Survival of Patients with the Automatic Implantable Cardioverter Defibrillator

Between May 1982 and May 1988, 37 patients (28 males and 9 females, mean age 57.6, range 16–76 years) of approximately 600 evaluated for sustained ventricular tachycardia and/or fibrillation (VT/VF) were treated with an automatic implantable cardioverter defibrillator (AICD). Twenty-eight of the patients had coronary artery disease, 7 had nonischemic cardiomyopathy, 1 had amyloid hear disease, and 1 had rheumatic heart disease. The mean ejection fraction was 32.2 ± 12.9% (range, 9–64%). Eleven patients have died at a mean of 16.7 months after implantation. The cumulative survival rate was 81% at 1 year, 77% at 2 years. 68% at 3 years, and 53% at 4, 5, and 6 years. Considering only sudden deaths, the survival was 97% at 1 and 2 years, 90% at 3 years, and 80% at 4, 5, and 6 years. Twenty-one of the 37 patients received spontaneous shocks. If the first shock marks the time to death in the absence of an AICD, the cumulative survival rate would have been 56% at 1 year, 42% at 2 years, 29% at 3 years, and 14% at 4, 5, and 6 years. The maximum amount of time to a first appropriate shock was 39.7 months. Thirty-nine devices have been explanted; 28 for battery depletion; 5 for infections; 3 for improper sensing; 2 for electronic failure; and 1 at the time of cardiac transplantation. The average time to failure of the 28 units removed for battery depletion was 19.8 ±6.9 months. We conclude that in this group of patients, short-term survival is increased by AICD implantation, hut long-term survival remains poor. Incidence of sudden death is reduced to 3% at 1 and 2 years, 10% at 3 years, and 20% after 4 years. Other patient selection criteria may be useful in selecting those patients whose survival will be improved by the AICD.     

Direct and Telemetered Lead Impedance

Objectives: We undertook this study to determine whether telemetered lead impedance measurements (LIM) can be correlated with direct LIM and to determine the stability of LIM over time when measured directly and via telemetry. Methods: Direct LIM and telemetered LIM were measured in 91 patients; 101 leads during initial implantation and 40 leads during pulse generator replacement. Differences in direct LIM measured during initial implant and pulse generator replacement (direct-direct) were compared in 41 patients (28 atrial leads and 37 ventricular leads). The stability of telemetered LIM obtained immediately postoperatively, at 1 month and 1 year, postimplantation was assessed in 50 patients (23 atrial and 49 ventricular leads). Results: In atrial leads acute direct LIM was 633.9 ± 18.4 Ω versus 575.8 ± 18.5 Ω for telemetered LIM (r = 0.58), and chronic direct LIM was 670.9 ± 49.3 Ω versus 607.0 ± 36.3 Ω for telemetered LIM (r = 0.87). In ventricular leads acute direct LIM was 747.3 ± 16.9 Ω and 684.7 ± 16.4 Ω for telemetered LIM (r = 0.69), and chronic direct LIM was 674.8 ± 29.9 Ω and 625.2 ± 28.5 Ω for telemetered LIM (r = 0.68). The mean direct–direct UM rose 124 Ω (P < 0.001) in atrial leads and 10 Ω (P = NS) in ventricular leads. Telemetered LIM for atrial leads was 581.0 ± 27.6 Ω immediately postimplantation compared to 625.7 ± 34.8 Ω at 1 month and 754.1 ± 43.0 Ω at 1 year. Telemetered LIM for ventricular leads was 661.3 ± 17.5 Ω at implant, 684.6 ± 20.7 Ω at 1 month and 724.7 ± 22.7 Ω at 1 year. Conclusions: There is a good but limited correlation between direct and telemetered LIM. Mean direct LIM obtained at initial implantation is similar to that measured at pulse generator replacement. The telemetered LIM is stable over the first month postimplantation but tends to rise during the first year of follow-up and substantial changes in impedance are not uncommon in individuals with normal function. There is a tendency for LIM to rise with lead maturation. If telemetered LIM is to be followed over time, a baseline telemetered value should be obtained immediately postoperatively.     

Performance of Implantable Cardiac Rhythm Management Devices

Our data represent use, follow-up, and management decisions from seven independently functioning centers and most importantly, actuarial survival of ICRMDs that have been implanted for sufficient time period to allow assessment of time versus failure. General patterns of possible target durations for adequate performance for present or future generations of similar clinical devices may be suggested by the data that we have presented. However, it would be inappropriate to conclude from these data that any presently implanted ICRMD would have a particular functional reliability. These data, furthermore, only summarized device hardware performance and cannot and must not be used to determine either short-term or long-term individual patient status, management, or outcome.     

Simplifying Pacemaker Follow-up by the Implementation of the NASPE Standards on Computer Interface

One of the basic limitations for the development and use of more sophisticated pacemakers is the difficulty to provide adequate instrumentation and methodology for the follow-up in the clinical environment. To address this problem, the NASPE (North American Society of Pacing and Electrophysiology) Computer Committee has developed an interface standard to allow communication with the pacemaker programmer through many computer systems. The document recommends the use of the RS 232/c (CCITT V.24) interface with baud rates variable from 300 to 19,200, 7 bit word, no parity and Xon/Xoff protocol. The actual message format consists of an SOH code, a function code describing the action to he taken, a variable code defining the parameter on which the action is to be taken, and the actual numerical data. The CHECKSUM is calculated by summing all the ASCII codes beginning after the SOH code and ending with the last byte preceding the checksum itself. The last seven bits so calculated are attached to the message, and a consecutive ETX code ends the communication. At the receiving end, the checksum is recalculated and compared to the value received. If the two match, the message is accepted. Using this format, compatible computer software and pacemaker programmers can be developed independently and function as a unit. The protocol is capable of real-time ECG transmission.     

Treadmill Assessment of an Activity-Modulated Pacemaker: The Importance of Individual Programming

Maximum benefit from a rate-modulated pacemaker requires individualized programming of rate response settings. We tested an externally strapped activity-sensing pacemaker (Activitrax-Medtronic 8400) in eight healthy volunteers, to assess the pacing responses of the different rate response and activity threshold settings. Five males and three females, aged 20 to 70 years (mean 40), performed a total of 67 treadmill exercise tests, using a specific protocol designed to assess the activity-sensing unit. The external unit was compared to implanted units in four patients, to validate its accuracy. A reproducible sinus response to the treadmill protocol was observed, against which pacing responses were compared. The activity threshold determines the degree of activity required to elicit a pacing rate response, whereas the rate response setting determines the rate attained. Rates of 140 bpm were rarely achieved, despite vigorous exercise. The sensor responds rapidly to activity, not to physiologic demand; to increase in speed, not grade. Four patients performed repeated limited treadmill tests to determine their optimum program setting, with symptomatic status and the healthy volunteer sinus response as guides. These results, and those from the external Activitrax unit, suggest that LOW 6 and MEDIUM 6-10 settings will prove optimum for most patients.