Transesophageal echocardiography has become the standard of care for various cardiac related clinical applications and is used in the echo laboratory, outpatient settings, as well as the operating room. The clinical dependence on the proper operation of these probes is increasingly expanding due to both the clinic utility of TEE and significant technological advancements in transducer design.
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Almost all manufacturers' operator's manuals say that electrical leakage testing should be done before every patient exam. If your healthcare facility is an Intersocietal Accreditation Commission (IAC) accredited institution you are required by the standard of care to conduct an electrical leakage test on all TEE probes prior to high-level disinfection. The IAC standard states, "The manufacturer's guidelines must be followed for the appropriate care and cleansing of the TEE transducer and adhere to the appropriate infectious disease standards to prevent the transmission of disease. Effective December 31, 2015, the structural and electrical integrity of the transducer must be checked between each use, using an ultrasound transducer leakage tester, "Passed" or "Failed" must be documented in the routine TEE probe cleaning/maintenance log along with action taken if failed."1 The CS Medical HLD TRACKER offers healthcare professional the ability to record this information as well as cleaning and high-level disinfection data in one central location for later review and audit compliance. The TEE ultrasound machine, when in-use, is directly connecting 120 or 240VAC electrical source to the patient. If the TEE probe has a crack or break in the outer coating, bending rubber or distal tip it will result in an electric shock to the patient. This electrical shock has the potential of resulting in death. Electrical current flowing near the heart, even a very small amount, might result in the loss of heart rhythm, resulting in death. Large currents flowing through the extremities, while not fatal, can result in severe burns. To make sure electrical equipment is not hazardous, electrical leakage current testing is required. The main reason for electrical leakage testing of the TEE probe is that it always involves patient contact. TEE, intra-cavity and intraoperative probes are especially important because they have a lower resistance connection to tissue and are often used in the proximity of the heart. Electrical leakage testing can improve your ability to identify minor probe damage so as to obtain repair before the probe: leaks electricity in a dangerous amount, is damaged beyond repair, has reduced image quality or has corrosion of the internal components. Leakage current is measured to ensure that direct contact with the medical equipment is highly unlikely to result in electrical shock. The test is designed to simulate a human body coming in contact with the insertion shaft of the TEE probe. The measured leakage current values are compared with acceptable limits. These limits are based on the TEE probe manufacturer's recommendation for acceptable levels. As a function of repeated use, a TEE probe can develop punctures or cracks somewhere along the insertion tube. These conditions could likely expose the patient's esophagus to chassis leakage current and cause the patient to be at the chassis ground potential. If this occurs, then the probe is no longer functioning as a class BF device and would fail a leakage current test. The patient would then be at ground potential and leakage currents from other devices within the patient's environment could travel through the patient, compromising the safety of both the patient and the operator.2 One leakage testing device User's Manual also states that… For more information, please visit Hengfeng. Although a TEE probe does not actually come into direct contact with the heart, it is only separated from the heart by a very thin membrane. As it takes very little current to excite the muscle of the heart and potentially cause ventricular fibrillation, an obvious life-threatening condition, it is vital that TEE probes are tested for electrical leakage testing prior to every use. Electrical leakage testing is required by IAC and is necessary to ensure TEE probe and patient safety. Conducting electrical leakage testing can be accomplished with CS Medical's ULT-PC-31 electrical conductive probe within the TD 100® Automated TEE Probe Disinfector or TEEClean® Automated TEE Probe Cleaner Disinfector prior to high-level disinfection. CS Medical offers the BC Group ULT-2020 ultrasound leakage tester and respective adapters that are specific to each TEE probe manufacturer. Conducting the electrical leakage test, with the TD 100, is simple and requires less set up and supplies to complete prior to each high-level disinfectant when compared to other solutions.
What is electrical leakage current and what is the potential hazard?
How does testing leakage current affect patient and user safety?
"In addition to verifying that the ultrasound transducers are safe for patient use, the test makes it possible to reduce expensive repairs. Identifying transducers that exceed safe leakage currents early may allow for repairs to be made before a transducer becomes non-repairable."3
What are some manufacturer's recommendations for TEE testing?
TEE probe manufacturer's recommendations for testing frequency are:
What is the solution for electrical leakage testing?
References:
I agree that all persons in the chain have normal responsibilities, but not for "type testing" and for standards compliance, as this is impractical.
Consider a medical system of devices A and B where both devices contribute to the accuracy of measurement, for example B is the sensor and A is the main unit. Each are made by different manufacturers with no contractual agreement.
Our instincts say that the only way to be sure the system of A+B is accurate, safe, that EMC/EMI is OK, usable etc etc is to test the whole system. FDA and the IEC 60601 series agrees with this in many places. It seems obvious.
But I can say with 100% confidence, this instinct is wrong.
Consider that the system target accuracy is ±5% as specified in IEC 60601-2-999.
Let's say A and B independently use ±5% as their design target (obviously this is wrong, but I have seen this many times).
So, the FDA and IEC say we must have a test report showing a single sample of system A+B meets ±5%.
In theory, the result could range from ±10%, but statistically more than half of the combinations will be OK (likely around 70-80%, depending on the design margins used). So in a single random sample there is a good chance that the result will be OK. However, in the market, there may be 10-20% of combinations that exceed the limit. Something doesn't add up here.
In fact, A and B could use ±10% (terrible design) and still a random, single sample of A+B could be OK, for example A could be +8% and B is -6% and they cancel to give an overall result of +2%, which seems good. Crazy? Well I have seen this kind of thing in real life, in a round about way: system A+B fails a test in an independent lab, so the manufacturer of A tweaks the software of A so that the particular sample A+B complies, rather than addressing the underlying accuracy issues. Obviously this sounds terribly irresponsible, and the true reason is manufacturer A is under the pump timewise and wants to get the FDA 510(k) moving so they just want a report (any report!) that says the system is OK and they quietly plan to fix the issue later.
But it does beg the question: why does the FDA and IEC allow a test on a single sample (or a system) as evidence of conformity?
The answer lies in a higher level document called the "declaration of conformity". This is the most important document legally, not the test report. In this document, an entity declares that the object (which could be a system), as manufactured, conforms to a specification. A declaration of conformity is not just a glorified addition to a test report (i.e. I declare this apple is OK), it adds the extra dimension of regular production (I declare all my apples are OK).
A declaration of conformity can be made for a system, and it can also be made by persons who are not the manufacturer. This is not the issue.
But, the catch is this test report on a single sample. As we have seen, a single sample is statistically irrelevant. If we have no knowledge of the underlying design, we would need to test a large volume of random combinations and do a statistical analysis in order to be confident that actual systems placed in the market are compliant. This is an option, but it's not really practical.
And this is where the manufacturer has an advantage: they know the design and assuming good design practice, margins, reliable parts, production controls, production testing there is the potential to be confident that a test report on a single sample is representative. This means they can sign declaration of conformity with legal confidence. The test report on the single sample is really just a rubber stamp, a final check that everything is OK, the icing on top of a large cake of design and production controls.
Thus while it is possible to cover a system in a declaration of conformity, in practice it is not an option.
How do we solve the issue of A+B and the ±5% specification in IEC 60601-2-999? In practice each manufacturer should research, experiment, use historical norms and decide what is a reasonable breakdown of system specifications to each individual device, and any interface specifications that might arise (for example, limiting the current to a sensor to avoid excessive heating, dimensions of mating connectors). To an outsider it sounds dodgy but it works in practice, it's basic engineering and happens all the time. For example, A and B could assume 50:50 split and then round down to 2%, an approach that ensures the system of A+B is well within 5%.
Now, here's where the FDA, IEC and those that insist on system testing have not only got it wrong, but are dangerously wrong.
If A and B each decide to use 2% for their device, what about the declaration of conformity? If the IEC standard says 5%, do they declare 5% or 2%?
In my opinion, it is legally important that the 2% appears in the declaration. If IEC 60601-2-999 says 5% for the system, then the declaration for the device (e.g. A) should include a breakdown of any system specifications and assumptions for other parts of the system that are necessary for compliance. If A declares 5% and only informally uses 2%, there is a risk that in design changes, moving factories etc that somebody uses 5% instead of 2%, because the standard/declaration says 5% is OK. It is also important in various stress testing, such environment (temp/RH/press), mechanical (shock, vibration), EMI, water ingress, defib proof and other normal condition stress tests that the accuracy criteria for used for testing each device is 2%, not 5%. For testing device A, for example, a special dummy sensor with 0.1% accuracy can be used to replace B which then allows us to verify any errors associated with device A alone, against the 2% criteria.
Or better still, the IEC should stop writing standards for systems! And the FDA should get their act together. This is not a new concept. The older EN, ISO, JIS (Japan), AAMI (USA) often wrote very good standards that were based on components of a system (e.g. just for a sensor) rather than the overall system. The system test is a lazy surface level thinking, without working through the implications.
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