Sometimes by shutting off the ambulance and killing the inverter, etc, so no power is on I have retaken and gotten a better quality. If not I have had to go with what I have. If they have other S/S to match then its probably still accurate to say MI. If not I would say can not rule out MI and treat as possible MI. I have no data as to how accurate the EKG is when you get that advisory. But one extra attempt does not delay transport by more than a minute and might give you more to work with.