Changes in Q-T and Q-aT intervals at rest and during exercise with different modes of cardiac pacing

Dual chamber versus single chamber ventricular pacemakers for sick sinus syndrome and atrioventricular block

BACKGROUND

Dual chamber pacing or single chamber atrial pacing ('physiologic' pacing) is believed to have an advantage over single chamber ventricular pacing in that it resembles cardiac physiology more closely by maintaining atrioventricular (AV) synchrony and dominance of the sinus node, which in turn may reduce cardiovascular morbidity and mortality thus contributing to patient survival and quality of life. However, a significant proportion of pacemakers currently implanted are single chamber ventricular pacemakers.

OBJECTIVES

The objective of this review was to assess the short- and long-term clinical effectiveness of dual chamber pacemakers compared to single chamber ventricular pacemakers in adults with AV block, sick sinus syndrome or both. An additional objective was to assess separately any potential differences in effectiveness between dual chamber pacing and single chamber atrial pacing. The clinical effectiveness of single chamber atrial pacing versus single chamber ventricular pacing was not examined.

SEARCH STRATEGY

The Cochrane Controlled Trials Register (The Cochrane Library Issue 3, 2002), MEDLINE (1966 to 2002), EMBASE (1980 to 2002) and the Science Citation Index (1980 to 2002) were searched on 19th August 2002. Citation lists and web sites were checked and researchers in the field contacted.

SELECTION CRITERIA

Parallel group or crossover randomised controlled trials of at least 48 hours duration comparing dual chamber pacing and single chamber ventricular pacing, and investigating cardiovascular morbidity, mortality, patient related quality of life, exercise capacity and complication rates.

DATA COLLECTION AND ANALYSIS

Data was extracted onto pre-piloted data extraction forms. Quality assessment was undertaken using a checklist, with a sub-sample of quality data independently extracted by a second reviewer. Where appropriate data was available, meta-analysis was performed. Where meta-analysis was not possible, the number of studies showing a positive, neutral or negative direction of effect and statistical significance were simply counted.

MAIN RESULTS

Five parallel and 26 crossover randomised controlled trials were identified. The quality of reporting was found to be poor. Pooled data from parallel studies shows a statistically non-significant preference for physiologic pacing (primarily dual chamber pacing) for the prevention of stroke, heart failure and mortality, and a statistically significant beneficial effect regarding the prevention of atrial fibrillation (odds ratio (OR) 0.79, 95% CI 0.68 to 0.93). Both parallel and crossover studies favour dual chamber pacing with regard to pacemaker syndrome (parallel: Peto OR 0.11, 95% CI 0.08 to 0.14; crossover: standardised mean difference (SMD) -0.74, 95% CI - 0.95 to -0.52). Pooled data from crossover studies shows a statistically significant trend towards dual chamber pacing being more favourable in terms of exercise capacity (SMD -0.24, 95% CI -0.03 to -0.45). No individual studies reported a significantly more favourable outcome with single chamber ventricular pacing.

REVIEWERS' CONCLUSIONS

This review shows a trend towards greater effectiveness with dual chamber pacing compared to single chamber ventricular pacing, which supports the current British Pacing and Electrophysiology Group's Guidelines regarding atrioventricular block. Additional randomised controlled trial evidence from ongoing trials in this area will further inform the debate.

Effect of Physical Training on Corrected QT Dispersion in Patients With Nonischemic Heart Failure

BACKGROUND

The effect of physical training (PT) on QTc dispersion and ventricular tachycardia (VT) remains unclear in patients with nonischemic heart failure.

METHODS AND RESULTS

Eight patients with nonischemic heart failure performed PT using a bicycle ergometer and their exercise tolerance increased (4.9+/-1.8 to 7.0+/-2.5 METs, p<0.05) and QTc dispersion decreased (71+/-22 to 48+/-24 ms, p<0.05). However, PT did not change the frequency of VT.

CONCLUSION

Physical training could improve QTc dispersion in patients with nonischemic heart failure, possibly by improving the autonomic nerve system.

Improvement in corrected QT dispersion by physical training and percutaneous transluminal coronary angioplasty in patients with recent myocardial infarction

The aim of the present study was to assess whether physical training and percutaneous transluminal coronary angioplasty (PTCA) improve the corrected QT (QTc) dispersion in patients with recent myocardial infarction (MI). Twenty-four patients with recent MI were allocated to one of 3 groups: training (n = 8), PTCA (n = 7) or controls (n = 9). Physical training as well as PTCA decreased QTc dispersion, whereas QTc dispersion increased in the control group. Changes in QTc dispersion after physical training or PTCA were inversely correlated with exercise-induced ST depression at the baseline test. These observations suggest that physical training, as well as PTCA, could improve QTc dispersion and electrical instability in patients with recent MI, possibly due to improvement of myocardial ischemia.

Developments in sensor-driven pacing

This article reviews the recent major developments in the field of rate adaptive pacing. Including, the improved instrumentation of existing sensors, the use of multiple sensors to enhance sensor specificity or sensitivity, and the automation of sensor calibration. The physiologic benefits and programming of rate adaptive pacing are reviewed.

Exercise prescription for patients with pacemakers

When developing an exercise program for pacemaker patients, basic information about the pacemaker must be understood. Atrial, ventricular, and dual-chamber devices can produce varying exercise responses and impact the exercise prescription. The type of rate adaptive sensor the pacemaker has will affect the nature of heart rate response, and therefore, must be taken into account when prescribing exercise. While rate modulation is used with most chronotropically incompetent patients, individuals with VVI pacemakers will also benefit from regular exercise. Although the value of exercise testing pacemaker-dependent patients for ECG interpretation may be limited, it is useful in determining exercise capacity and ensuring proper pacemaker function. Participation in a supervised exercise training program can greatly enhance the follow-up and management of pacemaker-dependent patients as well as afford them the opportunity to experience the physical and psychologic benefits typically associated with cardiac rehabilitation.

Reduction in QT interval dispersion by successful thrombolytic therapy in acute myocardial infarction. TEAM-2 Study Investigators

BACKGROUND

QT dispersion (QTd, equals maximal minus minimal QT interval) on a standard ECG has been shown to reflect regional variations in ventricular repolarization and is significantly greater in patients with than in those without arrhythmic events.

METHODS AND RESULTS

To assess the effect of thrombolytic therapy on QTd, we studied 244 patients (196 men; mean age, 57 +/- 10 years) with acute myocardial infarction (AMI) who were treated with streptokinase (n = 115) or anistreplase (n = 129) at an average of 2.6 hours after symptom onset. Angiograms at 2.4 +/- 1 hours after thrombolytic therapy showed reperfusion (TIMI grade > or = 2) in 75% of patients. QT was measured in 10 +/- 2 leads at 9 +/- 5 days after AMI by using a computerized analysis program interfaced with a digitizer. QTd, QRSd, JT (QT minus QRS), and JT dispersion (JTd, equals maximal minus minimal JT interval) were calculated with a computer. There were significant differences in QTd (96 +/- 31, 88 +/- 25, 60 +/- 22, and 52 +/- 19 milliseconds; P < or = .0001) and in JTd (97 +/- 32, 88 +/- 31, 63 +/- 23, and 58 +/- 21 milliseconds; P = .0001) but not in QRSd (25 +/- 10, 22 +/- 7, 28 +/- 9, and 24 +/- 9 milliseconds; P = .24) among perfusion grades 0, 1, 2, and 3, respectively. Similar results were obtained comparing TIMI grades 0/1 with 2/3 and 0/1/2 with 3. Patients with left anterior descending (versus right and left circumflex) coronary artery occlusion showed significantly greater QTd (70 +/- 29 versus 59 +/- 27 milliseconds, P = .003) and JTd (74 +/- 30 versus 63 +/- 27 milliseconds, P = .004). Similarly, patients with anterior (versus inferior/lateral) AMI showed significantly greater QTd (69 +/- 30 versus 59 +/- 27 milliseconds, P = .006) and JTd (73 +/- 30 versus 63 +/- 27 milliseconds, P = .007). Results did not change when Bazett's QTc or JTc was substituted for QT or JT or when ANOVA included adjustments for age, sex, drug assignment, infarct site, infarct vessel, and number of measurable leads. On ANCOVA, the relation of QTd or JTd and perfusion grade was not influenced by heart rate.

CONCLUSIONS

Successful thrombolysis is associated with less QTd and JTd in post-AMI patients. The results are equally significant when either QT or JT is used for analysis. These data support the hypothesis that QTd after AMI depends on reperfusion status as well as infarct site and size. Reduction in QTd and its corresponding risk of ventricular arrhythmia may be mechanisms of benefit of thrombolytic therapy.

Long-term follow-up of patients treated with the QT interval sensing rate-responsive pacemaker

Thirty-seven patients (mean age 70 years) with QT-interval sensing (TX) rate-responsive pacemakers were followed for a mean of 27 months. This pacemaker measures the QT interval of the paced beat and, if the QT shortens, the pacing rate increases according to a programmed relationship, the 'slope'. With TX pacing the heart rate was 56% and the exercise tolerance 15% higher than with fixed-rate ventricular-inhibited (VVI) pacing during exercise. Holter ECG monitoring showed a physiological rate variability. Six patients (16%) have died during follow-up, and programming to the VVI mode was necessary in six patients (16%). The remaining patients were alive with a subjectively adequate TX function at the end of follow-up. Including the five patients who were on TX pacing at the time of death, satisfactory TX pacing was obtained in 84% of the patients. Although the TX pacemaker has been reliable and provides an adequate rate response during exercise, complaints of worsening angina pectoris and the somewhat complicated programming of the device are problems with the current models.

Effect of propranolol on the QT intervals of normal individuals during exercise: a new method for studying interventions

A new method was used to study the effect of a single dose of propranolol on the QT intervals during exercise in 11 normal volunteers. They exercised maximally on a bicycle ergometer and repeated the test after taking propranolol (40 mg) by mouth two hours before. Electrocardiograms were continuously recorded on magnetic tape and the cardiac cycle length (RR interval) and the QT interval were measured every five seconds by a computer aided method. The RR-QT data from each test during the exercise phase were analysed by an exponential formula, QT = A - B x exp (-k x RR) and by Bazett's formula, QT = K x square root of (RR). Three reference QT intervals, QTc1, QTc2, and QTc3, estimated at RR = 400, 700, and 1000 ms respectively from the regression curves of both formulas were compared. The exponential formula, which consistently gave a better fit with the data, showed that propranolol had a biphasic action on the QT intervals during exercise. It significantly prolonged the mean (SD) interval at longer cycle lengths (from 287 (27) to 305 (18) ms at RR = 1000 ms and shortened it at shorter cycle lengths (from 198 (14) to 179 (16) ms at RR = 400 ms). In contrast, Bazett's formula did not show any significant effect when the same raw data were used. The exponential formula can be adapted to study other interventions or conditions that affect QT intervals.

Changes in QT and Q-aT intervals induced by mental and physical stress with fixed rate and atrial triggered ventricular inhibited cardiac pacing

We have investigated the influence of mental stress and physical stress, i.e., exercise, on the QT and Q-aT intervals (measured from the pacemaker stimulus to the end or the apex, respectively, of the T wave). The study was made on ten patients with high degree atrioventricular block treated with AV universal (DDD) pacemakers. These were programmed to a fixed rate ventricular (VVI) or an atrial triggered (VDD) function for different parts of the study. An arithmetic mental stress test and a bicycle exercise test were performed with each mode of pacing. In the VVI pacing mode, the atrial rate increased by 11% during mental stress and by 46% during exercise. There was a significant shortening of QT and Q-aT intervals with both types of stress. With VDD pacing, mental stress induced a 12% increase in rate and a significant shortening of QT and Q-aT. The paced rate increased by 50% during the exercise test. This increase in ventricular rate was associated with the most marked changes in QT and Q-aT intervals. Thus, both types of stress cause a significant shortening of the QT and Q-aT interval even in the absence of a simultaneous increase in ventricular rate. When the latter is allowed to increase during VDD pacing, both intervals shorten considerably more. There was a marked inter-individual variability in the response to both types of stress. These findings are of importance with regard to the QT sensing rate responsive pacemaker which can be expected to respond to mental stress in most patients, but that response might be unpredictable in the individual.

QT-related rate-responsive pacing during acute myocardial infarction

We studied the findings in a patient who had a TX 915 pacemaker and who was suffering from an acute inferior myocardial infarction. On admission, we observed pacing at the upper rate limit due to excessive adrenergic tone. Shortly thereafter a strong vagal reaction occurred, and the pacing rate fell back to the lower rate limit. This condition was improved with atropine.

CONCLUSIONS

(1) In the described condition the TX pacemaker behaved in a very "physiological" fashion; (2) Sudden strong cholinergic activity lengthened the QT interval; (3) Programming of a steep slope must be avoided in coronary artery disease; (4) Stimulus-T-wave intervals at maximal adrenergic and cholinergic stimulation should be taken into account when programming the slope and the T-wave sensing window.

Importance of lead selection in QT interval measurement

The influence of lead selection on QT estimation in the 12-lead electrocardiogram was assessed in 63 patients (21 control subjects, 21 with anterior myocardial infarction, 21 with inferior myocardial infarction). QT estimates varied between leads. The variation was greater in patients with myocardial infarction than in control subjects (mean dispersion of QT: control subjects, 48 +/- 18 ms [+/- standard deviation]; anterior myocardial infarction, 70 +/- 30 ms; inferior myocardial infarction, 73 +/-32 ms). The maximum QT in any lead (QTmax) was determined and the deviation of each lead from this maximum value calculated. In all 3 groups, anteroseptal leads (V2 or V3) provided the closest approximation to QTmax. Interlead variability was found to be mainly due to variation in timing of the end of the T wave, rather than the onset of the QRS complex. The variability due to leads was considerably greater than the variability due to cycles, observers or measurement error. Implementation of a variety of current lead selection practices resulted in widely divergent estimates of QT interval. It is concluded that there is a need for standardization of lead selection practice for QT measurement. If measurements are confined to one or a few leads, anteroseptal leads provide the closest approximation to QTmax.