Stability of far field R wave signals in different conditions.

AIMS: The presence of far field R wave sensing (FFRS) is usually evaluated in patients with dual chamber pacemakers in supine position. To check if this approach is valid, we tested whether FFRS is consistent both in terms of amplitude threshold and timing characteristics in different daily life conditions. METHODS AND RESULTS: In 42 patients with a DDD pacemaker, the presence, amplitude threshold and timing parameters of FFRS were therefore determined, with patients supine, standing and at peak exercise. Measurements were made of paced and sensed R waves, in unipolar and bipolar sensing configurations (at peak exercise only paced R waves and bipolar sensing). After paced R waves (bipolar sensing) amplitude thresholds/time of FFRS after V pace were 0.32+/-0.18 mV/119-139 ms (supine), 0.32+/-0.16 mV/114-130 ms (upright) and 0.27+/-0.13 mV/121-136 ms (exercise) - with unipolar sensing, this was 0.49+/-0.27 mV/101-150 ms (supine), 0.51+/-0.29 mV/100-144 ms (upright). After sensed R waves (bipolar sensing) amplitude thresholds/time of FFRS after V sense were 0.27+/-0.18 mV/24-42 ms (supine), 0.29+/-0.16 mV/18 to 41 ms (upright) - with unipolar sensing, thresholds were 0.59+/-0.32 mV/3-50 ms (supine), 0.59+/-0.36 mV/2-58 ms (upright). CONCLUSION: given the lower FFRS thresholds with bipolar sensing, bipolar sensing is superior in avoiding FFRS compared with unipolar sensing. No differences were found in terms of amplitude thresholds and timing characteristics with patients supine, standing and at peak exercise. Thus, measurements made in the supine position are basically sufficient to predict the presence/absence of FFRS under different conditions.

Mode switching failure during atrial flutter: the '2:1 lock-in' phenomenon.

AIMS: To evaluate incidence and mechanism of a special form of automatic mode switching (MS) failure in patients with atrial flutter. METHODS AND RESULTS: Retrospectively the charts of 134 patients implanted with dual chamber pacemakers with MS algorithms were reviewed. Seven patients (5.2%) were identified that presented with sustained rapid ventricular pacing resulting from atrial flutter with failure of automatic MS. Since this form of MS failure implies 2:1 tracking of atrial flutter, it was coined '2:1 lock-in'. A theoretical timing model was developed to clarify the mechanism of this special form of MS failure. Prerequisites for the '2:1 lock-in' phenomenon are: (1). the sum of the AV delay and the post ventricular blanking (PVAB) must be longer than the cycle length of the atrial flutter, (2). the tachycardia detection rate must be higher than half the atrial flutter rate and (3). the maximum tracking rate (MTR) must be higher than half the atrial flutter rate. Recommendations for programming in order to avoid this specific form of MS failure are made accordingly and parallel algorithms for flutter detection are discussed. CONCLUSION: '2:1 lock-in' is a typical form of MS failure in patients with atrial flutter and the mechanism is closely linked to the typical atrial sensing windows.

Superovulation of goats with purified pFSH supplemented with defined amounts of pLH.

The superovulatory response of goats treated with purified pFSH supplemented with 30, 40 or 50% pLH was compared. Sixty-four Boer goat does were synchronized by progestagen-containing ear implant, randomly allotted to 3 groups and, beginning 2 d before implant removal, treated with purified pFSH supplemented with 30, 40 or 50% pLH. Each animal received 16 Armour Units of pFSH administered in 6 descending doses at 12-h intervals. Along with the last 2 injections, the does received 5 mg PGF(2alpha). Embryos were flushed either surgically or after slaughter on Day 5 or 6 after the last day of standing estrus. The percentage of animals responding to treatment was not different among groups treated with pFSH supplemented with 30, 40 or 50% pLH (76, 71 and 63%, respectively). The corresponding data for number of ovulations was 11.3 +/- 1.6, 16.3 +/- 1.8 and 16.4 +/- 2.6, for number of ova and embryos recovered 8.1 +/- 1.9, 12.0 +/- 1.5 and 13.5 +/- 2.9 and for number of transferable embryos 6.6 +/- 1.9, 9.1 +/- 1.5 and 7.1 +/- 2.1 (x +/- SEM). Results confirm the earlier finding of a good response of goats to pFSH preparations with a high FSH:LH ratio, and, although group differences were statistically nonsignificant (P > 0.05), they suggest that supplementation with approximately 40% pLH may be close to the optimum.

Clinical observations with a dual sensor rate adaptive single chamber pacemaker.

The Topaz model 515 (Vitatron B.V.) is a dual sensor rate responsive pacemaker for single chamber stimulation. It can be driven by activity counts (ACT) and QT interval measurements. Inappropriate rate modulation due to one sensor can be corrected by "sensor cross-checking." It was implanted in ten patients (20-86 years) of whom seven had complete heart block and atrial arrhythmias. After implantation T-wave amplitude ranged from 0.9 mV-3.5 mV. T-wave sensing ranged from 88%-99% in 9/10 patients at the follow-up of 3 weeks. Eight patients remained in default setting of the activity threshold, after evaluation with a short walking test. An exercise test was performed on all patients. In one test, QT sensing was marginal because of lead implantation in the right ventricular outflow tract. Therefore, this pacing rate was only modulated by ACT sensing. All others were tested with equal contribution of information from both sensors (ACT = QT). In 7/9, rate response was satisfactory. When the treadmill was repeated with ACT in five of these seven patients, rate generally accelerated too fast. In one patient the setting was adjusted to "QT > ACT," because of inappropriate acceleration due to activity sensing, in another it was adjusted to "QT < ACT" because of delayed response to activity. The pacing rate and the ACT during treadmill tests in "QT = ACT" mode were more closely correlated in the first 3 minutes, compared with the last 3 minutes. We feel that rate modulation with this new pacemaker is adequate. Sensor blending and sensor cross-checking are of clinical importance.

Catecholamine levels and pacing behavior of QT-driven pacemakers during exercise.

It is thought that increasing catecholamine levels in the heart are partly responsible for shortening of the repolarization time and so indirectly for the pacing behavior of the QT driven pacemaker. Adrenaline and noradrenaline (NA) plasma levels were determined at rest, during symptom-limited exercise, and during recovery more than 1 month after the implantation of a 919 or a Rhythmyx pacemaker (Vitatron, The Netherlands) in eight patients (age 54-85 yrs). Significant increases were detected in NA level (from 0.57 +/- 0.23 ng/mL to 2.15 +/- 0.76 ng/mL), but not in the circulating adrenaline level. The correlation coefficient of the mean pacing rate and the mean NA level during exercise and recovery was 0.963 (P less than 0.0001), the correlation coefficient with the mean oxygen consumption was 0.888 (P less than 0.01). No correlation with the adrenaline level was observed. The correlation coefficient of the changes of pacing rate and the changes of NA level during exercise and recovery was 0.882 (P less than 0.005). The pacing rate of the new generation of QT driven pacemakers is closely correlated with the noradrenaline spillover in the plasma, not with the adrenaline level. A short delay (less than 1 minute) is observed in the adaptation.

Pacing and sensing: how can one electrode fulfill both requirements?

Pacing and sensing are two different functions which can be accomplished by one and the same electrode. Optimal pacing requires a high tissue resistance in order to minimize the stimulation energy, making a small surface electrode highly desirable. For adequate sensing, however, the tissue resistance should be as low as possible which requires a larger electrode surface area. Decreasing the electrode surface area results in an increased polarization impedance. As this latter should be low for both pacing and sensing, an electrode with a large surface area should be used. How can these opposing needs be met by one electrode? The combination of a small geometrical surface and a large porous microstructure along with the choice of low polarizable materials meets both the requirements of pacing and sensing.

Prediction of Wenckebach behavior and block response in DDD pacemakers.

At higher atrial rates, the behavior of a DDD pulse generator will depend on the atrial rate or spontaneous atrial interval (SAI) and the settings of the pacemaker: upper rate interval (URI), atrioventricular interval (AVI), and atrial refractory interval (ARI). An algorithm was developed enabling the prediction of the degree of Wenckebach block using the parameters mentioned above. In the absence of the programmed settings of the pacemaker, these parameters can be determined by noninvasive methods. AVI can be measured by application of a magnet over the pulse generator, while URI and ARI can be estimated during chest wall stimulation by progressively increasing the frequency of the external extrastimuli. The use of the formula in combination with chest wall stimulation allows the evaluation of the proper functioning of any DDD pacemaker during exercise and in patients with atrial rhythm disturbances, even when no information about the pacemaker settings is available.