 |
Description  |
|
|
The present invention relates to electrode-bearing catheters, and more
particularly to catheters of the type which are inserted along a blood
vessel in order to position its electrodes in the region of a patient's
heart. The electrodes may be used for sensing cardiac electrical signals,
applying electrical stimulation for diagnostic testing or the like, or
applying treatment signals, such as tissue ablation signals. The catheter
may include other structures, such as a lumen and a delivery system, for
applying light, thermal energy or chemical agents, or a sampling system
for sampling tissue, forming images of tissue or withdrawing a specimen of
the surrounding fluid.
To position such an electrode-bearing catheter at a desired site within the
patient's body, one or more catheter insertion and radiographic
visualization steps are usually required in the course of bringing the
catheter to its general target area. Once the catheter has been placed in
position, further control and interaction by the surgeon is generally
required to assure that the catheter is precisely positioned and properly
oriented to perform its intended functions.
For example, when the catheter is an RF tissue ablation or coagulation
treatment catheter, it may be necessary to perform various preliminary
electrical measurements or mapping operations to assure that the power
electrode has moved into contact with a specific tissue region that is to
be treated, such as a cardiac lesion, that is initiating arrhythmias.
Among the prior art known to applicant, one cardiac catheter of this type
is configured as a special purpose mapping catheter, and utilizes a
multi-electrode structure to generate characteristic tissue impedance
responses at different tissue sites, from which the relative position of
the catheter can be determined. Specifically, U.S. Pat. No. 4,911,174
shows such an electrode structure wherein the impedance is measured by
successive pairs of electrodes to detect when an electrode has advanced
beyond a blood/tissue interface. An abrupt shift in phase of the detected
impedance is noted at the electrode positioned by the interface. The
device of that patent appears to be intended for the very specific purpose
of determining the size of a patient's ventrical, or more precisely,
positioning a catheter having a plurality of surface ring electrodes in
the ventrical in such a way that the relative locations of the electrodes
are known and it becomes possible to map an impedance distribution from
which ventricle volume is derived.
Other forms of catheter impedance measurement are known, generally
involving the measurement of localized or whole body impedance paths for
the purpose of setting or controlling the level of power delivery of an RF
coagulation electrode during electrosurgery. To applicant's knowledge such
systems have no application to catheter positioning.
It is therefore desirable to develop more accurate ways of positioning or
orienting a catheter by the use of electrical signals detected at its
electrodes.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a positioning system
for a cardiac ablation catheter.
It is another object of the present invention to provide a catheter system
useful for mapping and ablation of tissue.
It is another object of the invention to provide an improved system for
monitoring electrical activity of cardiac tissue.
These and other desirable objects are achieved in accordance with the
present invention by providing a multi-electrode catheter having pairs of
electrodes, stimulating at least one pair of the electrodes to provide a
sensing signal responsive to regional tissue variations, and combining two
or more sensing signals. Preferably the combined sensing signal is a
differential signal developed from several sets of electrodes such that
the magnitude of the combined signal is null except when two sets of
electrodes reside in or near regions of differing tissue type. The circuit
connection is such that the signal vanishes when the electrodes all reside
in similar tissue environments. By forming a differential signal from
multiple sets of electrodes in this way, the invention eliminates the
thresholding, normalizing or averaging and other complex signal processing
operations formerly necessary to obtain a meaningful impedance
measurement. Furthermore, by selection of particular ones of the catheter
electrodes for polling, the precise orientation or position of the
catheter is determined.
In a preferred embodiment, the catheter is a cardiac ablation catheter
having a tip electrode for applying ablation energy to a tissue site.
Tissue impedance is measured by applying probe signals between the tip and
a downstream electrode, sensing the return current through two or more
intermediate ring electrodes located below the tip, and combining the
sensed return current to define a differential signal. So long as each
electrode resides in blood, the inter-electrode impedance path
characteristics of each electrode pair are similar, and after being
differentially combined with a gain factor to correct for electrode
geometry, they yield a null signal. Thus, the electrode is self-zeroing
without calibration in an external saline cell. However, when the catheter
resides in a blood vessel and the tip electrode contacts tissue, the
differential signal rises sharply. Similarly, should a proximal electrode
contact the vessel wall, a similarly discernible signal will occur, but of
reversed polarity. The differential output signal thus provides a
dependable indication that the catheter has assumed an effective position
for applying electrosurgical power.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be understood from the description of illustrative
embodiments below, taken together with the drawings, wherein
FIG. 1 is a perspective view of a multi-electrode catheter for the practice
of the present invention;
FIG. 2 and 3 show catheter signal coupling arrangement for the practice of
the present invention;
FIG. 2A shows a detail of electrode signal sampling circuitry;
FIG. 4 shows a representative catheter signal traces; and
FIG. 5 shows catheter signal processing elements.
DETAILED DESCRIPTION OF THE INVENTION
A multi-electrode mapping and ablation catheter 10 is shown in FIG. 1, and
is characterized by an elongated insulating body 8 having a rounded tip
electrode 12 mounted at its tip, and a plurality of axially-spaced ring
electrodes 13a, 13b . . . mounted along its surface. Each of the ring
electrodes has dimensions and an exposed surface area A.sub.r identical to
the others, and they are preferably equi-spaced along the body 8. The tip
electrode 12, which may also take other forms, such as a split wire, or
bipolar electrode, has a somewhat larger area A.sub.t. Catheter 10 is
mounted at the end of a flexible but axially incompressible tube 20 which
is used for manipulating and inserting the catheter along a vessel, and a
plurality of electrode signal leads 30 extend from the various electrodes,
through the catheter body 8 and tube 20 to a control circuit located
outside the body. Leads 30 allow each electrode to be separately connected
to the control circuit or they may be connected to sense the propagation
of the tip electrode signal.
For example, tip electrode 12 may be connected to a relatively strong
source of RF power which is adjustably controlled to perform tissue
ablation, or is operated at a lower level to provide a monitoring signal
for determining tissue impedance measurements. Ring electrodes 13a, 13b .
. . are connected to sensing and signal processing circuity for sensing
muscle discharge potentials and mapping the locations of cardiac lesions
or arrhythmia-generating nodes.
In a conventional mapping protocol, the catheter may be slowly advanced
along an intracardial vessel, and local arrhythmias stimulated by a pulse
signal emitted at the tip electrode may be detected and mapped by an
analysis of the signal as detected at each of the plurality of ring
electrodes. Once a tissue site responsible for initiating an arrhythmia is
mapped, the catheter is then repositioned to coagulate the arrhythmia site
by placing the tip in contact with the site and applying an RF signal of
effective tissue coagulating power thereto.
In using a multi-electrode mapping probe in this manner to first identify
and then coagulate unhealthy tissue, it is necessary that the tip
electrode 12 be contacting, or at least in very close proximity to the
target tissue site. This is because the region of effective RF power
delivery drops off sharply away from the immediate surface of the
treatment electrode. Moving tip 12 even a few millimeters away from a
tissue site can reduce the applied energy to an ineffective level, or can
cause coagulation of blood to occur in the vessel rather than ablation of
adjacent tissue.
In accordance with the present invention, the relative position of a probe
is determined with respect to surrounding tissue by applying a test
signal, for example, a continuous-wave, low power, AC signal, between a
pair of outer electrodes arrayed along the catheter, and pairing each
outer electrode with an inner electrode to develop a sensing signal
characteristic of impedance for the tissue between the electrodes. By
"outer" is meant an electrode which lies axially at one extreme of a set
of the electrodes. Thus, in FIG. 2, showing the tip electrode and
successive first, second and third ring electrodes, the tip electrode and
the third ring electrode are the outer electrodes of the set, and an AC
pulse signal is applied between these two electrodes to establish an
impedance chain or bridge extending through the tissue spanning all of the
intermediate electrodes. Thus, the tissue acts as an impedance dividing
bridge, allowing local impedance to be sensed between any pair of
intermediate electrodes.
As shown in FIG. 2, signal amplifiers 31, 32 are each connected to one pair
of electrodes to develop a signal that is essentially proportional to the
product of the electrode area times a function of the impedance of tissue
lying between the two electrodes of the pair. A normalizing gain 9 is
applied to one amplifier, preferably to amplifier 32, to correct by a
scale factor the contribution of the larger (tip) electrode. This gain is
a constant that, for a given general probe size and shape, depends only on
the relative area and spacing of electrode 12, and may be readily set, for
example, when the electrodes are immersed in a sample saline solution. The
gain factor normalizes the amplifier outputs, so that when both pairs of
electrodes are immersed in the same tissue, e.g., blood, the outputs of
the respective amplifiers will be equal. The outputs of both amplifiers
31, 32 are applied as the inputs to a final or second stage amplifier 40.
Amplifier 40 produces an output signal proportional to the difference in
the signal potentials appearing at its input terminals, so that it has a
net output only when the outputs of amplifiers 31, 32 differ, i.e., when
different tissue types are positioned near to the two different electrode
pairs, producing distinctly different sets of impedance paths.
Preferably, as shown in FIG. 2A, the apparatus further comprises a
multiplex switch 42a, 42b which operates, either with manual selection by
a user, or preferably as shown under program control by a state selector
43 to selectively connect the differential impedance sensing circuitry to
different pairs of the catheter electrodes 13i, 13k. By coupling the state
of the multiplex switch 42a, 42b to a suitable display, the system
indicates detection, for example, of tissue impedance changes that occur
within a region spanned by an arbitrary one of the sensing electrode
pairs, rather than simply the frontmost pair as described above for the
sensing electrode configuration of FIGS. 1 and 2.
Switching unit 42a, 42b may also be controlled to vary the spatial
resolution of the impedance detection circuitry, by selecting the sensing
signals across pairs of electrodes that are separated from each other by
one or more electrodes, or the signals from two pairs of electrodes
wherein the electrodes of one pair are spaced from the electrodes of the
other pair by one or more intervening ring electrodes 13.
An alternative construction (not illustrated) dispenses with the switching
unit 42a, 42b, and directly attaches each pair of adjacent electrodes to
its own amplifier, with each adjacent pair of amplifiers having its
outputs fixedly attached to a second stage amplifier. In that embodiment,
an output switching unit 42c may be employed to sample the outputs of
either the first stage or the second stage amplifier, rather than to
switch the electrodes between inputs of the first stage amplifiers. This
alternative construction, while lacking the flexibility to monitor an
impedance path extending between widely-separated ring electrodes, may
quickly identify the precise region of tissue change.
FIG. 3 shows an alternative circuit for the practice of the present
invention. The probe 10 is illustrated schematically, with only the tip
and three ring electrodes shown. The ring electrodes may, for example, be
the first, second and third rings, or first, third and fourth, or any
three rings spaced in order along the catheter. They are therefore simply
labeled A, B, C and D to indicate their order of appearance along the
catheter axis, for clarity of discussion. An RF generator 52 is
transformer coupled to the probe across the outer electrodes, to apply a
biologically safe probe signal at a level of approximately two volts RMS.
In this embodiment, a first pair of electrodes, A and B, are connected to
a current transformer winding S.sub.1, and a second pair of electrodes, C
and D, are connected to a separate winding S.sub.2 in the opposite sense.
A common primary winding or series connected pair of windings, denoted P
in FIG. 3, is magnetically coupled via transformer core C to the electrode
sensing windings S.sub.1 and S.sub.2 so that the winding P develops a
signal which is essentially a phase-delayed multiple of the difference in
the electrical signal sensed by the two electrode pairs. The sensing
windings S.sub.i for all adjacent ring electrodes 13.sub.i, 13.sub.i+1,
are all similar, but the sensing winding S.sub.t attached to tip electrode
12 preferably is wound such that the ratio of the number of turns in the
winding S.sub.t to those in winding S.sub.i is inversely proportional to
the effective areas of the respective electrode pairs, times a distance
factor reflecting the geometry and spacing of the irregularly spaced tip
electrode from the ring electrode with which it is paired. The winding
ratio is selected so that the sum of the sense and anti-sense impedance
signals in coils S.sub.t, S.sub.i is nominally zero when both pairs of
electrodes are fully immersed in blood.
As shown, transformer winding P is connected to a discrimination/control
circuit T, which may, for example, integrate the magnitude of the signal
detected across winding P and put out a trigger enable signal when the
integrated value exceeds a predetermined threshold indicative of electrode
tissue contact. Alternatively, circuit T may put out a normalized sense
impedance value, which may serve as a basis for determining the relative
disposition of the catheter or the type of tissue which surrounds or
contacts the catheter. It will be understood that the circuit of FIG. 2A
may also be employed in a transformer coupled circuit of this type. In
that case, it is preferable to have the multiplex switching units
selectively connect the catheter electrode pairs to the transformer
sensing coils in an arrangement whereby plural different electrodes are
selectively connected to a single pair of windings of one coupling
transformer.
FIG. 4 shows typical signal traces of a signal when all of the sampled
electrodes are disposed in a homogeneous medium, as detected between
electrodes A, B (trace A of the Figure), and electrodes C, D (trace B of
the Figure). Trace C shows the signal of trace A normalized by a constant
scale factor to correct for the different area and spacing of tip
electrode A, and trace D shows the combined signal, as appearing, for
example across the winding P of the transformer in FIG. 3 or the output
stage amplifier 40 of FIG. 2.
Traces E-H of FIG. 4 show the signals corresponding to those of traces A-D,
respectively, when the catheter has moved to a position such that the body
of the catheter resides in blood, but tip electrode 12 (FIG. 1) has been
brought into close proximity or contact with a vessel wall or heart muscle
tissue. As illustrated, the combined output signal (trace D or H) becomes
non-null and develops a discernible peak, upon contact of the tip with
heterogeneous tissue. This peak is more or less well-defined, depending
upon the degree of proximity to, or area of contact between, electrode 12
and the surrounding tissue structures.
It will be understood that the non-zero differential sensing peak need not
be a positive-valued signal as shown, but may include negative dips or
have some other shape or polarity, caused by phase inversion or the like
which occurs as a consequence of the locally varying tissue structures.
Further, it will be understood that the sensed signals need not be fully
representative of the tissue impedance function, but rather are sensed
signals from which impedance is derivable. For example, when impedance
itself is to be monitored in order to determine a substantive physical
property of the surrounding tissue, e.g., the type of tissue or its degree
of coagulation, the instantaneous voltage and current of an electrode pair
may be sampled, processed and compared to an RF probe signal that has been
applied by signal generator 50 (FIG. 1), to provide an actual impedance
function. In general, however, the benefits of the invention are achieved
in simply detecting the magnitude of a well-defined difference signal,
lines F-H, when it is only desired, for example, to confirm that tissue
contact by the probe tip has occurred.
FIG. 5 is a schematic block diagram for the operation of a catheter
positioning system 100 for controllably positioning an ablation catheter
in the cardiac region. As shown in FIG. 5, a processor 120 is
interconnected with the sensing and treatment electrodes 12, 13i of
catheter 10 via a switching unit 130 comparable to switching unit 42a, 42b
of FIG. 2A, connected to a signal conditioning circuit 135. Processor 120
controls the switching unit to select spaced electrodes, preferably two
pairs as described above, selected from three or more electrodes, for
connection to the signal conditioning circuit, and then receives and
evaluates the output of the signal conditioning circuit 135 to detect
either the presence of a tissue impedance change at a particular selected
electrode, or the value of the local tissue impedance. Signal conditioning
circuit 135 may include an arrangement of differential amplifiers as
described above, and indicated schematically in the Figure. Alternatively,
circuit 135 may include circuit elements only for filtering and amplifying
the inter-electrode signals. In that case, the outputs of circuit 135 may
be digitized and the microprocessor may digitally combine the output
signals to obtain a differential impedance function.
In a variation as described above, an alternative embodiment may have its
electrodes fixedly connected to an array of differential signal
amplifiers. In that embodiment, the outputs of the signal conditioning
amplifiers are switchably sampled via an n:4 switching unit under the
control of the processor 120 and similar to unit 130, but located between
the signal conditioner 135 and processor 120.
In either case, processor 120 correlates the detected electrode signals
with particular ones of the electrodes, preferably by polling them in an
ordered sequence, and drives and synchronizes a display 140 to represent
the detected tissue characteristics. Display 140 may, for example, depict
the probe with a variable marker--such as a LED bar display or a plurality
of discrete LEDs mounted on a graphic representation of the catheter. The
LEDS are actuated by the processor to display the processed information.
Specifically, display 140 may show the site along the probe at which the
differential impedance change has been detected, by illuminating a LED at
that location; or may show which electrodes are in contact with tissue by
illuminating a special colored light at the corresponding electrode
location. It may also show information such as displaying the relative
portion of the probe which has been inserted into the heart. This may be
done by selectively illuminating LED markers along that portion of the
displayed image of the probe corresponding to one or more detected
impedances or signal conditions which indicate that a longitudinally
extending region of the probe body has passed a heart valve, or resides
inside a closed tissue structure.
Processor 120, in addition to executing one or more logical programs to
determine and display probe disposition corresponding to the detected
differential electrode signals, preferably is part of a cardiac ablation
or electrosurgery control system. In that case the processor also provides
a signal along line 142 to the RF power control unit 45 of the system, for
controlling the level of power applied to the probe tip. Some forms of
coagulation or ablation power control are conventional, for example,
programmed control to apply a sufficient power level, based on sensed
whole body tissue impedance, to destroy tissue locally at the tip
electrode without causing damage to non-involved tissue. In accordance
with the coagulation control aspect of the present invention, the
processor detects the electrode disposition, and provides a signal to
power control unit 45 to adjust the level of localized power based further
on the area of contact of the tip electrode, which is derived, by
microprocessor 120, from the magnitude of the differential impedance
signal. Thus, for example, when the tip achieves only partial contact, as
indicated by a detected charge curve that has a sharply defined shape but
still has a relatively high impedance value, the ablation power may be
reduced from the conventionally-selected level, to a lower level that
limits the amount of ablation energy applied to the contact point. Thus,
rather than allowing coagulation to extend into the bloodstream as could
occur if the power were delivered based on the assumption of full
electrode contact to the vessel wall tissue, a smaller pulse of energy is
applied to affect only the smaller area of tissue actually contacting the
electrode.
Other variations and modifications adapting the differential signal sensing
electrodes and system of the present invention to known cardiac mapping or
ablation systems and probes will occur to those skilled in the art, and
all such variations and modifications are considered to be within the
spirit and scope of the invention to which patent rights are sought, as
set forth in the claims appended hereto.
* * * * *
|
|
 |
|
 |
Description |
|
