David H. Bennett

Comparison of Resting Hemodynamic Indices and Exercise Performance During Atrial Synchronized and Asynchronous Ventricular Pacing

Resting hemodynamic indices and exercise tolerance were measured during atrial synchronized (VAT) and asynchronous ventricular pacing (VOO) in 35 patients with implanted pacemakers which could be externally programmed to function in either pacing mode. Cardiac output and mean systemic arterial pressure were significantly greater during VAT pacing (VAT: 4.5 ± 1.21 /min, 115 ± 28 mmHg; VOO: 3.7 ± 0.8 1/min 105 ± 25 mmHg respectively), although there was no difference in pulmonary artery end‐diastolic pressure. Maximal exercise performance was assessed using the Bruce protocol in both pacing modes. Neither the patient nor the supervising physician was aware of the preselected pacing mode; a second physician monitored the electrocardiogram and blood pressure but influenced the point of exercise termination only if a potentially dangerous arrhythmia or hypotension occured. Blood pressure responses were superior and atrial rates lower during VAT pacing, In all but five patients, exercise tolerance was improved by VAT pacing. This amounted to 33 percent or more in 23/35 patients. This improvement was shown to be maintained in the 20 patients who had repeat exercise tests several weeks later. Ventricular arrhythmias, hypotension, and lightheadedness frequently complicated exercise during asynchronous pacing but occurred rarely with atrial synchronized pacing. Resting hemodynamic indices did not predict the extent of improvement gained by physiological pacing.

Reliability of the Evoked Response in Determining the Paced Ventricular Rate and Performance of the QT or Rate Responsive (TX) Pacemaker

The TX pacemaker uses a conventional transvenous electrode to sense T‐waves of paced ventricular complexes and it adapts the pacing rate to varying physioiogical demands by responding to changes in the QT or, more correctly, the stimulus artifact‐to‐T‐wave (stimulus‐T) interval. This pacing system was assessed in 13 patients. The relation between heart rate and stimulus‐T interval and the effect of programming on the performance of this pacemaker were studied on several occasions in each patient. Treadmill exercise performance during TX pacing mode was compared with atrial synchronized ventricular (VAT) and asynchronous ventricular demand (VOO and VVI—70 beats per minute) pacing modes. T‐wave sensing problems arose in three patients. In one. this was overcome by reducing the pulse amplitude from 5.0 to 2.5 V. In another patient, spontaneous recovery of T‐wave sensing occurred 5 months after pacemaker implantation. T‐wave sensing deteriorated with the passage of time in most patients. Satisfactory rate response as assessed by treadmill exercise testing and Holter monitoring was achieved in 12 patients through adjustments of two programmable parameters: the slope that defines the alteration in heart rate in response to a millisecond change in stimulus‐T interval and the “sensing window” that is the interval during which T‐waves can be sensed and a rate response is possible. Exercise performance was significantly better during rate responsive pacing (TX) mode as compared with VVI pacing but was comparable to that during VAT pacing. The resting heart rate/stimulus‐T interval can be described by the following linear regression equation; stimulus‐T interval = 466 – 1.68 × paced‐rate, r2=–0.62. This relation, however, was subject to wide inter‐ and intra‐patient variation. Consequently, given identical programmed parameters and exercise protocol, the chronotropic response differed significantly from patient to patient and in the same patient from one occasion to another. Our results show that a physiologically beneficial chronotropic response can be achieved in most patients. However, reprogramming, based on results of exercise tests and Holter monitoring, may be necessary to adjust for changes in T‐wave sensing and the heart rate/stimulus‐T interval relation and, thus to ensure that the pacemaker continues to function optimally.

Safety and acceptability of implantation of internal cardioverter-defibrillators under local anesthetic and conscious sedation.

Background: Implantation and testing of implantable defibrillators (ICDs) using local anesthetic and conscious sedation is widely practiced; however, some centers still use general anesthesia. We assessed safety and patient acceptability for implantation of defibrillators using local anesthetic and conscious sedation.

The Impact of a Manufacturer's Revision of Pacemaker Service Life on the Workload of a Large Pacing Center

In certain batches of a manufacturers pacemakers, inadequate insulation of the battery cell feed‐through mechanism led to internal short‐circuiting and premature battery depletion. We describe the impact of the consequent reduction in service life and recommended close supervision of these generators on the follow‐up clinics and inpatient workload at a large pacing center. One‐hundred and ninety patients had Cordis Gamma series pacemakers implanted between September 1981 and July 1985; 135 have had to undergo premature generator replacement. Eleven of these pacemakers had developed early depletion of rate and 12 no‐output failure. Within 3 months of their generator change, 12 patients developed wound sepsis, three of them requiring fiirther system replacement. We calculate that at least 540 extra outpatient appointments, 400 in‐hospital patient days and 12 full working days in one of our catheterization laboratories were required to deal with the problems. The pacemaker industry, as well as doctors and technicians involved in pacemaker purchase and implantation, should be aware of the consequences of a major recall not only on workload but also on patient morbidity and anxiety.

Effect of Transvenous Atrioventricular Nodal Ablation on the Function of Implanted Pacemakers

We report on seven patients with implanted pacemakers who underwent transvenous ablation of the atrioventricular junction using direct current shocks of 200 to 350 joules. Pacemaker impulse duration and rate were unaffected, but one rate responsive (TX) pacemaker was reprogrammed by a 300‐joule shock. Transient increases in stimulation threshold did occur in two patients, and exit block for 2–15 seconds developed on four occasions. Chronic stimulation thresholds were unaffected. We conclude that it is preferable to carry out ablation before pacemaker implantation, but it is possible to perform transvenous ablation of the atrioventricular junction without damage to an implanted pacemaker; however, a transient rise in stimulation threshold or even exit block may occur, and pacemaker function should be carefully assessed after the procedure.

Comparison of Vitreous Carbon and Elgiloy Transvenous Ventricular Pacing Leads

In a randomized prospective study of 150 patients. we compared two porous‐tipped, finned leads, one with a vitreous carbon lip (surface area = 12 mm2) and the other with an Elgiloy tip (surface area = 8 mm2). We assessed the acute and chronic stimulation thresholds and the ease of implantation of the two leads. The acute current thresholds for the vitreous carbon and Elgiloy‐tipped leads were 1.7 ± 0.5 (±SD) mA vs. 1.1 ± 0.4 mA (0.25 ms pulse width), 1.3 ± 0.5 mA vs. 0.9 ± 0.3 mA [0.5 ms], and 1.0 ± 0.3 mA vs. 0.8 ± 0.3 mA (0.75 ms), respectively (P < 0.0005). Impedance at implantation was 473 ± 121 ohms and 716 ± 285 ohms (P < 0.0005) for the carbon‐tipped and Elgiloy‐tipped leads, respectively. The R‐wave amplitudes were 10.2 ± 5.1 volts and 6.8 ± 3.0 volts, respectively (P < 0.0005). Both leads were easy to implant and showed low stable chronic thresholds from 3 weeks after implantation, at which time the stimulation threshold was 1.9 ± 1.0 volts vs. 1.5 ± 0.6 volts with the implonted voltage‐variable generators and 1.8 ± 0.8 mA vs. 1.6 ± 0.6 mA with the implanted current‐variable generators. Reoper‐ationfor displacement was required for two of the 87 (2.3%) carbon‐tipped and 2 of the 67 (3.2%) Elgiloy‐tipped leads. There was no exit block nor any other lead‐related complication. We conclude that both leads have similar and satisfactory performance.

Experience with 407 Transvenous, Finned Pacing Leads with a Sintered Porous-surfaced Electrode

Experience with 407 transvenous ventricular finned pacing leads with a sintered porous‐surfaced electrode is reported. A low complication rate was encountered: 97% of patients were free from lead‐related complications. Four patients (1%) required re‐operation: two for lead displacement, one for failure to sense spontaneous ventricular activity and one for exit block. A further patient developed exit block which resolved spontaneously. The vast majority of patients had low chronic stimulation thresholds; for example at six months after implantation, the mean threshold for a group of pacemakers with a pulse duration of 0.5 ms was 1.4 ± 0.7 V and for another group with a pulse duration of 0.8 ms it was 1.9 ± 0.7 mA. However, six patients developed a transient high stimulation threshold during the month after implantation, and a high threshold developed in another patient at five months and persisted. Mean acute stimulation and sensing thresholds were satisfactory: 0.7 ± 0.2 V at 0.5 ms pulse duration and 7.6 ± 2.4 mV, respectively.