I'd be greatful if you fellow forumers would answer the above question on what you use to verify whether you infusion pumps, syringe drivers, etc are accurate enough. Do you use devices like the IDA2/4 , a burette, weighing the solution or something different? And why did you choose that method?

The reason I ask you is as follows:

I'm an upper-sixth form student and am currently doing a project, being given a bursary by the Nuffield Foundation. I've been having quite a few us and downs in deciding which piece of equipment that i could do a project upon and end up with an outcome and an accomplished objective.

I started with pulse oximeters (HERE - click me) , researched on angiography and been behind the scenes in MRI and CT scans.

I've now ended up on infusion device analysis as I feel this is one field in which it would be possible to carry out reasearch into and be able to come out with an outcome. My main objective is to find ou which is the most accurate by performing various analysis using the different techniqes. It would also be quite easy to build a new device for this task in the EBME dept. rather than messing with X-Rays and magnetic fields all day .

Your cooperation is greatly appreciated

Thanks,
Ash

HERE (clicky) is where it all started.....

Ash
Medication is a very impreceise art. (Not a science at all in most cases) A dose of paracetamol for instance is 1gram (2 tablets) for an adult, which could be a 10stone or a 20stone person, so the dose per unit body weight is double for the lighter person.
Drugs given by pump tend to be more critical but the tollerences of a pump these days is a few per cent, add on to this inaccuracies in syringes and other external things and you are unlikely to be over 10% inaccurate, this is usually close enough.
If you are looking at accuracies of drug delivery systems look at the whole system not just the electrical pump. IV pump accuracy often depends on the diameter and elesticity of a piece of tube, I would guess this would be more prone to inaccuracy than the pump, especially as time passes and the properties of the tube material changes.
Pumps are so accurate these days that the commercial testing apparatus is sometimes more inaccurate than the devce it is testing. I remember from way back some rules of thumb about test aparatus needs to be orders of magnitude more accurate than the device it is testing.
As I have said the accuracy of testing a pump is not really a priority, the ease of use of the equipment is more of a concern. Can you set it up simply, leave it running and come back and get meaningful results that are easy to interpret?
Have a look in to ergonomics, that is an area where things often fall down.
Robert

KITT,

I think you have a good topic to research. We do use the IDA and the reason being it is a set up and wait test device.It doesn't matter how long you test, it provides a readout of the delivery rate and volume.

If you were to use a burette (we sometimes do) You need to be present to record the time and exact amount.

IDAs are fine for continuous infusion devices but can be innaccurate for peristaltic devices.

An area to develop could be a burette type device that measures the volume(or mass) and gives a readout of the time taken to reach a set level i.e 100ml delivered. The need is to automate the stop watch.

For performance of volumetric pumps I use 2x25mLx0.1mL Class A burettes linked together. For syringe drivers I use a dial guage to check plunger travel. Using a stop watch I am also able to check incremental delivery and the overall time of delivery, although the overall time is not normally a problem. These techniques I feel take me back to basic measurement with a good degree of accuracy without having to worry about the design or performance accuracy of another piece of equipment.

Correct me if I'm wrong but surely that's exactly how the IDA works. It times how long it takes for the fluid to rise in the fixed volume tube within the device. :p

Ash (KITT)
Just a bit of information, the IDA will give you lots of spot reading which when graphed with the appropriate software (IDA4plus plus hydrograph software) gives a quick to read result you can eyeball for problems, but as has been said pulsatile flow causes it problems.
A burette and stop watch will give longterm overall results even with pulsatile flow but will not pick up and changes over a medium time scale. What is needed is a combination of the two, something that can cope with pulsatile flow but can resolve short term flow changes.
I have seen nice waves on a hydrograph plot caused by a pump being dropped on to its top and bending the lead screw but the overall accuracy was correct.
Robert

Papa Doc,

With out giving too much away, I was hoping to give KITT (Ash) a pointer to look at a device to time how long it takes to fill a receptacle with fluid.

The IDA is not best for peristaltic devices so a device that will signal ‘how long it takes to fill with water’ taking over from a person using a burette and a stopwatch might be a way for him to go.

The IDA operates slightly differently. I digress: This site once again changes slant from offering advice to criticising posts.

We have a student here who wants to use this forum to evaluate why we test infusion devices. I do not know the level of his expertise, but I am all for helping him.

I wonder if fibre-optics could be used to measure depth of fluid, hence the rate of change in volume over time (or delivery rate), of fluid in a container of fixed dimensions? Techniques using pulses of different wavelengths can be used to detect where the optical characteristics of a fibre change along its length can't they? Characteristics of fibres change when the ratio of the index of refraction of fibre core or sheath changes. What is the ratio for glass/air and glass/water? How are breaks, losses in transmission or step-changes in fibre characteristics along a fibre measured in fibre-optic sytems? I wonder if fast processing methods could resolve small, rapid, fluctuation in rate deivery over all ranges of measurement to improve on current systems. Or whether the method could minimise errors by adapting the measurement technique for different ranges of measurement?

Kitt
One of the most accurate and simplest methods is to weigh the infused liquid. If you used pure water as the infusion liquid then for every 1ml you have 1g. This could prove to be the basis of a very accurate system. Simple. I leave the rest for you to ponder!

Ash (KITT)
At what temperature is Paul's definition correct? And be careful with this definition it is used to calculate the cc which by modern definition of SI units is slightly different to a ml. Are these differences going to be of significant to the close accuracies of your research? An avenue to look down
As I emphasised, to accurately measure something your measuring apparatus needs to be very accurate. Can you find a measuring device Paul is alluding to accurate enough? Another line of investigation.
Have a look around previous threads on this site to see other discussions on measuing pumps.
Robert

PS How is the talking car? Rusted away yet?

It is possible with a stopwatch and burettes/dial guage to determine short and medium term variations in flow rate (and I have detected bent lead screws using this technique) but it requires monitoring the delivery test. The advantages of using an IDA are, I think, that it will give you a printed record of the result, important in some situations, and you do not have to monitor the test, you can get on with something else.

Not exactly the basis for a new thread of research though - and not particularly interesting if its been done before a hundred times.

Pump Testing

Gentlemen, Are we getting a bit too pedantic here?
Surely with any infusion device, in particular syringe drivers, all we need to measure is the accuracy of delivery (volume) over a range of therapeutic time periods (1 to 24 hours) taking into account different sizes and types of syringes.
You could also add into the equation different viscosities of fluid, again representing the different drugs that are used, as well as time to reach specified delivery rate (trumpet curve).
The IDA is probably the most widely used device for measuring most of the above, but for more info on testing equipment talk to Teresa Dunn at the Bath Institute.

Unfortunately research is a bit pedantic I think that's the objective.

Personally, for bench work, I'm happy with a burette that measures volume very accurately over a period I can measure pretty accurately. E.g. uncertainty in measurement on a 50ml f.s. burette of 0.1ml resolution is 0.lml hence +/-0.2% accuracy (it all depends on the raio of total volume measured to resolution of course but at least this is within our control). Add this to the system accuracy of a syringe driver and the is +/-2.2% the acceptable tolerances for volumetric delivery for a syringe driver with a 2% volumetric accuracy specified.

Errors in volumetric measurement using a burette can be made negligible for benchwork really. A range of burettes of different volumes allows pedants to maintain volumetric accuracy for delivery of a range of volumes without thinking about accuracy when doing the work at the bench.

Do the volumetric measurement with an IDA with uncertainty in measurements of +/-2% accuracy add this to the +/-2% specified system volumetric accuracy of a syringe driver and the potential error in delivery that is indicated can be +/-4%.

For a random pump and your IDA you tell me where the biggest error lies if you get a reading of -4%. Is it in the IDA or syringe driver? Where the IDA wins is in providing an indication of rate-delivery dynamically.

Gravimetric is the best of both worlds in my opinion: probably the best absolute volumetric accuracy and the capability to provide dynamic measurement of rate-delivery is very useful to indicate the delivery profile of pumps i.e. delivery rates over specified observation windows.

Actually the system accuracy should take all this into account since the fluid, set, pump, etc should be specified with this for repeatability of testing. Pure water at a specified temperature with calibrated scales interfaced to a PC is what's used for type-testing I think - that says it all.

But do you want to know how accurate the pump is under controlled laboratory conditions or the accuracy when delivering different drugs with different types of syringes over different periods of time in the clinical situation?

I want to have enough knowledge to indentify the pros and cons of each measurement method and choose the appropriate one - whatever that is. Working on the bench I would prefer a zero-error measuring instrument, whatever the device, anyday, then I wouldn't have to consider uncertainties in the measurement or the methods used to make the measurements repeatable and meaningful for the next pedant that comes along.

If the error in the IDA were 20% and the system error of a pump were 2% would the measurement be of any value? What if it were 10%, 5%, 2% where's your subjective threshold for the accuracy of an instrument used to give you confidence that a syringe driver is delivering within specification?

My view is that,for volumetric measurements, 2% uncertainty is not good enough when I've a burette that makes measurement uncertainties negligible. However if I needed an indication of the consistency of delivery then the IDA would win hands-down. If I was type-testing then I'd probably be inclined to use published methods in standards i.e. gravimetric checks.

Is there any specific reason that the IDA is inaccurate for measurng the volume of liquid infused by perilstaltic devices?

I think devices like the IDA work by timing how long a small internal volume takes to fill i.e. the IDA integrates lots of small volumes to measure volumetric accuracy. During the period this small volume is filling I guess if the rate fluctuates on the pump then the time taken to fill the volume varies i.e. ml/s changes but the IDA can't detect it. If the IDA can't resolve this fluctuation then this is an error I think -it will probably be cumulative so may become significant over time and at different rates so I guess the error will fluctuate depending upon rate, the total volume delivered and how pulsatile the pump delivery is.

Anyone know how an IDA really works?

What do you think on this prototype of a new kind of IDA?


CLICK HERE TO VIEW
Admin edit- Image removed as it was too big


I think the pic is self explanatory, although im not sure if it would be financialy practical to produce.

Im not sure how accurate this would be either, but producing a simple model (not as accurate as the finished product and on a smaller scale) to use in my presentation would be practical enough.

I just want some comments on my idea. Is this idea feasable, or a load of rubbish?

I have posted a private message to you.

We use an IDA4, also a biurette (?? can't spell) and high accuracy weighing scales. DEspite certain infusion pump manufactureres saying the IDA4 isn't any good, we can test their pumps on all three of the above methods and get almost identical results.

Which pumps do you use? We use(d) infutest on Graseby 9000 series and Baxter Colleague - I wasn't convinced with the results.

KITT
My understanding is that the IDA from Ultramedics works via looking into a fine burette with a series of leds at fixed intervals up the burette.

As the water creeps up the burette the beam is refracted and picked up by a sensor opposite and the device increments its volume count. I don't know how it purges the burette between measurement cycles.

The main difference between the old IDA 2 Plus and the newer IDA 4 is that there are more leds closer spaced along the burette giving a higher resolution and more accurate performance.

The system is prone to slight optical deterioration from contaminants in the water.
My understanding is that the annual service of these devices includes the fitting of a new burette to keep the system clean.

Your design look superficially a bit like the Ultramedic system. Instead of using optical interference from refraction though you are using optical interference from an opaque solution.

The unit from Metron has a pump chamber within it and in response to the fluid input pressure allows a measuring chamber to fill, then purges it and restarts in the manner of a very tiny Imed 900 series piston pump, or perhaps in more contemporary terms like a Graseby 500 boxer mechanism. The units accuracy is principally therefore determined by the measuring chamber accuracy and its resolution is determined by the measuring chamber volume. The unit makes a rather alarming racket from its pump mechanism but is really quite excellent in use.

Your ideas are excellent but you need really to know how pump tester manufacturers test their pump testers. After all if you're going to develop a test method you will need to be able to test the methods you develop and compare their performance.

My guess is that they use a burette! If the volume measurement is accurate and the system clock is accurate then they can infer that the incremental volume measurements will in practice be accurate.

As regards currently used test methods no one has yet mentioned potentiometric measurement of syringe pump travel. Ultramedic market that as well. A large potentiometric device is fitted into the syringe drive and converts linear travel to resistance and this can then be converted and interfaced to a PC

Our trust has 3 service bays. One uses burette, one uses an IDA 2 Plus and one uses a digital scale. Each naturally prefers their own technique (and for very good reasons).

For something a little bit different why not think about using turbine rotation as a sensing method. I don't think anyones used that yet (probably for good reason too).

Marc

I dont think a turbine system would work as you have problems like dynamic friction, etc and maintainance would be much more than in a system without moving parts. Maybe a dynamic weiging system would work?

Hi Colleagues,

Infusion pump measurement is a very interesting discussion.

At North Middlesex Hospital we've used the two manual methods to test our Arcomedical Volumed uVP5005 pumps i.e the measuring cylinder (burrett? - I can't spell either!) and also very accurate weighing scales (e-Accura SC131), always using distilled water as the pumping medium to measure the total volume delivered. In addition, the total time of delivery is measured using an electronic bench stopwatch so that the rate of delivery can be checked. The method is to connect the stopwatch to the "nurse call" socket on the pump. Start the pump and the stopwatch together and when the pump finishes it's delivery it automatically stops the stopwatch. Hence, the total time that it takes to deliver the set volume can be read (without sitting watching the pump with a manual stopwatch!) Therefore, the average rate can be checked as total volume/time (ml/h etc)
Just reading the volume in the measuring cylinder isn't very accurate at all. Our 100 ml measuring cylinders have only 1 ml graduations and are only accurate to +/- 0.5 ml @ 20 deg C. Moreover, it is very difficult to read the level accurately from the cylinder by eye. Using the cylinder with the accuate scales is much better. Moreover you can "zero out" (tare) the weight of the cylinder when it's empty and hence get a direct reading from the scales of the weight of the volume of dist. H2O delivered.(This method is clearly a cheaper option than purchase of an IDA device.)
What I'm not clear about is, do we need to measure the instaneous rate as opposed to the average rate delivered as you would with an IDA-2 or IDA-4 device? The use of of this type of testing device is obviously much better, especially if your testing lots of pumps.
However, it seems to me that with all testing/checking we should only carry tests that the manufacture recommends to ensure that the equipment is working within its specification. Volumed do not specify that to test their pumps you must use an IDA device. In fact their engineer has only one rate check on his test sheet i.e. 100ml/h with VTBI = 50 ml. We check this with +/- 1% accuracy (49.50 - 50.50 ml with an expected period of 29.40 - 30.20 min). Possibly, too accurate a measurement?
Finally, it's obviously important to always use a new or hardly used giving set when testing pumps.
Having said all of the above I'm sure that our unit would benefit from having a new IDA device given the number of infusion pumps that we test. (We've got a very old IDA-2 sitting around gathering dust. I think I've seen it used more than a couple of times about 10 years ago as it always had problems!)
Ian Rez

I think for practical purposes, i.e. outside the physics lab, it's generally accepted in engineering circles that the uncertainty in a measurement should, ideally, be about 1/10th of the stated accuracy in whatever parameter you're measuring. Then you can avoid, i.e. not have to get involved in, discussions about whether accuracy is an issue

Bravo Ian,

Do we need another thread to discuss occlusion pressure testing – or should it come under this one also?

The “North Mid model” is a sound one, based as it is on physical principles. However, as described it does not “test across the whole range”, as it were.

But what I like about the IDA’s, Infutest’s or what-have-you is that you can obtain a quick check as to whether a pump is generally working OK (or not). You can quickly test it over a range of flow-rates, and also check the occlusion pressure. And you get the all-important printout for your equipment records files!

Incidentally, in my experience, most IDA problems have been due to impurities in the fluid (water) being used (ie, not being flushed out from the sensing mechanism). Follow the manual and all is usually well!

I thought the context of the thread was aimed at a "novice" who wanted some background advice. All this is old hat and a bit tedius for me as well but if people are not prepared to occasionally state the obvious to "newbies" then there's no point in having the forum.

Personally speaking my trivial contributions are just to highlight what should be well-known issues RE: uncertainties in measurements and the problems that have been experienced with accuracy, in the past, with IDA-like analysers - particularly when used to measure pulsatile-delivery. Things that are not necessarily obvious to "schoolies" looking for a project perhaps.

In general, syringe pumps are used to deliver drugs and IV pumps are used to deliver fluid volume.
I think you do need to have a reading of short term accuracy for syringe pumps. not just whether it can deliver the syringe full in a given time. If you are giving a drug with a short half life such as inotropes, fluctuations in the flow rate will cause physiological changes. These drugs have half lives measured in a few minutes so you need to be able to measure for consistancy of flow rate over similar timescales.
I have seen a patients BP fluctuating because their inotrope was given with a pump with a bent lead screw. But on average the pump was accurate for the whole syringe.

If you are giving plain fluid through and IV pump it does not really matter if a patient gets 90mls one hour and 110mls the next if it is set to 100mls/hr.

Should we be adopting two flow measuing systems depending on the pump and its clinical use?

For those with the luxury of an IDA4 with hydrograph or a similar setup I would suggest you have that facility.

As to the initial question as to what is used, here are the many answers. As to what is required here are many suggestions.

What is needed is an easy to use system that gives the information required in an easy to analyse format with little operator input. You need to be able to set the system up to run for several hours then return and read the results quickly and easily. We do not have time to baby sit it, timimg and counting.
If you can fulfil our needs with a system that does it all well done and thank you.
Robert

We test all infusion device on an IDA-4 Plus, with the hydrograph software. The two reason for this.

1: It gives you nice written report of how the infusion device has run, plus a nice software archive of all the test you have done.
2: We also use a mains timer, which cuts the power to the infusion device in the middle of the night for three hours. Good check of the battery life.

If the pump fails any of these tests you can fall back to the manufactory recommended instruction. Be it glass burettes or dial test indicator. The other thing we do is after all the test are complete is to do a data download from the pump, so any one can see what test were run and what alarms have shown.

Good luck with the project.

A.M

K.I.T.T.

Hows about using a capacitive strip sensor as used in fuel tanks. They are seen medically in urodynamics investigations when measuring the total volume and rate of flow of urine. I've no idea what degree of depth acuracy these give.

I know with urodynamic systems the receptacle has to be primed with a small amount of fluid before starting and they must be cleaned and dried after use and not left soaking overnight. I think that is because of the corrosive powers of the urine and the contaminants that will coat the sensor if left soaking rather than any requirement for the sensor to be stored out of fluids.

Accuracy would almost certainly be inadeqaute for pump calibration purposes but would probably be ok for IDA type testing and particularly for checking linearity of delivery. In fact given the continuous nature of the sensor's function it could give a superior graphing function and would eliminate problems with artefacting due to intermittent nature of readings from a conventional IDA.

It seems to me the fundamental problem for you is choice of transducer.

Marc

K.I.T.T.

Hows about using a capacitive strip sensor as used in fuel tanks. They are seen medically in urodynamics investigations when measuring the total volume and rate of flow of urine. I've no idea what degree of depth acuracy these give.

I know with urodynamic systems the receptacle has to be primed with a small amount of fluid before starting and they must be cleaned and dried after use and not left soaking overnight. I think that is because of the corrosive powers of the urine and the contaminants that will coat the sensor if left soaking rather than any requirement for the sensor to be stored out of fluids.

Accuracy would almost certainly be inadeqaute for pump calibration purposes but would probably be ok for IDA type testing and particularly for checking linearity of delivery. In fact given the continuous nature of the sensor's function it could give a superior graphing function and would eliminate problems with artefacting due to intermittent nature of readings from a conventional IDA.

It seems to me the fundamental problem for you is choice of transducer.

Marc

K.I.T.T.

Hows about using a capacitive strip sensor as used in fuel tanks. They are seen medically in urodynamics investigations when measuring the total volume and rate of flow of urine. I've no idea what degree of depth acuracy these give.

I know with urodynamic systems the receptacle has to be primed with a small amount of fluid before starting and they must be cleaned and dried after use and not left soaking overnight. I think that is because of the corrosive powers of the urine and the contaminants that will coat the sensor if left soaking rather than any requirement for the sensor to be stored out of fluids.

Accuracy would almost certainly be inadeqaute for pump calibration purposes but would probably be ok for IDA type testing and particularly for checking linearity of delivery. In fact given the continuous nature of the sensor's function it could give a superior graphing function and would eliminate problems with artefacting due to intermittent nature of readings from a conventional IDA.

It seems to me the fundamental problem for you is choice of transducer.

Marc

I thought we were being asked what methods we use to measure the accuracy of our infusion pumps that's why I gave a faily detailed response! I'm sorry if this has upset anyone.
Clearly, any new method that someone can come up with would be most welcome. In my view, it needs to be simple to operate and maintain and give the required accuracy. Moreover, with pressure on budgets it need to be cheap!
However, let's go back and ask ourselves why we need to measure the accuracy of any item of medical equipment? In the case of an infusion pump we want to be sure that the volume delivered and the rate at which it's delivered is as close as possible to settings carried out by the nursing staff when it was initially set up i.e. "it does what it says on the tin" (I'm sorry if this is stating the bloody obvious!)Again, how accurate does this measurement have to be? "I refer the honorable gentleman to my previous answer" IT NEEDS TO BE WITHIN THE MANUFACTURERS SPECIFICATION. Therefore, we should always use this spec. when we make measurements and/or use the test equipment which the manufacturer specifies. For example, in the case of the Arcomedical Volumed uVP5005 infusion pump (early service manual - I can't find the later one at the moment) the deviation in flow-rate using the correct giving set was specified as +/- 5%. The delivery accuracy in volume was specified using a measuring beaker and measured at 25 ml with a rate of 250 ml/h over a six minute period This should be 25 cm3 or 25 g +/- 5% of water. Hence, if the pump meets this spec. it's OK.
I rest my case.

No definitely not - total worst-case error or uncertainty in the system is the worst-case uncertainty in the measuring device added to the worst-case uncertainties in the device under test (IUT). That's a fundamental concept of measurement and intrumentation theory.

For example if you have a measuring instrument capable of measuring a parameter to an accuracy of +/-20% then the worst-case error will be added to the worst-case accuracy of the IUT, say 2%.

If you have an uncertainty in the measurement instrument much greater than the accuracy of the IUT, say 15% (within its +/- 20% tolerance), how much of this 15% error (from the nominal reading obtained from the IUT) is IUT error? Think about it. The 2% potential error in the IUT is "buried" in the "noise" of the measuring instrument uncertainty.

Limit the measurement instrument error to a fraction of the IUT system error and it becomes negligible - then the majority of error in the reading is IUT error added to a tiny proportion of measurement error. Think why measurement standards always are highly accurate i.e. measure to a high precision and resolution. It's to make the measurement more representative of the actual uncertainty, i.e. error, in the parameter being measured.

In your example worst-case error could possibly +/-10%, overall. If you had a syringe driver with a spec of 2% and got an error of 7% how much of this is actual pump error and how much actual cylinder error?. Both pump error and burette error will vary, statistically speaking, from measurement to measurement (burette more so since error-band is greater relatively speaking) so you can't say the burette error will always be +/-5% or pump error will always be +/-2%. So how do you know the contribution of each to the overall reading?

Take a +/-0.2% accurate measuring device and use it to measure a +/-2% specified accuracy syringe driver and the worst-case error will be +/-2.2% overall error. The actual measured pump accuracy will consistently be within +/-0.2% when measurements are repeated; not potentially anywhere between +/-5% between measurements as in your example.

Overall Err(Ind)/Act Bur Err(5%spec)/Act Pump Err

-3%/-5%/+2%
1%/+2%/-1%
1%/-3%/+2%
1.5%/+0.5%/+0.1%
3%/+5%/-2%


Imagine overall error is what you see and calculate from readings on the burette. This has to be actual burette error plus actual pump error - the errors are additive in the real world.

Now you don't know pump error (that's why you need to measure it) or burette error (it varies in the real world between measurements, of diff volumes, technique, etc) between measurements (you only know worst-case error for sure). Imagine that we do know these unknowns, i.e someone has measured pump and burette measurement accuracy after each measurement (there will be variation).

Cover up the two right-hand columns (the unknowns) and now tell me what information pertaining to the pump accuracy and burette error the overall error gives you. You will probably say that as long as the error is less than the worst-case burette error plus worst-case pump error then everything's ok (+/-7% error acceptable max). This doesn't tell you what the likely pump error is though and the next time the pump is tested the value is likely to be significantly different again since the maginitude of measurement instrument errors are relatively large in comparison to pump error.

Now:


Overall Err/Act Bur err (0.2%spec)/Act Pump err

+2.2%/+0.2%/+2%
1%/-0.2%/1.2%
-1%/-0.05%/-0.95

Cover up the unknowns in the two columns on the right; now using the overall error what can you tell me about the pump accuracy, i.e it's likely value and maximum error? Now, since burette error is never >10% of pump error (at optimum volume delivered of course since accuracy of a burette is down to the ratio of volume delivered to resolution andd ability of operator to read it) the error more closely represents the pump accuracy and will on the next test and the test after that - more than the 5% accuracy instrument will.

We are interested in absolute accuracy, i.e. with specified tolerancesm and repeatability if we are to compare actual performance of a pump to specified performance and get results that are meaningful for the next pedant that comes along. Otherwise it's not worth using test instruments if they produce a significant number of readings that are meaningless.

Oredered 2 burettes yesterday and they arrived this morning (1x100 ml, 1x25 ml = £65 ). Ran a test on them today using a baxter flo guard (cant remember which model.... )

anyway, set it up to infuse at a rate of 150 ml/hr and a total volume of 75 ml. I got 79.00 ml on the burette and 79.80 using the IDA4. Not as i'd had in mind....I thought thered be a larger difference

Anyway, that was just a trial really. Ill start proper measurements on moday using a calibrated tube (on the giving set)...

KITT

I think BIME use a system where they have a laboratory grade balance scale where they measure the amount of water that is infused based on the assumption that 1KG of water is 1000ml at STP. I think they also use deoxygenated water and also have a layer of parafin/oil in the collection beaker to minimise evapouration of water (not sure how quickly water evaporates but might be an issue in your burette set when its hot i.e. summer. Also might be an issue in the winter when the air is dry and the heating is on). Could explain why your burette reading is lower than your IDA one.

Contact Teresa Dunn at BIME where I'm sure she will share with you what happens there.

Presuming the usual tolerance of +/-5% on volumetric pumps (excluding enteral feed types) - the actual pump accuracy tolerance just passes using the burette +5.3% and fails using the IDA4 at +6% then. Is this the case? Probably not since you need to add/subtract the worst-case calculated/estimated errors of the burette (resolution divided by nominal volume delivered) at the total volume delivered and that specified in the IDA4 manual to these values to arrive at a range of the likely combined pump/giving-set volumetric tolerances (system accuracy).

The difference or error between the actual reading (that measured) and the nominal reading (that expected) consists of the error in the pump plus the error in the burette or IDA4). If burette reading has 5.3% overall error (pump system error +/-burette error) and calculated accuracy is 0.13% (75ml/0.1ml - I asume 0.1ml resolution?) then estimated pump system tolerances are 5.43% - 5.17%. For IDA4 if reading has 6% error overall error (pump system error +/- IDA error) then estimated pump system tolerances are 4% - 8%. Do you think this reasoning is valid?

Limits of accuracy for measuring instruments are indicated so we can do this jiggery-pokery to arrive at an estimated figure of the actual tolerances in the parameter or device we're measuring; which is preferable to providing a figure that includes the measuring instrument uncertainties. Hopefully manufacturers do all this thinking for us when they specify the measuring method, instrument and tolerances on the measuring instrument. However sometimes this is not the case.

There will always be uncertainty but when two sources of error are combined we can at least estimate the limits of accuracy or tolerances in the unknown source of error, with a level of confidence, if the limits of accuracy of the other source (measuring instrument) is known. Particularly if a trial of measurements are taken, i.e. statistically speaking. Single one-off measurements are a bit tricky thus eliminating sources of error in measurements or measurement technique is always preferable under this circumstance.

Hence setup is important - bag-height (@ 30"?), new set, outlet of giving set/burette datum/IDA ports @ mid pump mechanism height, etc, etc. Of course a scientific investigation would perform a statistical analysis and comparison between the two methods. Repeatability and comparability of tests is essential of course.

I think it's likely that using an appropriate choice of burette to measure a particular volume and infusing at a rate and for a time that is indicated in the trumpet and startup curves for the pump (to ensure the driver/pump is infusing at the rate specified, as accurately as it can, for the test to be consistent and to meet manufacturers specification), it is possible to make measurement errors almost negligible for all practical purposes. This includes syringe drivers.

The result of all this waffle is that that the reading on the burette will be very close to the likely pump system delivered volume. Thing is; can the same be said of the IDA-like devices?

Basic Accuracy of a Burette? Minimum volume that can be resolved by eye divided by full-scale value. Remember the accuracy will vary with total volume delivered (since the minimum volume resolved increases as a proportion of the volume delivered as total volume delivered is reduced)so choice of burette, volume delivered, grade of burette and resolution is important. Are you using Grade A or B burettes KITT?

Otherwise, for type-testing and evaluation, gravimetric methods, as described by Bioman, seem to be the "gold-standard" method of measurement since very accurate mass measurements can be achieved and the tests carried out under very carefully controlled conditions (as specified in British standards) I suspect.

The problem is, that the 100 ml burette has a resolution of 0.2 ml Which would most likely have caused it to fail the test on that as well. My 10 ml burette has an resolution of 0.02 ml, so i think using that and comparing it to an IDA would be a good idea, although i find it hard to think of a situatio where an infusion pump wuld be used to deliver such a small amount of fluid.

For the burette you might have to choose different delivered volumes to maintain the level of accuracy you require using a particular accuracy burette, e.g. if you deliver 50mls into your 100ml burette then accuracy is essentially 0.2ml/50ml or +/-0.4% (of reading) because the basic accuracy of your burette is at full-scale. Same thing as with accuracy on analogue gauges and suchlike - accuracy can be stated as a percentage of full-scale or of reading.

Don't you see that the basic full-scale accuracy of the 10ml burette is just the same as the full-scale accuracy of the 100ml burette? Actually a 0.2% basic accuracy is less than 10% of the stated system volumetric accuracy of a volumetric-pump (usually 5%). This means that the figure you read off the burette is basically pretty close to that of the actual delivered volume. In a practical workshop setting you can pretty much say that if you can ensure the accuracy of the measuring device is less than 10% of the tolerances of the device you're measuring then you can ignore the measurement inaccuracy and take the reading as the delivered volume (including errors in the pump only).

What makes you think your test has failed? What are you trying to achieve from the measurement? Your burette has a basic volumetric accuracy of 0.2ml /100ml*100% or 0.2%. Thus it has a volumetric accuracy of 0.2ml /75mlor 0.26% if 75ml is delivered. Overall the %age error indicated by the burette in your test was: (75-79/75)*100 = +5.3%

The indicated error is a combination of pump error in delivery due to tolerances in performance and burette error due to measurement uncertainty (inaccuracy). Hence error in pump delivered volume is: 105.3% +/-0.2% = 105.1-105.5% delivered by the pump. If I measured this initially I would look to the pump or the test setup since 5.1-5.5% over-delivery indicates accuracy is marginally unacceptable (assuming a spec of 5%). I would probably repeat the test, possibly average 5 results (Baxter suggest this in some manuals) and if the values for pump tolerance do not improve then I'd look for wear and tear or damage to the pump.

Your initial results show that the IDA gave an indication for pump accuracy that is potentially worse than that indicated by the burette. This may be because the potential for measurement uncertainties could be greater with the IDA, i.e. the specified volumetric measurement accuracy is >0.2%. What is the stated volumetric accuracy of the IDA? Is this stated accuracy consistent at all rates/volumes delivered?

At different rates the manufacturer should tell you the specified accuracy of the IDA and you can determine the accuracy expected of the burette yourself. Hence given the overall error from a volumetric measurement (at the same rate, over the same time, with the same setup and method) you should be able to estimate the tolerances in pump delivery. Assuming the same pump is used at the same rate, over the same observation time, for each method, of course.

Then if there is a discrepancy between the estimated pump tolerance between the burette and IDA measurements, even when you take into account the specified errors in the burette and IDA measurement, then you need to investigate whether the IDA, the burette or the method you're using on either has a problem, i.e. does not meet it's specified/estimated tolerance for accuracy, or whether there's variation in the same pump between each test.

If you're trying to compare accuracy of one method to another, i.e. burette to IDA, then I would have thought that you need a benchmark or gold-standard to compare them both to, i.e. gravimetric method as used in type-testing and evaluation of infusion devices. But it's pretty obvious really that if you use one method of measuring volume that has a basic accuracy that is better than another then it's likely that this will produce an indication of delivered volumes that are more representative of the actual delivered volumes of the pump hence there's going to be some discrepancy between indicated readings obtained by measuring instruments that have different system accuracies.

In this case the error in actual delivered volume and the nominal volume expected is predominantly due to tolerances in the pump: Error = (actual reading - nominal reading/nominal reading)*100% plus/minus the worst-case accuracy of the measuring instrument as a percentage. Therefore with any measurement you end up with %age pump accuracy +/- %age worst-case measurement uncertainty. For your pump 105.1-105.3% delivered volume assuming 0.2% accuracy of the burette or 104%-108% delivered volume assuming 2% volumetric accuracy of the IDA. What is the specified accuracy of the IDA you’re using?

This aside; once you've established that the volumetric errors in the burette and IDA are within their specified tolerances then I suppose you can then determine which method gives the most appropriate readings over the range of rates/volumes you require to test pumps at.

If you're trying to determine whether burette or IDA is most suitable for pumps and/or syringe drivers given typical rates they operate at then perhaps you need to select one pump and one syringe driver and then select a high, medium and low rate tests for each using trumpet and startup curves to determine the minimum observation windows you need to do the measurement over. You then obtain results using each method that you can compare to each other.

Perhaps you could obtain a pulsatile delivery pump and then do a comparison between the estimated pump volume delivered using the burette (minus burette measurement tolerances) and the volume indicated by the IDA (minus IDA measurement tolerances).

Richard, a very good treatise of the subject of measurement. One comment; I have always considered that with a burette you are making two measurements for each test, a zero setting 'measurement' and a volume delivered measurement, and so the inherent accuracy, resolution/test volume, should be muiltiplied by two. Is this correct?

In practice, the zero or datum reading should be taken from the bottom of the fluid meniscus each time. The zero error is usually slightly positive with respect to the zero-line, if anything, since you can actually see if the reading is above or below it. You add the potential zero "error" to the volume delivered since this is, in effect, a zero-offset not a span-error. When considering the zero-error we would need to “subtract” it from our overall measurement if we were being pedantic.

100ml burette, 0.1ml resoultion, basic accuracy 0.1%. E.g. Zero error somewhere between zero and 0.1ml. 75ml delivered (estimated accuracy of burette 0.13%) 79ml indicated (including zero and span error) hence error overall is ((79-75)/75)*100 = +5.3%.

The fixed worst-case zero error is 0.1ml (and usually positive) hence delivered volume is actually between 79-78.9ml by subtracting this +ve offset error from the volume delivered. The overall error in delivered volume taking into account fixed zero offset is ((78.9-75)/75)*100 =+5.2% Assuming accuracy of 0.13% then tolerance in the measured parameter, i.e. pump, is +5.3%+/-0.13%(basic accuracy of burette) -0.13% (fixed zero error) = +5.07 to -5.43% of pump nominal (75ml).

Just multiplying the error by 2 does not take into account the zero being positive (i.e. it would include -ve errors) which it usually is in practice otherwise the measurment would not be adequately zeroed.

Gentle Men,

There is but one thing that strikes me as being more important than the accuracy or otherwise of the test methods given much coverage by many learned people on this quest by K.I.T.T.

Our exalted Sage, Monsignor RoJo hit it on the head on day one of this request for help, in fact he was indeed the first to reply.

As Monsignor RoJo said:

“If you are looking at accuracies of drug delivery systems look at the whole system not just the electrical pump. IV pump accuracy often depends on the diameter and elasticity of a piece of tube, I would guess this would be more prone to inaccuracy than the pump, especially as time passes and the properties of the tube material changes.”

Hit on the head why don't you, spot on!

This plastic tube, especially the RMC9608 Solution Administration Set supplied by Baxter has been proven over many years to be very inconsistent in accuracy. Syringes by various manufacturers are also prone to large variations in accuracy and stiction.

Having just repaired a Flogarde 6201 Volumetric Infusion Pump I placed it on test using our IDA4+ with Hydrograph, using a new standard admin set as above type. One hour later the displayed graph and printout showed a flow rate, which equated to an error of minus 4.53%. This was not believed because as we all know the 6201's are inherently accurate. Not like the Colleague!

The test can be done, my learned student friend, by using a standard Baxter Flogard Administration set or Baxter calibration tubing; my preference is to use the standard admin set because that is used on the patient.

Having from experience decided this test was inaccurate and was in fact probably the solution admin set, I then used an old set – used twice before – same parameters/set-up. Result – error now minus 1. 23%. Pump's a good un! Test equipment is also accurate, but the solution set wasn't, and consistently are not, even in the same batch.

This however K.I.T.T., is not always the case with other similar products of different manufacture. Having just also calibrated an Alaris Signature Gold I.V. Pump, using the manufacturer's specially supplied Volume Calibration Set; I then tested it using a standard – ward use – solution set.

The calibration set gave a pump volumetric accuracy of plus 0.15%. With the standard solution set – using the same test parameters – it gave a volumetric accuracy of minus 0.75%.

This test K.I.T.T., was done gravimetrically, using a glass flask on a 0.1g resolution, high precision top pan balance, which has a tolerance of +/- 0.3g, [actual deviation upto 500g however is nil.] This is calibrated in-situ annually by the manufacturer - Avery Berkel. [Now Avery Weigh-Tronix] To ensure good delivery – no bubbles or other fluid errors, the pump is run for a short time to prime the system and the balance is tared out before starting the test proper.

What you could do K.I.T.T. To make an accurate gravimetric testing device is to use the output from the RS232 interface of the balance to a computer.

Then write a programme that gives a real-time graphic display, with auto tare on start, programmable start/finish times, date/time annotation, user entry fields for: set rate, set volume, pump tolerance for rate and time, and who is doing the test & on what balance, etc. With the final test results given as an average rate [ml/h], % rate accuracy, delivered volume [ml], % volume, and delivery time accuracy.

You might then be close to our in-house developed system which, may I add, the IDA4+ compares very closely to. [With-in +/- 1%.]

Best of luck.
Ian

Most if not all manufacturers specify system accuracy under stated test conditions (these include giving set, temperature of test, rates, etc, etc) so any element that can affect system accuracy, taken in isolation, is irrelevant. The pump is actually required to create a positive pressure and regulate the fluid through the giving set (to within system tolerances)otherwise we would just rely on the giving set on it's own and have one less inaccuracy (pump) to think about. Any giving set out of the box with any pump designed to use it should work within the system tolerances - that's what it's all about - otherwise why bother.

The whole point of making accurate measurements is to find these problems. I think you're missing this point Ian, which is why measurements are carried out - to able to detect the sort of problems you're discussing with sets. Of course there will be sifferences between sets but how much and is it acceptable (tolerances)? Use a burette, IDA or gravimetric method, with adequate accuracy for the job, and you should be able to determine system tolerances, i.e. if pump delivery is outside tolerances then you have a problem with the pump or the giving set.

Whether it's a burette (direct volumetric measurement), IDA or weighing scales (derived mass measurement) - the volume of the fluid delivered is what's used to measure system volumetric accuracy - fullstop. The point I've been trying to put over for KITT's benefit is that measurement of volumetric accuracy and above all the methods you use to achieve this is important whether you're comparing one method to another or just trying to make an accurate measurement for the sake of repeatability.

As per my previous posting and highlighted by Ian, the disposables will have a bearing on the "accuracy" of any measured flow rate etc.
All medical plastic components, in particular tubing, have an engineering tolerance with regard to I.D. and shore hardness (elasticity). In particular, shore hardness can have a tolerance of +/-5 which equates to over 7.5% for typical PVC tubing.
Therefore, you can take two new IV sets from the same batch and there could be a difference in ID and elasticity which could have a marked bearing on the flow characteristics.
I think the whole scenario shows that if we want "accurate" readings there are going to be inaccuracies which can be factored in or not depending on how pedantic you want to be.

Ian,

I can't understand the point of your previous posting - lots of red-herrings and all it pointed out to me was that pumps can operate within a band of tolerances specified by the manufacturer when we change system components. That is patently obvious if the manufacturer actually quotes a volumetric tolerance, e.g. +/-5% and specifies the conditions the tests should be done under. Irrespective of measurement method aren't all of the errors you discussed earlier actually within the manufacturers specification for a floguard pump? Or does it deliver to a system volumetric accuracy of 2% or 3% or 4%? When you stated it was inaccurate did you mean to say the results were not typical of the majority of floguards?

When we do use appropriate test-kit and make measurements we're not really interested in fluctuations within these tolerances are we? If so that is being pedantic since system volumetric accuracy (of the pump system not measurement system necessarily) likely to change for a whole host of reasons - the most significant ones being giving set, temperature, rate that the volume is delivered at, etc, etc. All that matters is that we perform an accruate assessment (one that we can justify) of the pump system accuracy with a standard giving set to give confidence (and provide a record) that the pump is operating within tolerances after servicing, PPM repairs, whatever.

The original gist of this thread was about whether IDA and other devices are up to the measurements we are using them for or not - I think KITT may have been "on a mission" to compare burette accuracy to IDA accuracy. Personally I don't think worst-case accuracy of +/-2% such as the specification on our infutest IDAs is good enough for measuring syringe drivers with a system accuracy of 2% and I've explained why previously but its precisely because of worst-case tolerances and suchlike. If your IDA is within +/-1% of your gravimetric tester then its just over +/-1% accuracy (for that single measurement not taking into account worst-case tolerances) that may just be acceptable for testing sryinge drivers but is the +/-1% you estimated the manufacturers specified tolerances?

The only thing that would concern me about choosing any measurement method is if it produces significant errors that mean we can't determine whether the pump is actually operating within manufacturers tolerances or not, i.e. measuring instrument tolerances are greater or approaching the pump system tolerances as I've mentioned before.

Mr Ling,

To refresh your memory, from the very first posting on 09 August 2005 by K.I.T.T. who started this discussion thread.

I quote:

“I'd be grateful if you fellow forumers would answer the above question on what you use to verify whether your infusion pumps, syringe drivers, etc are accurate enough. Do you use devices like the IDA2/4, a burette, weighing the solution or something different? And why did you choose that method?

The reason I ask you is as follows:

I've now ended up on infusion device analysis as I feel this is one field in which it would be possible to carry out research into and be able to come out with an outcome. My main objective is to find out which is the most accurate by performing various analyses using the different techniques. It would also be quite easy to build a new device for this task in the EBME dept. etc.”

Richard, the red herrings my friend, percentage wise, [100%] are all of your own making, with much verbal fluidity, and bovine waste!

Good luck K.I.T.T. [or is it Ash] with your task.

Ian

"To whom much is given, much will be expected.'

Ian,

I discussed the burette, some problems with it, how I use it and inaccuracies with it - you discussed the gravimetic method. Any of these methods is suitable if applied correctly although have reservations about 100% automated methods, e.g. IDA-like devices, used with some devices, e.g. syringe drivers (on the limits acceptable accuracy) and pulsatile delivery devices (problems with accuracy fullstop).

I think what I've posted is actually quite accurate even if it's wasted on you Ian and I certainly wouldn't say what you posted is bullsh*t, which is what you've said about what I've posted. It was longwinded because that's how I am and anyway it was intended for someone with no experience to read and digest for future reference if they think its valid.

Having developed your own gravimetric system I guess you're biased towards it to the extent that you feel the use of burettes and understanding of instrument errors is beneath you.

However I think it's important to have an appreciation of the basics beyond the "if I have a 5% tolerance pump system then I use a 5% tolerance measuring instrument" attitude which is a bit slack in my opinion.

It was intended for KITT hopefully as a means for him to get the most accurate measurements from his £65 pounds worth of burettes. Even though you and I know that the gravimetric method is the way to go for absolute accuracy and repeatability.

All we need to do on the bench is choose a method that allows us to say with confidence what the pump system tolerances are within spec. You have not respnded to my previous post regarding your findings with floguard and ALARIS pumps thus I can only assume that your pumps are all within specification despite the diatribe about variation in giving sets.

If anyone thinks what I've posted is bull**** and more importantly inaccurate or mis-informed then please comment - it will be taken in the spirit intended.

Richard,
I think kitt appreciates your comments and everyone elses. It is up to him how he uses the advice he gets from this forum. Ian, I like the fact that we allow 'free speech' on this forum but please (and anyone else who feels the inclination) refrain from cryptic and possibly offensive remarks. As an administrator for this forum, I do not think it is productive and could put off some members from joining in.

Thank you, John.

Aha John – riposte!

Antiquis temporibus, nati tibi similes in rupibus ventosissimis exponebantur ad necem!

Damnant Qvod Non Intelligvnt. Animadvertistine, ubicumque stes, fumum recta in faciem ferri?

Errare humanum est! Et tu John.

Ian.

I would scrub that post if I were you John the mans a savage.

Richard,
If i knew what it meant i might do that. I would appreciate the english version Ian. It does interest me - the meaning that is... i assume there is some humour in the transcript. Anyway, if this topic is exhausted i will close it. Another 24 hours....

It possibly comes under the but it's quite subjective being Latin and all that.

"Errore" not "Errare" I think Ian.

If thats latin no wonder its almost a dead language. Trying to translate it is a pig. I'm not supprised the Romans changed to italian as chatting up birds in latin would be impossible you would get a slap after the first word.

Leonius,

You're right there, but it does have a sort of ring to it don't you think.

Something some verbose people should enjoy really!

[John - please check your e-mails for a private riposte!]

Ian

Pumps still in spec Ian? Or are you still looking for a wonder-set that will give you 0% errare? Or should that be 0 percenti?

Red-Herring - to introduce an irrelevant issue into a discussion. As in why discuss set-tolerances when the system runs in spec. Ahh, of course, just to introduce the fact that you're clever and have designed your own weighing scales. Doh! Is that verbose enough or shall I try harder?