A doctor currently working in the intensive care unit has been asked to insert an arterial line into a patient that has just been reviewed on the ward round.
1. What are the main indications for inserting an arterial line?
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The two main indications for the insertion of an arterial line are:


  • Continuous, beat-to-beat blood pressure measurement. (e.g. patients requiring inotropic support)
  • When frequent arterial blood gas analysis is required (e.g. patients with respiratory failure and/or severe acid/base disturbance)
2. Which artery should be the first choice for the placement of the arterial line? Why is this the chosen site?
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The first-choice site for intra-arterial cannulation is the radial artery at the wrist. This is the best site as it has low complication rates compared with other sites and is an easily locatable, superficial artery that aids insertion. Its superficial nature also makes it easily compressible for haemostasis.

3. Name a test that should ideally be performed before placement at this site.
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Allen’s test is recommended before the insertion of a radial arterial line. It is used to determine collateral perfusion between the ulnar and radial arteries to the hand: poor collateral perfusion is thought to be present in around 10-15% of people. If ulnar perfusion is suboptimal and a cannula occludes the radial artery, blood flow to the hand may be reduced.

Allen’s test is performed by first asking the patient to clench their hand into a fist. The ulnar and radial arteries are then occluded with digital pressure, and the hand is then unclenched, and the pressure over the ulnar artery is released. If good collateral perfusion is present, the palm should flush in less than 6 seconds.

4. Which insertion techniques are commonly used?
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There are three common insertion techniques used:

Direct cannulation: this insertion technique is similar to that used to insert a venous cannula. Extension of the wrist moves the artery closer to the surface. The stabilisation of the wrist in this position with the help of an assistant, makes insertion easier. The radial artery is then palpated and the cannula is inserted, aiming to hit the middle of the artery at an angle of approximatelt 30 degrees to the skin. When there is free flow of arterial blood back into the hub of the cannula, the cannula sheath can then be safely advanced over the needle into the artery.

Transfixion: after obtaining a flashback, the cannula is advanced through the posterior wall of the artery. The needle is removed, and a syringe is attached. The cannula should be slowly and carefully withdrawn while simultaneoulsy aspirating. Once free aspiration is achieved, the cannula can be advanced proximally along the artery.

Guidewire (Seldinger) technique:A guidewire may be used if the advancement of the cannula sheath over the needle proves difficult, for example, in atheromatous disease. The guidewire is inserted through the cannula sheath after removal of the cannula needle (or through needles that are provided in some arterial cannulation sets). The guidewire should advance freely along the artery. The cannula sheath is then advanced along the artery, and the guidewire is removed.



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