Table of Contents
Significant Events in the 2000s:
- 2001: World Trade Center (NY), Pentagon (DC) and United Flight 93 attacked by terrorists; 3,000 killed
- 2001: United States war in Afghanistan and Iraq (2003)
- 2001: World's first self-contained artificial heart transplanted
- 2002: Euro becomes European currency
- 2003: Space shuttle Columbia disaster
- 2004: Boston Red Sox win the World Series, first since 1918
- 2004: Tsunami in Far East kills more than 100,000
- 2005: Pope Paul John II dies; Benedict XVI appointed
- 2005: Hurricane Katrina devastates southern US
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Detection Of Asymptomatic Atrial Fibrillation By Implantable Devices
In patients with atrial fibrillation (AF), the clinical judgment whether antiarrhythmic drug therapy is successful or if anticoagulation can safely be withdrawn relies on patients' reports on symptoms associated with AF and ECG documentation of AF recurrences. However, symptom perception and serial ECG recordings may underestimate the recurrence rate of AF. In 110 patients with a history of AF, a pacemaker with dedicated functions for AF detection and electrogram storage was implanted and antiarrhythmic drug treatment was optimized. Patients were regularly followed over a mean of 19 months with evaluation of AF-related symptoms, resting ECG and interrogation of device memory. While the resting ECG showed AF recurrences in 46% of patients, review of stored electrograms confirmed the documentation of AF in 88% of patients. Device memory revealed AF recurrences lasting >48 h in 50 patients of whom 38% were completely asymptomatic and in sinus rhythm at the subsequent follow-up visit. In 67 patients with device-confirmed freedom from AF for at least 3 months, AF recurrences lasting >48 h recurred subsequently in 16%. This study suggests that AF recurrence under antiarrhythmic therapy may be much more frequent than previously suspected, even in the presence of sinus rhythm at follow-up visit and in the absence of any symptoms associated with AF. AF relapses >48 h remain completely asymptomatic in a significant proportion of patients. Freedom from AF for =3 months does not preclude subsequent long-lasting (asymptomatic) AF recurrence and thus does not represent a criterion for safe withdrawal of anticoagulation. Israel CW, Groenefeld G, Ehrlich JR, Li YG, Hohnloser SH. Long-term risk of recurrent atrial fibrillation as documented by an implantable monitoring device: Implications for optimal patient care. J Am Coll Cardiol 2004;43:47-52
Fig. 1: Lack of AF Detection with Serial ECGs. This example of device memory displays the amount of AF detected by the device in hours (hrs) and episodes per day between September 2001 and November 2002. At the time of follow-up visit (arrows: October 2001, April 2002, October 2002), the patient reported that no symptoms associated with AF had occurred, the ECG showed normal sinus rhythm. However, device memory suggests that numerous AF recurrences have occurred. In October 2002, persistent AF develops and the patient attends the hospital due to signs of congestive heart failure in 4 weeks later.
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Alternate Site Cardiac Pacing Is Feasible With Use Of Novel Lead And Catheter Technology
Several studies have suggested potential benefits from permanent cardiac pacing at alternate sites, such as the interatrial septum and the ostium of the coronary sinus for atrial pacing and various positions in the right ventricular (RV) outflow tract for ventricular pacing. A novel pacing lead was designed as a bipolar, active fixation, thin (4.1 F) and compact (bearing no stylet) steroid-eluting lead. Delivery was achieved through a steerable catheter. A total of 161 pts participated in a multicenter European study, during which these novel leads were positioned at 4 randomized sites: inter-atrial septum, high anterior RV outflow tract, coronary sinus os and low septal RV outflow tract (Figure). Selective site pacing was feasible with successful lead placement at all pre-specified atrial and ventricular sites in the majority (78-93%) of patients, with satisfactory lead and catheter handling scores, while there were no unanticipated adverse events and those occurring were managed appropriately. In 44 patients, followed for 12 months, the long-term performance of this novel lumenless pacing lead implanted at alternate right atrial and RV sites was very satisfactory with excellent pacing and sensing thresholds maintained at 12 months and absent late adverse events.
In conclusion, alternative site pacing proved feasible with the use of novel pacing technology, including a prototype lead and a special guiding catheter system, in the majority of patients in this multicenter European study. The excellent intraoperative pacing and sensing measurements were maintained during long-term 12-month follow-up for all 4 atrial and ventricular sites.
Manolis AS, Simeonidou E, Sousani E, Chiladakis J. Alternate sites of permanent cardiac pacing: A randomized study of novel technology. Hellenic J Cardiol 2004;45:145-149
Figure: Fluoroscopy view (45 degree left anterior oblique) of the novel pacing leads implanted at alternate sites: interatrial septum/high anterior right ventricular outflow tract.
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Entrainment And Interruption Of Reentrant Tachycardias
The concepts of transient entrainment of reentrant rhythms evolved from the failure in 1973 of an attempt to use overdrive atrial pacing to interrupt atrial flutter (AFL) and restore sinus rhythm in a post-open heart surgical patient. Subsequent studies demonstrated that a critical pacing rate and duration of pacing were required to interrupt AFL. Further overdrive pacing studies during AFL, ventricular tachycardia, atrioventricular (AV) reentrant tachycardia, AV nodal reentrant tachycardia, and atrial tachycardia refined the understanding of what occurs during overdrive pacing of a reentrant tachycardia, and permitted a mechanistic understanding of entrainment as continuous resetting of a reentrant tachycardia to a pacing rate that is faster than the rate of the tachycardia, but which fails to interrupt it. Demonstration of the unique activation sequences during entrainment and interruption of a tachycardia during overdrive pacing establish the presence of ordered reentry, and differentiate reentry from other arrhythmic mechanisms. Four entrainment criteria have been established, any of which, if demonstrated, provide a reliable tool to establish the presence of reentry. The short cut descriptions of these four criteria are that during overdrive pacing 1) the last captured beat is entrained but not fused; 2) there is progressive fusion at different overdrive pacing rates; 3) localized conduction block occurs at termination of the tachycardia; and 4) an electrogram equivalent of progressive fusion occurs. The principles of entrainment also have been applied clinically to assist in the effective application of antitachycardia pacing, and catheter ablation techniques.
Waldo, AL: From bedside to bench: Entrainment and other stories. Heart Rhythm 2004;1:94-106
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Remote ICD Monitoring
Trans-telephonic monitoring has been an indispensable component of pacemaker follow-up for more than three decades, but only recently has remote monitoring been added to the armamentarium of the clinic following ICDs. In light of the exponential growth of ICD implants, remote monitoring may address some of the challenges faced by pacer-ICD physicians, with particular emphasis on increasing time constraints and growing numbers of patients living far from the pacer-ICD clinic. The feasibility of remote monitoring of ICDs was first demonstrated in a multicenter study evaluating an internet-based system. The network is comprised of a patient monitor, a secure server, and clinician and patient websites. Comprehensive device data were captured by the monitor, transmitted and thereby available for access and review; this included stored episodes, parameters, diagnostics including stored intracardiac electrograms and real-time electrograms. Ease of use and functionality were confirmed for both patients and physicians alike. Clinician review of data transmissions revealed several clinically significant findings, including silent AF discovery, assessment of antiarrhythmic drug efficacy in a previously diagnosed AF patient, previously unobserved atrial undersensing, and ventricular tachycardia. Remote monitoring of ICD devices is therefore feasible, may serve to facilitate ICD follow-up, and lead to the earlier diagnosis of device malfunction and/or change in a patient's condition. Multiple technical, ethical, regulatory and financial challenges are posed in the full implementation of remote monitoring. What remains unclear is to what extent such technology should complement (as opposed to supplant) traditional ICD follow-up, who should perform and be able to access remote monitoring, how to universalize this technology, and how will remote monitoring of ICDs impact upon cost effectiveness of device follow-up. Prospective trials and the development of medical guidelines will be required to guide implementation of this exciting new technology.
Schoenfeld MH, Compton SJ, Mead RH, Weiss DN, Sherfesee L, Englund J, Mongeon LR Remote monitoring of implantable cardioverter defibrillators: a prospective analysis PACE 2004;27:757-763
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Repolarisation Fingerprint
The interest in the relationship between the duration of QT interval and heart rate can be traced into the pre-electrocardiography era when the proportions between the duration of systole and cardiac cycle were studied using mechanical apexographs. Since the initial publication of Bazett's and Fridericia's formulae in 1920, numerous attempts have been made to propose a universally applicable and accurate QT correction formula. None of these attempts have ever been successful. Every proposal of a new correction formula was shortly followed by another publication showing that the previous proposal was not sufficiently accurate and that a different approach is needed.
The reason for all these discrepancies has recently been explained by the Electrophysiology and Electrocardiography team at St. George's Hospital Medical School in London. In detailed studies of long term 12-lead ECG monitoring, it has been found that there is no 'physiologic' QT/RR relationship since the QT/RR patterns differ very substantially between different subjects (examples in the top panel). Note, for instance, that when changing RR interval between 600 and 800 ms, QT interval changes by 20 ms in some healthy subjects and by 70 ms in other subjects. In principle, there cannot be a mathematical formula that would fit all such changes with a reasonable accuracy.
Moreover, the QT/RR relationship and its inter-subject differences is not a result of random chance. The relationship stays stable within each individual (the bottom panel shows such an intra-subject stability in 4 healthy individuals recorded repeatedly after 1 day, 1 week, and 1 month). Thus the properties of QT/RR relationship, together with other dynamic parameters of cardiac repolarisation, create a pattern that may be portrayed as an electrophysiological fingerprint.
References:
Malik M, Färbom P, Batchvarov V, Hnatkova K, Camm AJ. Relation between QT and RR intervals is highly individual among healthy subjects: implications for heart rate correction of the QT interval. Heart 2002;87:220-228
Batchvarov VN, Ghuran A, Smetana P, Hnatkova K, Harries M, Dilaveris P, Camm AJ, Malik M. QT-RR relationship in healthy subjects exhibits substantial intersubject variability and high intrasubject stability. Am J Physiol 2002;282:H2356-H2363
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Technical Aspects of Radio Frequency Pacing
Electromagnetic energy, in the form of radio frequency, has been used in various ways to "wirelessly connect" implanted cardiac pacing components to externally situated devices. These devices can be separated into several general categories:
- A totally passive circuit, consisting of an antenna pick-up coil, a capacitor, a rectifier, and a pacing lead, are encapsulated and implanted. The implanted circuit does not contain a battery or other self-contained power source. The electrical energy that produces the life-sustaining pacing spike is supplied from an integrated external pulse generator and radio frequency transmitter unit. This external unit is battery powered. In this category, RF plays the dual role of providing both the energy and the timing for the spike. Examples of pacemakers that fall into this category are: Lucas (Abrams-Lightwood) Inductively Coupled Cardiac Pacemaker; the Cammilli-Grassi RF Pacemaker; Airborne Instruments (Glenn) Cardiac Pacemaker; Suma-Togawa Induction Pacemaker (commercially available from Medical Electronics Division Nippon Electric Company1)
- A battery powered pulse generator, which includes a pick-up coil and associated circuitry to periodically charge the battery, is encapsulated and implanted. The battery in the implanted unit is recharged periodically by inductively coupling energy sourced from the external recharge unit. The recharge unit has an inductive antenna coil that is place on the skin over the implanted receiving coil. With these units, the frequency of the coupled energy is generally lower, and the only role is to recharge the internal battery.
- Implanted pulse generator with self-contained power source. The implanted pacemaker can be controlled by an external unit via induction coil, to temporarily increase rate. The GE model A2070AA and A2070AB fall into this catagory1. In this case, the RF signal is a used to control the implanted device.
The RF Pacemakers that fall into the first group provide several attractive benefits. Since the implanted portion is passive and contains no power source, there is no need to periodically explant the unit due to depletion in energy supply. In addition, with so few internal electrical components, the probability of failure of the implanted portion is very low. The external battery can be easily replaced as needed, with a replacement being as close as the corner store. The pacing rate and pulse width are also readily adjustable externally. Of course, these devices lack the sophistication of modern pacemakers.
The RF pacing experiment conducted by Verzeano, Webb and Kelly2, used the technique discussed in Category 1 above.
References:
1. Cardiac Pacemakers - Appendix
2. Radio Control of Ventricular Contraction in Experimental Heart Block, Science Vol. 128, 24 October 1968
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