U.S. patent number 3,590,322 [Application Number 05/010,736] was granted by the patent office on 1971-06-29 for medical instrumentation system.
This patent grant is currently assigned to Sci Systems, Inc.. Invention is credited to Travis C. Carr.
United States Patent |
3,590,322 |
Carr |
June 29, 1971 |
MEDICAL INSTRUMENTATION SYSTEM
Abstract
A device for protecting patients against excessive currents
which might flow through the patients from medical electrical
equipment. A full-wave diode bridge rectifier circuit is connected
between the patient and ground and provides a low-impedance path
between the patient and ground. A constant current source is
connected to the bridge circuit to forward-bias the diodes. The
current allowed to flow between the patient and ground is limited
to the current supplied by the current source.
Inventors: |
Carr; Travis C. (Houston,
TX) |
Assignee: |
Sci Systems, Inc. (Huntsville,
AL)
|
Family
ID: |
21747154 |
Appl.
No.: |
05/010,736 |
Filed: |
February 12, 1970 |
Current U.S.
Class: |
361/42; 128/908;
607/63 |
Current CPC
Class: |
H02H
9/08 (20130101); H02H 9/00 (20130101); Y10S
128/908 (20130101) |
Current International
Class: |
H02H
9/08 (20060101); H02H 9/00 (20060101); H02h
003/08 () |
Field of
Search: |
;128/2.1,419,387,423
;317/10,18D,52 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller; J. D.
Assistant Examiner: Moose, Jr.; Harry E.
Claims
I claim:
1. A patient protection device for use in medical electrical
instrumentation, said device comprising unidirectional conduction
means, a constant current supply connected to supply forward bias
current to said unidirectional conduction means, and means for
connecting said unidirectional conduction means between a patient
and ground to limit the amount of current which can flow between
the patient and ground.
2. A device as in claim 1 including at least one physiological
electrical instrument with a ground terminal connected to said
ground and at least one signal terminal connected to said
patient.
3. A patient protection device for use in medical electrical
instrumentation, said device comprising a full-wave diode bridge
rectifier circuit, a constant current supply connected to two
opposite corners of said bridge circuit so as to provide forward
bias current to each of the diodes in said bridge circuit, and
means for connecting one of the other corners of said bridge
circuit to ground and the remaining corner to a patient.
4. A device as in claim 3 in which said current supply includes two
separate current sources, one connected to one of said opposite
corners, and the other connected to the other of said opposite
corners, said sources being connected together in series, and the
common connection between them being connected to ground.
5. A device as in claim 3 in which the positive terminal of said
constant current supply is connected to the anodes of two of said
diodes, and the negative terminal of said constant current supply
is connected to the cathodes of the other two diodes,
6. A device as in claim 3 including at least one physiological
electrical instrument with a ground terminal connected to said
ground and at least one signal terminal connected to said
patient.
7. A medical instrumentation system, said system comprising a
plurality of electrical physiological devices, each of said devices
having an instrument ground lead, means for connecting each of said
ground leads together to from a common instrument ground point,
unidirectional conduction means, a constant current supply
connected to supply forward bias current to said unidirectional
conduction means, and means for connecting said unidirectional
conduction means between said common instrument ground point and at
least one patient to limit the amount of current which can flow
between the patient and said ground point.
8. A system as in claim 7 in which said unidirectional conduction
means comprises a full-wave diode bridge rectifier circuit, and a
constant current supply connected to two opposite corners of said
bridge circuit so as to provide forward bias current to each of the
diodes in said bridge circuit, and said connecting means includes
means for connecting one of the other corners of said bridge
circuit to ground and the remaining corner to a patient.
9. A system as in claim 7 in which said connecting means includes
means for connecting said unidirectional conduction means between
said common ground point and a plurality of patients.
10. A system as in claim 8 in which said current supply includes
two separate current sources, one connected to one of said opposite
corners, and the other connected to the other of said opposite
corners, said sources being connected together in series, and the
common connection between them being connected to ground.
11. A patient ground circuit protection device for use in medical
instrumentation, said device comprising variable impedance means
for presenting a relatively low impedance between two terminals to
currents below a predetermined maximum level, and for changing said
impedance to a relatively very high value to currents above said
level, and means for connecting said terminals of said variable
impedance means between a patient and ground.
Description
This invention relates to medical instrumentation equipment; more
particularly, this invention relates to devices for protecting
patients from receiving electrical shocks from medical
instrumentation equipment.
The problem of preventing patients from receiving shocks from
electrical medical instrumentation is both serious and difficult to
solve. The problem is serious because a substantial number of
injuries and deaths occur due to patent electrocution in hospitals
every year. The problem is difficult to solve because the
constantly increasing proliferation of medical electronics
equipment currently in use greatly increases the potential sources
of shocks to the patient. Furthermore, with equipment utilizing
probes inserted into the flesh or blood vessels of the patient,
much less current flow through the patient is required to cause
permanent damage or death to the patient than if external probes
were used. Thus, currents through the patients must be limited to
very low values at all times for maximum safety. However, much
medical electronic equipment requires the provision of a
low-impedance path between the patient and ground. This requirement
makes it difficult to limit the current to very low values.
In accordance with the foregoing, it is a major object of the
present invention to provide medical instrumentation apparatus
which limits the flow of electric current through a patient's body
to a very low level, and yet provides satisfactory ground
connections for the electrical equipment. It is another object of
the present invention to provide such equipment which is relatively
simple in construction, reliable in operation, and inexpensive to
build and maintain.
In accordance with the present invention, the foregoing objects are
met by the provision of a patient protection device in which
unidirectional conduction means are connected between the patient
and ground in a medical instrumentation system. A constant current
source supplies forward bias current to the unidirectional
conduction means, and the current flow between the patient and
ground is limited to the amount of the forward bias current.
The foregoing and other objects and advantages of the inventions
will be set forth in or apparent from the following description and
drawings:
In the drawings:
FIG. 1 is a schematic circuit diagram of a medical instrumentation
system constructed in accordance with the present invention;
and
FIG. 2 is a schematic circuit diagram of another embodiment of the
present invention.
FIG. 1 shows a medical instrumentation system 10 including a ground
current protection device constructed in accordance with the
present invention. The ground circuit protection device comprises a
current-limiting circuit including a constant current source 12 and
a full-wave diode bridge rectifier circuit 14. A medical instrument
28 has measurement leads 30 connected to a patient 32. It also has
an instrument ground connection 24, and a power source ground
connection 26. The body of the patient 32 is connected to
instrument ground 24 through the rectifier bridge 14.
The bridge circuit 14 consists of four silicon diodes 16, 18, 20
and 22 connected in a bridge arrangement. The positive output
terminal of the current source 12 is connected to the corner of the
bridge which is connected to the anodes of the diodes 16 and 18.
The negative terminal of the source 12 is connected to the cathodes
of the diodes 20 and 22. Thus, the current source 12 supplies
forward bias current for all of the diodes in the rectifier bridge
circuit 14. The junction between the cathode of diode 16 and the
anode of diode 22 is connected to the instrument ground lead 24,
and the opposite corner of the bridge is connected to the patient
through a lead 34.
A low-impedance path from the patient to ground often is required
by modern medical electronic equipment. Such equipment includes
electrocardiograph recorders, electroencephalograph recorders, etc.
The equipment identified by the reference numeral 28 can be
considered to be any one of such instruments. When the bridge is
balanced, as it is when there are no currents flowing from the
patient to ground, the impedance through the bridge between the
patient and ground is equal to the forward impedances of diodes 16
and 18 in parallel with the forward impedances of diodes 20 and 22.
This impedance is low, and the bridge thus provides, effectively,
the low-impedance path needed by the electronic equipment 28. For
example, typically the forward impedance of each diode at 5
microamperes bias current is 20,000 ohms. Thus, the total impedance
in each branch of the path through the bridge is 40,000 ohms, and
the total bridge impedance is 20,000 ohms. This impedance is
relatively low, so that the path through the bridge is the low
impedance path needed by the equipment 28. When a diode is
back-biased, its impedance and the bridge impedance both increase
to a very large value, e.g. 100 megohms or more.
If current starts flowing from the patient 32 to ground 24, the
current flow in diodes 18 and 22 decreases while the current in
diodes 16 and 20 increases until finally diodes 18 and 22 become
back-biased and present a very high impedance to the flow of
additional current. The current at which this occurs is equal to
the bias current supplied by the current source 12. Similarly, if
current flows from ground 24 to the patient 32, the diodes 16 and
20 become back-biased when the current reaches the level of current
supplied by the source 12, and the current does not increase
significantly after that. Thus, the flow of current between the
patient and ground is limited to the bias current supplied by the
current source 12.
It has been determined that in order to protect patients against
electrical shock, the current through their bodies from electronic
instrumentation should be limited to as low as 10 microamperes (10
millionths of an ampere). Since the circuit 10 limits the flow of
current through the patient's body to the output current of the
current source 12, the output current of source 12 preferably is
maintained at or below 10 microamperes.
FIG. 1 shows, in dashed outline, a fault voltage source 36 which is
connected to power supply ground over a lead 37. Such a fault
source might be, for example, a faulty power supply of another
instrument connected to the patient. In such an instance, rather
than being electrocuted, the patient will suffer no ill effects
because the current-limiting device limits the current through the
patient's body to 10 microamperes.
The power ground connection is provided, for example by the usual
60-cycle wall outlet found in hospitals and homes, or other known
means.
The constant-current source 12 can be any of a number of well-known
high-impedance devices, such as the well-known constant-current
sources using field effect transistors or bipolar transistors.
The rectifier bridge circuit 14 can be composed of discrete
components, or it can be one of a number of full-wave rectifier
devices which are sold as a complete unit. One such unit which has
been found to be satisfactory is sold by the Varo Corporation under
the identification number "VE 48." The breakdown voltage of the
bridge should be at least equal to the maximum signal voltages to
be encountered in the system. A breakdown voltage of at least 400
volts has proved to be satisfactory.
The patient protection circuit described above performs the dual
function of providing a low-impedance ground connection to the
patient which is required by many types of modern medical or
electronical equipment, and yet limits the flow of current through
the patient's body, in case of faults or other potential sources of
electrical shocks, to currents which are well below maximum safe
levels. The patient protection device is simple, relatively
inexpensive and reliable.
FIG. 2 shows an alternative embodiment of the present invention
which is the same as that shown in FIG. 1, with certain exceptions.
The same reference numerals are used to identify the same
components in both figures of the drawings.
One way in which the system shown in FIG. 2 differs from that shown
in FIG. 1 is that a constant current supply is formed by two
separate constant current sources 12 connected in series, with the
common point between the two current sources connected to ground 24
through a lead 38. Each of the sources 12 produces a constant
current of 10 microamperes, and the bridge circuit 14 operates as
described above. The use of two sources 12 instead of one can
reduce the cost of system in that it allows the use of commonly
available, relatively inexpensive power supplies which produce
positive and negative voltages with respect to ground rather than
the sometimes more expensive floating supply which is needed for
the system shown in FIG. 1.
FIG. 2 also shows, in addition to the patient 32, a second patient
42, and shows a second medical electrical instrument 28 connected
to the patient 42, as well as third medical instrument 40 also
connected to the patent 42 through a pair of signal leads 44. The
instrument 40 is connected to its instrument ground through a lead
48.
The power ground connection for the unit 40 is shown as if it were
broken at 50. Under such circumstances, currents might be induced
from instrument 40 through patient 42 to power ground 26. However,
the current-limiting circuit of the present invention prevents this
from occurring and thus prevents injury to the patient 42.
The present invention can be used in several different ways to
protect significant numbers of patients in hospitals or similar
places. One such way is to protect each patient with his own
individual limiting circuit, as is shown in FIG. 1. Another way is
to provide only one ground connection and only one limiter in the
ground connection, as is shown in FIG. 2. In either arrangement,
the invention provides maximum safety for the patients.
The above description of the invention is intended to be
illustrative and not limiting. Various changes or modifications in
the embodiments described may occur to those skilled in the art and
these can be made without departing from the spirit or scope of the
invention as set forth in the claims.
* * * * *