Hi
You just talk about a sympathetic block but I assume you mean a lumbar sympatheric block and not a localised nerve block which would normally be done in the limb itself.

In the UK the usual preferred method for a lumbar sympathetic block is not lying face down. They have you lie on your side and inject to L1/L2 from the side of your trunk down and in towards your spine. This is different to the usual protocol in the US where it is more common for practitioners to have you lying face down. Normally, if you have it done on your side you can pull your legs up a little towards a foetal position for comfort and balance and they will support you with pillows. This way, there is no strain on your lower back for the procedure. I had to lie for 30 or 40 minutes for mine (no sedation etc) and it was fine.

If its not this type of block you are interested in then you might want to clarify and someone who has had a norve block will doubtless help you. Good luck.