I'm not an expert, but from my personal knowledge heat does kill certain nutrients, yet it also activates certain parts of a plant so it depends on what you're looking for. perhaps if the instructions tell you to do a hot water infusion you could do a hot water infusion and then throw it on ice? Ultimately though an infusion probably won't be very powerful and it would be a better idea to get the raw herbs used in the infusion and boil them for a few hours. I would look into the traditional uses of the herbs used in the infusion to ultimately decide whether to do a cold or hot extraction.