Method and apparatus for reliably producing pacing pulse trains

A method and apparatus for reliably producing a pulse train includes a control system which automatically selects a secondary pulse generator circuit when high frequency pulses are needed or whenever the use of alternating pulse generators would be desirable. This secondary pulse generator may be provided for other functions or it may be dedicated to providing alternate pulses, for example, to increase the frequency of the primary pulse generator without the loss of amplitude. This system may be useful for many purposes including implementing a noninvasive programmed stimulation operation or for providing antitachycardia arrhythmia therapy.

FIELD OF OUR INVENTION 
Our invention relates to cardiac pacemakers, and more particularly to 
cardiac pacemakers which are capable of reliably delivering pacing pulse 
trains of desired amplitude. 
BACKGROUND OF OUR INVENTION 
Implanted cardiac pacemakers are employed to assist patients suffering from 
severe bradycardia or chronotropic incompetence. A cardiac pacemaker 
captures the heart by delivering an electrical pulse to the myocardium of 
a selected heart chamber during an interval in the cardiac cycle in which 
the cardiac tissue is excitable. These electrical pulses cause 
depolarization of cardiac cells and consequently, contraction in the 
chamber, provided that the energy of the pacing pulse as delivered to the 
myocardium exceeds the threshold value. 
Pacemakers may have a pre-defined pacing rate or pre-defined range of 
pacing rates. Other pacemakers may be rate responsive or rate adaptive in 
which case the pacing rate may be adjusted based on sensed physiological 
parameters. For example, when the patient is undergoing emotional or 
physical stress, the pacing rate may be increased to accommodate the 
enhanced biological demands. 
The delivery of fast bursts of pacing pulses may be used for 
antitachycardia therapy or non-invasive programmed stimulation ("NIPS"). 
In antitachycardia therapy, a fast burst of pacing pulses can be used to 
capture a certain region of the heart in order to terminate an arrhythmia. 
In non-invasive programmed stimulation, a fast burst of pacing pulses may 
be generated in a clinical setting to produce a tachycardia arrhythmia for 
diagnostic purposes. 
Delivery of such fast bursts of pacing pulses imposes a tremendous burden 
on the voltage multiplication and regulation circuits used to charge tank 
capacitors in typical pacemakers. This burden often makes it difficult to 
produce trains of closely separated pacing pulses with consistently high 
output amplitude. This burden is further increased as the internal 
impedance of the pacemaker battery increases with increased battery 
depletion. 
When producing high frequency pulse trains, the tank capacitor from which 
the energy is delivered must be recharged fast enough between pulses to 
ensure that all the pulses in the train have consistently high output 
amplitude. This is necessary to ensure that the desired stimulation of the 
heart muscle is achieved. 
Generating high frequency bursts, however, is problematic for typical 
pacemakers, and this is especially so for those designed to treat 
bradyarrhythmias. One reason for this is that the internal impedance of a 
typical lithium-iodide pacemaker battery is relatively high even when it 
is new. As the battery is depleted, its internal impedance increases to 
the point where the end of battery life is reached. In typical batteries 
the internal impedance of the battery may run from hundreds of ohms 
initially to hundreds of kiloohms near the end of battery life. 
As a result of increasing battery impedance, the amount of charge that can 
be drawn within a given amount of time by the pacemaker's circuitry from 
the battery to fully recharge the tank capacitor decreases. Eventually, 
with sufficiently increased battery impedance and sufficiently high pulse 
frequency, not enough charge can be drawn from the battery to fully 
recharge the tank capacitor. As a result, the amplitude of pulses in the 
pulse train may be reduced beyond the point where reliable stimulation of 
heart muscle is achieved. Thus, the performance of the high frequency 
pacing therapy may be reduced or completely mitigated. 
Consistently maintaining the desired pulse amplitude may be a problem at 
normal pacing rates when battery impedance is excessive. The same pulse 
amplitude problems that occur at higher frequency can occur at lower 
frequencies because the battery impedance may be high enough that complete 
recharge of the tank capacitor can not be accomplished in the available 
time period for recharge. 
If the amplitude of the pacing pulses is sufficiently diminished, it is 
possible that the pacing pulses will not capture the heart muscle. This 
could have severe effects on the patient. 
Therefore, it would be highly desirable to provide a system which enables 
pulses to be reliably produced with consistently appropriate amplitude. 
SUMMARY OF OUR INVENTION 
We have invented an implantable pacemaker that is capable of reliably 
producing pulse trains of consistently appropriate amplitude. The 
pacemaker uses two pulse generators which operate in alternate cycles to 
allow recharging of each pulse generator tank capacitor while the other 
tank capacitor is being discharged. In this way a pulse is provided 
alternately by each of the pulse generators. The tank capacitor of each 
pulse generator therefore has effectively almost twice the recharge time. 
As a result, even when the impedance of the battery is substantially 
increased as a result of the approach of end of battery life, pulses of 
consistently high voltage amplitude may be produced because of the 
additional time available to recharge the capacitors. 
In many instances, it is advantageous to provide an additional pulse 
generator to implement the present invention. However, there are some 
instances where at least two pulse generators are already used. In such 
cases, the second pulse generator would not conventionally have been used 
to produce the pulse trains. For example, in a dual chamber stimulator, 
one of a pair of pulse generators may provide ventricular stimulation 
while the other provides atrial stimulation. In accordance with the 
present invention, the second pulse generator may be co-opted into 
providing alternate pulses when necessary. 
Where two pulse generating systems are utilized to provide pulse trains 
that maintain the desired pulse amplitude, it is advantageous to have a 
way to automatically enable the cycling between the first and second pulse 
generators on alternate cycles. Whenever necessary, the pacemaker includes 
circuitry to perform automatic source toggling between the primary tank 
capacitor and an additional tank capacitor for generating successive 
pacing pulses in the train. In this way, while one of the capacitors is 
being used to deliver a pacing pulse, the other is being recharged.

DETAILED DESCRIPTION OF OUR PREFERRED EMBODIMENT 
We will now describe the preferred embodiment of our invention with 
reference to the accompanying figures. Like numerals will be used to 
designate like parts throughout. 
Referring now to FIG. 1, an implantable pacemaker, generally designated 10, 
is illustrated in schematic fashion with connection to the human heart 12. 
The present invention is applicable to pacemakers with atrial sensing, 
ventricular sensing, ventricular pacing and atrial pacing or any 
combination thereof. In addition, the features of our invention could also 
be combined with an implantable defibrillator/cardioverter. 
With this understanding, the illustrated pacemaker 10 comprises a 
microprocessor 14 which executes various control programs to regulate the 
action of the pacemaker. The microprocessor 14 may be connected to 
additional memory 16 which stores programs and data as needed. 
Conventionally, one or more internal clocks may be provided to permit 
timing of various events. For example, an A-V interval timer 18 may be 
provided. Similarly, a V-A interval timer 20 may also be provided as known 
in the art. 
The microprocessor may also be provided with a telemetry circuit 22 to 
enable communication by the antenna 24 with an external pacemaker 
programmer (not shown). Telemetry permits an attending physician to obtain 
data and information from the pacemaker and to control the pacemaker by 
setting various selectable parameters. 
Our invention is amenable to implementation with pacemakers using either 
bipolar or unipolar leads. The illustrated pacemaker 10 may be connected 
to the heart 12 through a first lead 26 to an electrode 27 in the atrium 
and through a second lead 30 to an electrode 31 in the ventricle 32. An 
indifferent electrode (e.g. the pacemaker can) is provided to complete the 
electrical circuit through the body. In the illustrated embodiment, a can 
43 or outer casing of the pacemaker serves as the indifferent electrode. 
Atrial electrogram sensing, through an atrial sense circuit 34, and 
ventricular sensing through a ventricular sense circuit 36, provide 
information to the microprocessor 14 concerning the condition and 
responsiveness of the heart. In addition, pacing pulses are provided to 
the ventricle and/or the atrium from the atrial/ventricular stimulus 
generator 38.. However, it is clearly with the scope of those skilled in 
the art to provide cardioversion/defibrillation capabilities in response 
to the detected condition of the heart. 
Stimulation of the heart is passed through coupling capacitors 40 and 41 in 
a conventional fashion. The switches 73 and 74 and resistors 75 and 76 may 
be used to actively discharge the coupling capacitors 40 and 41. 
To control the pulse rate of the ventricular stimulus generator 38, the 
microprocessor may acquire information on the condition of the heart 
through an impedance circuit 42. The impedance circuit 42 detects changes 
in impedance primarily due to the changing shape of the heart, which may 
be related to the physical shape of the heart as it beats and pumps blood. 
This information can be used to derive a measure of the stroke volume or 
ejection fraction or end diastolic volume of the heart. Furthermore, the 
shape of the impedance waveform can provide further information on other 
cardiac timing parameters such as isovolumetric contraction time or 
pre-ejection period. One exemplary impedance circuit is described in U.S. 
Pat. No. 5,531,772 to Prutchi, which is expressly incorporated by 
reference herein. 
In addition to the measurement of impedance, a sensor 44 may also be 
provided to obtain an indication of physiologic need and adjust the pacing 
rate. Such a sensor may be an accelerometer, as described by Dahl, U.S. 
Pat. No. 4,140,132, a temperature sensor as described Alt, U.S. Pat. No. 
4,688,573, or any other suitable sensor parameter which may be correlated 
to physiological need of the patient. 
The atrial/ventricular stimulus generator 38, shown in FIG. 2, includes a 
battery 56 connected to a pair of regulators 58 and 60. The regulator 60 
is connected to a tank capacitor 62 while the regulator 58 is connected to 
a tank capacitor 64. The regulators 58 and 60 include multiplication and 
regulation circuitry used to charge the tank capacitors 62 and 64. 
The tank capacitor 64 is connectable by a switch 66 to a node 70 while the 
regulator 60 is connected by a switch 68 to the same node 70, which 
connects via the lead 30 to the ventricle 32 of the heart 12. The tank 
capacitor 64 is also connectable via the switch 67 and the lead 26 to the 
atrium 28 of the heart 12. 
The pace sequencer 72 controls the switches 66, 67 and 68 to utilize the 
regulator 58 and tank capacitor 64 to augment the pulses produced by the 
regulator 60 and tank capacitor 62 when needed. In particular with the 
switch 66 open and the switch 68 closed, a normal pacing pulse may be 
produced by the tank capacitor 62 to ventricle 32. 
At the same time with switch 67 closed, a pulse train may be delivered via 
the lead 26 to the atrium 28. After each pulse is created, and transmitted 
to the heart tissue 12, the switches 67 and 68 may be opened allowing the 
capacitors 62 and 64 to be recharged by the regulators 58 and 60 and 
battery 56. 
When it is desired to augment the ventricular pacing pulse train, the 
switch 67 is opened and the switches 66 and 68 may be alternately opened 
and closed at a desired frequency by the pace sequencer 72. This allows 
additional time for the capacitors 62 and 64 to be recharged. Namely, 
while the capacitor 64 is being discharged to produce a pacing pulse, the 
capacitor 62 may be recharging and vice versa. 
The pace sequencer 72 may be a state machine which is programmed to provide 
switching sequences. The sequencer 72 is connected for control by the 
microprocessor 14. However, a variety of other conventional techniques may 
be used to control the switches 66,67,68,73 and 74. 
With the present invention it is possible to produce a normal pulse 
frequency through the regulator 60 and capacitor 62 and then when 
selected, produce a higher frequency pulse train without concern for loss 
of reliability. Because of the extra time provided for recharging of the 
tank capacitors 62 and 64, the possibility of incomplete charging is 
lessened and therefore the likelihood that pulses of full amplitude will 
be produced is increased. 
As one example of the application of the present invention, the regulator 
60 and capacitor 82 can produce normal frequency pacing pulses. When it is 
desired to undergo a noninvasive programmed stimulation cycle, one or more 
bursts of high frequency pulses may be produced for ventricular analysis 
and diagnostic purposes. This may be done by alternately producing pulses 
using regulator 60 and capacitor 62 and the regulator 58 and capacitor 64. 
This stimulation cycle can be implemented through telemetry by the 
physician. A signal received by the antenna 24 and telemetry circuit 22 
may be passed to the microprocessor 14 which in turn sends an appropriate 
control signal to the sequencer 72. In the same way the stimulation may be 
terminated when sufficient data has been obtained. 
Similarly, it may be desirable to counteract a detected tachycardia 
arrhythmia by producing a high frequency burst cycle. A tachyarrhythmia is 
detected by the atrial sensor circuit 34 or the ventricular sense circuit 
36. The microprocessor 14 then signals the pace sequencer 72 to implement 
a high frequency burst cycle to the ventricle using both tank capacitors 
62 and 64. Once the arrhythmia has been countered, the pace sequencer 72 
may automatically revert to a normal pacing frequency. 
In accordance with still another embodiment of the present invention, tank 
capacitors 62 and 64 are used to produce a combined pulse train when low 
battery condition is detected by the conventional monitor 80 or low pulse 
amplitude has been detected by a monitor 82. That is, upon detection of a 
low battery or low pulse amplitude, the microprocessor 14 directs an 
appropriate control signal to the sequencer 72 to operate switches 66 and 
67 to co-opt the regulator 58 and tank capacitor 64 to produce alternate 
pulses of a pulse train supplied to the lead 30. 
The monitor 82 may be implemented as disclosed in a copending application, 
by the same inventors, filed on the same date as this application, titled 
"Method and Apparatus for Detecting Amplitude Loss in Cardiac Pacing 
Pulses", which is hereby expressly incorporated by reference herein. 
Our invention may be embodied in other specific forms without departing 
from the spirit or central characteristics thereof. The foregoing 
description is, therefore, to be viewed in all respects all illustrative 
and not restrictive. The scope of our invention is defined solely by the 
appended claims.