heart) differs between units; consistency and documentation matter more.
🇮🇳 India notes
- Disposable transducer kits are costly in India; reuse is unsafe. Where arterial lines are not affordable, frequent automated NIBP (every 2-5 min) is an acceptable alternative for stable patients.
- 20G radial arterial cannulae (with or without guidewire) are standard; ultrasound guidance improves first-pass success.
References
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Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.