Essential Performance Case Studies
Case #1: Infusion Pump. Infusion pumps have their own product-specific basic standard: IEC/EN 60601-2-24. Although this standard makes some modifications to the requirements of basic standard IEC/EN 60601-1-2, the essential performance, risk analysis, and their effects on the EMC immunity test plan are representative of issues for all medical devices.
The essential performance of the pump was identified as providing fluid at a flow rate that remained within tolerance (the tolerance was defined based on technical judgment from within the manufacturer's expertise in this specific medical application). In addition, the pumps ability to detect certain error conditions (such as air in the line or line occlusions) and create an alarm was also considered a part of essential performance. This meant that two modes had be tested — normal mode (with the pump infusing at a predetermined rate) and alarm mode (with various types of errors introduced into the system).
To address the flow-rate evaluation, the manufacturer first considered the use of a scale to determine the volume infused over a certain period of time. This method was considered inappropriate because short duration changes in the instantaneous flow rate might go unnoticed when the weight of fluid delivered was averaged over the duration of a test. To counter this, the manufacturer developed an additional device to measure flow rate and to give a visual indication if the flow rate went out of tolerance. The flow meter had to be tested for immunity prior to performing the test to determine whether there were any susceptibilities prior to testing the final product to ensure that any flow-rate alarms were because of the device susceptibility and not due to the monitoring equipment.
The alarm conduction tests required no special test equipment since the error conditions could be recreated by manually injecting air into the line or pinching off the fluid line. As these conditions could not be recreated on a continuous basis, the manufacturer followed our recommendations to run these tests at spot frequencies for radiated and conducted immunity, with the frequencies selected based on the operating frequencies of ISM and radio devices in the hospital environment. For transient tests (ESD, surge, voltage dips, and interruptions), as the transient phenomena are all short duration events and the error condition would be a long duration event, the primary concern was that these transients did not damage the error-detection circuitry or inhibit the alarm. The ability to detect the error conditions was verified before and after applying each phenomenon, while also verifying that the transient phenomena did not reset the alarm condition.
Prior to implementing the above test plan, there was some discussion on the possibility of having a method of monitoring the alarm circuitry during “normal” operation to verify that they were not susceptible to the applied phenomena (for example, by indication of the analog output from the circuit). This was not possible on the current product line but may be implemented in future products. While it may not alleviate testing in the alarm mode, it might reduce the number of tests required by allowing selection of reduced tests on the mode based on the areas where the circuits showed susceptibility.
Case #2: Telemetry System. This device contained a sensor and would periodically send data acquired by the sensor to a remote data logging device. In normal operation, the device would transmit every 2 minutes; however, the interval could be adjusted for the device to transmit every 90 seconds. It was not possible to reduce this sampling time any further because the device required this as a minimum sampling time to avoid producing invalid data.
Although there were no complicated test configuration issues with this device, the fact that the dwell time could be reduced from 120 to 90 seconds represented a significant test time savings. This case also brought up the issue of an exclusion band for testing. The 4th edition states that degradation in performance around the receiver operating frequency of the device under test can occur during radiated RF immunity tests, so this is allowed. However, the basic safety and essential performance requirements must still be met.
Case #3: Ablation System. This piece of medical equipment is designed to use RF heating to cauterize during surgical procedures. The risk management process resulted in identifying two possible problems that might result in an unacceptable safety risk during operation — the first was if the power setting should change or if the device should power-on unintentionally. It was determined that the RF switching off unintentionally was not a risk provided that the instrument indicated that the RF was off and did not re-initiate without manual control.
Two test modes were, therefore, tested. The first was with the power set at a midpoint to allow the displayed power to be monitored for an unacceptable increase or decrease. The second mode was with the device operating in a standby mode to ensure that it did not switch on the RF unintentionally. By providing two samples, radiated immunity was performed on both modes simultaneously to reduce test time.
Case #4: Home Use Patient Monitoring System. The risk management process for this product concluded that there were no functions that were considered essential to the safety of the patient. Any information obtained by the system would be evaluated on a case-by-case basis, and the system was to be used to provide additional patient information between outpatient visits to the physician. Any abnormal data recorded by the system would be evaluated by a physician who, in turn, would run additional tests before making a diagnosis and prescribing treatment.
In this case, the requirements state that it is not necessary to perform immunity tests to declare compliance. However, the manufacturer still wanted to know whether the product was susceptible, so testing was performed. The performance criteria were not related to essential performance, but to anticipated customer expectations.
For most products, performing the assessment against the 4th edition will require testing and updates to documentation supplied to the end user. The importance of the risk management process has been highlighted in this article.
Product-specific knowledge and, in many cases, medical expertise are required to clearly identify basic safety and essential performance and minimize the number of functions and test times that need to be used when evaluating the product for susceptibility to the EMC phenomena described in IEC/EN 60601-1-2. The compliance of radios must also be part of an assessment of the product both with regard to basic safety and essential performance and EMC.
This article was written by David Bare, a chief engineer at NTS in the company's Fremont, CA, lab. For more information, Click Here.