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Keywords: electrocardiograph, protection circuitry, RFI, ESD, EMI, EMS, radio susceptibility,
electromagnetic interference, electromagnetic susceptibility
APPLICATION NOTE 5724
LIGHTNING BOLTS, DEFIBRILLATORS, AND
PROTECTION CIRCUITRY SAVE LIVES
By: Bill Laumeister, Strategic Applications Engineer
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.
A similar version of this article appeared July 2014 in Electronic Design.
Introduction
Victims of sudden cardiac arrest (SCA) can be saved with a small, prompt lightning bolt (i.e., a defibrillator
shock) to the chest. The shock (3kV to 5kV and 50A) stops the heart from unproductive fluttering
(fibrillating) which fails to pump blood to the brain and other organs. This lightning bolt allows the heart to
restart orderly pumping of blood. In hospitals it is common to monitor the heart using an electrocardiograph
(ECG) with a separate defibrillator. The ECG leads (i.e., electrodes) are on the patient when the defibrillator
delivers the shock. With no warning, the ECG must withstand this lightning bolt and continue working
properly.
According to the American Heart Association (AHA) nearly 383,000 out-of-hospital sudden cardiac arrests
occur annually, and 88 percent of cardiac arrests occur at home. Sadly, less than eight percent of people
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who suffer cardiac arrest outside the hospital survive. These are sobering statistics. In medical terms a
heart attack is much different than an SCA. An SCA has no warning signs; a person just collapses. A heart
attack has multiple, generally understood warning signs preceding it.
Without our protective skin a patient’s heart is vulnerable to very small currents. In the electrically
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susceptible patient, moreover, even minute amounts of current (10µA) can cause ventricular fibrillation.
Remember that with an ECG and separate defibrillator it is not uncommon to have several pieces of
equipment attached to the patient at the same time. Clearly, the total leakage current must remain below
the threshold that can harm a human heart.
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