[trux]

Brian, I am not sure if I understand well that you need to hold your breath for possibly long periods of time because it gives you some relief from the back pain. If it is so, and if you do it only dry, then probably the easiest way to delay the contractions would be hyperventilating prior the breath-hold. Doing it with oxygen would allow possibly even longer apneas, but it bears some additional risks, so you probably don't want to go there.

Normally I would not advise hyperventilating, and especially not if you hold the breath under water, but if it is used as a form of medical relief and only dry, then the disadvantages of hyperventilation are not that bad.