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BACKGROUND OF THE INVENTION
Whenever an electrode at the end of an EKG or EEG lead is applied to a
patient's body, a potential electrical shock hazard to the patient is
created. Electrodes are generally designed to provide a low resistance
connection to the patient so as to facilitate the detection of low
amplitude physiological signals from the patient's body. However, this low
resistance connection also facilitates the application of hazardous
voltages to the patient and the flow of current to and from the patient,
thereby exposing the patient to electrical shock. Hazardous voltages and
currents may arise from static discharges, from contact of the patient or
of an electrode with surrounding electrical equipment, from EKG or EEG
equipment failure, from ground loops, or from stray capacitance which
often develops between the lead and grounded objects to which the lead is
coupled.
Many voltage and current limiting devices such as isolator circuits and
non-linear series circuits have been developed to protect the patient from
electrical shock. Isolator circuits generally comprise considerable
floating or non-grounded circuitry, including modulators and transducers
or couplers of the optical, magnetic, electric field or acoustic type.
However, these circuits are usually complex and costly, and because these
circuits are usually incorporated into biomedical equipment located away
from the patient, they do not adequately protect the patient from
electrical shock such as from long-lead shunt currents that arise from
stray capacitance in the lead. Also, isolator circuits do not adequately
eliminate artifacts that sometimes reside in the physiological signal
received from the patient. Such artifacts are often caused by large
electrostatic common mode voltages arising from multiple electrodes placed
on the patient's body.
Non-linear series circuits also have certain inherent disadvantages which
cause them to provide inadequate shock protection to the patient. These
disadvantages are described below. Most known non-linear series circuits
are semiconductor circuits using Field Effect Transistors (FET), Bipolar
Transistors, or Diodes.
FET circuits are unduly complex and costly and can be damaged easily when
large overload voltages are applied such as electrocautery and
defibrillation voltages thereby providing inadequate patient protection.
Electrocautery voltages are generally about 2 kilovolts in magnitude while
defibrillation voltages are generally about 8 kilovolts in magnitude.
Bipolar transistor circuits, which are often constructed having a plurality
of bipolar transistors, a battery, and one or more resistors, having the
same disadvantages as FET circuits mentioned above plus the added
disadvantage of limited battery life.
Diode type current-limiting circuits generally comprise diode bridge
circuits, hot carrier diode circuits, germanium diode circuits or silicon
diode circuits. Diode bridge circuits do not adequately protect the
patient from shunt currents that occur in long leads because these
circuits, by reason of their spatial requirements or bulk, are generally
located in the biomedical equipment itself and away from the patient. For
example, these circuits often require space to accommodate both a positive
and a negative voltage source and at least four connecting leads, a signal
source lead, a signal output lead and two voltage source leads.
Hot carrier diode circuits provide decreased shock protection as
temperature increases. For example, the conductivity of these circuits
increases by a factor of 14 (i.e., increases to fourteen times its
original value) for each 25.degree. C increase in temperature.
Furthermore, hot carrier diode circuits inadequately limit the amount of
current flowing to a patient. Typically, these circuits permit as much as
5 milliamps per lead, or 60 milliamps via a typical configuration of
twelve leads, to pass to the patient.
Germanium diode circuits often have a low breakdown voltage that is
inadequate to protect the patient when line voltages of 220 volts or
higher are applied. Also, because the reverse leakage current of germanium
diodes is relatively high, typically from one to twenty microamperes at
room temperature, and doubles every 8.degree. C above room temperature,
the shock protection afforded to patients by this type of circuit
decreases by 50 percent each time the temperature rises 8.degree. C. The
cause of a rise in temperature may be, for example, the hot lights or
other heat sources in an operating room near to the circuit.
Silicon diode circuits have a high series resistance and low conductivity
due to their small reverse leakage current. Because a patient's
physiological signals are typically low amplitude signals requiring for
conduction a device with high conductivity, the high-resistance
low-conductivity characteristic of silicon diode circuits makes these
circuits unsuited to conducting these signals.
SUMMARY OF THE INVENTION
According to the illustrated embodiment of the present invention, an
optically excited diode current limiter circuit is placed in series with a
lead having an electrode attached to a patient. The circuit conducts the
patient's physiological signals picked up by the electrode while limiting
the amount of current flowing through the lead so as to protect the
patient from electrical shock.
The circuit includes two optically excitable silicon diodes connected back
to back to each other, in series with the lead. One diode is connected to
the patient electrode for receiving input signals picked up from the
patient. The other diode is connected to a signal processing device for
outputting patient signals to the device. The circuit also includes a
variable light source for illuminating the diode junctions thereby to
increase the reverse leakage current and, hence, the conductivity of the
diodes, and an enclosure which prevents external light from reaching the
diodes. As the intensity of the light source is increased, the
conductivity of the diodes is increased sufficiently to allow small signal
currents, below a predetermined level not hazardous to the patient, to
pass through the diodes. Currents above this level are blocked by one of
the diodes when flowing to the patient and by the other diode when flowing
from the patient.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic illustration of one embodiment of the current
limiter of the present invention.
FIG. 2 is a waveform diagram illustrating the current-voltage
characteristics of the current limiter of FIG. 1.
FIG. 3 is a waveform diagram illustrating the current-voltage
characteristics of the current limiter of FIG. 1 when light from a varying
light source is incident on the current limiter.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows an optically excitable diode current limiter 11 having back to
back silicon diodes 13, 15 connected in series with a lead 17, and having
a variable light source 19 such as a potentiometer 20 controlled light
source for optically exciting the diodes 13, 15, and an enclosure 21 to
prevent external ambient light from reaching the diodes 13, 15. The light
source may also be a light-emitting terminus such as the end of a fiber
optic, or a light source that emits non-visible light such as X-rays.
Signals picked up by an electrode 23 at the end of the lead 17, are
selectively transmitted by the current limiter 11 via lead 18 to a signal
processing device 25 for processing. Device 25 may be any physiological
signal processing device such as an electrocardiograph.
The current limiter 11 may be placed near to the electrode 23, for example
within 6 inches of electrode 23. When so placed, lead 17 between the
patient and the current limiter 11 is short, thereby eliminating the
hazard of shunt currents that can arise from long leads between the
patient and the current limiter.
When light from the light source 19 is incident on the diodes 13, 15, which
may be two diodes as shown in FIG. 1 or a greater number of diodes, free
charge carriers are formed at the diode junctions. This reduces the
resistance of the diodes 13, 15 to current flow. When small signals with
voltage levels in the range from -25 millivolts to +25 millivolts are
applied to the diodes the diodes 13, 15 act much like two resistors with a
constant value producing a linear signal voltage-current relationship over
this voltage range. When acting like resistors, the diodes conduct signals
with small, non-hazardous currents such as physiological signal currents.
FIG. 2 shows the relationship between applied voltages (V) and current
flow (I) through the diodes 13, 15. For a selected amount of light
incident on the diodes, 13, 15, the magnitude of the current conducted by
the diodes 13, 15 varies linearly from -I.sub.LIMIT1 to I.sub.LIMIT1 as
the applied voltage varies from -25 millivolts to +25 millivolts. As
explained below, I.sub.LIMIT1 represents the reserve leakage current of
the diodes, 13, 15 and is the maximum magnitude of current that can flow
serially through both diodes 13, 15, for a selected amount of incident
light. FIG. 3 shows that as the light source 19 is varied, for example by
varying the potentiometer 20 thereby increasing the intensity of light
incident upon the diodes 13, 15, I.sub.LIMIT1 increases in magnitude to
another limiting value I.sub.LIMIT2.
The diodes 13, 15 conduct small currents flowing to or from the current
limiter 11. The back to back orientation of the diodes has no appreciable
effect on the direction of flow of such small currents when the diodes 13,
15 are acting like resistors. Consequently, a physiological signal flowing
from a patient 27 to the current limiter 11 would not be blocked by diode
15; the current would be allowed to flow to the signal processing device
25.
However, as FIG. 2 shows, when the voltage of the applied signal is outside
of the above-mentioned range, i.e., less than -25 millivolts or greater
than +25 millivolts, the diodes 13, 15 act like typical diode
semiconductors, each conducting current in one direction and blocking
current greater in magnitude than I.sub.LIMIT1 flowing in an opposite
direction. Signals having current levels greater in magnitude than
I.sub.LIMIT1, for example, electrocautery, defibrillation, ground loops,
stray capacitance or other signals occasioned by equipment failure, are
deemed hazardous to the patient and are blocked by the current limiter 11
from flowing to or from the patient, i.e., no current greater than
I.sub.LIMIT1 in magnitude is allowed to pass the current limiter 11. Diode
15 of the current limiter 11 blocks currents of hazardous signals that
flow from diode 13 to diode 15. Diode 13 blocks currents of hazardous
signal that flow from diode 15 to diode 13. A hazardous signal applied to
diode 15 reverse biases diode 15. This causes diode 15 to conduct signals
with current levels less than or equal to I.sub.LIMIT1 in magnitude, and
to block or not conduct signals with current levels greater than
I.sub.LIMIT1 in magnitude. Similarly, a hazardous signal applied to diode
13 reverse biases diode 13, thereby blocking the passage of all currents
greater than I.sub.LIMIT1 in magnitude and permitting the flow of currents
less than or equal to I.sub.LIMIT1 in magnitude.
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