Note: Descriptions are shown in the official language in which they were submitted.
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T' e: STEERABLE CATHETER WITH ROTATABLE TIP
ELECTRODE AND METHOD OF USE
FIELD OF THE INVENTION
This invention relates to an improved catheter which can be
used both for applying a radio frequency (RF) ablation current to a body
part, and for electrogram recording. The invention also relates to a
method of applying an RF ablation current and a method of electrogram
recording. An important application of the invention is in the field of
cardiology.
BACKGROUND OF THE INVENTION
A healthy heart is normally caused to contract and relax in an
orderly fashion by a spreading wave of electrical excitation originating
from the sinoatrial (SA) node in the right upper atrium. The wave
initiated at the SA node spreads over cardiac fibers until it reaches the
atrioventricular (AV) node. The AV node then relays the wave over
specialized cardiac fibers known as the bundle of His, to the ventricles.
The cardiac fibers over which the impulses are conducted have a refractory
period, so that once stimulated they cannot be restimulated for a short
time period. This normally serves as a protective mechanism.
However some people are born with an accessory pathway of
cardiac fibers extending from the ventricle near the area of the AV node
back to the atrium. The accessory pathway allows the excitation wave
from the AV node to retrograde or travel back to the atrium.
In some cases, if the retrograde wave reaches the atrium just
after the end of a refractory period, it can then travel back to the AV node,
stimulating the AV node prematurely and producing an oscillatory loop.
Various other mechanisms, e.g. partial damage to atrial or ventricular
heart muscle, can also result in an oscillatory loop. The oscillatory loop
causes abnormally rapid heart action (tachycardia). This is usually self
limiting, but in cases where it is not, it may be fatal. Therefore the
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condition requires treatment.
Tachycardia and other arrhythmias have sometimes been
treated with medication. However the medication is not always effective
and may have serious side effects. .
A second method of treating the condition has been open
heart surgery, to cut the tissue (e.g. the accessory pathway) which forms
part of the feedback loop, thus opening the feedback loop. However open
heart surgery is a serious and costly operation.
Therefore, for about the last fifteen years cardiologists have
attempted to deal with the condition by inserting catheters containing
electrodes into the interior of the heart. They have attempted to locate the
accessory pathway or other tissue in question and then to apply RF energy
to ablate the tissue by coagulation. The catheters are pushable and
steerable, and are guided to the approximate location by x-rays for general
guidance, and then by the use of electrograms to the exact location for fine
localization. For example, the fibers known as the bundle of His,
emanating from the AV node, are close to the accessory pathway, so
cardiologists often look for electrograms with His activity to determine
that the catheter is close to the accessory pathway.
Numerous catheters have been designed to perform the
above functions. Examples are shown in U.S. patent 5,242,441 to Avitall,
U.S. patent 5,125,896 to Hojeibane, and U.S. patent 5,190,050 to Nitzsche.
Various designs compete on the basis of which is more easily steerable.
In an article by L.T. Blouin and F.I. Marcus, in Pace, Vol. 12,
January, 1989, part 2, pages 136 to 143, it was disclosed that by insulating
part of the tip of the catheter, and applying the bare (conductive) portion of
the tip against dog ventricular muscle in vitro, larger lesions could be
produced with lower RF power. However so far as is known, this
technique has not been used to date in any practical catheters in animal or '
human subjects.
Regardless of which catheter is used to date, a serious
problem has existed and currently exists in determining whether the
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catheter is in the right location to form an ablation. It is common for the
surgeon to guide an ablation catheter to a location which is believed to be
correct, then to apply RF power to create an ablation, and then to observe
that nothing has happened. In that case the surgeon then moves the
catheter elsewhere and tries again. In many of these cases, the catheter
may in fact have been properly positioned, but because most of the tip of
the catheter is bathed in blood rather than lying against the tissue, and
since blood has a lower electrical impedance than the tissue, the RF power
has effectively been short circuited through the blood and an adequate
ablation has not been formed. Problems of this kind can greatly increase
the time required for catheterization procedures and can decrease the
likelihood of a successful result.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention in
one of its aspects to provide an improved catheter which can more readily
be guided so that its tip can be placed in the desired location. In one aspect
the invention provides a catheter comprising:
(a} a handle,
(b) a longitudinally extended shaft connected to said
handle,
{c) a distal end connected to said shaft, said distal end
being bendable with respect to said shaft,
(d} a tip electrode mounted on said distal end for axial
rotation thereon, and at least one ring electrode
mounted on said distal end adjacent to said tip,
(e) said tip electrode having a conductive portion and
an insulated portion,
(f) a first control extending from said handle through
said shaft and connected to said distal end and being
operable for bending said distal end relative to said
shaft,
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(g) a second control extending from said handle
through said shaft to said tip electrode and being
operable for axially rotating said tip electrode, so
that said conductive portion of said tip electrode ,
may be placed against tissue and said insulated
portion may be oriented to face a patient's
bloodstream.
In another aspect the invention provides a method of
positioning a catheter in a desired manner over selected tissue within a
patient, said catheter having a handle, a longitudinally elongated shaft
connected to said handle, a bendable distal end connected to said shaft, a
first control connected to said distal end for bending said distal end
relative
to said shaft to steer said catheter, an axially rotatable electrode on said
distal end, and a second control connected to said electrode for axially
rotating said electrode, said electrode having a conductive portion and an
insulated portion, said method comprising:
(h) steering said catheter so that said electrode is
positioned over said selected tissue,
(i) operating said second control to axially rotate said
electrode while said electrode is positioned over
said selected tissue,
(j) monitoring the impedance between said electrode
and a reference level as said electrode is axially
rotated,
(k) determining when said impedance reaches a peak
value, said peak value being indicative that said
conductive portion of said electrode is lying against
said tissue, and;
(1) thereby positioning said electrode with said
conductive portion lying against said tissue.
Further objects and advantages of the invention will
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appear from the following description, taken together with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is a top plan view of a catheter according to the
invention;
Fig. 2 is a side view of the catheter of Fig. 1;
Fig. 3 is a side view of a portion of the distal end of the
catheter of Fig. 1;
Fig. 4 is a view similar to that of Fig. 3 but with the pull
wires included;
Fig. 5 is a top plan view of the portion of Fig. 4;
Fig. 6 is an enlarged view of the tip of the catheter of Fig.
1;
Fig. 7 is a side sectional view of the distal end of the
catheter of Fig. 1;
Fig. 8 is a top plan sectional view of the distal end of the
catheter of Fig. 1;
Fig. 8A shows a modification of the arrangement of Figs.
7 and 8;
Fig. 9 is a cross-sectional view of the distal end of the
catheter of Fig. 1, taken along lines 9-9 of Fig. 8;
Fig. 10 is a cross-sectional view of the catheter of Fig. 1,
taken along lines 10-10 of Fig. 8;
Fig. 11 is a cross-sectional view of the shaft of the catheter
of Fig. 1;
Fig. 12 is a diagrammatic top plan view, partly in section,
of the handle for the catheter of Fig. 1;
Fig. 13 is a side view, partly in section, of the handle of
Fig. l2;
Fig. 14 is a diagrammatic view of a computer display and
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control for the catheter of Fig. 1;
Fig. 15 is a graph showing variation of impedance versus
rotation angle of the catheter tip;
Fig. 16 is a top plan view of a modified handle for the .
catheter of Fig 1;
Fig. 17 is a side sectional view of the handle of Fig. 16;
Fig. 18 shows a visual control display for the catheter of
Fig. l;
Fig. 19 is a side view of a modified ring electrode for the
catheter of Fig. 1;
Fig. 20 is a plot showing variation of impedance with
heartbeat; and
Fig. 21 is a diagrammatic view of a modification of the
computer display and control shown in Fig. 14.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Organization of Catheter
An ablation catheter according to the invention is shown
generally at 10 in Figs. 1 and 2. Catheter 10 has three sections, namely a
control handle 12, a shaft 14, and a distal or free end 16. The shaft 14 may
typically be about 100 cm long and the distal end 16 may typically be about 8
cm long.
The distal end 16 typically contains four platinum-
iridium alloy electrodes, namely a tip electrode 18 (which may be e.g. 4
mm long) and three ring electrodes 20. The ring electrodes are typically 1.5
mm long, with 2.5 mm spacing between their edges. However all
dimensions mentioned can be varied as required.
Both the shaft 14 and the distal end 16 typically have an
outside diameter of 7 French (2.34 mm) so that they can be guided through '
blood vessels into the heart.
The handle 12 contains a J-bending control knob 22 and
an axial rotation control knob 24. Using knob 22 the distal end 16 can be
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deflected laterally into a J-shape (as shown in dotted lines in Fig. 1) to
assist
in steering the catheter toward a desired location. As will be explained, a
portion of the tip 18 of the catheter is insulated, and the axial rotation
control knob 24 is used to rotate the tip, to bring the bare (uninsulated)
portion of the tip against the heart wall and the insulation toward the
bloodstream once a desired location has been reached.
Distal Catheter End 16
The catheter distal end 16 is shown in more detail in Figs.
3 to 10 inclusive. Reference is first made to Figs. 3 to 5, which show that
the distal end 16 includes an assembly of three very thin planar stainless
steel ribbons 30, 32, 34. Middle ribbon 32 is larger than ribbons 30, 34 and
thus has a projecting proximal end 36 and a projecting distal end 38.
Central ribbon 32 is also welded on each side to the proximal ends of upper
ribbon 30 and lower ribbon 34, the weld locations being indicated at 40.
The ribbons are not connected to each other except at the welds 40, so that
they may slide at their distal ends. The ribbons thus determine a preferred
bending direction (in a plane orthogonal to their flat surfaces) for the
distal
end 16 (i.e. J-bending).
The projecting proximal end 36 of the middle ribbon 32
narrows into an elongated tab 42 {Fig. 5) which inserts into a stainless steel
coil 44 (Figs. 7 and 8).
Three retainer rings 46, 48, 50 are attached (welded) to the
ribbon assembly. Rings 46, 48 are attached to the upper ribbon 30 while
ring 50 is attached to the projecting distal end 38 of middle ribbon 32. The
rings 46, 48, 50 form a guide channel for a stainless steel torque wire 52
(Figs. 7 to 10) which is coated with polytetrafluoroethylene (PTFE). The
use of the rings results in minimal friction and column stiffness and yet
allows free rotation of wire 52 as torque is applied to it (as will be
described)
to produce axial tip rotation. A length of polyimide or similar tubing 51
{Fig. 8A), attached e.g. by glue to upper ribbon 30, can be used in place of
rings 46, 48, 50 to provide similar benefits. (In Fig. SA primed reference
numerals indicate parts corresponding to those of Figs. 7 and 8.)
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_g_
Torque wire 52 also serves as an electrical connection to
the tip electrode 18. .
The extent of tip rotation is made sufficient to bring the
insulated portion (to be described) of the tip 18 to face the blood and then
slightly beyond, regardless of the starting position of the tip and its
direction of rotation. Preferably there is a range of axial rotation of ~300.
Reference is next made to Figs. 6 to 8 inclusive. As
shown in Fig. 6, the tip electrode 18 is partially covered with an
electrically
insulating coating 56 {typically a polyimide coating), which coats more
than 50% of the electrode surface. This leaves a single rectangular bare
area 58, whose length is the length of the tip electrode (typically 4 mm) and
whose width can be described in cross section by the extent of the arc in a
direction circumferentially of the bare portion. The arc describing the
lateral edges of the bare portion 58 will be less than 180 and is typically
between 90~ and 120. However it can be less than 90~. The shape of the
bare area may be modified, e.g. hourglass instead of rectangular.
The tip 18 is generally bullet shaped but at its proximal
end includes a reduced diameter proximally extending portion 60 {Figs. 7,
8) having a circumferential groove 62 therein. The groove 62 retains a
sealing ring 64 made of silicone. An outer silicone sleeve 66, which forms
a jacket for the catheter distal end, is pushed over sealing ring 64 to abut
the proximal end of tip 18 and is then glued in position to provide a fluid
tight seal against the tip 18.
The tip 18 also includes a central channel 68 in which the
torque wire 52 is firmly soldered. Central channel 68 also houses a
thermocouple heat sensor 72 (Fig. 7) , from which lead wires 73 protrude.
Alternatively, a thermistor heat sensor could be used.
The ring electrodes 20 (not shown in Figs. 7 and 8 for
simplicity) are glued to the outer silicone sleeve 66, and indent the sleeve
so that their outer surfaces form a relatively smooth surface with that of
the outer sleeve. Before gluing, ring electrode wires 74 (Fig. 9) which have
been threaded through sleeve 66 and out appropriately positioned holes
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f I. r ., I f f f
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9 ,..
(not shown) in the sleeve are soldered to the ring electrodes. As the ring
electrodes 20 are then moved into position over the sleeve 66, the wires 74
are pulled from the handle end of the sleeve leaving some slack in the
wires 74 (as in thermocouple wires 73) to allow for bending and rotation.
This operation is of course performed before the tip electrode 18 is
attached.
Four stainless steel PTFE coated pull wires 76, 78, 80, 82
are provided, each terminating in an enlarged disk 76a, 78a, 80a, 82a. The
enlarged disks are welded to the projecting distal end 38 of the middle
stainless steel ribbon 32. Two pull wires are located on each side of the
ribbon assembly as best shown in cross section in Fig. 9. The pull wires 76-
82 serve to bend the assembly of ribbon 30, 32, 34, and hence the catheter
distal end 16, in either of~its two preferred directions for bending. It will
be
apparent that the bending will be in a J-shape.
Although not specifically shown in the drawings, torque
wire 52 tapers in diameter at its distal end to provide increased flexibility
to
permit easier lateral deflection of the distal end 16 of the catheter.
As best shown in Figs. 7, 8 and 10, the pull wires 76-82, the
tab section 42 of middle stainless steel ribbon 32, and the torque wire 52 are
all inserted into the end of the stainless steel coil 44.
As shown in Fig. 9, the pull wires 76-82, the ribbon
assembly 30, 32, 34 and the torque wire 52 are held together adjacent the
catheter distal end 16 with three layers of material (which for clarity are
not shown in Figs. 7 and 8). These layers consist of an inner layer of PTFE
shrink tubing 86, a middle wrapping 88 of aramid fiber to provide strength
against tearing during bending, and an outer layer of PTFE shrink tubing
90. The three layer covering extends distally about 0.025 inch (0.635 mm)
beyond the ribbon assembly 30, 32, 34 and extends proximally to butt
against the end of the stainless steel coil 44.
Located between the outer PTFE shrink tubing 90 and the
silicone outer sleeve 66 are the thermocouple wires 73 and ring electrode
wires 74 (Fig. 9).
AMENDED SHED
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A polyimide sleeve 92, shown in Figs. 7 and 8 and in cross
section in Fig. 10, bridges the junction of the catheter shaft 14 with the
distal end 16, for reinforcing purposes. The sleeve 92 extends over the
proximal end of the three layers 86, 88, 90 (which as mentioned are for
clarity not shown in Figs. 7, 8). As shown in Figs. 7 and 8, the proximal
end 94 of thin silicone sleeve 66 extends over a recessed area 95 of the
catheter outer shaft wall 96 (which wall is described below).
Catheter Shaft
The catheter shaft 14 is shown in cross section in Figs. 10
and 11. Fig. 10, which is a cross section close to the junction of the shaft
14
with the catheter distal end 16, shows the tab section 42 of the middle
stainless steel ribbon 32 inside the stainless steel coil 44.
The outer shaft wall 96 (Figs. 10 and 11) is constructed of a
stainless steel braid coated with a polyether block amide (PEBA)/nylon
compound. This construction confers pushability and column stiffness to
the shaft 14, while still allowing shaft flexibility. Wall 96 is fixed to the
handle 12 and extends to a position over sleeve 92 (Fig. 7), to abut and
support proximal end 94 of silicone sleeve 66 as described above.
Stainless steel coil 44 runs from inside the catheter
handle 12, where it is rigidly fixed, along the entire length of the shaft
(within wall 96) and slightly into the catheter distal end 16 (Figs. 7 and 8).
The distal end of the stainless steel coil 44 is the point against which the
pull wires 76-82 produce the lateral bending of the catheter distal end 16.
As shown in- Fig. 10, thermocouple wires 73 and ring
electrode wires 74 are located between the stainless steel coil 44 and the
outer shaft wall 96. The four pull wires 76-82 and the torque wire 52 are as
mentioned contained within the coil 44.
Control Handle
The details of the control handle 12 are shown in Figs. 12
and 13. As shown, the J-bending control knob 22 includes a fluted or
ridged shaft 98 rotatably mounted on handle wall 100 and having an
interior shaft portion 102. The pull wires 76-82 are secured as shown to
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projections 104 from the interior shaft portion 102, so that turning of the
knob 22 produces equal winding and unwinding of pull wire pairs. The
resultant J-bending of the catheter distal end 16 is maintained with a
locking lever 106. Lever 106 is adapted to slide forwardly in slots 107, 108
in the handle wall 100, to engage with the fluted or ridged control knob
shaft 98. Locking lever 106 is then locked in position in the laterally
extending portion 109 of slots 107 (which is L-shaped) to maintain the
selected bend.
A wire bundle 110, consisting of the thermocouple wires
73 and ring electrode wires 74, extends through the handle 12 and
terminates in a male electrical connector 112 which may be connected by
an appropriate female conductor and cable 113 to a computer based
instrument 114 (Fig. 14). Torque wire 52 passes through an axial opening
115 in the acrylic (non-electrically conductive) shaft 116 of axial rotation
control knob 24. Torque wire 52 is fixed to shaft 116 by a set screw 116a.
After passing through shaft 116, torque wire 52 terminates
in a metal pin 117 which rotates within a fixed socket 118 which in turn is
attached to the electrical connector 112. Electrical continuity of torque wire
52 is preserved in this manner, but other suitable methods of providing a
rotating electrical connection may be provided. Ablation current (typically
500 KHz) when needed is supplied, under instrument 114 control, from
radio frequency (RF) section 119 and through switch 119a to wire 52 and
hence to tip electrode 18.
It will be seen from the above that whether the catheter
distal end 16 is bent into a j-shape or is straight, the tip electrode 18 can
be
rotated through a considerable angle, essentially without altering the
location of the tip electrode. In other words, if the tip electrode 18 is over
particular tissue section, it will normally remain there even when the tip
electrode 18 is rotated. The rotation does twist silicone sleeve 66, which is
flexible and resilient for this purpose.
The Com~uter_ Instrument 114
Instrument 114 contains, in addition to the RF section
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119, an impedance section 200. Impedance section 200 is conventional and
generates a 50 KHz excitation current required to measure the impedance.
It also produces an impedance measurement which is transmitted to a
computer section 146 containing a CPU (not shown). Switch 119a allows .
the user to add RF current to the tip electrode to begin lesion making.
However, impedance monitoring continues during application of the RF
current, because a high frequency blocking filter 202 is located after the
impedance section to filter out the RF current.
Instrument 114 also includes a standard electrogram
amplifier 204 which is connected through a high frequency blocking filter
206 directly to leads 52, 74. Amplifier 204 receives the electrogram signals
(whose frequency spectrum is all under 1 KHz, and mostly under 500 Hz),
and amplifies these signals, and transmits them to the computer section
146 for recording and display. Filter 206 removes the higher frequencies of
the impedance current and the ablation current. Thus the electronic
signals and the impedance can be, and preferably are, monitored
simultaneously, e.g. on monitor 147. (Note that the impedance section 200
need not be protected from the electrogram signals because their upper
frequency limit (1 KHz) is appreciably lower than the 50 KHz used by the
impedance section.)
Finally, instrument 114 includes a conventional
thermocouple section 208 connected to thermocouple leads 73. Section 208
receives the thermocouple current, amplifies it, and transmits an
appropriate signal to computer section 146 for recording and display.
The use of the catheter 10 will now be described.
Use
Reference is made to Fig. 15, which shows a curve 120 of
impedance (on the vertical axis) versus tip angle rotation (on the
horizontal axis), for various angles of rotation of the catheter tip electrode
18. Shown above the plot in Fig. 15 are representations 121a to 121d of the
catheter tip electrode 18 showing the bare (uninsulated) portion 58 of the
tip in various positions with respect to the myocardial wall 122. The
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impedance shown is that between a reference electrode (not shown)
connected to the patient and the wire 52 connected to the tip electrode 18.
As shown, when the uninsulated portion 58 faces the
bloodstream (representations 121a and 121d), the impedance shown at 124,
125 is relatively low since blood is more conductive than myocardium.
When the bare portion 58 is fully shielded from the bloodstream by lying
directly against the myocardial wall 122 (representation 121c), the
impedance reaches a peak as indicated at 126. Therefore the rotational
position of the bare portion 58 of the catheter can be readily determined.
When the impedance reaches a peak, then (assuming the catheter has been
properly positioned over the arrhythmic pathway), RF energy can be
efficiently and reliably applied to form the desired lesion.
In use, the impedance between catheter tip electrode 18
and the reference electrode (not shown) can be continually read on a
graphic display such as that shown in Fig. 15. When the catheter has
advanced to the desired location, as determined by electrograms picked up
by ring electrodes 20 or tip electrode 18, the physician rotates the catheter
tip using knob 24, meanwhile watching the display of Fig. 15, until the
impedance has reached a peak. Impedance can also be displayed
numerically or as a rising bar graph or other increasing graphical display.
It can also be input to an audio source whose frequency increases as
impedance increases, or can be displayed in any other desired manner. ItF
power is applied to the tip electrode 18 when impedance is greatest.
If desired, potentiometers may be connected to knobs 22,
24 as shown in Figs. 16, 17, where double primed reference numerals
indicate parts corresponding to those of Figs. 1 to 13. In Figs. 16, 17 a spur
gear 130 is formed on the end of shaft 116" of the axial rotation knob 24".
Gear 130 drives a gear 132 on the shaft of potentiometer 134 mounted on
wall 100". Potentiometer 134 has two leads 136 connected to connector
112".
Similarly, a semi-circular ring gear 138 is formed on
interior shaft portion 102" of J-bending central knob 22". Gear 138 drives a
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bevel gear 140 connected to the shaft of a second potentiometer 142 having
leads 143 connected to connector 112". The resistances of the
potentiometers 134, 142 will indicate the axial rotation position of tip
electrode 18 and the degree of J-bending of distal catheter end 16,
respectively.
It will be realized that the relationship between the
position of knob 24 and rotation of the tip electrode is not linear because
friction on torque wire 52 within the distal catheter end 16 increases as J-
bending increases. For example when the catheter distal end 16 is straight
(no J-bending), one unit of rotation of knob 24 produces approximately one
unit of axial rotation of the tip electrode 18. However when the catheter
distal end is bent into a J-shape, there are significant torque losses and it
may take (for example) two units of rotation of knob 24 to produce one
unit of tip rotation. The actual relationships can be determined after the
catheter has been manufactured, and can then be provided in the form of a
look-up table in the read-only memory (ROM) 144 of the computer section
146 of the computer based instrument 114. The look-up table provides to
the CPU in the computer section 146 the data representative of the actual
degree of axial rotation, so that true tip rotation may be displayed
graphically and/or numerically on a monitor 147.
Several forms of display are possible, e.g. on the monitor
147. For example, as shown in Fig. 18, the tip electrode 18 may be displayed
in cross-section as icon 150 with an arrow 152 indicating the number of
degrees of rotation of the uninsulated portion 58 from a reference
(unrotated) position 154 marked as 0°. The tip can as mentioned be
rotated
in either direction from the 0° position.
Above or beside the icon 150 for the tip rotation is a
display 160 showing impedance. The impedance display consists of a
shaded bar 162 which rises and falls as the impedance varies, with an ohm '
scale beside it on the vertical axis, and with the numeric value of the
impedance at any given time displayed at 164. Thus, the operator of the
catheter can see exactly how far the tip electrode 18 has been rotated from
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its 0° or reference position, and at the same time can see from display
160
(by rotating the tip electrode 18 back and forth) when the impedance
reaches a peak.
The graphic display of Fig. 18 can also include a display
168 showing the amount of J-bending. This is indicated by an icon 170
representing the catheter distal end 16 and showing the amount of J-
bending in the distal end 16. The amount of J-bending to the left or right is
reproduced in the icon, with the alphanumeric value and the direction of
bending displayed at 172.
An alternative or additional form of display for the
rotational position of the tip electrode 18 is shown as icon 174 in Fig. 18.
The icon 174 is simply one of the displays 121a to 121d from Fig. 15 and
displays the rotational relationship of the tip 18 relative to the tissue wall
122. As the impedance rises or falls, the displayed position of the tip 18
rotates. Such display can be generated by the computer instrument 114,
which can be programmed to rotate the representation of tip 18 dependent
on whether the impedance (indicated by graph 120) is at a peak or valley,
or between those two extremes. The orientation of the insulated portion
56 (whether it is at the top or bottom of the drawing) can be determined by
ZO the program from whether the impedance rises or falls when the tip is
rotated clockwise or counterclockwise.
While the impedance is normally measured between the
tip electrode 18 and a reference electrode connected elsewhere to the
patient, the impedance can if desired be measured between the tip
electrode 18 and any of the ring electrodes 20. However it is preferred to
use a larger reference electrode located elsewhere on the patient for more
consistent results.
While the catheter described is particularly useful for
ablation purposes, it may also be used simply for monitoring purposes, to
detect signals from any of its electrodes (to record electrograms), i.e. it
may
be used as a diagnostic catheter. Diagnostic catheters are normally identical
to ablation catheters except that the tip electrode 18 is shorter (e.g. only 2
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mm long) in a diagnostic catheter. There may also be more than four
electrodes. In diagnostic mode electrograms may be recorded between the
tip electrode 18 and a reference electrode, or between the tip electrode 18
and any of the ring electrodes 20, or between any ring electrode and any
other ring electrode or a reference electrode.
Further, if desired one or all of the ring electrodes 20 may
be partially insulated, such insulation being shown at 176 in Fig. 19. The
arcuate extent of the uninsulated portion 178 may have the same range as
the uninsulated portion 58 of the catheter tip electrode 18.
In use of this last version, as the catheter is being
positioned a four channel read-out can be displayed on a multichannel
monitor or on monitor 147. If one of the ring electrodes 20, for example,
begins picking up an electrogram feature of interest, indicating that such
ring electrode is over a desired site, then the catheter as a whole, or simply
that ring electrode, can be switched to impedance mode. Alternatively as
mentioned, by use of the electronic filtering described, impedance and
electrograms can be monitored simultaneously. Since as mentioned the
frequency spectrum for the electrogram is under 1 KHz, while the
impedance excitation current is 50 KHz, filtering suffices in each section to
remove unwanted signal. In impedance mode, the impedance is read
between the catheter electrode in question and the reference electrode (or
between each catheter electrode and the reference electrode if all four
channels are switched to impedance mode).
When impedance mode is activated, the impedance
between the selected ring and the reference electrode is displayed, or four
channels of impedance can be displayed, between each catheter electrode
and the reference electrode. The catheter distal end 16 is then rotated to
bring the uninsulated portion 178 of the ring in question as fully as
possible against the tissue wall, at which time the impedance will be a
maximum and the signal being picked up by the ring in question will be
optimized in size and quality, i.e. more detail will be visible in the wave
form of the electrogram signal. The increase in size and detail of the
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electrogram signal as the catheter is rotated confirm the information from
the impedance signal that the uninsulated portion 178 is fully against the
tissue wall.
Even if the rings are not partially insulated, the
electrogram signal from tip electrode 18 will show an increase in size and
quality when the tip is rotated so that its uninsulated portion is fully
against the tissue wall, and this will constitute confirmation that the tip
has been rotated to the correct position. Thus, if desired, one channel of
impedance and four channels of electrograms (one electrogram channel
for each electrode) can be displayed simultaneously (where only the tip is
partially insulated), or (where the rings are also partially insulated)
several
impedance channels (up to four) and up to four channels of electrograms
can all be simultaneously displayed.
It will be seen that rotating the partially insulated tip so
that no uninsulated portion is exposed to the circulating blood prior to
applying lesion current has a number of advantages for an ablation
catheter. The procedure prevents loss of RF current to the blood and
thereby increases the efficiency of the lesion making process. It also
improves the reliability of the process because of the greater certainty that
a
lesion has been made. In addition, because less RF current is needed, it
reduces the risk of blood clot formation from the RF current, and it also
reduces possible risk of changes in blood coagulation factors produced by
exposure to the RF current.
For a diagnostic catheter a major advantage of tip rotation
is that the signal detected can be optimized. In all cases the use of
impedance readings allows the operator to rotate the distal end to the
desired axial position. Rotation is realized best with tip electrode 18, and
progressively less with ring electrodes 20 as their distance from the tip
increases since their rotation is produced by twisting of the outer silicone
sleeve 66, to which they are attached.
Impedance readings are dependent on waveform
frequency and current magnitude. (The current magnitude dependence
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can be eliminated if a tetrapolar impedance measurement system is used,
but this would be difficult within the body.) The 50 KHz waveform from
impedance section 200 is supplied at a constant current level by constant
current source 200a to prevent variations in impedance due to current
magnitude, thereby making this system more accurate. This can be useful
during RF ablations using ablation catheters with either insulated or non-
insulated electrode tips where changes in impedance will be followed or
observed as a method of determining whether or not a satisfactory lesion
has been produced. Typically the heating from RF current produces an
initial liquefaction of tissue, causing a slight decrease in impedance (e.g.
from an initial 150 ohms down to 130 ohms), followed by a larger
impedance rise as the liquefied tissue coagulates into a solid (e.g. up to 300
ohms.) The initial fall in impedance is an important signal that good
lesion formation will follow, indicating the desirability of monitoring
impedance before application of the RF current because the decrease is
often small and could be missed if no pre-lesion value was available.
Current use is to obtain impedance measurements only during lesion
making by taking the ratio of rms (root-mean-square) RF voltage to rms
RF current. Typically, RF current changes during lesion making -- this
factor itself affecting impedance readings -- and therefore is likely to
obscure the initial true fall in impedance.
With the invention, where impedance measuring
current is supplied at a different frequency (e.g. 50 KHz) from the ablation
current (e.g. 500 KHz), and because of the filter 202, impedance can be
monitored both before and during the lesion making process, as described
above.
Finally, it has been discovered that even when the
electrode tip has been properly rotated against the heart wall so that only
the uninsulated portion 58 contacts the heart wall, nevertheless the
impedance as measured by the impedance section 200 and transmitted to
the computer section 146 will be subject to variability. This variability has
two components. One component is due to the motion or beating of the
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heart. It is found that the measured impedance will vary as the heart wall
moves with each beat. A typical variation is shown in Fig. 20, which is a
plot of impedance against time. In Fig. 20, curve 300 shows the impedance
measured by impedance section 200 as varying (typically) between 200 and
220 ohms as the heart beats. It is assumed that the heart is beating at the
rate of 90 beats per minute, so the time between each peak in curve 300 is
.667 seconds.
Conventional methods of measuring the impedance are
to cause impedance section 200, under software controlled by computer
section 146, to sample single values of the impedance at constant intervals
and to display each value, or alternatively to take the average of a fixed
number of samples during regular intervals such as AB, BC, CD, etc. (Fig.
20) and to display the average values. Irt either case, an apparent random
variability in impedance due to the heartbeat will be observed, making it
difficult to follow the impedance variation caused by rotation of the
partially insulated tip.
Various methods can be adopted to solve the problem of
impedance variability due to the heartbeat. One method is to measure and
display impedance only at the peaks 302 of curve 300. This can be
accomplished by using a conventional electronic peak detector (not
shown). Alternatively, and preferably, the impedance section 200 can
sample the impedance at a frequency much higher than the heartbeat, e.g.
10 times per second. These impedance measurements are directed to a
peak detection software function indicated at 304 in Fig. 21 (where
corresponding reference numerals indicate parts corresponding to those of
Fig. 14). The peak detection software function 302 causes the display
_ monitor 147 to display only the peak impedances.
Another alternative is simply to display impedance at the
occurrence of the troughs 306, using a trough detector software function as
indicated at 308 in Fig. 21. Peak and trough detector software functions are
well known to those skilled in the art.
Other alternatives may also be used. For example, the
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time of the impedance measurement, or of an impedance display, can be
synchronized with an event within the heart cycle. Typically the R wave
of the electrocardiogram (generally the largest wave in the
electrocardiogram) may be used as a synchronizing event. This may be
accomplished under software control by connecting a synchronizing
software function 310 (Fig. 21) an ECG signal obtained from the body
surface, so that the impedance section 200 will measure and the monitor
will display (or the monitor will simply display) the impedance when the
synchronizing software function 310 determines that an R wave event has
occurred.
Alternatively, the time of the impedance measurement
can be synchronized with a cardiac-related non-electrical event, for
example the pressure wave recorded in an arterial catheter, or with pulse
or volume change recorded by various means on the body surface, or by
changes in optical density recorded at the fingertip (or by using other
suitable synchronizing events).
The second component of impedance variation can be
caused by respiratory motion. This is a much slower variation than that
caused by movement of the beating heart, although the two components
are superimposed together. The reason for impedance variation due to
respiratory motion is that impedance is typically measured between the
catheter tip (in contact with the heart wall) and a distant, large reference
electrode. A common position for this reference electrode is against the
trunk or abdomen. The current used for measuring tip impedance must
therefore flow through at least part of the lungs in order to reach the
reference electrode. The impedance of this pathway varies as the air
content of the lungs changes during respiration, becoming maximum at
peak inspiration.
The impedance variability caused by respiratory motion
can be eliminated in various ways. One preferred way is to measure the
impedance between the tip electrode 18 and one of the ring electrodes 20.
This limits the current used for measuring impedance to a local circuit,
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avoiding its flow through the lungs and thereby eliminating this source of
variability.
While preferred embodiments of the invention have
been described, it will be appreciated that various changes may be made
within the scope of the invention without departing from the invention
itself.
