Understanding the Test Method of Defibrillation

S

sankui

#1
Hi all,

In 3rd edition, the standard requires that except for PERMANENTLY INSTALLED ME EQUIPMENT, the ME EQUIPMENT is to be tested with and without the PROTECTIVE EARTH CONDUCTOR connected.

However, the rationale of 8.5.5.1 states that NORMAL USE includes the situation that a PATIENT is defibrillated while connected to the EQUIPMENT and, at the same time, the OPERATOR or another person is in contact with the ENCLOSURE. The possibility of this occurring at the same time as the SINGLE FAULT CONDITION of a defective PROTECTIVE EARTH CONNECTION is very unlikely and is therefore disregarded.

From mentioned above (especially the phrase underlined), it seems that the rationale conflict with the requirement. I am confused that whether the PROTECTIVE EARTH CONDUCTOR interrupted when conducting the defibrillation test for class I equipment (NON-PERMANENTLY INSTALLED).

Please kindly advise it. Thanks in advance.
 
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Peter Selvey

Staff member
Super Moderator
#2
Hi Sankui,

Looks like you found a technical mistake in the standard!

But, in practice it is not so much of an issue.

The most critical equipment for this test are patient monitors, which are normally battery operated and hence need to be tested without an earth anyway.

Secondly, most equipment has a Type BF/CF isolation barrier which can withstand a 5kV defib pulse without breakdown nor letting any significant energy across.

Some manufacturers bridge this isolation barrier with a spark gap, which can breakdown from a 5kV pulse. However, the spark gap is normally rated for at least 3kV to avoid breakdown in the 1.5kVrms test (technically, a spark gap should be removed, but for convenience e.g. in production, it normally remains in circuit).

Modern biphasic defibrillators typically use much lower voltages less than 3kV, which means breakdown is unlikely in practice.

So, for a potential hazardous situation to occur we would need (a) earth open, (b) a spark gap to be fitted, and (c) the spark gap is rated less than the defib pulse.

Basically, it won't happen.

So, it's safe, but not because of the reason in the standard. It could be be dangerous situation for a Type B defib proof applied part, but I've never seen a such a rating in practice.
 
S

sankui

#3
Hello peter,

Thanks for your technical explanation. I totally agree with your point. However, the standard allows DEFIBRILLATION-PROOF TYPE B APPLIED PART, though we have not seen such device so far.

Therefore, I believe that a mains operated device without internal power source (e.g. battery) shall be tested with earth (the rationale clearly shows that interruption of earth is unlikely while conducting the defib test).
 

Peter Selvey

Staff member
Super Moderator
#4
Yes, but the standard rationale does not fit with basic risk management.

Statistics taken in Australian hospitals show that around 1-2% of earths are open at any time, and that is in a fairly controlled environment with regular service tests. The moderately high result is due to the heavy mechanical abuse cords and connectors get in hospitals from equipment being moved around all the time.

Defibrillation is considered normal condition. We can expect equipment in critical care environment to be exposed to several defib events / year.

So, we have an overall probability (frequency) of ~0.1 events/year/device that a defib pulse will occur when the earth is open.

In the world of safety, a probability of 0.1 is quite high, even for low to moderate severity of harm.

I would suggest that it is worth testing, but we can predict most equipment will pass due to BF/CF isolation barrier.
 
S

sankui

#5
Hi Peter,

How do you calculate the overall probability? The value of 0.1 is really high.
 

Peter Selvey

Staff member
Super Moderator
#6
Actually there is a bit of a mistake in ISO 14971 to use probability as the basis of risk.

The correct parameter is frequency of (adverse) events.

When the frequency is << 1 (e.g. 0.1 or less), probability and frequency are usually the same. But, when frequency is high relative to the period of analysis (e.g. several times a year), it is no longer appropriate to use probability for estimates of risk.

Compare the following calculations in probability (P) and frequency (F):

Defib events, per year: Pd = 1 (100%); Fd = 5
Broken earth, per device: Pe = 0.02; Fe = 0.02

Combined event (defib + open earth):
Pc = Pd x Pe = 0.02 events / year
Fc = Fd x Fe = 0.1 events / year

These are quite different results. But I can assure you the frequency based result is the correct one.

The alternate would be to calculate on a shorter time span (e.g. per day or per use), in which case both probabilty and frequency would yeild the same results.

But from a manufacturer's and the operator's point of view, number of events per year is resonable basis on which to make decisions.
 

Roland chung

Trusted Information Resource
#7
I would think it is not suitable to regard the frequency as the basis of risk.

The frequency is a random variable. Unless the frequency is small enough, the results shall be very different if one uses frequency to calculate the occurrence of events. Based on your example, I would assume that:

1) Defib events/ year: Fd = 5; Broken earth/ device: Fe = 0.02 --> Fc = Fd x Fe = 0.1 events/ year

2) Defib events/ year: Fd = 50; Broken earth/ device: Fe = 0.02 --> Fc = Fd x Fe = 1 events/ year

For case 2), defib event assumed to be 50 times per year may be reasonable in large hospital (large hospitals have more patients). The result (1 events/ year) means that the defib test with defective earth is an inevitable event.

This is clearly not correct. It shall be overestimated. The probability of the real world is 0.02 events / year.
 

Peter Selvey

Staff member
Super Moderator
#8
By frequency I mean the average rate of occurrence, which is not random.

Probabilities are also based on the average rate of occurrence, but by definition, a probability can never exceed 1, whereas frequency can and often does exceed 1, especially over longer periods like 1 year.

Of course, it is not expected that the final probability (or frequency) of harm will ever be close to 1 in risk management.

However, harm is often the result of a sequence of events, and individual events in that sequence may have a frequency exceeding 1. In that case, calculations should be based on frequency not probability.

Calculations are normally on a per device basis which will be the same regardless of the size of the hospital. And in a large hospital we can expect these events to be inevitable regardless the method of calculation.

If we have a hospital with 100 devices, each device gets a defib pulse 5 times a year on average, and on average 2% of devices have open earths, using probability you would get 2 events per year (defib + open earth), using frequency would be 10 events per year. Either way, it's high and testing (defib + open earth) is warranted. But it should be clear the value of 10 events / year is the correct one on which risk decisions should be based.

In this particular case, even if we have a defib pulse + broken earth, harm still requires a number of other unlikely events, including most significantly, breakdown of the patient isolation barrier.

If the equipment is properly designed for Type BF/CF (4mm / 1.5kVrms), the probability of breakdown is extremely low. So although defib + broken earth is an inevitable event, it's not something that is expected to cause harm.
 
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