If you are having it with Dr J it is to see what type of compression is happening at the BP. I assume it is investigative. He will inject your scalenes and some other muscles adn ask you how you feel after each, then he will observe by ultrasound if there is arterial occlusion with arm position.

the effects of the block will last ~12-24 hours and you will be asked to do provocative maneuvers and rate your pain for 24 hours following the block to determine if it was successful.

Dr Ahn uses it as an indicator for surgery.