The perfusion index (PI) ratio can be used to predict a successful supraclavicular nerve block.

A successful nerve block is associated with vascular dilation due to reduced sympathetic stimulation.


The perfusion index is the ratio of pulsatile and non-pulsatile blood flow as measured with a special pulse oximeter.



(1) PI at 10 minutes after block

(2) PI at baseline


PI ratio =

= (PI 10 minutes after) / (PI at baseline)



• A perfusion index ratio > 1.4 indicates vascular dilation and predicts success of the limb block.

• A limb without a nerve block has a PI ratio in the range of 0.9 to 1.1.

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