Direct measurement of blood pressure and Allen's test

Invasive arterial pressure monitoring provides beat-to-beat information about the patient.

Equipment

Arrow arterial products
An indwelling Teflon arterial cannula (20/22 G) is used. The cannula may be sited in the radial, ulnar, brachial, posterior tibial, femoral or dorsalis pedis artery. The radial artery is the preferred site. The cannula has parallel walls to minimise turbulent flow.

Allen’s test

Before cannulating the radial artery, Allen’s test should be carried out
(inadequate collateral flow exists in 3% of hospitalised patients)

Causes of inadequate ulnar circulation

[i] Normal anatomic variant
[ii] Hypothenar hammer syndrome

Technique

1) Patient elevates their hand and makes a fist for 20 seconds
2) Firm pressure is held against both the radial and ulnar arteries
3) Patient opens their hand and it should blanche white
4) Examiner releases ulnar compression only

Normal result

Hand colour flushes within 5 to 7 seconds

Abnormal result

1) Inadequate collateral circulation
2) Hand remains white until radial pressure released
3) Risk of serious hand ischaemia if radial vessel spasm

Hence, if abnormal result (negative Allen’s test) do not cannulate the radial artery.

[i] Allen's test and systolic arterial pressure in the thumb.
Husum B, Berthelsen P.
Br J Anaesth 1981; 53(6): 635-7