We evaluated the capability of a new pacemaker-based rate-smoothing algorithm (RSA) to reduce the irregular ventricular response of AF. RSA prevents sudden decreases in rate using a modified physiological band and flywheel feature. Twelve patients (51 +/- 21 years) with hemodynamically tolerated AF of 4 months to 20 years duration were studied. Atrial and ventricular leads were connected to the external pacemaker device in the electrophysiology laboratory. Consecutive RR intervals during AF were measured at baseline and after ventricular pacing with RSA ON. Ventricular pacing with the rate smoothing algorithm reduced maximum RR intervals (1,207 +/- 299 vs 855 +/- 148 ms, P = 0.0005), with no significant change in the minimum RR interval (401 +/- 55 vs 393 +/- 74 ms, P = 0.292). A small shortening of the mean RR interval (634 +/- 153 vs 594 +/- 135 ms, P = 0.007) was seen with no change in the median RR interval (609 +/- 153 vs 595 +/- 143 ms, P = 0.388). There was a 43% reduction in RR standard deviation (145 +/- 52 vs 82 +/- 28, P = 0.0005), 49% reduction in mean absolute RR interval difference (MAD) (152 +/- 64 vs 77 +/- 34, P = 0.0005) and MAD/mean RR ratio (0.23 +/- 0.05 vs 0.13 +/- 0.04, P = 0.0005). We conclude that rate-smoothed pacing effectively reduces RR variability of AF in the acute setting.
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 referred 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.
Increasing pacemaker memory allows integration of heart rate monitoring into the pacemaker. Two main methods can be distinguished. 1. Heart rate monitoring in histograms. 2. Heart rate monitoring in the time domain (heart rate holter). Method 1 is useful in antitachycardia and diagnostic pacemakers when short specific events must be detected (tachycadia, bradycardia). For the analysis of a rate adaptive pacemaker this method is less appropriate as it does not give any information about the dynamics of rate changes or its time relations. For this purpose Method 2 will give more information about the functioning of the pacemaker as it does not only store the heart rate but also the timing of the heart rate so that changes in heart rate can be correlated to the activity of the patient. An algorithm was developed to store the average heart rate over 7.8 minute periods in a pacemaker. On interrogation of the pacemaker the information will always reveal the heart rate over the 24 hours prior to interrogation. This monitor can also be temporarily programmed to store the average heart rate in 20 second intervals to monitor the response to an exercise test for a period of 1 hour. The time needed for a standard follow-up procedure of a rate adaptive pacemaker can be dramatically reduced to a value close to the follow-up time of a standard VVI pacemaker.
We investigated the possibility to use the interval from an atrial stimulus to the ventricular R wave (A-R interval) as an indicator of physical stress, in 16 patients with pacemakers implanted for severe atrial bradycardia but with intact AV conduction. The A-R interval was studied during incremental atrial pacing at rest and during exercise with a constant workload. In addition, the atrial pacing rate was kept constant just above spontaneous sinus rate and the dynamics of the A-R interval were studied during exercise with a low constant workload and during a maximal exercise test with increasing workload. Incremental atrial pacing prolonged the A-R interval and this response was blunted during exercise (P less than 0.003). Atrial pacing at a constant rate and during a constant workload resulted in an almost direct shortening of the A-R interval. When the workload was increased but the atrial rate kept constant, a pronounced shortening of the A-R interval was noted (P less than 0.0001). It is concluded that changes of the A-R interval during different kinds of exercise were prompt and predictable in patients with sinus node dysfunction but intact AV conduction. In these patients the shortening of the A-R interval during exercise may be a suitable indicator for rate adaptive atrial pacing.
IntroductionDual chamber pacing systems have been in use for more than ten years and have proven to provide superior hemodynamics compared to single chamber pacing systems. Since their introduction only minor modifications to tbe basic bebavior have been introduced. Tbe main principle of most dual cbamber pacemakers still consists of following all sensed atrial events, providing tbe ventricular rate does not exceed a certain, programmable upper rate limit. Pacemakers that control their upper rate by sensor information are becoming available, but so far only one pacemaker automatically reduces the upper rate limit when a pathological atrial rhythm is detected. A next step in dual cbamber pacing may consist of reliable beat-to-beat diagnosis of atrial sensed events. Tbis information in turn can influence the pacemaker response. Adaptive mode switching, whereby on a beat-to-beat basis tbe optimum pacemaker response is determined, is intended to provide maximum tracking of sensed atrial beats, with elimination of synchronization to pathological atrial rhythm.In tbis study beart rate variation is investigated and an atgoritbm that should reliably detect pathological rhythms based on tbe analysis of heart rhytbm alone is tested.
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