Paul A. Levine

Industry Viewpoint: St. Jude Medical: Pacemakers, ICDs and MRI

Magnetic resonance imaging was introduced and began to gain popularity in the early to mid 1980s. The concern at that time was that any implanted metal object might be pulled out of the body or permanently damaged by the very strong magnetic field. However, both pacemakers and ICDs utilize nonferrous metals and as such, a device ripped out of the body has never been realized. Based on in vitro and in vivo studies,1,2 the fluctuating magnetic fields generated by the MRI systems were demonstrated to potentially inhibit and/or trigger pacemakers, result in false detection of tachyarrhythmias leading to inappropriate ICD therapy or induce a current in the lead resulting in rapid cardiac stimulation well above the runaway protection limit of these devices.2 Most of the time, the induced currents were subthreshold and of no clinical consequence.3 There were a number of anecdotal incidents, commonly reported to the FDA and not published in the peerreviewed medical literature, in the early days of MRI studies noting both device malfunction as well as patient death associated with MRI studies. These usually occurred in patients undergoing an emergency study without the attending staff being aware that the patient had a pacemaker. The patients’ implanted systems were not evaluated prior to the study nor were their rhythm monitored during the MRI study. It is not known if the deaths were due to an adverse interaction between the MRI and the implanted system or due to an unrelated event with the coincidental presence of an implanted electronic device. However, based on the anecdotal reports and studies involving earlier generation devices, the initial recommendation of the radiology community and the MRI manufacturers was to proscribe the performance of an MRI study in any patient with an implanted pulse generator or ICD. Over the ensuing 20 years, the protective circuitry and resistance to external interference of implanted devices have improved. At the same time, the MRI systems have become more sophisticated and are being used for multiple diagnostic purposes. The prohibition with respect to MRI

Automatic adjustment of pacing output in the clinical setting

BACKGROUNDnAutoCapture (AC) is a programmable feature that enables the pacemaker to both track the capture threshold and automatically adjust the output on a beat-by-beat basis. Although AC safely and significantly reduces the current drainage, some authors have argued that the longevity benefit of such a system is overstated. This study aims to estimate the longevity extension that can be obtained, in the clinical routine, by turning the AC on in comparison to pacemakers programmed to operate at the shipped and manually optimized output.nnnMETHODSnWe selected 83 consecutive patients who received implanted St Judes Affinity pacemakers >6 months earlier. Eight patients died or were lost to follow-up and in 9 subjects the AC could not be turned on. In the remaining 66 patients, current drain and estimated longevity were compared in 3 situations: (1) AC on; (2) AC off, optimized programming (100%-150% voltage threshold); (3) AC off, shipped output (3.5 V).nnnRESULTSnFive patients had large variations (>1 V) of the AC threshold. Current drainage was 8.0 +/- 0.9 mA in the AC group, 8.7 +/- 1.8 mA with AC off and optimized programming, and 11.3 +/- 2.3 mA at shipped output (P <.01). Estimated longevity was significantly extended (P <.01) by AC (12.1 +/- 1.0 years) when compared to shipped (8.9 +/- 1.7 years) and optimized programming (11.3 +/- 1.4 years).nnnCONCLUSIONnReprogramming the pacemaker output significantly enhanced its estimated longevity; AC added a moderate but significant extension over manual reprogramming and was associated with increased safety in patients with large ventricular threshold variations.

Documentation of acute rise in ventricular capture thresholds associated with flecainide acetate.

FORNIELES‐PÉREZ, H., et al.: Documentation of Acute Rise in Ventricular Capture Thresholds Associated with Flecainide Acetate. Treatment of paroxysmal atrial fibrillation with flecainide acetate resulted in a 4‐fold increase in ventricular capture thresholds. The detailed time course of the threshold increase and decrease was documented using the AutoCapture algorithm and graphic summary.

Enhanced rate response algorithm for orthostatic compensation pacing.

Upon orthostatic stress after a period of rest, the heart rate increases rapidly to maintain cardiac output and minimize the fall in arterial pressure. Pacemaker patients are often prone to a deficient response to orthostatic stress. This may cause lightheadedness and, in rare patients with autonomic dysfunction, syncope. To alleviate these undesirable consequences, an enhanced rate response algorithm was developed using an accelerometer. The pacemaker generates two signals from its accelerometer: instantaneous activity level (Act) and long‐term change in activity level (ActVar). Low values of both Act and ActVar indicate a resting state. An increase in Act while ActVar remains low indicates the onset of motion after prolonged rest. Upon detecting this transition, the algorithm increases the pacing rate to a programmable orthostatic compensation rate for a programmable duration. A taped‐on pacemaker with this algorithm was evaluated in three healthy women and two healthy men, 36 ± 8 years of age. Electrocardiogram and ventricular pacing pulses were recorded by a 24‐hour ambulatory system. Each trigger of the orthostatic compensation rate was verified against a >10 beats/min increase in heart rate, a response classified as appropriate. The overall specificity of the algorithm among the five subjects was 78%. The nocturnal specificity (10 pm to 7 am) was 98%, considerably higher than during daytime (72%). In conclusion, a pacing algorithm to alleviate orthostatic stress was developed, which was highly specific during the night hours.

Serial changes in right ventricular apical pacing lead impedance predict changes in left ventricular ejection fraction and functional class in heart failure patients.

Pacing impedance has been proposed to monitor the clinical status of patients with congestive heart failure (CHF). This study examined whether changes in right ventricular (RV) pacing impedance correlate with changes in left ventricular ejection fraction (LVEF) and New York Heart Association (NYHA) functional class during long‐term follow‐up in pacemaker recipients with CHF. The study included 67 patients, 70 ± 12 years of age, in NYHA class II or III, and with a mean LVEF = 29 ± 8% at implant. LVEF, NYHA class, and bipolar pacing impedance at the RV outflow tract (RVOT) and apex (RVA) were measured at implant and at 3, 6, 9, and 12 months of follow‐up. At implant, impedance was similar in RVOT (548 ± 115 Ω) and RVA (571 ± 174 Ω). Between implant and 3 months, mean impedance decreased (P < 0.0001) at both the RVOT (472 ± 62 Ω) and RVA (488 ± 86 Ω), LVEF increased (43 ± 14%, P < 0.0001), and the NYHA class decreased from 2.4 ± 0.5 to 2.1 ± 0.6 (P = 0.0001). Changes in RVA impedance correlated with changes in LVEF (r = 0.45, P = 0.002). A 50 Ω decrease in RVA impedance corresponded to a 3% decrease in LVEF. RVA impedance decreased significantly as NYHA class increased from I to IV (P = 0.04). There was no correlation between impedance measured at the RVOT and LVEF or NYHA class. A decrease in bipolar pacing impedance at the RVA was associated with worsening LVEF and the NYHA class. The use of pacing impedance to monitor the clinical status in CHF is dependent on the RV pacing site.

Device Eccentricity: Postmagnet Behavior of DDDR Pacemakers with Automatic Threshold Tracking

LEVINE, P.A., et al.: Device Eccentricity: Postmagnet Behavior of DDDR Pacemakers with Automatic Threshold Tracking. The normal pacing system function associated with the AutoCapture Pacing System algorithm triggered concern on the part of the clinical staff caring for the patient when the initiation of a threshold search sequence was detected during a transtelephonic follow‐up evaluation. The analysis of the rhythm demonstrates that the behavior of the system is normal and consistent with the design of the AutoCapture algorithm. As a variety of new algorithms are introduced, similar unexpected but normal behaviors can be anticipated.

Activity-Controlled Circadian Base Rate

The current pacing rates are clustered around a fixed base rate since pacemaker patients are usually sedentary, resting, or sleeping most of the time. This fixed base rate is either too low for daytime hemodynamic support or too high for nighttime rest and recovery. Multiple Holter studies involving normal individuals have suggested that the resting base rate fluctuates during the course of the day. The circadian base rate (CBR) algorithm was designed to provide patients with a circadian change in paced resting rate and a normal rate distribution. The CBR algorithm, using a sophisticated accelerometer sensor, was developed and tested using the downloaded activity data from patients implanted with Trilogy DR+ pacemakers. Twenty‐five patients (19 men, 6 women, age 72 ± 9 years) were studied. Trilogy DR+ is able to record the detailed sensor and system behavior data for a week. During outpatient visits, the pacemaker was interrogated and the data accumulated in the pacemaker memory were downloaded. The CBR algorithm was applied to the activity variance histogram to calculate the base rate and to construct its histogram. The base rates in the CBR histogram are generally below 100 ppm with a distribution that mimics the natural sinus rate distribution of normal subjects. The CBR algorithm provides the highest daytime rates for hemodynamic support and the lowest nighttime rates for cardiac recovery, with a smoothly changing base rate modeling the normal circadian variation in heart rate.

Implementation of automatic mode switching in Pacesetter's Trilogy DR+ and Affinity DR pulse generators

Zusammenfassung Diese Arbeit erläutert die Grundlagen und die wichtigsten Eigenschaften der Algorithmen zur automatischen Betriebsartumschaltung (Automatic Mode Switch) der beiden frequenzadaptiven Zweikammer-Herzschrittmacher Trilogy DR+ und Affinity DR einschließlich der feinen Unterschiede zwischen beiden Geräten. Von besonderer Bedeutung erscheinen die unabhängig voneinander programmierbaren Werte der oberen Grenzfrequenz der maximalen Sensorfrequenz und der atrialen Tachykardie-Erkennungsfrequenz sowie die postventrikuläre atriale Ausblendzeit. Es werden Hinweise zur Beurteilung der An- oder Abwesenheit von Fernfeld-R-Wellen gegeben, die Einzelheiten der Berechnung des zur atrialen Frequenz gehörenden Intervalls (FARI) für zu- und abnehmende atriale Frequenzen, sowie der Einfluß dieser Berechnung auf den Wechsel in die Betriebsartumschaltung oder aus ihr zurück werden erläutert. Unter Zuhilfenahme der zahlreich vorhandenen Ereigniszähler, wie das Histogramm der automatischen Betriebsartumschaltung, das Zustandsdiagramm, die Zustandsdiagnostik und die vom Patienten markierten Episoden/Schnappschüsse werden verschiedene Methoden zur Bewertung des Verhaltens des Mode-Switch-Algorithmus vorgestellt. Die Interpretation dieser Episodenzähler bei der Bewertung der Effektivität jeglicher therapeutischen Optionen wird diskutiert, seien sie medikamentös oder apparativ in Form einer Veränderung der Stimulationsparameter des Herzschrittmachers.Summary This paper reviews the rationale for and key features of the automatic mode switching algorithms in the Pacesetter Trilogy DR+ and Affinity DR dual chamber rate-modulated pulse generators along with the subtile differences between these two devices. Critical parameters include independently programmable maximum tracking, maximum sensor and atrial tachycardia detection rates and the post-ventricular atrial blanking period. Guidance is provided as to the assessment of the presence or absence of a far-field R-wave, details as to how the filtered atrial rate interval is calculated in the presence of increasing and decreasing atrial rates and the impact of this calculation on entering or exiting mode switching. Guidelines are provided in programming the sensor using the prediction model. Various methods of assessing the behavior of the mode switch algorithm are provided using multiple different event counters including the automatic mode switch histogram, the event histogram, the event record, and the patient triggered event record and/or event snapshot. The use of these event counters to assess the effectiveness of any therapeutic intervention, be it pharmacologic or a change in the pacing parameters, is discussed.

Effect of Pentaphasic Pulse Sequence as an Impedance Sensor on Standard Electrocardiographic Recordings

Two advances in cardiac pacing have resulted in an internal conflict in some pacemakers. One is the development of a standard lead physiological sensor and the other is protection from electromagnetic interference (EMI). One popular type of standard lead sensor uses sub‐threshold pulses to measure intracardiac and intrathoracic impedance changes, i.e., minute ventilation. Recent clinical observations and extensive in vitro testing have verified that digital cellular phones can be troublesome. Large feedthrough capacitors (FCs), effective in blocking the EMI, will preclude sensing of the standard impedance‐based signals. A variety of pulse configurations were studied that might be effective for a sensor‐based impedance signal while allowing the pacemaker to continue to use large Fcs protecting them from environmental EMI. In comparison to both monophasic and biphasic pulse sequences, a pentaphasic pulse sequence was effective as an impedance sensor, still allows large FCs to function as an effective filter for environmental EMI, and would not produce artifacts on surface ECG.

RE: Sauer et al, PACE 2006; 29: 1028‐1030

The recent case study by Sauer1 and associates referenced an earlier article in PACE by Suri et al.2 as an example where even beat-by-beat monitoring of capture may be associated with “adverse clinical outcomes.” That reference is correct but the inference is disingenuous. The behavior reported by Suri and colleagues was the result of a software anomaly in St. Jude Medical’s first-generation Ventricular AutoCapture (VAC) algorithm in a dualchamber pacing system (AffinityTM). This case and a couple of others had been reported to St. Jude Medical, the cause identified and the software corrected. Indeed, this behavior was corrected by a software download at the time and this problem did not recur. The appropriate software capable of being downloaded into all of our dualchamber pacemakers with VAC capability was incorporated in the next FDA-approved release of the programmer software such that it was automatically downloaded into every pulse generator, even those that did not display adverse behavior so that it would not occur in the future. The reported issue was therefore related to an anomaly in the initial implementation of the algorithm and not an issue related to the design of the algorithm itself. Dr. Sauer and colleagues make a specific recommendation that “the safety of using algorithms that automatically measure pacing threshold and lower pacing outputs should be considered for each individual patient before this feature is activated.” I absolutely agree with this recommendation and when shipped, VAC is not enabled. The default is a bipolar fixed-output setting. If one wants to enable AutoCapture, there is a specific evaluation to be performed and the system will report whether or not the key parameters are appropriate to allow VAC to be safely enabled.