IEC 60601-1-10 Scope (Physiological closed loop controllers)

The standard IEC 60601-1-10 applies to physiologic closed loop controllers (PCLC): devices that try to control a physiological parameter through feedback. A simple example is an infant incubator where the system controls body temperature using a sensor placed on the infant.

Despite being obviously applicable for many devices, there are bound to be some borderline situations where it is unclear if the standard should apply.

In Annex A of the standard, several examples are given as systems which are not PCLCs. Example #3 is a ventilator, which can control parameters like flow, pressure and volume and breathing rates. The conclusion of the committee was that pressure is not measured “from” the patient, and therefore a  pressure controlled ventilator is not a PCLC.

But it is cited as being a difficult example to analyze, and it is worth to look closer why. The problem is that volume, pressure, flow and breathing rates are all parameters that could be considered physiological. Why then are the PCLC requirements not applicable?    

The key may come down to the part of the system called the “patient transfer element”, which is shown as element “P” in Figure 1 in the standard (refer to the standard for context / definitions):

According to the rationale (Annex A, sub clause 1.1), the “…standard was developed [in part] to address … the difficulty in adequately characterizing the PATIENT TRANSFER ELEMENT”. This suggests the nature of the patient transfer element is important in determining the scope.

In the case of, for example, a blood glucose controlled insulin pump, there is a complex body process between the injection of the insulin through to the final blood glucose level.  The modelling of this process is obviously critical to the overall loop performance, and should be well documented.

Similarly for an infant incubator (in “baby controlled mode”), the output of the control system is heat (power, in Watts), which the infant’s body converts into body temperature (°C).

In these cases, we can clearly see that there is a non-unity transfer element (P ≠ 1) between the equipment’s output (m) and the physiological parameter we are trying to control (y).

In contrast, for a ventilator there is in effect a unity patient transfer element – parameters like airway flow and pressure are both the system output and the parameters used for control, as well as being (potentially) a physiological variable. In this case, m = y, and P = 1. Although the patient influences the system, it acts only as a variable load, and does not form part of the feedback loop.

Another way to look at this is to say that a patient’s physiologic variable just happens to coincide with a non-physiological parameter that used for control. For example, a pacemaker may output a fixed rate of 60bpm, which under normal conditions coincides with the patient’s heart rate, a vital physiological parameter. Nevertheless, the pacemaker does not use the patient’s heart rate for control, and it does not form part of a feedback loop.

So an improved definition may be that PCLC is a system where a physiologic variable is used for feedback, and where there is an identifiable non-unity patient transfer element (m≠y) that forms part of the control loop.

Unfortunately the above interpretation cannot be directly taken from the normative part of the standard. If a system exists where P = 1 (and m = y), nothing could be found in the scope statement, and definitions which would clearly exclude such a system from the standard.

However, the interpretation can be supported by noting that all the examples of PCLCs in standard have non-unity patient transfer elements. Further, it helps to better explain why example #3 in Annex A is not considered a PCLC.  Finally, the title of the standard refers to a “closed loop”, it is logical to expect that the physiologic variable necessarily forms part of the loop. The fact that a loop variable is the same as a physiologic variable is not enough condition to be considered a PCLC.

The standard should remain be an important reference where the patient or environment forms a variable load on the output of a control loop. The ability of control loops to respond to disturbances (e.g. door opening in an infant incubator operating in air controlled mode) is often not well documented in risk management, specifications or general design control. Nevertheless, the fact that a system is susceptible to external disturbances should not be a criteria for determining if the standard IEC 60601-1-10 is applicable.

IEC 60601-1-8 Alarms, Sound level measurement

Material here is transferred from the original MEDTEQ website, last updated 2015-10-27

Many medical devices include alarms, and both collateral and particular standards specify sound level measurements.

But, there is a trap: the beeping sounds from alarms often have strong tones. When a sound is made up of tones, reflections create an interference pattern where the difference between spatial maximum and minimums in the region of interest can be in the order of 10dB.

You can experience this by simply playing a 1kHz tone from your PC (click on the MP3 file right). As you play this file, walk around the room very slowly and note the changes in sound level. The interference pattern is caused by reflected sound from walls and the floor which can either add or subtract with the direct  sound depending on the phase difference at the point in space. If you are careful, you can find places where the sound almost disappears as the reflected waves cancel out the direct wave. For a 1kHz waveform, these peaks and troughs should around 15-20cm apart. As the frequency increases, the space between the peaks and troughs will reduce.

 

You might reasonably expect that this problem can be eliminated by using the “free field over a reflecting plane as specified in ISO 3744” , referenced in IEC 60601-1-8 and a number of particular standards. But it turns out that the strongest reflection will actually come from the “reflecting plane” (the floor), and this reflection alone can cause a large interference pattern. The above graphs are actually simulated using only a single reflecting plane (floor). The graph on the left is the expected variation as a meter is kept at 1m above the floor and moved away from the sound source. The graph on the right is the variation with the sound level meter 1m away, and the height from the floor for both the source and meter is varied. The location of the peaks and valleys will also change with the frequency of the source.

Moreover the test sample, and any supporting device such as a table is not a point source. Simulations show that a rectangular object emitting sound in free space will still create an interference pattern, simply due to the different time that sound from different parts of rectangle reach a certain point in space.

To put it bluntly, trying to measure sound pressure is a mugs game for sounds with strong, discrete tones.

So what is going on? How can we have a fixed requirements when the test method is hopelessly unrepeatable? 

It turns out there is a fundamental error in medical standards that reference ISO 3744. The standard ISO 3744 is intended to estimate the sound power level from the source. It does this by taking many sound pressure measurements (up to 20) around a surface which encloses the noise source against the reflecting plane. These measurements are then integrated to find the power level from the source itself. The spacing of these measurements is such that interference patterns are largely canceled out, so the test environment does not have to be a perfect anechoic chamber, rather the test is intended for use in a broad range of locations. The standard also correctly assumes that sound will not emanate from a single point (i.e. the speaker), but rather that all parts of the enclosure will be involved in the transmission of sound to the outside world; again multiple measurements help to "collect" all the sound power irrespective of the shape of the test item. The reflecting plane(s), while causing an interference pattern, also help to reflect the sound power to the measurement points.

What's the difference between sound power and sound pressure? It's a bit like power and temperature. Think of a 10 ohm resistor in a power supply which has 2Arms flowing through it. We can estimate the power of the resistor quite accurately as 40W. The power emitted is relatively independent of the environment. But the temperature of a part 10cm away from the power resistor is a complex function of many factors, the physical layout, material properties, air flow and so on. We know that increasing the power increases the temperature, but we cannot predict the actual temperature without measuring it. And most importantly, even though we measure the temperature of one part 10cm away, it does not mean that all parts at 10cm have the same temperature.

In the same way, a speaker will emit a fairly constant amount of sound power irrespective of the environment. However, the sound pressure will be complex function depending on the environment, including construction of the medical device itself (in much the same way as the speaker's box also influences sound pressure). A measurement a one point 1m away will not be representative of another point 1m away.   However, by taking many sound pressure measurements around an object, we can use the properties of air and the surface area of the measurement framework to estimate the sound power from the source.  

One reason why we quickly become confused between sound power and pressure is they both use a dB scale. However, the underlying units are the watt (W) and Pascal (Pa). The dB scale simply reflects a wide range over orders of magnitude. Sound power and sound pressure are as different as resistor power and resistor temperature - they are related but definitely not the same thing.  

It appears the IEC 60601 series has originally fallen for the trap of treating sound power and sound pressure as the same thing, and in doing so ignored many of the issues associated with measurement of sound pressure. 

Fortunately, the committee for IEC 60601-1-8 has finally woken up to this, and in the latest amendment 1 issued in 2012 now a number of measurements are required according to Annex F in ISO 3744, with the measurements averaged. Although they missed the final step of converting sound pressure to sound power, the process of averaging in effect acheives a similar purpose. A word of warning though to the unsuspecting; averaging sound pressure is not simply averaging the readings in dB. To get the correct average, the readings should be in pressure, which means first converting to a non-logrithmic representation, averaging, and then convert back to dB. Mathematically it is not necessary to convert to any particular unit of pressure, the main point is that the average should be performed on the "anti-log" representation. For example, three values 72, 70 and 62dB incorrectly averaged in dB gives 68dB, but if averaged in pressure and converted back is 69.75dB, roughly 2dB higher. 

While the 2012 amendment is an improvement, technical issues remain: 

  • Annex F is intended for measurements at least 4m away from the source. The calculations of the measurement points in Table F.1 refer to Table F.2, which only has values for 4m and above. Since IEC 60601-1-8 clearly specifies a measurement radius of 1m, it appears to be a technical oversight making the standard impossible to use. Further checking in the standard reveals that Annex F is intended for outdoor noise measurements, where reflections are not expected (e.g. not for indoor). Even ignoring the technical mismatch with Table F.2, it seems possible the measurements under Annex F will not yield repeatable results. 
     
  • For the measurement of harmonics, the standard specifies a single point measurement 1m away from the equipment. However, both theoretical analysis and experimental evidence show that harmonics vary greatly with the small changes in the position of the sound level meter. This is because the interference pattern will be different for each frequency (again, even the reflecting floor will produce interference patterns). The requirements for harmonics are relative, so ignoring reflections the exact position of the meter is not critical. To eliminate the effect of reflections, it is recommended the measurements should be made with the meter positioned fairly close to the equipment's speaker, e.g. 5cm away. Experiments with this found the relative harmonics to be consistent when measured close to the equipment, irrespective of the exact position of the meter.
     
  • For relative priority (high > medium > low), the measurements rely on Annex F at 1m, which is still influenced by reflections and high uncertainty. Designers often only have a small difference between the sound level of different types of alarms, so errors in the test method can produce false negative results. Since the measurements are relative, it again makes sense to use a single point measurement at a point close to the equipment where the influence of reflections and the interference pattern are negligible.  
     

In general, a formal test for sound level measurement according to ISO 3744 can be expensive yet still give poor quality results. Also, a major point in the whole test is missing; an appropriate criteria. Criteria have been specified for harmonics, pulse characteristics, and relative priority and these are fairly easy to measure with relatively at low cost (noting the technical points above). But the main criteria in the standard for the ISO 3744 sound level measurement is simply to match what is disclosed in the instructions for use. This does not address the original point of the risk control: getting the operator's attention. According to the fundamentals of risk management, we need to judge if a risk control is effective, which in turn will require a judgment on whether the sound level is sufficient. This is certainly difficult and complex subject, but nevertheless, unless a criteria is developed, there seems little point in making expensive and complicated sound level measurements.

So what is the solution? 

One possibility is to formally change the criteria to sound power, rather than sound pressure, with a simple reference to measurements under ISO 3744. There is no need to specify the position or radius of measurement and so, use of Annex F etc, these are all handled in ISO 3744 and are judgments made by test lab.

Experiments could then be performed to develop appropriate ranges for sound power for typical situations, e.g. operator always near the equipment (within 1m); an operator in a quiet medium sized room (e.g. operating theater); noisy medium size room (e.g. general ward) and so on.

Finally, a manufacturer could choose to bypass the ISO 3744 test by estimating sound power from the speaker specifications. Although there is greater uncertainty with this method, it might be more reasonable if the sound level is adjustable. And given that sound power measurements are anyhow fairly rough, this method may yield similar uncertainty to actual measurements. This method also frees the manufacturer to make design or option changes without having to retest the equipment.

For example, a patient monitor may come with various optional modules, and the attachment of the modules will affect the acoustic properties. Under the current standard, a strict interpretation would require all versions to be tested at great expense, and as well any design changes that could affect the acoustic pressure. But if the limits are changed to power, as long as the speaker is not changed we can expect the acoustic power to be similar irrespective of the model or design variations.   

All of this needs some research and experiments before being put into a standard; but one thing is clear, something has to be done to improve the current situation and avoid unreasonable use of limited resources. 

IEC 60601-1-6 Usability Engineering - General comments

No fishing allowed

The most common mistake in applying usability standards is to assume that they are a kind of fishing expedition to find the weak points with the device, through extensive trialling with real users. For example, a touch screen on a medical device with a slow update to a new screen or confusing layout might cause the user to frequently press the wrong keys.   

This fishing expedition view is generally what you will find if you search for information or books on usability engineering or usability testing. It is an extremely good idea, with valuable experienced gained getting out of the design walls and seeing how the device works in the real world.

At the same time it sends a shiver down the spine of many small to medium manufacturers, thinking about the cost and also how to deal with the usability feedback, especially late in the design, or worse for existing products already on the market.  

Fortunately, the fishing expedition view is wrong.

The regulatory view

While valuable, field trials are often vaguely formatted, lack formal specifications, and allow users to provide feedback on any aspect they feel is important. In many cases, the designers are not even sure what issues they are looking for, they are just seeking broad based feedback. This feedback will be analysed, filtered and decisions made as to whether to change the design and specifications. Field trial feedback can be extensive and lead to significant upheaval in the design; meaning the device is far from stable. This lack of definition (specification, pass/fail result), weak records (device configuration, environment) and early stage in the design (relevance to the final marketed product) can make field trials largely irrelevant in the regulatory context. Although manufacturers may feel they must keep the records of field trials, the design changes that occur after the trial often make the results unrepresentative of the final marketed product - in other words, not usable as a regulatory record. The impulse to treat the records as formal comes from the false assumption that prototypes are "medical devices" and hence all design records must be kept.   

In a regulatory context, all standards should be viewed with respect to the medical device - the final product that is actually placed on the market. One of the frequent mistakes in regulation is to view prototypes as medical devices, making them the under scope of regulations. This is not correct - all results in the regulatory file must be representative of the final released medical device, and should be in the form of objective evidence against an approved specification, with a pass result. Under this rule, much of the data taken in the early stages of R&D are simply historic records with no regulatory significance. 

Consider for example the way that an electronics engineer handles the performance test which is intended to go into the regulatory file: the engineer would have gone through all the R&D work, including fishing expeditions, trial and error, debugging and refinement, finally arriving at a point of stability in which they are confident of meeting the performance specifications and the planned tests. In the early stages of development, while many records are created, few of these would meet the quality required for formal regulatory records, and most are irrelevant with respect to the final product due to the cumulative effect of design changes. In contrast, the final tests are performed in a controlled environment, with well defined specifications and test methods, and test records detailing the environment, test sample serial numbers, software revisions, hardware configurations, traceable test equipment, who did the tests and when, as well as the actual test result, and an awareness the result must represent the marketed device. This formal (and rather heavy) test environment is certainly not intended to go fishing to find bugs in the design.

The same concept should be applied to usability engineering - all of the fishing expeditions, field trials and the like should have been done well before embarking on the formal IEC 62366 test. The formal test should only be performed when the design is sufficiently stable and the designers are confident of the result. The specifications for usability should be written in a way that provides a clear pass/fail result, and most importantly the specifications should tie into the risk management file - wherever the file indicates the user is involved in managing the risk, the usability assessment forms the evidence that the risk control is effective. For example, if the risk management file says that a certain type of incorrect set up can lead to serious harm, and the risk control refers to the instructions for use (or easy assembly or labelling), these need to be validated through usability testing.

Time to relax

With this formal view of IEC 62366 in mind, designers of stable products can relax somewhat and set up usability targets focusing on risk controls with specifications that are reasonably expected to be met. If that still feels scary, chances are that the content of the risk management file is illogical - unfortunately another frequent problem is the use of instructions, warnings and cautions in the risk management file to sweep away issues that are actually dealt with in other ways in the real world. A careful review often finds that the user was never really relied on in the first place, and hence a usability assessment would be meaningless. In particular, for medium to high risk situations, there is almost always other means of controlling the risk since it is unrealistic to expect ≥99%  of users will follow instructions, and would require huge numbers of real users to validate the effectiveness. 

If a careful review of the risk management file still finds the user is genuinely the risk control, and the severity of harm is significant, the usability assessment needs to be done carefully with specifications and results that demonstrate the risk is acceptable. But this is expected to be the rare case. 

Legacy products

For products on the market, the approach is very similar, except in this case the objective evidence can be derived from market experience, as opposed to pre-market testing. Specifications are still necessary, reporting and the strong link to risk management remains. But the key point is that it is in principle OK to point to market experience as evidence that a user based risk control is effective.  

The final word

If any management system standard seems scary it is usually due to false assumptions about what the standard actually requires. In general, these standards are specification based (not fishing), flexible and resources can be adjusted to suit the situation. Responsible manufacturers that know their product well and are confident it is safe and effective should never fear these standards. Simply use the requirements in the standard as a checklist, and tick off each item one by one until the file is prepared. If the standard requires a record, make sure the record exists. If a requirement is subjective, document whatever the particular qualified responsible individual feels is appropriate for the situation. Avoid asking third parties for opinions, as they rarely know the product as well as you do.