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CROSS-REFERENCE TO OTHER APPLICATIONS
This application is related to the applications of Robert G. Bresler Ser.
No. 261,521, which was filed on even date herewith.
FIELD OF THE INVENTION
This invention relates to a method of and apparatus for determining
accurately the position of a device inside biological tissue, particularly
the position of an endotracheal tube in the trachea or windpipe of a
medical patient.
BACKGROUND OF THE INVENTION
It is necessary to insure that the breathing passageway of certain medical
patients, e.g., those in surgery or intensive care, is kept open at all
times. This is accomplished in the prior art by means of an endotracheal
tube which is inserted through the patient's mouth or nose and extends
through the patient's throat and into the patient's windpipe or trachea.
These prior art tubes are hollow and open at both ends, and the end that
extends outside the mouth or nose is anchored in place, usually with tape.
Air can then pass through the tube into and out of the patient's lungs.
The principal drawback of the prior art tubes is that the distal end of the
tube inside the patient must be inserted to and kept at a relatively
specific position which is at about the midpoint of the trachea. This is
because if the tube is inserted too far into the trachea, its distal end
may extend into the bronchial tree for one lung, and thus the other lung
will receive no air and may collapse. On the other hand, if the end of the
tube is not inserted far enough, it may interfere with the vocal cords, or
it may enter the esophagus, which opens near the bottom of the throat, and
air would not reach the lungs. In a normal adult the trachea is about 11
centimeters in length, and the distal end of the tube is generally
positioned at the trachea's approximate midpoint, and it may be anchored
in place by expanding a balloon attached to the tube. This positioning,
however, has much less margin for error in children or infants, whose
tracheas are much shorter in length. Furthermore, for both adults and
children, even if the tube is properly positoned initially, the movements
of the patient often cause the tube to move up or down, and therefore the
location of the distal end of the tube must be continuously monitored.
The prior art uses several methods for monitoring tube position. First, the
tube position can be determined by x-ray, but notwithstanding the possible
adverse effect of continued exposure to x-rays, the principal drawback
here is that by the time the x-ray is taken, developed and returned, the
tube may have moved again. Accordingly, two real-time monitoring methods
are in wide use. They involve listening to the chest to hear is both lungs
are filling and visual observation of the depth markings on the exposed
tube. Neither of these real-time methods, however, is very precise.
SUMMARY OF THE INVENTION
I have discovered a method and an apparatus for determining very accurately
the position of a device inside biological tissue, particularly the
position of an endotracheal tube in the trachea of a patient. The
invention comprises a detecting instrument which generates a magnetic
field which when disturbed causes the instrument to generate a signal, the
field being disturbed by a highly magnetically permeable metal attached to
the device inserted into the tissue.
In a preferred embodiment, the probe has a central coil and two end coils,
all identical. When the instrument is turned on, the central coil is
activated thereby setting up a magnetic field, and the voltage outputs
from the two end coils are compared in the detecting instrument. If the
end of the probe comes adjacent to the metal attached to the device in the
tissue, there is an imbalance in the field, and the instrument produces a
signal. At that point, the position of the probe on the skin is recorded
by using a slide or template which slips down the probe and is held in
place of the probe on the patient's skin, which is then marked by a pen
through a central hole in the slide or template. The position of the
device can then be monitored by replacing the probe on the indicated spot.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Drawings
I now turn to a description of the preferred embodiments, after first
briefly describing the drawings.
FIG. 1 is a side view in partial section of the invention along with the
endotracheal tube in place in a patient,
FIG. 2 is an enlarged sectional view of the probe of this invention, and
FIG. 3 is an enlarged sectional view of a coil arrangement of the
invention, and
FIG. 4 is a schematic diagram of the circuit of this invention, and
FIG. 5 is a schematic diagram of another circuit of this invention.
Structure
Referrring to FIG. 1, a locating apparatus is shown at 10. Locating
apparatus 10 generally comprises a probe 20 and a detector instrument 30,
attached together by a flexible electrical cable 40.
Probe 20 generally comprises three coils L1, L2 and L3 co-axially wound
around a core 22 which is disposed inside a cylindrical casing 24, which
is shaped to act as a pointer. The core 22 has three annular grooves 26,
27, 28 in which the coils L3, L1 and L2, each containing the same number
of windings, are located. However, coils with different numbers of
windings may also be used. The end grooves 26, 28 are equidistantly spaced
from the central groove 27. In the preferred embodiment, the distance
between grooves is about 1 cm, and the grooves are each about 2 mm wide
and 2 mm deep. The core itself has a diameter of about 9 mm. Separate
pairs of leads (not shown) are connected to each coil, and the leads are
carried to the detector instrument 30 by cable 40. The front end 29 of the
probe 20 is opposite the cable end, and a slide 21 is movably disposed on
the casing 24. Instead of the slide 21, the probe 20 may have a marking
element attached to its front end 29. The end of the core 22 is set
adjacent to the front end 29 of the casing 24, but it could be set at some
distance inside the casing away from the front end 29. The casing is
preferably made of a non-magnetic material such as plastic, while the core
22 is made of phenolic resin impregnated linen.
The detector instrument 30 generally comprises a box 32 which is of a size
that can easily be held in one hand. The box 32 has a switch SW1 on its
front end, a speaker opening 34 on its back end, and a series of lights 36
and a calibration control knob 38 on its top. Detector instrument 30
contains a circuit 50 for the probe 20, which circuit 50 is shown in FIG.
4.
The instrument 30 is detachable from the cable 40 at connector 42. This
allows the probe 20 to be sterilized when needed, which sterilization
process would otherwise damage the instrument 30 and its circuit 50. In
addition, since the probe 20 is detachable, different sized probes for
different types of patients, e.g., adults, infants, can be used with the
same instrument 30.
Referring to FIG. 4, the circuit 50 has a voltage source B1 connected
between ground and the switch SW1, shown schematically. The voltage source
B1 in the preferred embodiment is a 12.6 volt mercury battery.
A multivibrator U1 is connected across the voltage source B1 and switch
SW1. The duty cycle of the multivibrator U1 is set by resistors R1, R2 in
series with C1. Capacitor C2 stabilizes a reference point in the
multivibrator U1. In the preferred embodiment, multivibrator U1 is CMOS,
and preferably an Intersil ICM7555. R1 is 10K ohms. R2 is 330K ohms, and
C1 and C2 are 1000 picofarads, and 0.01 microfarads respectively.
The output from multivibrator U1 is connected through resistors R3 and R4
to the bases of transistors Q1 and Q2. Capacitor C3 is connected to ground
from between resistors R3 and R4. The transistors Q1 and Q2, which are NPN
and PNP respectively, are arranged as an emitter-follower circuit. In the
preferred embodiment, Q1 is a 2N2222A, and Q2 is a 2N2907A. Resistors R3
and R4 are 330 ohms and 220 ohms respectively, and capacitor C3 is 1000
picofarads.
The collector of transistor Q1 is connected through resistor R5, which is
22 ohms, to the positive side of the voltage source B1 (when switch SW1 is
closed) and to ground through capacitor C4, which is 47 microfarads.
Capacitor C5 and resistor R6 are connected to the positive side of voltage
source B1. Capacitor C5 is also 47 microfarads, and resistor R6 is 100
ohms.
The emitters of the transistors Q1, Q2 are connected to central coil
circuit 52. Central coil circuit 52 comprises capacitor C7 in series with
resistor R7 and central coil L1 of the probe 20. Capacitor C6 is connected
across the coil L1. Capacitors C6 and C7 are 0.1 microfarad and 6.8
microfarads respectively and resistor R7 is 150 ohms.
The emitters of transistors Q1 and Q2 are also connected to voltage doubler
circuit 54. Doubler circuit 54 is comprised of capacitors C8, C9 and
diodes D1 and D2. Diodes D1 and D2 are both 1N4148, and capacitors C8 and
C9 are both 47 microfarads. Also, a resistor R8 of 100 ohms and capacitor
C10 of 47 microfarads in series are connected across capacitor C9.
The coil L1 is, as previously explained, the central or transmitting coil
on the core 22 of the probe 20. Receiving coil L2 is the coil at the front
end 29 of the probe 20 and receiving coil L3 is at the opposite end. As
shown in the bottom portion of FIG. 4, coil L2 is connected through
capacitor C12 and resistor R11 to the negative input of differential
amplifier U2. This side of coil L2 is also connected to ground through
terminating resistor R9.
The coil L3 is connected to the positive input of differential amplifier U2
through capacitor C13 and variable resistor R12. This input circuit for
coil L3 also has a terminating resistor R10, and variable resistor R12 is
connected to ground through resistor R13. Resistor R14 is connected from
the wiper of the variable resistor R12 to ground, and capacitor C15 is in
parallel with resistor R14. The feedback loop for amplifier U2 comprises
the parallel combination of resistor R15 and capacitor C14.
The amplifier U2 is a Texas Instruments TL081. Terminating resistors R9,
R10 are both 220 ohms. Capacitors C12 and C13 are both 0.1 microfarads.
Resistor R11 is 1K ohms, and variable resistor R12 is 20K ohms. Resistors
R13 and R14 are 220K ohms each, and capacitor C15 is 120 picofarads. The
feedback loop resistor R15 and capacitor C14 are 220K ohms and 1000
picofarads.
The output of amplifier U2 is fed through capacitor C16 of 0.1 microfarad
and R16 of 220 ohms to the negative input of amplifier U3, the positive
input of which is tied to ground. Diode D3, a 1N4148, is connected between
the output of amplifier U3 and its negative input. The feedback loop for
the amplifier U3 comprises the parallel combination of resistor R17 and
capacitor C17 connected to the negative input and the output of the
amplifier U3 through diode D4, also a 1N4148. The amplifier U3 is a Texas
Instruments TL082, and resistor R17 and capacitor C17 are 220K ohms and
0.047 microfards.
The output from amplifier U3 is coupled to ground through capacitor C18 and
resistor R18, 6.8 microfarads and 10K ohms. This output is also fed
through R19 of 100K ohms to the positive input of non-inverting amplifier
U4, which is also a Texas Instruments TL082. This positive input is also
connected through resistors R21 to the wiper of variable resistor R20.
Variable resistor R20 is connected between negative voltage V- and through
resistor R33 to ground. Resistors R21, R20, R33, and R35 are 330K ohms,
20K ohms, 100K ohms, and 390K ohms, respectively.
The feedback loop for amplifier U4 which is connected between the negative
input and its output comprises the parallel combination of capacitor C20
and resistor R24, 1 microfarad and 100K ohms, respectively, and includes
resistors R22, R23. The negative input is also connected to ground through
resistor R22 and potentiometer R23, being 1K ohms and 50K ohms,
respectively.
The output from amplifier U4 is fed to display driver U5 which is connected
to light emitting diodes D5 through D14, which comprise the lights 36 of
detector instrument 30. Driver U5 is a National Semiconductor LM3915.
Resistor R25 of 470 ohms is connected to the driver U5, and filter
capacitor C11 of 47 microfarads is connected between the positive voltage
line to the driver U5 and ground.
The output from amplifier U4 also is fed through resistor R26,
potentiometer R27 to audio circuit 60. Audio circuit 60 comprises
unijunction transistor Q3, a speaker, (shown schematically), capacitor 21
and resistors R28 and R29. The transistor Q3 is 2N1671. Resistors R26 and
R27 at 4.74K ohsm and 20K ohms respectively. Resistors R28 and R29 are
4.74K ohms and 220 ohms, respectively, and capacitor C21 is 0.22
microfarads.
In the preferred embodiment, it is desirable to include a low battery
voltage circuit 70. Circuit 70 comprises a pair of transistors Q4 and Q5,
both TIS97. The base Q4 is connected to ground through resistor R34 of 47K
ohms and to the positive voltage through zener diode D16 and resistor R30.
Resistor R30 is 820 ohms. The emitter of transistor Q4 is connected to
ground, and its collector is connected to the base of transistor Q5 and
resistor R31, the latter being 10K ohms and connected to the positive
voltage source. The emitter of transistor Q5 is also connected to ground,
and its collector is connected to the positive voltage source through
resistor R32, of 820 ohms and light emitting diode D15.
Operation
The general operation of the invention is as follows. As shown in FIG. 1,
an endotracheal tube 100 is inserted into a patient's mouth, and is
extended down a desired distance into the trachea 200 in accordance with
clinical judgment. The exposed portion of the tube 101 is fixed in place.
It is desirable that the distal end 102 of the tube 100 be positioned at
the approximate midpoint of the trachea 200, as shown, between the vocal
cords 202 and the carina 207 of the bronichial tubes 206 to the lungs. The
tube 10 has a band 104 of metal foil near its end, which band 104 is
covered by plastic (not shown). The metal is preferably mu metal sheet
(available from Arnold Engineering of Fullerton, Calif.) or equivalent.
The distance between the band 104 and the distal end 102 is selected so
that for the particular type of patient, i.e., adult, child or infant, the
band 104 will be positioned above the sternal manubrium notch (not shown)
when the tube is in place.
In order to determine the positioning of the tube's distal end 102, the
detector instrument 30 is turned on and this induces a current flow
through the central coil L1 of the probe 20 thereby creating a magnetic
field encompassing the receiving coils L2 and L3. As the coils L2 and L3
are balanced in terms of windings and distance from the central coil, any
currents induced therein (in L2 and L3) will be the same. (This is not
precisely correct due to the inherent minor manufacturing differences in
coils intended to be identical).
The front end 29 of the probe 20 is then positioned perpendicular to the
patient's throat area. When the probe end 29 is very near the metal band,
which unbalances or distorts the magnetic field, the flux density through
coil L2 will increase, and the currents through coils L2 and L3 will be
unequal. This unequal current consequently causes the speaker to sound,
and one of the light emitting diodes D5-14 to light. Due to the speaker
sound, the operator can focus his attention on the patient and the probe
and does not have to look at the instrument to determine if the band has
been located. The frequency of the audible signal, and the diode lit
depends upon the amount of voltage difference (e.g., the last diode D14
will light and the sound will be at the highest frequency when the probe
is directly over the metal band).
The position of the band is then marked by sliding the slide 21 along the
probe until it contacts the skin. The probe is removed while the slide is
held in place on the skin. A pen is used to mark on the skin the location
of the band through the probe opening (not shown) in the slide.
To monitor the position of the tube 100, the front end of the probe is
placed on the marked spot. And if the maximum signal is not obtained, the
tube has moved. The skin area is then scanned by passing the probe end
over the surrounding area until the maximum reading is again obtained, and
if necessary the tube is re-positioned so that it corresponds to the
original or desired position. Alternately, the probe may be held at the
desired position, and the tube moved until a maximum reading is obtained
again. No reading (audible signal and lights) is obtained if the probe is
more than just above the skin or on the skin more than a centimeter
laterally away from the metal band. This is because the front receiving
coil L2 has a small cross-sectional area and is relatively close to the
transmit coil L1 and a relatively small field is used. As the receive coil
L2 is small, the presence of the small metal band in the field will induce
a substantial change in flux density through the coil L2. Also, as the
coil L2 is circular, the resolution of the device is improved as a maximum
signal is obtained when the band is adjacent to the center of the small
coil L2.
The specific circuit operation is as follows. The switch SW1 is closed to
turn on the instrument. Multivibrator U1 then produces an output which is
a squarewave having a duty cycle of slightly more than 50%. The frequency
of this signal is approximately 2 KHz for the preferred embodiment.
The multivibrator U1 is CMOS so as to minimize the current it draws (about
100 microamps). Accordingly, its output current drive capability is not
very high, and the emitter-followers, transistors Q1 and Q2, provide the
necessary current gain (of about one hundred in the preferred embodiment).
The interim circuit comprised of R3, R4 and C3 increases the rise and fall
time of the transistional edges of the squarewaves so as to reduce the
switching noise. Resistor R5 and capacitor C4 decouple the transistor Q1
from the positive supply.
The output signal from the emitters of transistors Q1 and Q2 drives the
coil circuit 52. Capacitor C7 a.c. couples the transistors' output (still
a squarewve) through resistor R7 and through the central coil L1 thereby
creating the magnetic field. Capacitor C6 of this circuit 52 damps some of
the ringing due to the length of the cable between the probe and the
detector instrument which houses the rest of the circuit 50 (except for
the coils L1, L2 and L3).
The two other coils L2 and L3 are connected to differential amplifier U2
which amplifies the difference between voltages across the two coils L2
and L3. The outputs from the coils L2, L3 are unequal even in a
no-metal-detected case due to the manufacturing tolerances of the coils of
the preferred embodiment. As R15/R11 determines the gain, the gain of the
preferred embodiment is 220. In the amplifier circuit, capacitor C14
defines the upper frequency of the bandpass of the amplifier U2.
As it is difficult to make coils L2 and L3 identical (as they are not,
there will always be a small voltage difference between them) variable
resistor R12 is used to correct out this error voltage so that the output
voltage of amplifier U2 is nulled to a minimum output signal in a
no-metal-detected condition. This can also be accomplished by mounting the
coils so as to permit adjusting of the field relative to the receive
coils. As shown in FIG. 3, for example, a transmit coil 80, receive coils
82, 84 are stationary, and mounted on hollow cylinder 86. Cylinder 86 is
internally screw-threaded, and a metal slug 88 is movably mounted therein.
The adjustment is made by inserting a screw driver through an opening 89
in the front end of the cylinder 86 and changing the axial position of the
slug 88 with respect to the coils 80, 82, 84.
The output voltage of U2, which is the amplified difference between the
coil voltages, is fed to the amplifier U3. Resistors R16 and R17 determine
the gain of this stage, which functions as a rectifier network. The diodes
D3 and D4 determine that the amplified output voltage from this stage is
always positive. The output signal from this stage is integrated by
capacitor C18, which has an additional discharge path through resistor R18
to reset more quickly the display lights when there is a reduced voltage
output from U3.
The output of amplifier U3 which is a positive rectified d.c. signal after
capacitor C18, is then fed to non-inverting amplifier U4. A negative d.c.
voltage from resistor R20, which is filtered by capacitor C19, is summed
with the amplifier U3 output before amplifier U4 giving a net voltage
close to zero volts for a no-metal-detected state. This summing serves to
eliminate any voltage due to the natural imbalance of the receive coils.
Resistors R22, R23 and R24 determine gain.
The output from amplifier U4 is fed to the display driver U5. Depending
upon the level of that output, one of the L.E.D.s. D5-D14 will illuminate.
These are set to light sequentially as the output from U4 increases from a
minimum to a maximum.
At the same time, the speaker circuit is activated, assuming a voltage
output from amplfier U4. This circuit is an R-C unijunction-type
oscillator which, as the output from U4 varies, so does the charging
current to capacitor C12 and the frequency of the tone of the speaker will
rise with increasing voltage signifying increasing proximity to the metal
band.
The low voltage battery circuit works as follows. As long as the battery
voltage is above the sum of the voltage drop across the resistor R30 plus
the zener diode voltage plus Vbe of Q4, the transistor Q4 stays on and Q5
is off. If the battery voltage drops, Q4 turns off and Q5 turns on,
lighting L.E.D. D15.
It should also be noted that V+ is supplied to the amplifiers in the
preferred embodiment by decoupling it from the positive supply by resistor
R6 and capacitor C5. Likewise, resistor R8 and capacitor C10 decouple the
V- from conventional voltage doubler circuit 54.
Other Embodiments
Referring to FIG. 5, another circuit for the instrument 30 is shown at 150.
An oscillator U6, having resistors R36, R37 and capacitor C22 as an input
stage, produces an alternating output current which is fed to a transmit
coil circuit 152. Oscillator U6 is an NE555, and resistors R36, R37 are
10K ohms and 330K ohms respectively. Capacitor C22 is 0.0018 microfarads.
Transmit coil circuit 152 comprises capacitor C23 in series with resistor
R38 and transmitting coil L1'. Oscillator U6, which draws about 10-12
milliamps, provides an output signal with sufficient output current
capability to drive transmitting coil L1' which produces a magnetic field
as in the preferred embodiment.
Also as in the preferred embodiment, two receiving coils L2' and L3' are
within the magnetic field when the circuit 152 is activated. Receiving
coils L2' and L3', however, are connected in the polarity shown so that
the net voltage output from the combination will be the difference in the
voltage dropped across each coil. This net voltage output is sent through
resistor R39 and capacitor C24 to the positive input of a non-inverting
amplifier U7. Capacitor C25 and resistor R40 are separately connected
between this input line and ground. The amplifier U7 is an LM747.
Resistors R39 and R40 are 10K ohms and 100K ohms respectively. Capacitors
C24 and C25 are 0.0068 microfarads and 0.015 microfarads.
The feedback loop for amplifier U7 has resistors R41 and R44, both being
100K ohms. Resistor R46 of 100 ohms is connected between the loop and
ground. Offset resistors R42, R43 are selected so that the output from
amplifier U7 will be zero if the voltage across the coils L2' and L3' are
equal. The sum of the resistors R42 and R43 is about 100K ohms for this
embodiment.
The a.c. output voltage (if any) from the amplifier U7 is fed through
capacitor C26 to the positive input for amplifier U8. As in the preferred
embodiment, the magnitude of this signal depends upon the amount of
difference in the voltages across the receiving coils, and that is
dependent upon any increase in flux density through coil L2' due to the
proximity of the metal band.
Amplifier U8 and its circuitry act as a precision rectifier. Diode D17 is
connected between the output and the negative input of amplfier U8.
Resistors R48 and R50 in series are connected between the negative input
of amplifier U8 and through diode D18 to the amplifier output. Diodes D17,
D18 are 1N4009. Resistors R48, R50 are 100K ohms 10K ohms respectively.
Also, resistors R47 and R49 are connected to ground as shown. These
resistors are 100K ohms and 330 ohms respectively.
The d.c. voltage output (if any) from the amplifier circuit is fed through
resistor R51 of 100 ohms to the display circuit 154. Display circuit 154
comprises four L.E.D. circuits, one each for L.E.D.s. D19, D20, D21 and
D22. Referring to the circuit for LED D19, the d.c. voltage from the
amplifier U8 is fed through resistor R52 to the base of transistor Q6,
thereby turning it on. A current flows through the transistor and D19 and
resistor R56 causing L.E.D. D19 to light. D20 lights in the same manner,
except that due to the presence of diode D23 in its L.E.D. circuit, the
voltage on the base of transistor Q7 must be somewhat higher to create a
current flow through diode D20. Diode 21 requires still more transistor
base voltage for Q8 as its circuit has two diodes in it, diodes D24 and
D23. Similarly, still more voltage is required for the three diode circuit
of L.E.D. D22. The net effect is that the L.E.D.s light in sequence, but
unlike the preferred embodiment, all will be on for a maximum voltage
condition (i.e., close proximity of the coil L2' to the metal band).
Transistors Q6-9 are all 2N5734. Resistor R52 is 1.8k ohms. Resistors R53
and R54 are both 1K ohms, and resistor R55 is 680 ohms. The diodes D23,
24, 25 are all 1N4009. Resistors R56, 57 are 470 ohms. Resistor R58 is 330
ohms and resistor R59 is 220 ohms. Filter capacitor C27 is 47 microfarads.
A circuit 156 comprised of resistor R60 in series with L.E.D. D26 and zener
diode 027 is connected between the +V and ground. While the +V level is
sufficient, L.E.D. D26 will light. Resistor R60 is 220 ohms and the zener
diode is a 4.7 volt diode.
Battery circuit 158 provides the +V voltage. Battery circuit 158 has a 9
volt battery B2, a switch SW2 and a filter capacitor C28 of 68
microfarads.
The -V voltage is provided by doubler circuit 160, which is similar to the
doubler circuit 54 of the preferred embodiment. Here, capacitor C29 is 22
microfarads and capacitor C30 is 47 microfarads. The diodes D28, D29 are
1N192.
Other uses and variations are possible. For example, in addition to use
with endotracheal tubes, the method and apparatus of this invention can
also be used with other medical devices, such as stomach tubes, tracheal
tubes, venus catheters, arterial catheters, surgical sponges, and other
types of catheters and devices, the position of which in tissue is
important. Further, other fields e.g., electrical fields with a dielectric
instead of a metal band on the tube, can be used in place of the magnetic
field and the metal band.
Other variations will occur to those skilled in the art.
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