Improved Pattern of Rate Responsiveness with Dynamic Slope Setting for the QT Sensing Pacemaker

We have recently described the electrophysiological basis of a new algorithm for the QT (TX) sensing rate responsive pacemaker. By using the new software program running on the standard programmer it has been possible to simulate the new algorithm in ten patients with complete heart block (seven patients had implanted TX units and three were paced with an external TX pacemaker) during routine exercise testing. In this way a single-blind, intra-patient comparison of the pattern of pacing rate change using both the existing and new algorithms was possible. In nine out of the ten cases the time taken to increase the pacing rate from 70 to 80 bpm was reduced significantly when the new algorithm was used (P = 0.037). Additionally, the correlation between the atrial and ventricular rates in those patients with normal sinus node function (seven patients) was determined. In all cases we have observed a significantly improved correlation between the atrial and ventricular paced rates during exercise with the new algorithm (P less than 0.001).     

QT Sensing Rate Responsive Pacing During Subacute Bacterial Endocarditis: A Case Report

A 63-year-old woman treated with a QT sensing rate responsive pacemaker following aortic valve replacement developed late subacute bacterial endocarditis. During febrile periods, associated with systemic upset, pacing was physiological as evidenced by an increased heart rate during pyrexia and a decrease when afebrile.     

Nonlinear Relationship Between Pacing and Evoked QT Intervals

The paced evoked response is an established biosensor which has been used in the design of a rate responsive pacemaker. The unit is capable of sensing the interval between the delivery of a pacing stimulus and the downslope of the evoked T wave. With fixed rate pacing this interval has been shown to shorten with exercise and the main cause of this effect is thought to be mediated by the increase in the plasma catecholamines which are released on exertion. The detection of a reduction in the stimulus-T interval results in an increase in pacing rate. The rate of change of pacing rate is re/erred to as the slope setting and this must be determined for each individual patient so that optimal rate responsive pacing can be effected. The algorithm underlying the slope setting is the pacing rate-evoked QT interval relationship. This relationship was implemented as a linear function, but this study, which was conducted to reevaluate it, has demonstrated nonlinearity between the pacing and evoked QT intervals. The degree of QT shortening is least at low heart rates. This finding has resulted in the development of a new algorithm for the pacemaker in the form of a new program for the pacing system. This should result in a more physiological rate of change of heart rate with exercise and less chance of sudden changes in rate. These postulates are the subject of current clinical trials.     

A Randomized Double-Blind, Cross-Over Study of the Linear and Nonlinear Algorithms for the QT Sensing Rate Adaptive Pacemaker

BAIG, M.W., ET AL.: A Randomized Double-Blind, Cross-Over Study of the Linear and Nonlinear Algorithms for the QT Sensing Rate Adaptive Pacemaker. We have compared the pacing rate responses during cardiopulmonary exercise testing in 11 patients (mean 59 years, six female) with implanted QT sensing rate adaptive pacemakers who were randomly programmed to 1-month periods in the linear and nonlinear algorithms using a double-blind, cross-over design. Exercise testing was performed at the end of each month block and symptoms were scored with the MacMaster questionnaire. With exercise, the time to a 10 beats/min increment in rate was significantly less with the nonlinear compared to the linear algorithm (126 sec vs 255 sec, P = 0.02) but there were no significant differences in exercise duration, the peak pacing rate, the peak VO₂, the VO₂ at the anaerobic threshold or the mean correlation coefficients of the pacing rate VO₂ relationship. Rate oscillation occurred in seven patients in the linear algorithm and in two patients in the nonlinear setting. Initial deceleration of the pacing rate at the onset of exercise occurred in seven patients in the linear algorithm and in four patients in the nonlinear setting. The nonlinear algorithm is associated with a faster response time during exercise and fewer instances of rate instability. However, it has not overcome the problem of a dip in the pacing rate at the beginning of exercise. The major difference in the function of the two algorithms is faster initial acceleration with the nonlinear algorithm. This is explained by the significantly higher values of the slope setting at the lower rate limit for the nonlinear versus the linear algorithm (6.3 ms/ms vs 5.1 ms/ms).     

Predictors of Early Mortality in Patients Age 80 and Older Receiving Implantable Defibrillators

Background: There are no upper age restrictions for implantable defibrillators (ICDs) but their benefit may be limited in patients ≥ 80 years with strong competing risks of early mortality. Risk factors for early (1‐year) mortality in ICD recipients ≥ 80 years of age have not been established. Methods: Two‐center retrospective cohort study to assess predictors of one‐year mortality in ICD recipients ≥ 80 years of age. Results: Of 2,967 ICDs implanted in the two centers from 1990–2006, 225 (7.6%) patients were ≥80 years of age and followed‐up at one of the two centers. Mean age was 83.3 ± 3.1 years and follow‐up time 3.3 ± 2.6 years. Median survival was 3.6 years (95% confidence interval 2.3–4.9). Multivariate predictors of 1‐year mortality included ejection fraction (EF) ≤ 20% and the absence of beta‐blocker use. Actuarial 1‐year mortality of ICD recipients ≥ 80 with an EF ≤ 20% was 38.2% versus 13.1% in patients 80+ years with an EF > 20% and 10.6% for patients < 80 years with an EF ≤ 20% (P < 0.001 for both). There was no significant difference in the risk of appropriate ICD therapy between those patients 80+ years with EF above and below 20%. Conclusion: In general, patients ≥ 80 years of age who meet current indications for ICD implantation live sufficiently long to warrant device implantation based on anticipated survival alone. However, those with an EF ≤ 20% have a markedly elevated 1‐year mortality with no observed increase in appropriate ICD therapy, thus reducing the benefit of device implantation in this population. (PACE 2010; 981–987)     

One-Year Follow-Up of Automatic Adaptation of the Rate Response Algorithm of the QT Sensing, Rate Adaptive Pacemaker

Optimal functioning of a rate adaptive pacemaker depends upon reliable sensing of the sensor and appropriate programming of the rate response algorithm. QT sensing pacemakers use data derived from the endocardial electrogram in the programming of the rate response algorithm. In the latest versions of these pacemakers, programming of the rate response algorithm may be performed using either a semiautomatic Fast Learn (FLJ procedure or by using the newly developed, fully Automatic Slope Adaptation (ASA) mechanism. We report our experience in a prospective study of 17 patients in the first year postimplantation. ASA was characterized by significant changes only in the values of the slope settings at the lower rate limit (3.7 msec/msec at time 0 to 5.77 msec/msec at 2 weeks, P < 0.001) during the first 2 weeks after its enablement. Further adaptation between weeks 2 to 4 was observed (5.77 msec/msec to 6.4 msec/msec, P = 0.2) but this was not significant. The slope settings derived using the FL procedure were also checked at 2 and 4 weeks and were reproducible. They were closest in value to the values attained by the automated mechanism at 4 weeks. This suggests that the final value of the slope setting at the lower rate limit using ASA is reached between weeks 2 to 4. Both methods of slope determination result in satisfactory and similar rate response profiles but the time to achieve slope stability will necessarily be slower with ASA.     

Multiple Sclerosis: Cells Secreting Antibodies Against Myelin-Associated Glycoprotein are Present in Cerebrospinal Fluid

We evaluated the B-cell response in cerebrospinal fluid (CSF) and blood by enumerating cells secreting antibodies to myelin-associated glycoprotein (MAG) and, for reference, to myelin basic protein (MBP), two myelin components which may constitute targets for autoimmune attack in multiple sclerosis (MS). Among 25 untreated MS patients, 12 had cells in CSF secreting anti-MAG IgG antibodies (mean value 1 per 1429 CSF cells) and three also had cells secreting anti-MAG antibodies of the IgM isotype but at lower levels. In CSF from 2 out of 10 MS patients examined, anti-MAG and anti-MBP IgG antibody-secreting cells were present concurrently. Antibody-secreting cells were less frequent in blood and bone marrow, reflecting compartmentalization to CSF. Anti-MAG antibody-secreting cells were found in CSF from only 1 out of 27 control patients. The intrathecal production of anti-MAG and anti-MBP antibodies may be important in the pathogenesis of MS.