Applicability of the stimulus-T interval for antitachycardia pacing

Inter- and intraindividual variations in shortening of ventricular effective refractory period after an abrupt decrease in pacing cycle length

After an abrupt decrease in pacing cycle length (PCL), the ventricular effective refractory period (VERP) shortens. The pacing protocol needed to determine accurate and reproducible values for the VERP during this process is elaborate and time consuming. In this study, steady-state values of VERP at 800 and 350 msec PCL and dynamic values of VERP due to an abrupt change in PCL from 800 to 350 msec were determined. This was done for 11 different dogs to test the interindividual variation and repetitively in the same dog to test the intraindividual variation. The results for steady-state and dynamic values of the VERP show a wide range for both groups. This means that accurate prediction of steady-state and dynamic values of VERP based on previous measurements is not possible.

Quantitative comparison of rate response and oxygen uptake kinetics between different sensor modes in multisensor rate adaptive pacing

Although multisensor pacing may mitigate the inadequacy of rate adaptation in a single sensor system, the clinical role of multisensor driven rate adaptive pacing remains unclear. The cardiopulmonary performance of six patients (mean age 63.5 +/- 10 years) who had undergone the implant of combined QT and activity VVIR (Topaz) pacemakers was assessed during submaximal and maximal treadmill exercise with the rate response sensor randomly programmed to either single sensor mode, QT and activity (ACT), or dual sensor mode, with equal contribution of QT and ACT (QT = ACT). The rate of response, the proportionality, oxygen kinetics, and maximal exercise performance of the various sensor modes during exercise were measured and compared. The ACT sensor mode "overpaced" and the QT and QT = ACT sensor modes "underpaced" during the first three quartiles of exercise (P < 0.05). The ACT sensor mode also gave the fastest rate of response with the shortest delay (13 +/- 1.5 sec vs 145 +/- 58 sec and 41 +/- 17 sec, P < 0.05), time to 50% rate response (39 +/- 2.7 sec vs 275 +/- 48 sec and 203 +/- 40 sec, P < 0.05), and time to 90% of rate response (107 +/- 21 sec vs 375 +/- 34 sec and 347 +/- 34 sec, P < 0.05) and a smaller oxygen debt (0.87 +/- 0.16 L vs 1.10 +/- 0.2 L and 1.07 +/- 0.18 L, P < 0.05) compared to the QT and QT = ACT sensor modes, respectively. These differences were most significant at low exercise workloads. Thus, different sensor combinations result in different rate response profiles and oxygen delivery, especially during low level exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Initial experience with a new single chamber, dual sensor rate responsive pacemaker. The Topaz Study Group

In August 1991, a new single chamber pacemaker became available that utilizes information from two sensors, activity and stimulus-to-T wave (QT) interval. We are reporting on the first 90 implants in 21 centers. T wave sensing was adequate at implantation in 88/90 patients, with a safety margin of > 100% in 86/90. Activity sensing was adequate in all patients. The contribution of each sensor (sensor blending) is programmable for each patient. Of 75 patients assessed at 1 month after implant, three have been programmed to "Activity-Only" mode, and 72 to dual sensor mode. Of these, 18 have been programmed to "QT < Activity," 48 to "QT = Activity," and 6 to "QT > Activity." Forty-five patients underwent exercise testing in dual sensor mode and a subgroup of 15 also underwent exercise testing in Activity-Only mode. The dual sensor mode produced a more gradual increase in pacing rate. Sensor Cross Checking satisfactorily prevented a sustained high pacing rate in tests of false-positive activity sensing (tapping, vibrating pacemaker, or static pressure). The maximum pacing rate on walking downstairs (94.2 +/- 7.2 ppm) was similar to that produced by walking upstairs (91.6 +/- 5.9 ppm). We conclude that initial assessment of this dual sensor, single chamber, rate responsive pacemaker confirms that the algorithm for combining data from two sensors functions satisfactorily. Dual sensor rate responsive pacing may offer significant advantages over single sensor devices, and further studies of this novel device are indicated.

Efficacy of pacemaker tachycardia termination algorithms: is electrophysiological testing alone adequate?

Selection of an optimal pacemaker tachycardia reversion algorithm is generally performed utilizing programmed electrical stimulation (PES). Multiple tachycardias are induced and various tachycardia termination protocols are tested for reversion success. However, PES may induce nonclinical tachycardia and result in an inaccurate assessment of subsequent reversion effectiveness for spontaneous tachycardia. To investigate this question, we compared tachycardia reversion protocol success for PES-induced tachycardia versus spontaneously occurring tachycardia in 16 patients with atrially placed Intermedics 262-12 antitachycardia pacemakers. The pacemaker has tachycardia response counters, and the reversion success was calculated from these counters. This was performed by comparing the percent of time 1 degree versus 2 degrees modality use occurred; crossover to the 2 degrees modality implied failure of the 1 degree modality to convert the tachycardia. PES was used to induce multiple episodes of tachycardia and spontaneous episodes of tachycardia were recorded over time by pacemaker counters. The pacemaker 1 degree modality success was then compared for spontaneous and induced arrhythmias.

RESULTS

A total of 53 discrete data comparisons of PES versus spontaneous tachycardia counters were performed in the 16 patients. PES reversion success occurred 85% +/- 22% of the time versus a spontaneous reversion success of 88% +/- 22%. However, the Spearman rank correlation coefficient test demonstrated nonsignficant overall correlation (P < 0.01), and Pearson correlation on an individual patient basis varied widely (r value from < 0.1 to 1.0).

CONCLUSIONS

When utilizing the same termination algorithm, the percentage conversion of tachycardias occurring spontaneously and induced by PES is similar but does not correlate well overall. This suggests that PES may not be a good linear predictor of the long-term success of antitachycardia pacing algorithms.

The range of sensors and algorithms used in rate adaptive cardiac pacing

Implantable sensors play an important role in physiological cardiac pacing. Sensors can be classified according to the technical methods in which sensing is achieved: the sensing of the evoked ventricular response, intrathoracic impedance and body acceleration forces, and the incorporation of special sensors on pacing electrodes. These sensors differ in their relative merits in terms of speed, proportionality, sensitivity, and specificity of rate response. The efficacy of a sensor can be significantly modified by the algorithm used in relating sensor signal to a pacing rate change. The currently available types of sensors and algorithms are summarized and compared in this review article. The relative merits of these sensors and algorithms form the basis for designing a multisensor pacing system.