Method and apparatus for delivering defibrillation therapy through a sensing electrode

A pulse generator circuit and method for using a defibrillation lead positioned close to the right ventricular apex to optimize energy delivery and sensing. This method includes operating a defibrillator-pacemaker system in a true bipolar sensing mode as long as high voltage therapy is not required. When required, the defibrillator-pacemaker system delivers a first high voltage therapy via an RV defibrillation electrode, and, using true bipolar sensing, determines whether the first high voltage therapy was successful. If the first high voltage therapy is deemed to be successful, then true bipolar sensing is resumed. Otherwise, the defibrillator-pacemaker system causes a ring electrode to be electrically connected to the RV defibrillation electrode, delivers a second high voltage therapy, and, using integrated bipolar sensing determines whether the second high voltage therapy was successful. In this way, because no new circuit elements are added within the lead, the lead size and complexity are not increased.

FIELD OF THE INVENTION 
The present invention relates in general to medical devices, and it more 
particularly relates to a lead switching matrix for use in an implantable 
cardiac pulse generator for the detection and management of cardiac 
arrhythmias. The invention more specifically relates to a method for 
activating a true bipolar sense electrode to deliver high voltage 
defibrillation therapies. 
BACKGROUND OF THE INVENTION 
In recent years there has been a great deal of interest and progress in the 
integration of implantable medical devices such as defibrillators and 
pacemakers. For the purpose of this application, "defibrillation" is used 
in a broad sense, as including the application of relatively high energy 
and high voltage shocks to the heart to terminate tachyarrhythmias 
including fibrillation and malignant tachycardias. Similarly, "pacing" is 
used in a broad sense, as including the application of relatively low 
energy and low voltage pacing pulses to maintain an adequate heart rate or 
to break a tachycardia by stimulating the patient's heart. One traditional 
approach to combining pacing and true bipolar sensing electrodes in a 
defibrillation lead is to provide a ring electrode located between the 
pacing tip electrode and the defibrillation electrode where the ring 
electrode is dedicated exclusively to sensing the heart's electrical 
activities. The space required for this ring electrode forces the 
defibrillation electrode to be set back from the RV apex, and, because of 
the size limitations of the right ventricle, decreases the length 
available for the defibrillation electrode. 
However, in the context of endocardial ventricular leads, it would be 
desirable to provide an electrode, or electrode pair, for sensing adjacent 
the ventricular apex, while still providing an electrode which also is 
located as close to the apex as possible. 
Exemplary attempts to accomplish such objective are described in U.S. Pat. 
No. 5,336,253, to Gordon et al., and U.S. Pat. No. 5,342,414 to Mehra, 
both of which are incorporated herein by reference in their entirety. The 
Gordon patent describes a combined pacing and cardioversion lead system 
with internal electrical switching components for unipolar or bipolar 
sensing of electrograms, pacing at normal pacing voltages and 
cardioversion or defibrillation. In bipolar embodiments, a ring electrode 
is coupled through the internal switching circuitry to a large surface 
area cardioversion electrode. In these embodiments, pacing and sensing are 
accomplished through a pair of conductors extending through the lead body 
to the tip and ring electrodes. When cardioversion shocks are delivered to 
the ring electrode, cardioversion energy is also directed to the 
cardioversion electrode through the operation of the switching circuitry 
in response to the magnitude of the applied cardioversion pulse. 
However, the lead system disclosed in the Gordon patent uses discrete and 
non-programmable internal switching components, such as the zener diodes 
in the arrangement of FIG. 3, or the surge suppressor and the resistor in 
the arrangement of FIG. 4. These internal switching components appear to 
indiscriminately and automatically connect the ring electrode to the 
cardioversion electrode upon the application of a cardioversion pulse 
exceeding a predetermined magnitude. As a result, the lead system 
described in this patent lacks the required flexibility to adapt the 
application of the cardioversion shocks to specific cardioversion 
conditions in progress. 
Wherefore, it would be highly desirable to have a new lead switching matrix 
for use in an implantable cardiac stimulator for the detection and 
management of cardiac arrhythmias. It would also be desirable to have a 
new method for activating a true bipolar sense electrode to deliver high 
voltage therapies. 
The transvenous defibrillation lead described in the Mehra patent is 
directed towards optimizing the size, spacing and location of the 
electrodes, and more specifically towards providing a bipolar sensing pair 
of electrodes having adequate interelectrode spacing to insure appropriate 
sensing of cardiac depolarization, while still allowing the placement of 
the electrode as close to the distal end of the lead body as possible. The 
lead includes a helical electrode, extending distally from the lead body, 
for use as the active electrode in cardiac pacing and for use in sensing 
cardiac depolarizations. A ring tip electrode or a cylindrical ring 
electrode is located at or adjacent to the distal end of the lead body and 
provides the second electrode for use in sensing depolarizations. The 
helical electrode is insulated from the point it exits the lead body until 
a point adjacent to its distal end. The defibrillation electrode is 
mounted with its distal end closely adjacent to the distal end of the lead 
body, such that its distal end point is within one centimeter of the 
distal end of the lead body. 
The leads described in the foregoing Gordon and Mehra patents do not 
provide for integrated bipolar sensing, wherein sensing is carried out 
between the cardioversion electrode and the tip electrode. One feature 
that distinguishes integrated bipolar sensing and true bipolar sensing is 
that integrated bipolar sensing lacks an electrode dedicated solely to 
bipolar sensing in conjunction with the pacing tip. Typically, in an 
integrated bipolar electrode, the same electrode used for bipolar sensing 
in conjunction with the pacing tip is used to deliver defibrillation or 
cardioversion therapies. There are two potential problems with integrated 
bipolar electrodes. First, because the integrated electrode must be large 
for efficient delivery of defibrillation or cardioversion energy, it may 
reduce the resolution of the sensed signal due to spatial averaging of the 
different potentials within the heart. Secondly, the integrated electrode 
serves also as a defibrillation electrode and is likely to have 
substantial residual charge at its interface after a defibrillation 
therapy pulse. The residual charge or polarization of the electrodes 
results in less accurate sensing immediately after therapy. The true 
bipolar sense electrode should not be subject to these potential problems. 
The size of the true bipolar electrode is not governed by the need for 
efficient energy delivery during therapy and can be optimized for sensing. 
Additionally, because a negligible current flows across the electrode 
tissue interface, there is no build-up of charge or polarization at the 
interface, enabling the accurate measurement of endocardial signals 
immediately following therapy. However, a drawback with true bipolar 
sensing exists because the sense electrode in a true bipolar lead is 
located adjacent to the pacing electrode, and thus the cardioversion 
electrode is generally positioned further away from the apex of the heart, 
thus disadvantageously reducing the delivered therapeutic energy. 
Therefore, it would be desirable to have a new lead which permits the 
optimal delivery of defibrillation and cardioversion energies, and the 
minimization of poor sensing due to polarization effect. It would also be 
desirable to optimize the electrode functionality without the complexity 
and dimensional constraints of circuit elements located within the lead. 
SUMMARY OF THE INVENTION 
The present invention is directed towards providing a method and apparatus 
for using a defibrillation lead to defibrillate and sense in close 
proximity to the heart ventricular apex. In particular, the invention is 
directed towards providing a new method which permits the optimal delivery 
of defibrillation and cardioversion energies, and the minimization of poor 
sensing due to polarization effect, without the complexity and dimensional 
constraints of circuit elements located within the lead. 
It is also an object of the present invention to provide a method for 
activating a true bipolar sense electrode to deliver high voltage 
therapies, while simultaneously allowing for the flexible application of 
these therapies to a particular arrhythmia condition in progress. 
Briefly, the foregoing and other objects of the present invention are 
realized by providing a new method for operating a defibrillator-pacemaker 
system in a true bipolar sensing mode as long as high voltage therapy is 
not required. When required, the defibrillator-pacemaker system delivers a 
first high voltage therapy via an RV defibrillation electrode, and, using 
true bipolar sensing, determines whether the first high voltage therapy 
was successful. If the first high voltage therapy is deemed to be 
successful, then true bipolar sensing is resumed. Otherwise, the 
defibrillator-pacemaker system causes a ring electrode to be electrically 
connected to the RV defibrillation electrode, delivers a second high 
voltage therapy, and, using integrated bipolar sensing determines whether 
the second high voltage therapy was successful. 
If the second high voltage therapy is determined to be successful, then the 
defibrillator-pacemaker system resumes true bipolar sensing. Otherwise, a 
third high voltage therapy is delivered, and a determination is made, 
using integrated bipolar sensing, as to whether this third high voltage 
therapy was successful. If it is determined that this therapy was not 
successful, then another high voltage therapy is delivered, and the 
routine of inquiring about the success of the therapy, delivering a high 
voltage therapy and conducting integrated bipolar sensing is repeated as 
many times as necessary. If, on the other hand, it is determined that the 
high voltage therapy was successful, then the defibrillator-pacemaker 
system switches back to, and resumes true bipolar sensing. It may 
alternatively be desirable to deliver a first defibrillation shock with 
the ring electrode electrically connected to the RV defibrillation 
electrode and/or to switch back to the true bipolar sensing configuration 
following delivery of the defibrillation shock(s).

DETAILED DESCRIPTION 
FIG. 1 (Prior Art) schematically illustrates one traditional approach to 
combining pacing and true bipolar sensing electrodes to defibrillation 
leads. The lead 10 is a right ventricular (RV) transvenous defibrillation 
lead with true bipolar sensing, whereby a bipolar electrode is dedicated 
exclusively for sensing the heart's electrical activities. Lead 10 
includes a tip electrode 12 mounted at the distal end of an insulative 
lead body 14, and coupled to a conductor 15, which, in turn, is 
electrically connected to a connector 16. 
Lead 10 further includes an indifferent sense electrode which is typically 
a ring electrode 18 surrounding lead body 14, a short distance from tip 
electrode 12. Ring electrode 18 forms a true bipolar sensing electrode 
pair with tip electrode 12, and is coupled to a connector 20 through a 
conductor 22. 
An elongated large surface area defibrillation or cardioversion electrode 
23 is coupled to a connector 25 through a conductor 27. Electrode 23 takes 
the shape of a space wound coil, wrapped around insulative lead body 14, 
,and extends for a preset axial distance therealong. This axial distance 
is selected such that the proximal end of electrode 23 generally 
terminates in the vicinity of the tricuspid valve when the distal tip of 
lead 10 is secured to the myocardium of the RV apex. The distal end of 
electrode 23 is closely positioned relative to ring electrode 18. 
Connectors 16, 20 and 25 allow the coupling of lead 10 to an implanted 
pulse generator. 
In operation, bipolar sensing and pacing may take place between tip 
electrode 12 and ring electrode 18 through conductors 15 and 22, 
respectively. When cardioversion shocks are delivered, connector 25 which 
is coupled to one of the output terminals of a cardioversion pulse 
generator transmits the cardioversion therapy in a shock or pulse form. 
The shock is delivered to the myocardial cells via electrode 23 and an 
indifferent electrode or a plurality of indifferent electrodes (not shown) 
such as a coil electrode located in the superior vena cava (SVC), an 
epicardial patch electrode positioned in the left chest wall, the pulse 
generator housing implanted in the lea pectoral region, or a combination 
of these. 
However, in the context of endocardial ventricular leads, it would be 
desirable to provide an electrode, or electrode pair, for sensing adjacent 
the ventricular apex, while still providing an electrode which also is 
located as close to the apex as possible. 
FIG. 2 provides a block diagram showing the general organization of an 
implantable combined defibrillator-pacemaker system 31. The system 31 
includes sensing, analysis and control circuitry 32, a voltage regulator 
circuit 33 and an 8-bit microprocessor 34. A static random access memory 
(RAM) 35 is used to store digitized catracardiac electrogram (EGM) 
waveforms. External connections from a pacing-defibrillation circuitry 36 
to the heart 37 are provided by two high voltage (HV) conductors HV1 and 
HV2, and a pair of pace/sense conductors P/S1 and P/S2 through which 
millivolt level EGM signals are sensed and which also carry pacing pulses 
to the heart 37. In a preferred embodiment at least conductors HV1, P/S1 
and P/S2 are enclosed within a single lead, while conductor HV2 may be 
optionally contained within that same lead. 
Telemetry to and from an external programmer is carried via a coil-to-coil 
link 38. System software within the microprocessor 34 determines whether 
the EGM parameters indicate an arrhythmia and, if so, the appropriate 
therapy is initiated. The raw EGM data can also be stored in memory 35 for 
later retrieval, or it can be telemetered out of the system 31 in real 
time. The general operation of one exemplary embodiment of the system 31 
is described in U.S. Pat. No. 5,111,816 to Pless et al., which is 
incorporated herein by reference in its entirety. 
FIG. 3 shows a block diagram embodiment of the pacing-defibfillation 
circuit 36 in accordance with the present invention, without its full 
complement of cardiac leads and electrodes. The heart 37 is defibrillated 
by high voltage therapy (i.e., pulses or shocks) which are delivered 
through the high voltage conductors HV1 and HV2 through their respective 
defibrillation electrodes, including a right ventricle (RV) defibrillation 
electrode and a superior vena cava (SVC) defibrillation electrode. As used 
herein, "high voltage therapy" can include defibrillation or cardioversion 
high voltage therapy, as well as lower sequentially delivered therapies. 
One embodiment of a defibrillation electrode is shown in FIG. 1 as the 
elongated high surface area electrode 23. Other exemplary defibrillation 
electrodes that can be used in conjunction with the present system 31 are 
illustrated in U.S. Pat. No. 5,014,696 to Mehra, which is incorporated 
herein by reference in its entirety. 
For illustration purpose and without intention to limit the scope of the 
present invention, the pacing-defibrillation circuit 36 shows lead 10 to 
include the pacing/sensing conductor PS/2 as conductor 55 for connection 
to the tip electrode 12 and pacing/sensing conductor PS/1 as conductor 54 
for connection to the ring electrode 18. In such an embodiment, the high 
voltage conductor HV2 is connected to the superior vena cava electrode 
(not shown in FIG. 1). 
The pacing-defibrillation circuit 36 includes pacemaker circuit 40, high 
voltage protection circuit 41, and defibrillation circuit 43, whose 
general functions and circuit components are described in U.S. Pat. No. 
5,111,816 to Pless et al., and U.S. Pat. No. 4,830,006 to Haluska et al. 
which is incorporated herein by reference in its entirety. 
A switching matrix 50 forms part of the pacing-defibrillation circuit 36 
and provides an external switching control between the various electrodes 
according to the defibrillation process of the present invention. Such 
defibrillation process is controlled by a special software program in the 
microprocessor 34. The switching matrix 50 includes at least one normally 
open switch 52 between the high voltage conductor HV1 and the ring 
electrode conductor 54. 
FIG. 4 shows another embodiment wherein lead 10 uses an integrated sensing 
mode. When closed, switch 52 connects defibrillation electrode 23 to tip 
electrode 12. 
FIG. 5 shows yet another embodiment having a normally open switch 56 that 
can be connected between the pacing/sensing conductors 54 and 55 during 
defibrillation, such that both the ring electrode 18 and the tip electrode 
12 are activated, thus further shifting the effective position of the 
defibrillation electrode toward the heart apex. 
Turning now to FIG. 6, a flow chart illustrates a first embodiment of the 
defibrillation method 70 as processed by the combined 
defibrillator-pacemaker system 31 of FIG. 2. As shown in steps 71 and 72, 
the system 31 would normally be operating in a true bipolar sensing mode 
as long as high voltage therapy is not required. If, however, high voltage 
therapy is determined to be needed, then, as shown in step 73, a first 
high voltage therapy (i.e., a shock or a pulse) is delivered via the 
defibrillation electrode 23. 
True bipolar sensing is then carried out at step 74, and a determination is 
made at step 75, as to whether the first high voltage therapy was 
successful, and the heart 37 has restored to its normal sinus rhythm If 
this therapy is deemed to be successful, then system 31 resumes true 
bipolar sensing at step 71. If, on the other hand, the therapy was not 
successful, the switching matrix 50, under the control of the 
microprocessor 34, causes the switch 52 to close, thereby electrically 
connecting the defibrillation electrode 23 and the ring electrode 18, 
before the delivery of the subsequent high voltage therapy. 
The inclusion of the ring electrode 18 as part of the effective area of the 
defibrillation electrode advantageously provides a defibrillation 
electrode which is effectively located in a more apical position and has 
an increased surface area. Another high voltage therapy is then delivered 
at step 77. While the preferred embodiment of the present invention 
includes the delivery of only one high voltage therapy, it should be 
understood to those with ordinary skill in the art that the microprocessor 
controller for the switching matrix 50 can be programmed to deliver 
successive high voltage therapies, i.e., one or more high voltage 
therapies, until a determination is made to "activate" the ring electrode 
18 at step 76. In other words, the device may be programmable such that 
ring electrode 18 does not become active for defibrillation until after 
two or more failed shocks, the number depending on what is programmed. 
Integrated bipolar sensing is then carried out at step 78, whereby the 
large surface area defibrillation electrode 23 and the coupled ring 
electrode 18 act, in connection with the tip electrode 12, as a sensing 
electrode. Based on this integrated bipolar sensing, a determination is 
made at step 79 as to whether the high voltage therapy delivered at step 
77 was successful. If it is determined that this therapy was not 
successful, then another high voltage therapy is delivered at step 77, and 
the routine of inquiring about the success of the therapy (step 79), 
delivering a high voltage therapy (step 77) and conducting integrated 
bipolar sensing (step 78) is repeated as many times as necessary. 
Practically, this sequence is repeated about three to six times before 
terminating the defibrillation process 70. If, on the other hand, it is 
determined that the high voltage therapy was successful, then the 
switching matrix 50 is caused by the microprocessor 34 to switch back to 
and resume true bipolar sensing at step 71. By limiting the "activation" 
of the bipolar sense electrode for high voltage defibrillation to those 
situations where the initial high voltage therapy, which was delivered at 
step 73, has failed, the problem of residual charge at the metal-tissue 
interface remaining on the sense electrode occurs only after the initial 
rescue therapy (step 73). 
FIG. 7 is a flow chart illustrating a second embodiment of the 
defibrillation method 80 as processed by the combined 
defibrillator-pacemaker system 31 of FIG. 2. The defibrillation method 80 
of FIG. 5 is substantially identical to the defibrillation method 70 of 
FIG. 4, with similar reference numerals indicating similar or identical 
steps. The defibrillation method 80 differs from the defibrillation method 
70 in that step 81 replaces step 78, whereby after the delivery of the 
high voltage therapy at step 77, the switching matrix is caused to switch 
back to true bipolar sensing (step 81) under the control of the 
microprocessor 34, by opening the switch 52; and further that if the 
result of the determination at step 79 is negative, i.e., the therapy was 
not successful, then the bipolar sense electrode is activated once again 
at step 76. 
The foregoing description of the preferred embodiments has been presented 
for purposes of illustration and description. It is not intended to be 
exhaustive or to limit the invention to the precise forms described. 
Various modifications of the system components and methods of operation 
may be employed in practicing the invention. It is intended that the 
following claims define the scope of the invention, and that the 
structures and methods within the scope of these claims and their 
equivalents be covered thereby.