Electronics patent of the month: The problem of spurious monitored signals

This month features the problem of spurious monitored signals in hospitals, which comes up in CareFusion Corporation’s GB patent. Michael Jaeger, patent attorney at leading UK patent and trade mark attorneys Withers & Rogers LLP, writes:

GB Patent Number: GB2476862
Granted to: CareFusion Corporation

I’ve certainly overheard some interesting discussions between my children recently…

“I don’t like drinking water” my daughter declared in the car last week.

“Your body is made up of 90% water, so if you don’t like drinking water then you don’t like yourself”, my son retorted.

So it turns out that humans are in fact around 60% water, but we are nevertheless pretty aqueous. This can be quite a sobering thought, given that the human body also directly reacts to electricity.

The issue of the electrical properties of humans comes up in CareFusion Corporation’s GB patent no. 2476862, granted on 23 December 2015. Hospital patients can (and in the not too distant future all of us will) be monitored and treated in a myriad of ways using some truly wonderful medical devices. CareFusion, has discovered, however, that monitoring devices can give spurious readings when used with some treatment devices.

For example, a hospital patient may be connected to an intravenous drip. It might be necessary to warm the drip line in order to raise the temperature of the fluid to be administered to the patient. To achieve this, resistive heaters can be attached to an intravenous liquid dispenser. Such heaters apply a current to a resistive load in a cyclical manner to turn the heating element on and off and maintain the correct temperature.

Low frequency transients

It turns out that when the device is powered on and off, it produces low frequency transients. This in itself is not dangerous, however, as the intravenous line connects the heater to the patient’s body, the low frequency transients are capacitively coupled into the patient. A similar phenomenon can occur when using bypass pumps, infusion pumps, warming blankets and other devices with a large enough footprint to have a high capacitive coupling to a patient.

These low frequency transients are not, per se, problematic. However, a patient may typically be connected to one or more monitoring devices. These might include neuromonitors, electro-cardiogram monitors, sleep diagnostic monitors and other devices that are used to detect signals. These devices may display spurious readings due to the interference from the capacitive coupling with the patient.

CareFusion’s invention has attempted to overcome this problem by providing an ingenious arrangement for powering electric devices, as well as reducing transients coupled to a patient.

The patent describes a device which has two identical DC voltage sources with each voltage source being connected to a switch. The outputs of the two switches are connected to a load, which in turn may be capacatively coupled to a monitored patient. When a switch is closed, a current is provided to its output. Alternately, when a switch is open, no current is provided. If the switches are controlled so that they are turned on and off simultaneously then no voltage is applied to the load, since the voltage at both terminals rises and falls at the same time, thus producing zero potential difference. On the other hand, if the switches are controlled to open and close out of phase, this will produce an alternating current through the load.

Capacitive coupling problem

This invention overcomes the capacitive coupling problem in two ways. Firstly, patient monitoring devices typically include low pass filters. Therefore, the switching frequency of the switches is chosen to be high enough to be above the pass band of the low pass filter.

Secondly, when the switches’ operation is changed to go from out of phase (AC current through the load) to in phase (no current through the load) a spike or transient may be produced by both switches. However, the second input to both switches is earthed (see 214 and 216 in the drawing). This means that since the transients have opposing polarity and occur simultaneously, they cancel each other out so that no current discharges through the IV line to the patient.

Elegant switching

In summary, by using a pair of switches to control dual DC power sources and using an elegant switching algorithm, the problem of spurious monitored signals can be banished from hospitals and directly assist the treatment of patients.

Michael Jaeger is a patent attorney at leading UK patent and trade mark attorneys, Withers & Rogers LLP.


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