Lightning Bolts, Defibrillators, and Protection Circuitry Save Lives

Abstract: There have been many studies concerning the safe current levels impressed across the heart. The standards for medical equipment have bounced around, and today safe levels are said to be less than 4µA to 10µA. With lives in the balance, a designer of defibrillators must understand the entire gamut of possible input protection methods and then choose the best defense at a reasonable cost. Victims of sudden cardiac arrest (SCA) can be saved with a small, prompt lightning bolt (defibrillator shock) to the chest.

With lives at stake, the design of medical equipment is very demanding with extremely tight margins. Remember too that it is not uncommon to have several pieces of equipment attached to the patient at the same time. So the total leakage current must remain below the threshold that can harm the patient’s heart. This app note discusses several ECG input protection methods including radio susceptibility (RFI), ESD, electromagnetic interference (EMI), electromagnetic susceptibility (EMS), and defibrillator protection.