First, you need to determine if your AP's / controller has the ability to allow 11b clients WITHOUT going into protection. Not all vendors will do this.
If they do, then the next step is to find out how much traffic your new 11b device will transmit. If it is a lot, it will cause excessive collisions.
Hey... does the EKG machine have an Ethernet port? If so, easy solution. Put an 11g work group bridge on it and then you have no worries.