Darth, thanks for the pictures. Reading through the letters I see that you have located the area of the trolley that is coupling itself to the internal 'electrics'. I had not seen this letter earlier and had written a whole lot for you today. Forget that as it is obvious that you have located the chunk of tolley metal interfering. One way around this might be, apart from getting another trolley, to find yourself some mu-metal to put on the trolley table -top surface between it and the area in the defib where the inductor is located. You might try also connecting an earth wire with a crock-clip from the earthing stub that is normally an equipotential earthing point at the back of the defib and the table-top metal to see if this corrects things. Obviously one cannot run off with the defib with the earth connected but it might be of some help while you do your daily tests if it has any useful effect - Nick

Thanks for the reply Nick.

The defibrillator and trolley is currenly located in the EBME workshop - quite a distance away from its original location. The Codemaster will pass the self-test if it is moved just a few inches above the trolley, and will work fine on the bench top. What we have discovered is that placing a large enough metal plate adjacent to the left side of the Codemaster will cause the self test to fail, so it seems that this is some sort of inductive/capacitive interference. What still confuses us is why has the trolley caused no problems for over a year, and why does an identical trolley we have in another department work fine?

The defibrillator behaves the same whether it is plugged into the mains or running of its internal battery. We are still waiting for a reply from Philips Medical, whilst Beaver Medical are sending a service engineer to investigate.

Sounds like magnetic interference. Perhaps the metalwork in the trolley has recently had something done to it to magnetise it (something in its location or someone placing a magnet right on the top of the trolley). If the other one hasn't suffered the same fate - it wont cause a problem !
Just a thought :p

Here are the images - sorry it took a while - I 'missed' them in my in-box

Are Beaver Medical sure that this is a dedicated defib trolley, looks like it was designed for an ECG recorder. I would have concerns about the defib staying on the trolley if its was push round a corner quite fast, which is possible during an arrest. The defib doesn't seem to secure, the foot print of the defib seems a lot smaller than the rectangular top of the trolley (i.e. ECG recorder shape and size). I'm sure any cardiac technician would be able to confirm what I am saying or tell me I'm talking nonsense.

Just a thought, it is almost as if the magnetic field from the inductor, though small and intermittent, has magnetised, over a long period of time(the year), the side piece of metal I can see on the picture (left back/side wall piece to keep defib on the trolley). The magnetised side panel might be now gently permanently magnetised and interfering with the neighbouring electronics. Test to see if the wall has any magnetism at all.
However, a good trolley is often stainless steel for medical applications. As has been said, this is not a defib trolley at all as if nothing else, it has no drawers on it nor a place for a back-board for the patient if required, to carry a plethora of things needed at an arrest. The other trolley though the same make, might be made of Aluminium perhaps?

I can't remember the path that the output-wiring follows or where the output-energy, wave-shaping components are situated in the Codemaster, off the top of my head. However; as you know, many monophasic, dc-defibrillators have a series resistor (wiring-resistance) and inductor combination in the output-circuit, to ensure a monophasic output waveform of a characteristic-shape.

What I was thinking is that maybe the wave-shaping inductor or output wiring are located parallel and in close proximity to the bits of the trolley that Nick refers to i.e at the back, or bottom, of the Codemaster case, close to the ferrous trolley material with the plastic case in-between. This is easy to check from a schematic.

At relatively low-energy (ie. 100J) during energy-discharge; high currents will still flow in the output circuitry, magnetising the trolley. The effective-value of the wave-shaping inductor (or load) on the output circuit might be changed due to the high magnetic susceptibility and magnetic permeability of the trolley metal, in close proximity and perpendicular to the inductor, thereby modifying the output waveform (and possibly the delivered-energy measurement, in the process) when the Codemaster is situated on the trolley, during discharge.

This sort of effect may cause significant variation compared to the usual 100J waveform's delivered-energy. The Codemaster might then be monitoring an invalid derived-energy, from measurement of load-voltage across it's internal-load, that is outside calibration-limits and indicate a problem to the users.

Alternatively if there are any high-current carrying conductors or coils associated with the energy-measurement or energy-generation and storage portions of the Codemaster a similar effect may be causing them to play-up. I basically agree with Nick's, Roy's and Bioman's comments, I think.

The flying-battery, described above, might have been pushed by motor-action (a force acting on a current-carrying conductor in a magnetic-field). A current passing through a battery supplying power to the defibrillator (the conductor) which flys out of the defib (has a force acting on it), as the defibrillator discharges (a pulse of electrical-energy producing a pulsed magnetic-field in the metal-trolley). Sounds a bit far-fetched to me, since current, magnetic field and force must have components which are perpendicular to each other but maybe the electrical properties of the steel used to manufacture the trolley increased the effect of magnetism on the battery.

Greetings,

Sorry for the delay, we have been extremely busy the past couple of weeks.

The Codemaster trolley issue is now solved. Although the trolley was not affecting the delivered output of the defibrillator (as far as we could measure), it was affecting the measured impedance. There is an inductor (the "patient coil") located in the rear of the defib, this is used for measuring the impedance & delivered energy of the defib. When the 100-joule test is selected, the Codemaster looks for a measured value of 50 ohms impedance and 85-115 joules. When not on the trolley the defib was reading approx 110 joules (within tolerance), whilst on the trolley is was reading 117 joules, causing it to fail. This is most probably caused by the metal of the trolley affecting the operation of the inductor, as mentioned previously.

The solution? We re-calibrated the Codemaster to read 100 joules when not on the trolley (as verified by our defib tester), this rose to 106-joules when on the trolley - which is well within tolerance.

Note that the patient inductor is only used to measure the impedance during a shock; it does not affect the delivered energy - which is calculated from the voltage present on the capacitor based on a nominal 50-ohm load.

Many thanks for the replies.