What's the use of an Arterial Line?
Table of Contents
- Information from arterial wave
- Common waveform patterns
- Indications for arterial cannulation.
- Contraindications to radial cannulation
- Allen's Test is misleading.
- Misinformation from arterial pressure.
- Complications.
- Troubleshooting hints.

Arterial pressure wave derived information.
Much clinical information can be derived from looking at intra-arterial pressure waveform:

1. Systolic, diastolic & mean systemic pressures (MAP); absolute and trend data.
2. Organ perfusion pressure ≈ MAP except left coronary artery perfusion ≈ diastolic BP.
3. Arterial blood pH, pO2, pCO2, lactate.
4. Heart rate.
5. Stroke volume estimate ≈ systolic AUC.
6. Arterial waveform patterns:
(a) Steep rise and fall ⇒ hyperdynamic state e.g. sepsis, anaemia, liver failure, pregnancy, thyrotoxicosis, aortic regurgitation, AV fistula.
(b) Increased systolic variation (pulsus paradoxus) during respiratory cycle from hypovolaemia, high intrathoracic pressure during MV, severe LV dysfunction, tamponade, or pneumothorax.
(c) Hypovolemia pattern: increased respiratory swing in systolic, lower dichotic notch, steeply peaked systolic wave:
(d) LV systolic dysfunction pattern = flattening of up-slope, reduced peak pressure, respiratory swing in systolic.
7. Myocardial O2 supply vs demand: comparison of systolic vs diastolic 'areas under curve'.

Indications for arterial catheterisation:
1. continuous arterial pressure monitoring
3. repeated / frequent blood tests
4. arterial blood analysis
5. Specific therapies: ECMO, IABP, etc.
6. Diagnostic and therapeutic access: angiography, PCI, endoluminal stents, etc.
Sites: radial (most common), femoral, brachial, ulnar, dorsalis pedis.
See YouTube video: insertion radial IA line

1. Buerger's disease
2. Severe Raynauds disease
3. Wegener's granulomatosis
4. AV shunt for dialysis
5. Absent collateral circulation.
Allen's Test was not designed to test safety of arterial cannulation. A positive Allen's test is NOT a contraindication to IA line. A negative Allen's Test does not minimise risk of ischemia. 25% will still get thrombosis. Anesthesiol59:42-7

Potential misinformation from arterial pressure monitoring includes:
1. Poor guide to perfusion (pressure vs flow)
2. Poor guide to cardiac performance (BP = HR X SV x TPR)
3. Systolic pressure amplification: reflected pressure waves result in pedal > radial > femoral > aortic systolic pressure.
4. Technical limitations: amplification & resonance result in wave distortion.
5. Over-estimation of pressure occurs with tachycardia, gas bubbles in transducer line.


Complications:
1. Thrombosis (10-20%) related to duration of use (5-10% after 2/7), size of cannulae (18G>20G), wrist size (=arterial diameter), female > male, catheter material (Teflon best), flush system, prolonged systemic hypotension, number of insertion attempts.
2. Haematoma (>50%), haemorrhage
3. Sepsis risk after 4-5 days: local site (0.7%), bacteraemia (0.13%)
4. Distal emboli (2-4%), thumb (below) or hand ischaemia (transient in 10%), proximal forearm ischaemia.
5. Aneurysm (0.1%), AV fistulae.
6. Inadvertent arterial drug administration.

Signal fidelity.
1. Equipment factors which optimise signal fidelity include: a high-frequency response transducer, short stiff non-compliant tubing, bubble free, slow continuous flush device (1-3ml/hr results in <2% error), transducer zero, and calibration against a mercury column.
2. Small air bubbles <0.25ml produce under-damping; larger air bubbles over damp.
3. Patient factors which reduce signal fidelity = rapid heart rate (resonance), high-frequencies reflected (elderly, high SVR, arteriosclerosis), site of catheter (radial vs femoral, axillary vs aorta).
Scheer, et al Crit Care. 2002; 6(3): 199–204.

1. Low pressure: check manual BP, check transducer is not above heart level, look for large air bubbles in transducer (damping), check cannula for kinks, check arm position for compression of brachial or subclavian artery.
2. High (systolic) pressure: check manual BP, check transducer is not below heart level, look for small air bubbles in transducer (resonance).

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