Optically Isolated Electro-medical Device

Abstract

An optically isolated electro-medical device. An electronic device is disclosed that is used for monitoring physiological functions of a patient. The device is electrically connected to the patient and establishes electrical isolation from other patient-connected circuitry by optical coupling. The isolation substantially reduces the hazards of electrocuting a hospitalized, bed-ridden patient who may be connected to several different pieces of electro-medical equipment simultaneously. In an illustrative embodiment, the optical coupling includes a light-emitting diode in operative connection with a light-sensing transistor (photo-transistor) and is arranged to minitor the EKG of a patient.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electro-medical equipment, and more particularly relates to monitoring equipment which is electrically conductively connected to a patient but is electrically isolated from: (1.) other circuitry necessary to provide a usable output and (2.) other simultaneously connected electro-medical equipment.

2. Description of Prior Art

Prior art in the area of electrically isolated electro-medical equipment includes battery-operated heart stimulators. These stimulators are isolated from other patient-connected circuitry and from line voltages by virtue of their batteries. An example of prior art that utilizes a battery is U.S. Pat. No. 3,554,198.

The patent also discloses a high-speed relay switch that closes upon external electrical command to provide a conductive path. Ordinarily, the relay switch remains open to provide isolation. This relay switch scheme of isolation does not permit linear coupling of signal between the isolated command and stimulator circuits. However, linear coupling is not needed in a heart stimulator (the shape of the heart stimulation pulse is not at all that critical). But, by contrast, substantially linear coupling is required for accurate monitoring and the present invention provides such coupling.

A problem may arise when monitoring a patient in a hospital bed when that patient is electrically connected to several independent pieces of electronic apparatus. For example, a heart monitor and a respiration monitor may be connected simultaneously to the patient where both monitors are powered from line voltage. If the equipment is not grounded properly, the patient may be placed in a ground loop. This could be a dangerous situation where the patient may be electrically shocked by current flow from one piece of equipment to another through the patient. Particularly, in the case of monitoring heart activity with implantable electrodes that are implanted into the heart itself, stray ground loop currents flowing through the heart can kill the patient.

If each piece of electronic equipment were individually battery operated, then ground loops could be avoided. But this is not feasible. Monitoring equipments require too much current, and the batteries would be cumbersome. A hospital usually uses ordinary 60 cycle line voltage and its monitoring equipment is powered in this manner. This gives rise to the possibility of creating dangerous ground loops through the patient.

The present invention is a solution to the isolation problem of electro-medical monitoring apparatus. The present invention includes optical coupling to provide electrical isolation.

SUMMARY OF THE INVENTION

An illustrative embodiment of the present invention is arranged to work with an ECG signal from the heart of a patient. A preamplifier receives and amplifies the ECG signal from the patient's heart. The output of the amplifier is used to modulate current flowing a light-emitting diode. All of this patient-connected circuitry is powered by an isolated power supply such as a battery or an output from a DC to DC converter.

The modulated light-emitting diode provides a light-energy output that linearly varies in intensity with modulating signal. The remaining monitoring circuitry is electrically isolated from the patient-connected circuitry. It is powered from a second battery or a conventional regulated power supply. The circuitry includes a photo-coupled device such as a photo-transistor. The photo-trasistor receives a light input from the light-emitting diode and provides an electrical input to the remaining circuitry. Thus, electrical isolation of the signal path is accomplished by optical coupling and electrical isolation of the power supplies is accomplished by using a DC to DC converter.

An advantage of the present invention is that it provides a simple and efficient way to monitor electrical signals representative of physiological functions of the body without danger of electrical shock.

A further advantage of the present invention is an inherent increase in reliability over other isolation schemes because of the few number of components used herein.

It is thus an object of the present invention to provide improved electro-medical monitoring apparatus.

It is another object of the present invention to provide an EKG output signal that is electrically isolated from circuitry connected to the patient.

It is a further object of the present invention to provide equipment which isolates the patient-connected circuitry from leakage currents that may be generated by other equipment attached to the patient.

Other objects and advantages of the present invention will become apparent to one having reasonable skill in the art after referring to the detailed description of the appended drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative embodiment of the present invention; and

FIG. 2 is a block diagram of a power supply to be used in conjunction with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the circuitry to the left of reference line 40 is hereafter referred to as "patient-connected circuitry." The circuitry to the right of reference line 40 is hereafter referred to as "optically coupled" circuitry. The patient-connected circuitry is electrically isolated from the optically coupled circuitry as will be explained herein below.

Referring first to the patient-connected circuitry, electrodes 10 and 11 are attached between the patient (not shown) being monitored and the input to amplifier A.sub.1. Electrode 12 is connected between the patient and isolated ground 13, about which ground additional description is presented in succeeding paragraphs. The output of amplifier A.sub.1 is coupled via conductor 14 to the base of transistor 15. The emitter of transistor 15 is connected to one end of resistor 16, the other end being connected to isolated negative 15 volts (-15 V.sub.I). The collector of transistor 15 iss connected to the cathode side of light-emitting diode (LED) 17, the anode of whicl is connected to isolated positive 15 volts (+15 V.sub.I). A description of the positive and negative isolated voltages is presented in succeeding paragraphs. Amplifier A.sub.1 is powered by .+-.15 V.sub.I.

Optical coupling from the patient-connected circuitry to the optically coupled circuitry is depicted by light energy symbol 18. Photo-transistor 19 is influenced by incident light energy 18. Light-emitting diode 17 and photo-transistor 19 may both be encapsulated together and this is depicted by block 28 representing a photo-coupled device.

The collector of photo-transistor 19 is connected to positive 15 volts (+15 V). The emitter of transistor 19 is connected to both capacitor 21 and one end of resistor 20. The other end of resistor 20 is connected to third wire ground 23. This ground differs from isolated ground 13 and will be explained more fully elow. The other side of capacitor 21 is connected to an input of amplifier A.sub.2 and to one end of resistor 22. The other end of resistor 22 is connected to third wire ground 23. The output of amplifier A.sub.2 is obtained on terminal 27. Resistors 24 and 26, and potentiometer 25 are in a series connections between the output of amplifier A.sub.2 and third wire ground 23. The wiper of potentiometer 25 is conductively connected to another input of amplifier A.sub.2 as a feedback path. Amplifier A.sub.2 is powered by .+-.15 V.

In FIG. 2, DC to DC converter 30 and regulated power supply 31 are shown as having various DC outputs and grounds. The voltages and grounds shown correspond to those shown in FIG. 1. DC to DC converter 30 is the power supply for the patient-connected circuitry of FIG. 1, is commerically available, and is of conventional design. DC to DC converter 30 is powered by a conventional regulated power supply 31 which provides +15 V and -15 V inputs to converter 30. In turn, supply 31 is powered by 115 .sup.V rms power. Supply 31 is the power supply for the optically coupled circuitry.

The circuitry internal to converter 30 includes transformer circuitry which provides Isolation between the various inputs and outputs. The voltage outputs +15 V.sub.I, -15 V.sub.I, and ground 13 are electrically isolated from voltage outputs +15 V, -15 V, and third wire ground 23. For example, in the DC to DC converter used in a model constructed in accordance with the principles of the present invention, current flow was less than 1 microampere when 115 volts RMS was applied between any 15 V terminal (either + or -) and any 15 V.sub.I terminal (again, either + or -), or between isolated ground 13 and third wire ground 23.

Thus, when converter 30 and power supply 31 are connected to the circuitry of FIG. 1 as shown, there are no conductive paths (between the patient-connected circuitry and the optically coupled circuitry) established through the DC to DC converter. Third wire ground 23 is the ground that other pieces of equipment (not shown) would be referenced to.

Now, consider the operation of the circuitry of FIG. 1. The monitored signal (in this case, an EKG signal but could be other signals) is fed to the input of amplifier A.sub.1 via terminals 10, 11 and 12. The signal is amplified in amplifier A.sub.1, and the output of A.sub.1 is applied to the base of transistor 15. Transistor 15 permits current flow there-through in accordance with the electrical signal input on its base. Current flows from +15 V.sub.I through the series circuit of LED 17, transistor 15, and resistor 16 to -15 V.sub.I. The flow of current through LED 17 causes emission of light 18 which is the optical or light input to photo-transistor 19.

The variation of current or voltage applied to the base of transistor 15 is linearly related to the current flow through LED 17. The current flow through LED 17 is linearly related to the intensity of light 18 that is emitted. And, the intensity of light 18 is linearly related to the flow of current through photo-transistor 19. Thus, an electrical signal in the circuitry to the right of line 40 linearly corresponds to the electrical signal in the circuitry to the left of line 40. The signals are "connected" or coupled by light-intensity variations. Electrical isolation is achieved to the extent that in equipment constructed in accordance with the principles of the present invention less than 5.0 microamperes typically will flow when 115 volts RMS is applied between any combination of input electrode terminals 10, 11, 12 and third wire ground 23 in FIG. 1.

Variations of current flow through photo-transistor 19 create voltage variations across resistor 20. This voltage variation, or signal, is A.C. coupled through capacitor 21, which removes any D.C. component. The resultant A.C. signal is equivalent to the signal obtained on terminals 10, 11, and 12 and is applied to amplifier A.sub.2. The amplitude of the signal is controlled by setting feedback potentiometer 25 as desired. It should be understood that amplifiers A.sub.1 and A.sub.2 may comprise considerable circuitry and may not necessarily be a single transistor or amplifying device.

Summarizing, amplifier A.sub.1 is a preamplifier which amplifies an ECG signal from the patient. The output of A.sub.1 feeds the base of transistor 15 which amplitude modulates the current passing through light-emitting diode 17. Because of the properties of light-emitting diodes, modulating this current modulates the light being emitted by light-emitting diode 17. This modulated light 18 is sensed by photo-transistor 19 inside photo-coupled device 28. Photo-transistor 19 reconverts modulated light 18 into modulated current. The modulated current develops signal voltage across resistor 20, which is a reconverted ECG signal. It is then coupled through capacitor 21 to remove any DC component, and amplified by amplifier A.sub.2 which makes up for any loss of signal amplitude. A.sub.2 also acts as a buffer amplifier to increase output drive capabIlity.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example ultrasonic coupling may be substituted for optical coupling, the substitution incorporating appropriate circuitry changes. A single piezoelectric crystal could be used with two pairs of electrical connections--one pair conductively connected to the patient and electrically isolated from the other pair conductively connected to the other equipment or apparatus.

The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

What is claimed is:

1. In improved medical-electronic equipment used for monitoring a physilogical function of a patient, said equipment being capable of simultaneous use with a plurality of other electrical apparatus, said equipment including an electronic amplifier and terminal means for conductively connecting the body of said patient to the input of said amplifier, the improvement comprising:

optical coupling means comprising a light-emitting diode in operative connection with a photo-transistor for electrically isolating the output signal of said amplifier from said plurality of other electrical apparatus and for linearly coupling said output signal to at least one of said plurality of other electrical apparatus,

first power supply means for supplying power to said equipment, and

second power supply means for supplying power in a continuous fashion and without being recharged to both said at least one of said plurality of other apparatus and said first power supply means and means electrically isolating the power generated from said first power supply means from the power generated from said second power supply means.