Not only are medical devices expected to function as intended, they must meet ergonomic, safety, FDA and functional requirements. They must be designed to function in adverse environments; sometimes in the operating room; sometimes in an emergency vehicle for example. If a device is patient connected, it is also expected to function within proscribed parameters in the presence of a defibrillation pulse. These parameters differ depending on the type of device. All devices must pass an isolation test designed to ensure that the pulse will not affect the device’s signal input part/signal output part (SIP/SOP) ports; and effective with the Third Edition of IEC 60601-1, they must demonstrate that they absorb less than 10 percent of the defibrillation pulse. ECG monitoring equipment either for hospital (IEC 60601-2-27) or emergency use (IEC 60601-2-25) is additionally subject to performance requirements after a defibrillation pulse is applied.
The verification program to ensure device behavior in response to a defibrillation pulse is straightforward in theory. The procedure is to pulse the device with a defibrillator connected directly across the patient connection and check the result depending on the test performed. However, in practice, the use of a defibrillator is problematic. First, defibrillators monitor the output pulse and can attenuate it if the desired result is obtained before the complete pulse is delivered. Devices must perform when a full defibrillation pulse of 360J is delivered, so this attenuation feature is not optimal. Second, defibrillators have a short life when used as pulse generators, with field reports of <2,000 delivered pulses before defibrillator failure. Test equipment capable of delivering a full 360J pulse at duty cycles of 30 seconds continuous and a lifetime of over 1M cycles are commercially available and use of one of these generators is recommended for verification tests. Test equipment lifespan becomes important for manufacturers who perform life testing to ensure continued compliance over many defibrillation pulses as part of the verification test package for the device. Consistent 360J pulse deivery is an important requirement to consider for any verification package.
Manufacturers of components which will be visible to the defib pulse may also benefit from verification testing for this requirement. If the component is actually used to deliver the pulse, then the verification test also is providing testing for a core requirement of the component. Life testing of the component will be fairly straightforward with a test program of simulation of the product use at full defibrillator output for a number of cycles deemed proper to demonstrate the use of the component over the life of the device it is used in. Defibrillator leads and their connections are components that are good examples of components that would benefit from surge testing, both from a safety aspect (isolation of the pulse) and a performance aspect (pulse delivery). Use of a 360J generator instead of a defibrillator is again recommended for the reasons stated above. Manufacturers can develop a test program for the leads that could focus on the ability to maintain full pulse delivery and isolation over the life of the device; and the availability of these generators would make it possible to test for the life of the device in a relatively short amount of time.
Different Medical Surge Tests All Use the Same Generator
As noted, there are three basic tests that are conducted, using the same 360J power supply as described in IEC 60601, AAMI EC-13, and others. There are two different waveshaping networks described in the tests. The power supply shown in the IEC/AAMI Standards will deliver 360J minimum to the waveshaping network chosen, if the components are within the stated tolerances. Other caveats as noted in the Standards are that the switch S1 must be able to deliver the power, and the inductor L1 must not be allowed to saturate.
Energy Reduction Test
Changes to the Surge Test in the Third Edition of IEC 60601-1 introduce the Energy Reduction test, which has been a part of IEC 60601-2-49 in previous editions. This test verifies that the device under test absorbs a maximum of 10 percent of the delivered defibrillation pulse power and requires either a scope with math functions, an Excel spreadsheet and a digital scope, or a commercially available surge generator that can display the result directly. In practice, two pulses need to be delivered for each test result.
Since any heating of the internal 100 Ω resistor “R 100 Ω” will decrease its resistance, a corresponding increase in the energy dissipated will also occur. This will cause the 90 percent threshold required for compliance to the Standard to be set artificially high, which could cause a marginally passing device to appear to fail the energy reduction test solely because of test equipment error. For example, since a 5 percent tolerance is allowed, a value of “R 100 Ω” between 105–95 ohms is acceptable. If the test with the DUT connected is conducted with “R 100 Ω” = 105Ω (energy captured is 356J; pass/fail point is .9(356)=320J), and the referee pulse is conducted with “R 100 Ω”=95Ω (energy captured is 397J; pass/fail point is .9(397J)=357J), the DUT will appear to fail no matter what the outcome of the test conducted with the DUT attached, solely due to heating of “R 100 Ω”. (See Figure 1)