A pacemaker automatically adjusts sensitivity as a function of metabolic demand. The amplitude of cardiac signals is naturally reduced during exercise. The pacemaker measures a physiological parameter to determine the patient's metabolic demand. In response to increases in the patient's metabolic demand the sensitivity is increased and in response to decreases in the patient's metabolic demand the sensitivity is decreased.

Cardiac signal sensing device having sensitivity automatically controlled in response to metabolic demand

A single pass ventricular lead with a dual chamber electrode system, designed for VDD pacing, was implanted in 17 patients (11 men, 6 women, aged 53 to 86 years, mean 74) for symptomatic bradycardia due to second-or third-degree AV block and normal sinus node function. Bipolar atrial electrodes, diagonally displaced along the lead axis and positioned within the right atrial cavity, are used to detect atrial activity that is then differentially processed within the pacemaker. P wave amplitude (amp) derived from a PSA-DAA device at implant was 1.38 +/- 0.28 mV. P wave signal amp derived from telemetered atrial electrograms was 1.29 +/- 0.22 mV at predischarge (n = 17), 1.31 +/- 0.24 mV at 3 months (n = 15), 1.30 +/- 0.24 mV at 6 months (n = 8), 1.51 +/- 0.34 mV at 9 months (n = 4), and 1.35 +/- 0.35 mV at 12 months (n = 2); and the far-field QRS signal measured at predischarge was of negligible voltage (0.17 +/- 0.07 mV). The susceptibility of the atrial sensor system to interference was noted with chest wall stimulation and only at higher sensitivities (0.1 to 0.3 mV) and not with isometric arm exercise. Intact VDD pacing function at rest and during exercise was established using Holter and periodic ECG monitoring. Postoperative complications included one lead displacement and one pocket hematoma. Three patients died postimplant of causes unrelated to pacemaker function. Advantages of the single-lead VDD pacing include: (1) elimination of second atrial sensing lead; (2) superior atrial sensing performance; (3) effective resistance to myopotential and far-field signal interference; and (4) stability of postimplant atrial signal amplitude.

Single-Lead VDD Pacing System

Pacemakers in children: An update

Atrial sensing performance of the single-lead VDD pacemaker during exercise

In an attempt to evaluate the prevalence and predisposing factors of bipolar ventricular far-field oversensing, 57 patients were studied who had a Medtronic dual chamber pacemaker implanted (models 7940: n = 6; 7960i: n = 41; 401: n = 3; 8968i: n = 7) and bipolar atrial leads with a dipole spacing from 8.6 to 60 mm attached to various parts of the atrial wall (lateral/anterior: n = 30; appendage: n = 10; atrial septum: n = 10; floating: n = 7). Median bipolar sensing threshold for P waves was 4.0 mV (2.8-4.0 mV, lower and upper quartile) with standard leads and 0.35 (0.25-1.4) mV with single pass (VDD) devices. At the highest sensitivity available, 43 of 50 DDD pacemakers but only two of seven VDD systems detected intrinsic R waves in the atrium (P < 0.01). Ventricular far-field oversensing occurred at 0.5 mV in 28 (56%) and at 1.0 mV in 16 of 50 DDD units (32%), respectively, and there was one observation in a septal implant at a sensitivity of even 2.8 mV. With ventricular pacing, VDD systems were as susceptible to far-field signals as DDD pacemakers. Outside the postventricular blanking period (100 ms), evoked R waves were detected by 27 of 57 systems (47%) at maximum atrial sensitivity, by 10 (18%) at 0.5 mV, and by 2 (4%) at a setting of 1.0 up to 1.4 mV, respectively. There was no definite superiority of any lead position, there was a trend in favor of the atrial free wall for better intrinsic R wave rejection, but just the opposite was the case for paced ventricular beats. Bipolar signal discrimination tended to be higher with short tip-to-ring spacing (1 7.8 mm) but the difference to larger dipole lengths (30-60 mm) was not significant in terms of the R to P wave ratio and the overall far-field susceptibility. In summary, bipolar ventricular far-field oversensing in the atrium is common with short postventricular blanking times and high atrial sensitivity settings that may be warranted for tachyarrhythmia detection and mode switching. A potentially more discriminant effect of shorter dipole lengths (< or = 10 mm) remains to be tested.

Bipolar ventricular far-field signals in the atrium.

To investigate the atrial sensing function of dual chamber pacemakers during exercise, we studied 57 patients aged 12 to 81 years (m = 65). They were paced for sinoatrial disorders (n = 15), second or third degree AV block (n = 37), or binodal disease (n = 11). The examination was performed 3-24 months (m = 11) after pacemaker implantation. Individual sensing thresholds were determined at rest in the supine position, and proper detection of atrial signals at the programmed sensitivity level was verified during a period of 5-9 minutes (m = 5.8). Without a change in the program of the unit, symptom-limited bicycle ergometry was performed at a maximum load of 25-200 Watts (m = 95) and 6-channel chest wall electrocardiograms were continuously recorded throughout the test, including recovery. During exercise, 25/57 patients (44%) exhibited poor atrial sensing of the pulse generator; after termination of exercise in 16 of the 25 patients, proper atrial sensing resumed within 1 to 7 minutes of recovery. In the remaining nine cases, ergometry was continued after lowering the sensitivity threshold to half the initial setting or, depending on the pacemaker model, by a value of 0.4 mV. This resulted in normal function of the pulse generator in all patients but one, who needed a sensitivity adjustment of another 0.4 mV. In a subgroup of 25 patients, telemetric atrial electrogram recordings were monitored during ergometry, 19 of which could be evaluated quantitatively. Besides random atrial signal variations, presumably due to ectopic beats or runs, a systematic decrease of the peak-to-peak amplitude by 0.1 to 1.6 mV (3-30%) was observed in 16/19 patients during exercise. Mean signal reduction amounted to 11.8% and was statistically significant at the 0.1% level. It is concluded from these findings that, besides testing of atrial sensing at rest, the follow-up of dual chamber pacemakers should include an exercise test.

Atrial sensing performance of AV universal pacemakers during exercise.

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Atrial sensing performance of AV universal pacemakers during exercise.