Classification of Applied Parts - Blood Measurement Sensors

R

RednBlack

#1
Hello,

I am looking for some guidance on the Applied Part classification of the measurement sensors used with our system. The application area is cardiac bypass and ECMO; our system consists of a monitor and measurement sensors which are clipped onto the PVC tubes carrying the patient's blood. There is also a temperature sensor which is inserted into a dedicated port in the bypass equipment.

The monitor and sensors are supplied by us, all other parts of the system are supplied by third parties and are outside our control.

The system is classed as a multifunction patient monitor so the particular standard 60601-2-49 applies.

The approach I have used so far is that none of the measurement sensors directly contact the patient's skin, so the decision to classify any of the sensors as Applied Parts is driven by the risk management process clause 4.6.

The temperature sensor is a thermistor enclosed in a stainless steel jacket. The sensor is supplied by a third party, so the class of insulation between the sensing element and the jacket (which contacts the blood) is unknown and must be assumed to be Basic only. Therefore this should be classified as Applied Part type BF and provided with isolation from the rest of the system.

All of the other sensors are plastic with no accessible conducting parts, and no path for fluid ingress (the measurements are performed optically or ultrasonically). I would argue that these should not be classified as Applied Parts at all. However I have seen at least one very similar product from another manufacturer where the ultrasonic sensor head is classed as type BF.

Another complication is that 60601-2-49 requires all type BF applied parts to be defibrillation proof. I can see why this is needed for ECG electrodes etc, but does this requirement extend to applied parts that have been classified as such through the clause 4.6 route?

Thanks in advance for any clarification on these issues.

R.
 
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#3
It is, perhaps, worthwhile to remember that applied parts can be plastic or otherwise non-conductive. If I remember correctly, where this is the case the standard calls for patient leakage current tests to use foil over the applied part as part of the measuring circuit.

I am not familiar with -2-49, but there is good guidance to the classification of applied parts in the informative annex to 60601-1 at sub-clause 3.8. It gives examples that include such things as contact with electrically conductive body fluid lines.
 

Peter Selvey

Staff member
Super Moderator
#4
The applied part itself is the part which touches the body. For patient monitors typicallly, only the ECG circuit is electrical, and the rest have some insulation barrier to the actual circuits, so the applied part is, for example, the plastic surface of the sensor which touches the body or the blood circuit.

However, the applied part is not really the issue here. In all cases involving safety insulation, you need to be aware of the physical location where the insulation barrier is, test that barrier and have reasonable controls in regular production. There are many different solutions, so it is really up to the designer to make a plan (isolation plan) so the parts which are designated as safety insulation can be identified and tested.

Option 1 in a patient monitor is to ignore the insulation in the sensor, and build isolation barriers inside monitor from the sensors to the other circuits (mains, secondary, earth, for Type BF/CF isolation), and also between the sensors for differential defibrillator proof protection. The benefit of this approach is that you don't need to control the sensors, you can ignore them. Testing is done directly to the circuits, the sensor insulation is bypassed. The disadvantage to this approach is increased cost, space and complexity (e.g. communication across isolation barriers).

Option 2 is to isolate the ECG circuit from everything else, and then have all the other circuits (Temp, Spo2, IBP etc) on a single circuit isolated from everything else (mains, secondary, earth). In this case, compliance with Type CF/BF requirement and common mode defibrillator protection is met independently of the sensors, but for differential mode defibrillator protection, the sensor insulation is relied on.

That is where things gets a bit murky. Strictly, you should concretely list every sensor which can be used and then test every combination. Plus you need some production controls to confirm regular production and trigger re-tests in case of design changes.

In practice, this rarely occurs. Patient monitor manufacturers generally buy off the shelf sensors, and have no contractual relationship with the sensor manufacturer.

But, it is typically the case that there is more than enough insulation for the differential mode test, which is not actually very tough (it is high voltage, but only for a few ms). Plus, in the real world, the sensors will never be stressed by 5000Vp; that's a historical value with a number of worst case assumptions. So, although it is likely that controls don't exist, it is not a major safety issue.

What should happen is that the IEC prepares standards for the sensors. That already happens for IBP sensors in the US (BP22) and Japan (JIS T 3323). The sensor standards should then have a test to confirm the insulation is OK for the defib test (5kVp) and/or F-type insulation (1.5kV), or if not declare on the labeling. That would fit the practical situation well.
 
S

sankui

#5
Option 2......, but for differential mode defibrillator protection, the sensor insulation is relied on.
I cannot understand very well why sensor insulation is relied on when the test comes to the differential-mode.
 

Peter Selvey

Staff member
Super Moderator
#6
The differential mode test stresses insulation between applied parts. For example, it is necessary to apply 5kV pulse to the saline solution running through the IBP sensor, while at the same time the Temp, SpO2 and other sensors are grounded.

The pulse should not damage the equipment, and there should be no significant loss of energy into the patient monitor. To meet this requirement, there needs to be either insulation or at least some method to limit current and clamp-down any overload which could damage circuits.

I forgot to mention another reason why in practice it is not problem: there will always be two layers e.g. between IBP and Temp there will be one layer in the IBP sensor and another layer in the Temp probe. Most IBP manufacturers know to design with 5kV pulse isolation; Temp probe manufacturers know to use 1.5kVrms isolation. So, practically, there is little chance of breakdown.

The weakest point is usually the disposable SpO2 sensors. Some of those have weak insulation between the applied part and the internal wiring. But still the other applied part will usually have some good insulation.
 
R

RednBlack

#7
OK, the issue I am having is that the insulation built into the sensor head is adequate for the dielectric strength test, but in the defibrillator test the transient couples into the measurement circuit capacitively and crashes the microcontroller (I'm speculating here, I haven't reproduced this yet. 'Measurement stops responding' was the report I had from the lab). As you say, the 5kV spec is unnecessarily harsh and the equipment will never see anything like this in real life. Do we have any scope for arguing that the defibrillation test should not be applicable, or should be applied at a lower voltage for this part?

The dielectric strength tests are applied using metal foil wrapped around the outside of the tubing fitted to the sensor head - the tubing is outside our control so is 'short-circuited' for the purposes of the test. Does the same requirement apply to the defibrillation test, or could we (eg) specify that it is tested using a tube filled with saline, and the test voltage is applied to a conductive stopper at the end of the tube? This will be much more representative of real use, and obviously the capacitive coupling will be much lower due to the 3mm or so of tubing wall thickness.

These tests have all been done by our NB at their lab. It looks as if we should also be doing our own pre-compliance tests (similar to EMC). How do most companies do this - buy an off the shelf defibrillator, or build the test circuit from sec 8.5.5 (bearing in mind the safety issues obviously!)
 
R

RednBlack

#8
As an aside, one thing I find confusing is that there are a number of other components of bypass and ECMO systems in common use which seem to me to be in conflict with 60601. An example is the oxygenator, this is the part which substitutes for the lung and diffuses O2 in and CO2 out of the blood. This part also typically has a heat exchanger to maintain the blood circuit at a set temperature by means of a secondary loop containing plain water. So the problem I see is:
1. The water heater is mains powered and has a connection to PE
2. There is a conductive connection through the water to the heat exchanger
3. The oxygenator material (usually polycarbonate or similar) which forms the two chambers of the heat exchanger is Basic insulation only. This part has to be as thin as possible for good heat transfer.
4. Therefore, there is a potential leakage current path from patient to earth in fault condition.
Even if the heat exchanger material does form adequate insulation (presumably this is covered in the manufacturer's RMF), I have seen at least one type where there is a deliberate electrical connection made between the two chambers using stainless steel electrodes. How is this compliant? Bearing in mind that the water heater is usually supplied by a different vendor so is outside the control of the oxygenator manufacturer.

Are there any product standards for bypass and ECMO oxygenators which address these issues?
 

Peter Selvey

Staff member
Super Moderator
#9
For the defib tester, there are two manufacturers I know, Compliance West in the USA, and also M&S in Germany. They are not cheap, but I guess to speed up design and figure out how to get around this problem it might be the best option.

Keep in mind the standard allows a few seconds to recover from the defib pulse. It's not the best option but if you have for example a software WDT that quickly gets the isolated module up and running again and sending data to the display it might be an acceptable solution.

For the bypass system ... I'm not familiar with this directly, but for example dialysis has the same problem, it is unavoidable that the patient ends up getting grounded in the equipment, due to heat exchanges, metal valves and sensors. For dialysis a Type B (non-isolated) applied part is acceptable. For other equipment like X-ray again Type B is acceptable.

Usually there is special attention to keeping the grounding reliable; also note that a earth leakage of 0.5mA is not lethal (far from it), and the requirement for floating applied parts (BF / CF) are based on hypothetical situation (shock from other equipment) which there was never any concrete evidence of being needed.

The floating requirement in patient monitors is good because it reduces interference between sensors (especially SpO2, ECG respiration, leads off detection), helps with defib protection and also ESU rejection. So while it may not be really important for mains isolation, it's still a good idea.
 
S

sankui

#10
The pulse should not damage the equipment, and there should be no significant loss of energy into the patient monitor. To meet this requirement, there needs to be either insulation or at least some method to limit current and clamp-down any overload which could damage circuits.
For differential-mode test, the hazardous electrical energies (exceeding 1 V) appears on, such as SIP/SOP, any other applied part or any other function of same applied part is unlikely. Right?
 
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