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The activitrax rate responsive pacemaker system
The Activitrax Rate ResponsivePacemakerSystem KAREL DEN MILK, MD, LEON BOUWELS, MD, FRED LINDEMANS, PhD, IAN RANKIN, PhD, PEDRO BRUGADA, MD, and HEIN J.J. WELLENS, MD
@ses/min, respectively). No paclq-induced arrftythmias were seen durln9 ambulatory mrdkgraphic monitorln9. At ht@ pacing rates sl@htly irregular pacing intervals were sometimes observed, whkh was due to polar&&n senskg. Sporadically, 1 pacln9 interval shortened to the upper rate value, because of a known and now resolved tImin anomaly. Neither anomaly was of clinical p and the first couki be resolved by reprogramming. lt is concluded that this rate responsive pacemaker is safe, easy to program and incmses heart rate appropriately durln9 exercise; the bipolar version can be used safely for atrial packrg; and rate reqxmse and activfty Weshokl values that give a pa&9 rate of about 100 pulses/min durbq casual wafkkrg provkfe appropriate changes in heart rate durkg normal dally life.
Bipolar Medtronk Actlvitrax rate respondve pacemakers were implanted in 31 patfents for ventrkular (28) or atrial (3) pachrg. Mean follow-up was 18 months (range 10 to 28). Twenty pacemakers were implanted after catheter ablation of the His bundle, 7 for skk sinus syndrome, 1 for attioventrkxrlar bbckand3forsickslnussy&romewlthatrioventrkular block. A rate response value was selected that gave a pack19 rate of about 100 pulses/min &ring walkkrg. 01 the 31 patients, all had 24-hour ambulatory elecbocardkgraphic monltortn9 with diary, 11 walked a 20-mltxfte &cult, lncludln9 a flight of stairs, and 20 had a treadmill exercise test. In 9 patients the pacing rate coukl be compared wlth the underlying slnus rate durtq exercise and was seen to match it very closely. In 12 patients ths pacirrg rate durin9 car drlvltg was found to be similar to the sinus rate of 5 volunteers under similar condttkms (mean minimum and maximum rate was 80 and 99
(Am J Cardloi 1988;81:107-112)
F
or patients with normal sinoatrial function and atrioventricular (AV] block, atria1 synchronized pacing (DDD, VDD, VAT) provides AV synchrony and rate responsiveness. However, patients with abnormal sinoatrial function or atria1 arrhythmias may not be able to benefit from such a system. In these patients, a parameter other than atria1 activity must be used to provide changes in heart rate appropriate to metabolic needs. Parameters currently being used or investigated in pacing systems include: pH, venous oxygen saturation, respiratory rate and volume, QT interval, temperature and physical activity.1-6 This article presents our experience with the Medtronic model 8400 Activitrax rate responsive pacemaker, which detects and tracks physical activity. This study covers several From the Department of Cardiology, Academic Hospital Maastricht, University of Limburg, Maastricht, The Netherlands, and Medtronic I.R.S.C. B.V. Maastricht, The Netherlands. Manuscript received April 20,1987;revised manuscript received September l&1987, and accepted September 12. Address for reprints: Karel den Dulk, MD, Department of Cardiology, Academic Hospital Maastricht, University of Limburg, P.O. Box 1918,620l BX Maastricht, The Netherlands. 107
aspects: evaluation of appropriateness of pacing rate during normal daily life, evaluation of a simple programming protocol of the rate control variables and detection of potential anomalies in pacemaker behavior.
Methods Pacemaker description: The Medtronic model 8400 Activitrax pacemaker is a bipolar, multiprogrammable, single-chamber atrial- or ventricular-inhibited pulse generator with automatic rate variability and telemetry. Four pacing modes, namely, VVI or AA1 + activity, VVI or AAI, VOO or A00 + activity and VOO or AOO, can be selected using the Medtronic model 9710 programmer. In the activity modes, the pacemaker’s stimulation rate is controlled by the activity-sensing detector and circuitry. The sensor, a piezoelectric crystal on the inside of the pacemaker, transforms the mechanical vibrations in the body resulting from physical activity into an electrical signal that is further processed for rate control. The rate can vary between the programmable “basic rate” (80, 70 or 80 pulses/min) and the “maximum activity rate” (100, 125 or 150 pulses/min).
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The programmable “activity threshold” (low, medium or high) establishes a level that must be exceeded by the output signal of the activity sensor in order to influence the pacing rate. The programmable “rate response” (10 settings] determines the relation between pacing rate and the output signal of the activity sensor when it exceeds the activity threshold. Rate response 1 provides the lowest, 10 the highest pacing rate at a fixed activity level. Each increment from 1 to 10 shortens the pacing interval by 25 ms. When the detected activity level suddenly increases from 1 steady-state level to another, the pacing rate will increase immediately, but it will take about 25 seconds to cover 90% of the required increase in rate. For decreasing the rate, it takes about 90 seconds to cover 90% of the required decrease in rate. Other programmable parameters are pulse amplitude (2.7 or 5.0 V], pulse width (from 0.05 to 1.5 ms), sensitivity (1.25, 2.5,5.0 mV or asynchronous) and hysteresis (in VVI or AA1 mode only]. The refractory period is 225 ms and is not programmable. All pacemakers were connected to bipolar Medtronic Model 4012 or 4512 Target Tip leads. Patients: Thirty-one Activitrax pacemakers were implanted. Sixteen patients were men and 15 were women, ages 40 to 79 years (mean 62). Follow-up ranged from 10 to 26 months (mean 15.8).In 28 patients the pacing lead was placed in the right ventricle, and in 3, in the right atrium. In 20, the pacemaker was
FIGURE 1. Brief Inhibition ol the pacemaker during a treadmill pacemaker wlth the underlylng sinus rhythm.
exercise
implanted after catheter ablation of the bundle of His (atria1 fibrillation, 16; AV nodal tachycardia, 2; circus movement tachycardia, 1; atria1 tachycardia, 11.Other indications were: symptomatic sinus bradycardia (3 patients], replacement of a VVI pacemaker for total AV block (1 patient], bradycardia-tachycardia syndrome (4 patients) and symptomatic sinus bradycardia with AV conduction disturbances (3 patients]. Three patients with isolated sinus bradycardia were paced from the right atrium, the others received VVI pacing. All pacemakers were implanted in the pectoral muscle region. Associated cardiac disease: Eight patients had coronary artery disease, of which 5 had an old myocardial infarction; 6 patients had mitral regurgitation, 2 had a mitral valve prosthesis, 1 had an aortic valve prosthesis, 2 had congestive cardiomyopathy and 2 had ventricular tachycardia due to an old myocardial infarction. Eleven patients were taking antiarrhythmic drug therapy; 6 nitrate, 7 oral anticoagulant, 8 diuretic and 4 vasodilator therapy. Protocol: Underlying rhythm: During follow-up visits, the pacemaker was programmed to the VVI mode 40 pulses/min to study the underlying rhythm. Initial testing: To obtain an impression of the effects of changing the activity threshold and rate response parameters, we compared the pacing rate to the underlying sinus rate in 2 patients with a normal sinus node function during rest and casual walking. Since
test was done to compare the rate adopted by the actlvtty SenSlng
January 1, 1988
we considered a pacing rate of about 100 beats/min appropriate for casual walking, this formed the basis for all our initial postimplant programming of rate response and activity threshold. The appropriateness of this simple programming protocol was checked by a treadmill exercise test if possible, a 20-minute walking circuit that included a flight of stairs and X-hour ambulatory electrocardiographic monitoring (AEM) with diary. Five patients exercised according to the Naughton protocol in the VVI mode and in the activity mode as part of the European clinical investigation.7 Fifteen were exercised according to the Bruce protocol. Comparison to sinus rhythm: In order to compare the pacing rate to the underlying sinus rate during treadmill exercise testing, the pacemaker was inhibited briefly once a minute (if necessary), as illustrated in Figure 1. Patients with sick sinus syndrome, 1 to 1 ventriculoatrial conduction or supraventricular tachycardia at the time of comparison were excluded. Irregular pacing intervals: Because intermittent oversensing was observed, this phenomenon was checked during exercise testing, 24-hour AEM and at follow-up by tapping the pacemaker can with 2 or 3 fingers to reach the selected maximum activity rate while recording a 6-channel electrocardiogram. During testing for polarization, the pacemaker was programmed as follows: sensitivity 2.5 mV for ventricular use, 1.25 for atria1 use, output 5.0 V, pulse width 0.5 ms,
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with upper rate settings of 125 pulses/min (23 patients) or 150 pulses/min (8 patients].
Results Underlying rhythm: In all patients the heart rate was usually determined by the pacemaker. In 1 patient with paroxysmal AV nodal tachycardia and sick sinus syndrome, His bundle ablation resulted in a first-degree AV block with persistence of paroxysmal tachycardias. In this patient the pacemaker was also used to terminate tachycardia by tapping on the pacemaker can on the recognition of tachycardia, as described.a Initial testing: The effect of programming the activity threshold to low, medium or high was evaluated shortly after implant in 2 patients who bad normal sinus node function. Figure 2 illustrates the result in 1 of these patients for rate response 8. In order to obtain a pacing rate of about 100 pulses/min during casual walking [our target for initial programming of the pacemaker) while maintaining a pacing rate close to the selected basic rate at rest, the medium activity threshold was usually appropriate. Fine tuning was obtained by adjustipg the rate response value, and rate response 8 was used in 24 patients. In 6 patients with a soft gait (shufflers or tiptoers) (Figure 3) an activity threshold low was necessary to obtain the required rate of 100 beats/min during casual walking. Activity threshold high was never used, nor were rate response values <6.
MEDIUM
LOW 0
8
HIGH 8
120 -c-----p----SINUS
RATE
FIGURE 2. Comparison between pacemaker rate and sinus rate during casual walking, for activlJy thresholds low, medium, anfj high with rate response 8. 70 60-w: 0
FIGURE 3. Illustrates 3 walking styles. The shuffler (Mf), who generates lowamplltude s!gnals at a low rate as opposed to the quick step artist (right) who generates high-amplitude slgnals at a high rate are compared with a.presumed normal walker (animated sensor slgnal). As can be Fee? from the figure, a low threshold must be programmed for the shuffler, whereas a’ medluq threshold would be appropriate for the quick step artlst.
36
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108 .
1LL
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HIGH MEDIUM LOW
I
I
DETECTOR OUTPUT -4
+-
1 SECOND
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LOW MEDIUM HIGH
ACTIVITY
110
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PACING
Comparison to sinus rhythm during exercise: In 9 patients the pacing rate in activity mode could be compared with the underlying sinus rate during treadmill exercise. (Figure 4). In 3, this could also be done in the VVI mode with the pacemaker programmed to a rate of 70 beats/min. In Figure 4 the vertical axis represents the difference between pacing rate and underlying sinus rate expressed as a percentage of the underlying sinus rate. The horizontal axis represents the exercise time in minutes, followed by 3 minutes postexercise. The lower interrupted line of the graph represents the mean differences and standard deviations in the VVI mode: in rest, the sinus rate was faster than the pacing rate (mean difference -19%); this difference increased to -51% during exercise due to the increase in sinus rate while the pacemaker remained at 70 beats/min. After exercise, this difference gradually decreased to -34%. The top, solid line of the graph represents the comparison in the activity mode in 9 patients. At rest, the mean difference is -12% (standard deviation 15%], which decreases to mean values between 1 and 8% during the various stages of exercise (standard deviation between 6 and 9%) As can be seen from Figure 4, the pacemaker accelerates a little faster than the underlying sinus rate in the first minute from a mean difference of -12% in rest to t7% in the first minute. During exercise the correlation is very good, with a maximum mean difference in rate of only 8%. The maximum mean rate achieved was 122 beats/min. After exercise, the rate of the pacemaker decelerates quicker than the sinus rate when a difference of -21% is reached in the first minutes of rest. This quick deceleration did, however, not give rise to symptoms. Exercise time in VVI mode versus activity mode: Five patients performed a treadmill exercise test in the VVI mode as well as in the activity mode as described in the European clinical investigation report7 They were not informed about the mode in which the pacemaker was programmed. They showed a mean percent increase in exercise time of 37% and a mean percent increase in heart rate of 75%. All patients experienced the exercise test in the active mode as being easier and recovery much faster after exercise. One patient had severe coronary artery disease and preferred to have I
- - -
his pacemaker function in the VVI mode because he had less angina in this mode. During the exercise test, he experienced the activity mode as easier but only exercised for 1 minute longer. Both tests were terminated because of angina. Circuit: Eleven patients walked a fixed circuit in the hospital, with personnel to record the events. The circuit was walked in about 20 minutes and included periods of rest, casual walking and a flight of stairs. Continuous electrocardiographic recordings were made during this “tour.” The mean heart rate in rest was 74 beats/min. During walking, the rate fluctuated due to changes in walking tempo and brief interruptions. The mean maximum rate during walking was 103 beats/min, climbing stairs, it was 120, whereas coming down the stairs it was 130 beats/min. Ambulatory electrocardiographic monitoring: No pacing-induced arrhythmias were observed in the 31 patients monitored for 24 hours. The highest mean pacemaker rate was 122 beats/min. When relating the pacing rate to the reported activity, no inappropriate rate increase was seen except in 1 patient. There was an increase in rate from 100 to 125 beats/min when he leaned his elbow (same side as the pacemaker) against the doorpost during car driving. Because of this observation, pacemaker behavior during automobile travel was further analyzed. Car driving: In 12 patients data could be analyzed during car driving. In 3 of them the rate could be compared with the underlying sinus rhythm, which was higher in each case (sinus rate varied from 70 to 105 beats/min). In addition, 5 healthy volunteers were monitored during car driving. As shown in Figure 5, their heart rates varied from 70 to 140 beats/min, mean minimum rate was 85 and maximum was 112 beats/ min. As depicted in Figure 5, the pacing rate during car driving varied from 72 to 110 beats/min, Mean minimum rate was 80 and maximum 99 beats/min. From these data, it would appear that the pacing rate adopted by the pacemaker during car driving is increased but not inappropriately, except in the 1 patient when resting his elbow against the door. Arrhythmias: In 2 patients the activity pacemaker could be used for rate responsive pacing as well as to terminate reentry tachycardia by tapping the pace-
ACTIVITY MODE VVI MODE
T
T
T
FIGURE 4. Graph comparlng slnus rate (SR) and rate adopted by the pacemaker during exercise testing. See text.
-6O+
1 1
I I
I I
PRE
1
2
11’1, I
1
3
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I
I
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I I
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January 1, 1988
maker can to overdrive the tachycardia.8 In 1 patient AV conduction was modified after His bundle ablation for atria1 fibrillation. Although the ventricular rate during atria1 fibrillation was low, she found the paroxysmal irregular rate very unpleasant. This was solved by programming the basic rate to 80 pulses/min, and an aggressive rate response that gave a continuously paced, regular rhythm. Irregular pacing intervals: In 7 patients the pacing interval varied by about 100 ms at rates >115 pulses/ min. This phenomenon was most likely due to electrode polarization potentials, which were detected at the beginning of the noise sampling window, thus resetting the activity-determined pacing interval 100 ms after the previously paced event. In 4 of the 7 patients it occurred incidentally, in 1 it occurred every fifth to seventh beat. In 1 patient it occurred at a sensitivity value of 1.25 mV but not at 2.5 mV. In 1 patient it was seen in doublet form (Figure 6). In all patients this phenomenon was absent at half amplitude output (2.7 V), supporting our hypothesis that electrode polarization was the underlying cause of this phenomenon. Timing anomaly: Figure 6 also shows a timing anomaly of the device. The narrow arrow indicates a sensed premature beat followed by a paced ventricular complex with a coupling interval of 400 ms, which corresponds to the selected maximum activity rate of 150 pulses/min. This occurs when a sensed event and a timing signal from the activity sensing circuit occur within 8 ms of each other. It was seen occasionally and did not give rise to symptoms.
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Discussion This rate responsive pacemaker is safe and increases heart rate effectively and appropriately during exercise, as was seen when comparing the pacing rate during exercise with the underlying sinus rate. It is difficult to correlate rate response to activity on a minute-to-minute basis from the AEM recordings with di-
CAR DRIVING 150 N =5
N = 12 140
/ 130 / l204
IlOloo-
+
go-
80-
+
7060
MIN.
MAX.
MIN.
MAX.
ACTIVITRAX HEALTHY VOLUNTEERS FIGURE 5. Graph illustrating minimum and maximum heart rate and mean rate durlng car drlvlng. The left side represents the rate adopted by the actlvlty-sensing pacemaker. The right slde depicts the minimum and maxlmum rates of 5 healthy volunteers.
wr
FIGURE 6. Simultaneous &channel electrocardiogram during exercise showing 2 anomalles. First, the paced rhythm shows lntermlttent prolongation of the actlvlty escape Interval by about 100 ms, due to electrode polarlzatlon. Second, the lnterva! between a sensed ventricular premature beat (fhfn arrow) and the paced ventricular event thereafter Is 400 ms, which equals the programmed upper rate Interval. See text.
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PACING
ary. However, when activity lasted several minutes, this could be done and the pacing rate was never inappropriate for the activity reported. Car driving did not result in an inappropriate increase in heart rate except in 1 situation, when the patient rested his elbow against the car door. The device is easy to program, as demonstrated by the success of the simple protocol we used. It does not require extensive testing or adjusting during followup. A medium activity threshold was usually used unless the patient had a very soft or slow gait. Rate response curves 1 to 5 were never used. The patient merely walked up and down the room a number of times to see if the programmed settings gave a pacing rate of approximately lOO/min during casual walking. An upper rate response of 100 pulses per minute was selected in 1 patient with angina, most of the others were programmed to 125. No pacing-induced arrhythmias were seen during exercise testing or 24-hour AEM. Irregular pacing intervals due to electrode polarization were seen infrequently. It was not of clinical importance and did not give rise to symptoms. Electrocardiographically, a slightly irregular rhythm was seen with intermittent increase of the pacing interval by about 100 ms, as shown in Figure 6. The timing anomaly in which the pacing interval shortens to the programmed upper rate interval when a sensed event and a timing signal from the activity sensing circuit coincide within a ms occurred incidentally. This group of patients did, however, have a pre-
dominantly paced rhythm. Selecting a maximum activity rate of 125 pulses/min will limit the coupling interval to 480 ms while still allowing adequate rate response. This anomaly has been corrected in presently manufactured devices. AA1 rate responsive pacing was used in 3 patients successfully. This mode of pacing has the advantage of preserving AV synchrony as well as providing rate response in patients with symptomatic sinus bradycardia and intact AV conduction. Far field sensing of the QRS complex or T wave was not a problem with bipolar leads in the atria1 appendage but might affect pacemaker timing with unipolar models.
References 1. Cammilli L. Alcidi L. Shapland E, Obino S. Results. problems and penpectives with the autoregulating pacemaker. PACE 1983:6:488-493. 2. Wirtzfeld A, Heinze R. Liess HD, Stangl K, Ah E. An active optical sensor for monitoring mixed venous oxygen-saturation for on implantable rate-regulating pacing system. PACE 1983;6:494-497. 3. Rossi P. Plicchi G. Canducci G. Roenoni G. Aina F. Resoiratorv rate OSo deierminont of optimal pacing rdte. FACE 1983;6:502-507: ’ 4. Richards AF, Norman J. Relation between QT interval and heart rate. New design of a phydologicajly adaptive cardiac pacemaker. Br Heart 1 1981; 45:56-61. 5. Griffin JC, Jutzy KR, Claude JP. Knutti JW. Central body temperature as a guide to optima1 heart rate. PACE 1983;6:498-501. 6. Humen DP, Anderson K. Brumwell D, Huntley S, Klein G]. A pacemaker which automatically increases its rate with physical activity. In: Steinbach K, Glogar D. Laszkovics A, Scheibelhofer W, Weber H, eds. Cardiac Pacing. Darmstodt: Steinkopff Verlag, 1983:259-264. 7. Rankin I, Lindemans F. Activitrax Clinical Report. Medtronic Clinical Department, Research and Support Ceptre. Kerkrode, The Netherlands, May 1985. 8. deli Dulk K, Brugada P, Wellens HJJ. Tachycardip termination with o rate responsive ppcemaker. Am J Cardiol 1987;59:1424-1426.
@ses/min, respectively). No paclq-induced arrftythmias were seen durln9 ambulatory mrdkgraphic monitorln9. At ht@ pacing rates sl@htly irregular pacing intervals were sometimes observed, whkh was due to polar&&n senskg. Sporadically, 1 pacln9 interval shortened to the upper rate value, because of a known and now resolved tImin anomaly. Neither anomaly was of clinical p and the first couki be resolved by reprogramming. lt is concluded that this rate responsive pacemaker is safe, easy to program and incmses heart rate appropriately durln9 exercise; the bipolar version can be used safely for atrial packrg; and rate reqxmse and activfty Weshokl values that give a pa&9 rate of about 100 pulses/min durbq casual wafkkrg provkfe appropriate changes in heart rate durkg normal dally life.
Bipolar Medtronk Actlvitrax rate respondve pacemakers were implanted in 31 patfents for ventrkular (28) or atrial (3) pachrg. Mean follow-up was 18 months (range 10 to 28). Twenty pacemakers were implanted after catheter ablation of the His bundle, 7 for skk sinus syndrome, 1 for attioventrkxrlar bbckand3forsickslnussy&romewlthatrioventrkular block. A rate response value was selected that gave a pack19 rate of about 100 pulses/min &ring walkkrg. 01 the 31 patients, all had 24-hour ambulatory elecbocardkgraphic monltortn9 with diary, 11 walked a 20-mltxfte &cult, lncludln9 a flight of stairs, and 20 had a treadmill exercise test. In 9 patients the pacing rate coukl be compared wlth the underlying slnus rate durtq exercise and was seen to match it very closely. In 12 patients ths pacirrg rate durin9 car drlvltg was found to be similar to the sinus rate of 5 volunteers under similar condttkms (mean minimum and maximum rate was 80 and 99
(Am J Cardloi 1988;81:107-112)
F
or patients with normal sinoatrial function and atrioventricular (AV] block, atria1 synchronized pacing (DDD, VDD, VAT) provides AV synchrony and rate responsiveness. However, patients with abnormal sinoatrial function or atria1 arrhythmias may not be able to benefit from such a system. In these patients, a parameter other than atria1 activity must be used to provide changes in heart rate appropriate to metabolic needs. Parameters currently being used or investigated in pacing systems include: pH, venous oxygen saturation, respiratory rate and volume, QT interval, temperature and physical activity.1-6 This article presents our experience with the Medtronic model 8400 Activitrax rate responsive pacemaker, which detects and tracks physical activity. This study covers several From the Department of Cardiology, Academic Hospital Maastricht, University of Limburg, Maastricht, The Netherlands, and Medtronic I.R.S.C. B.V. Maastricht, The Netherlands. Manuscript received April 20,1987;revised manuscript received September l&1987, and accepted September 12. Address for reprints: Karel den Dulk, MD, Department of Cardiology, Academic Hospital Maastricht, University of Limburg, P.O. Box 1918,620l BX Maastricht, The Netherlands. 107
aspects: evaluation of appropriateness of pacing rate during normal daily life, evaluation of a simple programming protocol of the rate control variables and detection of potential anomalies in pacemaker behavior.
Methods Pacemaker description: The Medtronic model 8400 Activitrax pacemaker is a bipolar, multiprogrammable, single-chamber atrial- or ventricular-inhibited pulse generator with automatic rate variability and telemetry. Four pacing modes, namely, VVI or AA1 + activity, VVI or AAI, VOO or A00 + activity and VOO or AOO, can be selected using the Medtronic model 9710 programmer. In the activity modes, the pacemaker’s stimulation rate is controlled by the activity-sensing detector and circuitry. The sensor, a piezoelectric crystal on the inside of the pacemaker, transforms the mechanical vibrations in the body resulting from physical activity into an electrical signal that is further processed for rate control. The rate can vary between the programmable “basic rate” (80, 70 or 80 pulses/min) and the “maximum activity rate” (100, 125 or 150 pulses/min).
108
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SENSOR FOR RATE RESPONSIVE
PACING
The programmable “activity threshold” (low, medium or high) establishes a level that must be exceeded by the output signal of the activity sensor in order to influence the pacing rate. The programmable “rate response” (10 settings] determines the relation between pacing rate and the output signal of the activity sensor when it exceeds the activity threshold. Rate response 1 provides the lowest, 10 the highest pacing rate at a fixed activity level. Each increment from 1 to 10 shortens the pacing interval by 25 ms. When the detected activity level suddenly increases from 1 steady-state level to another, the pacing rate will increase immediately, but it will take about 25 seconds to cover 90% of the required increase in rate. For decreasing the rate, it takes about 90 seconds to cover 90% of the required decrease in rate. Other programmable parameters are pulse amplitude (2.7 or 5.0 V], pulse width (from 0.05 to 1.5 ms), sensitivity (1.25, 2.5,5.0 mV or asynchronous) and hysteresis (in VVI or AA1 mode only]. The refractory period is 225 ms and is not programmable. All pacemakers were connected to bipolar Medtronic Model 4012 or 4512 Target Tip leads. Patients: Thirty-one Activitrax pacemakers were implanted. Sixteen patients were men and 15 were women, ages 40 to 79 years (mean 62). Follow-up ranged from 10 to 26 months (mean 15.8).In 28 patients the pacing lead was placed in the right ventricle, and in 3, in the right atrium. In 20, the pacemaker was
FIGURE 1. Brief Inhibition ol the pacemaker during a treadmill pacemaker wlth the underlylng sinus rhythm.
exercise
implanted after catheter ablation of the bundle of His (atria1 fibrillation, 16; AV nodal tachycardia, 2; circus movement tachycardia, 1; atria1 tachycardia, 11.Other indications were: symptomatic sinus bradycardia (3 patients], replacement of a VVI pacemaker for total AV block (1 patient], bradycardia-tachycardia syndrome (4 patients) and symptomatic sinus bradycardia with AV conduction disturbances (3 patients]. Three patients with isolated sinus bradycardia were paced from the right atrium, the others received VVI pacing. All pacemakers were implanted in the pectoral muscle region. Associated cardiac disease: Eight patients had coronary artery disease, of which 5 had an old myocardial infarction; 6 patients had mitral regurgitation, 2 had a mitral valve prosthesis, 1 had an aortic valve prosthesis, 2 had congestive cardiomyopathy and 2 had ventricular tachycardia due to an old myocardial infarction. Eleven patients were taking antiarrhythmic drug therapy; 6 nitrate, 7 oral anticoagulant, 8 diuretic and 4 vasodilator therapy. Protocol: Underlying rhythm: During follow-up visits, the pacemaker was programmed to the VVI mode 40 pulses/min to study the underlying rhythm. Initial testing: To obtain an impression of the effects of changing the activity threshold and rate response parameters, we compared the pacing rate to the underlying sinus rate in 2 patients with a normal sinus node function during rest and casual walking. Since
test was done to compare the rate adopted by the actlvtty SenSlng
January 1, 1988
we considered a pacing rate of about 100 beats/min appropriate for casual walking, this formed the basis for all our initial postimplant programming of rate response and activity threshold. The appropriateness of this simple programming protocol was checked by a treadmill exercise test if possible, a 20-minute walking circuit that included a flight of stairs and X-hour ambulatory electrocardiographic monitoring (AEM) with diary. Five patients exercised according to the Naughton protocol in the VVI mode and in the activity mode as part of the European clinical investigation.7 Fifteen were exercised according to the Bruce protocol. Comparison to sinus rhythm: In order to compare the pacing rate to the underlying sinus rate during treadmill exercise testing, the pacemaker was inhibited briefly once a minute (if necessary), as illustrated in Figure 1. Patients with sick sinus syndrome, 1 to 1 ventriculoatrial conduction or supraventricular tachycardia at the time of comparison were excluded. Irregular pacing intervals: Because intermittent oversensing was observed, this phenomenon was checked during exercise testing, 24-hour AEM and at follow-up by tapping the pacemaker can with 2 or 3 fingers to reach the selected maximum activity rate while recording a 6-channel electrocardiogram. During testing for polarization, the pacemaker was programmed as follows: sensitivity 2.5 mV for ventricular use, 1.25 for atria1 use, output 5.0 V, pulse width 0.5 ms,
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with upper rate settings of 125 pulses/min (23 patients) or 150 pulses/min (8 patients].
Results Underlying rhythm: In all patients the heart rate was usually determined by the pacemaker. In 1 patient with paroxysmal AV nodal tachycardia and sick sinus syndrome, His bundle ablation resulted in a first-degree AV block with persistence of paroxysmal tachycardias. In this patient the pacemaker was also used to terminate tachycardia by tapping on the pacemaker can on the recognition of tachycardia, as described.a Initial testing: The effect of programming the activity threshold to low, medium or high was evaluated shortly after implant in 2 patients who bad normal sinus node function. Figure 2 illustrates the result in 1 of these patients for rate response 8. In order to obtain a pacing rate of about 100 pulses/min during casual walking [our target for initial programming of the pacemaker) while maintaining a pacing rate close to the selected basic rate at rest, the medium activity threshold was usually appropriate. Fine tuning was obtained by adjustipg the rate response value, and rate response 8 was used in 24 patients. In 6 patients with a soft gait (shufflers or tiptoers) (Figure 3) an activity threshold low was necessary to obtain the required rate of 100 beats/min during casual walking. Activity threshold high was never used, nor were rate response values <6.
MEDIUM
LOW 0
8
HIGH 8
120 -c-----p----SINUS
RATE
FIGURE 2. Comparison between pacemaker rate and sinus rate during casual walking, for activlJy thresholds low, medium, anfj high with rate response 8. 70 60-w: 0
FIGURE 3. Illustrates 3 walking styles. The shuffler (Mf), who generates lowamplltude s!gnals at a low rate as opposed to the quick step artist (right) who generates high-amplitude slgnals at a high rate are compared with a.presumed normal walker (animated sensor slgnal). As can be Fee? from the figure, a low threshold must be programmed for the shuffler, whereas a’ medluq threshold would be appropriate for the quick step artlst.
36
72
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+tt+++j,+~ 0 36 72 TIME
108 ILL 180 IN SECONDS
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++++t+&+t++ 0 36 72
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180
HIGH MEDIUM LOW
I
I
DETECTOR OUTPUT -4
+-
1 SECOND
I
I’
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ACTIVITY
110
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Comparison to sinus rhythm during exercise: In 9 patients the pacing rate in activity mode could be compared with the underlying sinus rate during treadmill exercise. (Figure 4). In 3, this could also be done in the VVI mode with the pacemaker programmed to a rate of 70 beats/min. In Figure 4 the vertical axis represents the difference between pacing rate and underlying sinus rate expressed as a percentage of the underlying sinus rate. The horizontal axis represents the exercise time in minutes, followed by 3 minutes postexercise. The lower interrupted line of the graph represents the mean differences and standard deviations in the VVI mode: in rest, the sinus rate was faster than the pacing rate (mean difference -19%); this difference increased to -51% during exercise due to the increase in sinus rate while the pacemaker remained at 70 beats/min. After exercise, this difference gradually decreased to -34%. The top, solid line of the graph represents the comparison in the activity mode in 9 patients. At rest, the mean difference is -12% (standard deviation 15%], which decreases to mean values between 1 and 8% during the various stages of exercise (standard deviation between 6 and 9%) As can be seen from Figure 4, the pacemaker accelerates a little faster than the underlying sinus rate in the first minute from a mean difference of -12% in rest to t7% in the first minute. During exercise the correlation is very good, with a maximum mean difference in rate of only 8%. The maximum mean rate achieved was 122 beats/min. After exercise, the rate of the pacemaker decelerates quicker than the sinus rate when a difference of -21% is reached in the first minutes of rest. This quick deceleration did, however, not give rise to symptoms. Exercise time in VVI mode versus activity mode: Five patients performed a treadmill exercise test in the VVI mode as well as in the activity mode as described in the European clinical investigation report7 They were not informed about the mode in which the pacemaker was programmed. They showed a mean percent increase in exercise time of 37% and a mean percent increase in heart rate of 75%. All patients experienced the exercise test in the active mode as being easier and recovery much faster after exercise. One patient had severe coronary artery disease and preferred to have I
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his pacemaker function in the VVI mode because he had less angina in this mode. During the exercise test, he experienced the activity mode as easier but only exercised for 1 minute longer. Both tests were terminated because of angina. Circuit: Eleven patients walked a fixed circuit in the hospital, with personnel to record the events. The circuit was walked in about 20 minutes and included periods of rest, casual walking and a flight of stairs. Continuous electrocardiographic recordings were made during this “tour.” The mean heart rate in rest was 74 beats/min. During walking, the rate fluctuated due to changes in walking tempo and brief interruptions. The mean maximum rate during walking was 103 beats/min, climbing stairs, it was 120, whereas coming down the stairs it was 130 beats/min. Ambulatory electrocardiographic monitoring: No pacing-induced arrhythmias were observed in the 31 patients monitored for 24 hours. The highest mean pacemaker rate was 122 beats/min. When relating the pacing rate to the reported activity, no inappropriate rate increase was seen except in 1 patient. There was an increase in rate from 100 to 125 beats/min when he leaned his elbow (same side as the pacemaker) against the doorpost during car driving. Because of this observation, pacemaker behavior during automobile travel was further analyzed. Car driving: In 12 patients data could be analyzed during car driving. In 3 of them the rate could be compared with the underlying sinus rhythm, which was higher in each case (sinus rate varied from 70 to 105 beats/min). In addition, 5 healthy volunteers were monitored during car driving. As shown in Figure 5, their heart rates varied from 70 to 140 beats/min, mean minimum rate was 85 and maximum was 112 beats/ min. As depicted in Figure 5, the pacing rate during car driving varied from 72 to 110 beats/min, Mean minimum rate was 80 and maximum 99 beats/min. From these data, it would appear that the pacing rate adopted by the pacemaker during car driving is increased but not inappropriately, except in the 1 patient when resting his elbow against the door. Arrhythmias: In 2 patients the activity pacemaker could be used for rate responsive pacing as well as to terminate reentry tachycardia by tapping the pace-
ACTIVITY MODE VVI MODE
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FIGURE 4. Graph comparlng slnus rate (SR) and rate adopted by the pacemaker during exercise testing. See text.
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maker can to overdrive the tachycardia.8 In 1 patient AV conduction was modified after His bundle ablation for atria1 fibrillation. Although the ventricular rate during atria1 fibrillation was low, she found the paroxysmal irregular rate very unpleasant. This was solved by programming the basic rate to 80 pulses/min, and an aggressive rate response that gave a continuously paced, regular rhythm. Irregular pacing intervals: In 7 patients the pacing interval varied by about 100 ms at rates >115 pulses/ min. This phenomenon was most likely due to electrode polarization potentials, which were detected at the beginning of the noise sampling window, thus resetting the activity-determined pacing interval 100 ms after the previously paced event. In 4 of the 7 patients it occurred incidentally, in 1 it occurred every fifth to seventh beat. In 1 patient it occurred at a sensitivity value of 1.25 mV but not at 2.5 mV. In 1 patient it was seen in doublet form (Figure 6). In all patients this phenomenon was absent at half amplitude output (2.7 V), supporting our hypothesis that electrode polarization was the underlying cause of this phenomenon. Timing anomaly: Figure 6 also shows a timing anomaly of the device. The narrow arrow indicates a sensed premature beat followed by a paced ventricular complex with a coupling interval of 400 ms, which corresponds to the selected maximum activity rate of 150 pulses/min. This occurs when a sensed event and a timing signal from the activity sensing circuit occur within 8 ms of each other. It was seen occasionally and did not give rise to symptoms.
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Discussion This rate responsive pacemaker is safe and increases heart rate effectively and appropriately during exercise, as was seen when comparing the pacing rate during exercise with the underlying sinus rate. It is difficult to correlate rate response to activity on a minute-to-minute basis from the AEM recordings with di-
CAR DRIVING 150 N =5
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ACTIVITRAX HEALTHY VOLUNTEERS FIGURE 5. Graph illustrating minimum and maximum heart rate and mean rate durlng car drlvlng. The left side represents the rate adopted by the actlvlty-sensing pacemaker. The right slde depicts the minimum and maxlmum rates of 5 healthy volunteers.
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FIGURE 6. Simultaneous &channel electrocardiogram during exercise showing 2 anomalles. First, the paced rhythm shows lntermlttent prolongation of the actlvlty escape Interval by about 100 ms, due to electrode polarlzatlon. Second, the lnterva! between a sensed ventricular premature beat (fhfn arrow) and the paced ventricular event thereafter Is 400 ms, which equals the programmed upper rate Interval. See text.
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ACTIVITY
SENSOR FOR RATE RESPONSIVE
PACING
ary. However, when activity lasted several minutes, this could be done and the pacing rate was never inappropriate for the activity reported. Car driving did not result in an inappropriate increase in heart rate except in 1 situation, when the patient rested his elbow against the car door. The device is easy to program, as demonstrated by the success of the simple protocol we used. It does not require extensive testing or adjusting during followup. A medium activity threshold was usually used unless the patient had a very soft or slow gait. Rate response curves 1 to 5 were never used. The patient merely walked up and down the room a number of times to see if the programmed settings gave a pacing rate of approximately lOO/min during casual walking. An upper rate response of 100 pulses per minute was selected in 1 patient with angina, most of the others were programmed to 125. No pacing-induced arrhythmias were seen during exercise testing or 24-hour AEM. Irregular pacing intervals due to electrode polarization were seen infrequently. It was not of clinical importance and did not give rise to symptoms. Electrocardiographically, a slightly irregular rhythm was seen with intermittent increase of the pacing interval by about 100 ms, as shown in Figure 6. The timing anomaly in which the pacing interval shortens to the programmed upper rate interval when a sensed event and a timing signal from the activity sensing circuit coincide within a ms occurred incidentally. This group of patients did, however, have a pre-
dominantly paced rhythm. Selecting a maximum activity rate of 125 pulses/min will limit the coupling interval to 480 ms while still allowing adequate rate response. This anomaly has been corrected in presently manufactured devices. AA1 rate responsive pacing was used in 3 patients successfully. This mode of pacing has the advantage of preserving AV synchrony as well as providing rate response in patients with symptomatic sinus bradycardia and intact AV conduction. Far field sensing of the QRS complex or T wave was not a problem with bipolar leads in the atria1 appendage but might affect pacemaker timing with unipolar models.
References 1. Cammilli L. Alcidi L. Shapland E, Obino S. Results. problems and penpectives with the autoregulating pacemaker. PACE 1983:6:488-493. 2. Wirtzfeld A, Heinze R. Liess HD, Stangl K, Ah E. An active optical sensor for monitoring mixed venous oxygen-saturation for on implantable rate-regulating pacing system. PACE 1983;6:494-497. 3. Rossi P. Plicchi G. Canducci G. Roenoni G. Aina F. Resoiratorv rate OSo deierminont of optimal pacing rdte. FACE 1983;6:502-507: ’ 4. Richards AF, Norman J. Relation between QT interval and heart rate. New design of a phydologicajly adaptive cardiac pacemaker. Br Heart 1 1981; 45:56-61. 5. Griffin JC, Jutzy KR, Claude JP. Knutti JW. Central body temperature as a guide to optima1 heart rate. PACE 1983;6:498-501. 6. Humen DP, Anderson K. Brumwell D, Huntley S, Klein G]. A pacemaker which automatically increases its rate with physical activity. In: Steinbach K, Glogar D. Laszkovics A, Scheibelhofer W, Weber H, eds. Cardiac Pacing. Darmstodt: Steinkopff Verlag, 1983:259-264. 7. Rankin I, Lindemans F. Activitrax Clinical Report. Medtronic Clinical Department, Research and Support Ceptre. Kerkrode, The Netherlands, May 1985. 8. deli Dulk K, Brugada P, Wellens HJJ. Tachycardip termination with o rate responsive ppcemaker. Am J Cardiol 1987;59:1424-1426.