Before sampling arterial blood from the radial artery the patient must be evaluated for the adequacy of collateral circulation. This is to ensure that infarction of all or part of the hand does not occur if a thrombus of the artery should occur after the arterial sampling.
Methods to evaluate the adequacy of collaterals in the hand - one or both of the following:
(1) modified Allen test
(2) ultrasonography of palmar circulation
Modified Allen test:
(1) The patient is instructed to make a tight fist with the hand whose radial artery is being evaluated.
(2) Digital pressure is applied to occlude both the ulnar and radial arteries.
(3) The hand is then partially opened but not fully extended. The palm and fingers should appear blanched.
(4) Pressure on the ulnar artery is then removed and the time required for the hand and fingers to flush is then counted in seconds.
(5) If adequate flushing is seen in <= 15 seconds, then the collateral circulation is considered adequate and the test is positive. If flushing takes more than 15 seconds, then collateral circulation is suspect and the test is termed negative.
Ultrasonography:
(1) An ultrasonographic probe is placed to assess flow through the superficial palmar vascular arch in the hand being evaluated.
(2) The ulnar artery and then the radial artery is compressed and flow through the arch assessed.
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Flow Through Arch During Compression
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Interpretation of Collateral Circulation
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only slightly reduced
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good
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markedly reduced
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poor
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absent
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absent
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(3) A reversal of direction in blood flow when the dominant artery was compressed is further evidence of a good collateral circulation through an intact arch.
Interpretation:
• If the tests demonstrate adequate collateral circulation, then radial artery sampling may proceed.
• If one or both tests indicates an inadequate collateral circulation, then an alternative site should be selected.
