The OERP amplitudes increased and the latencies shortened with increasing stimulus concentration. Furthermore, a difference between the groups was found, with higher OERP amplitudes and shorter latencies in healthy subjects compared with patients.

The Medtronic 6961 lead has been used in 14 patients for transvenous atrial sensing and/or pacing. This lead is furnished with small tines of silicone rubber at the distal end. The conductor coil material is space wound for flexibility. Thus, the lead lacks intrinsic elasticity and can be fastened within the right atrial appendage without a preformed J-shape. The clinical experiences with the lead are encouraging. The lead is easier to introduce and position in the right atrial appendage than the previously used tined J-shaped leads (Medtronic 6991). The small size of the new lead makes the choice of vein less critical and a normally-sized external jugular or cephalic vein permits the use of the same vessel for a second ventricular lead. By means of a lead extension wire, consecutive determinations of the P-wave amplitude, stimulation threshold of the right atrium, electrode resistance, and P/QRS-ratio were made for four weeks following electrode insertion. The mean P-wave amplitude at insertion was 4.9 +/- 1.5 mV (mean +/- SD). There was a significant decrease to a lowest mean level of 309 +/- 1.1 mV after one week. From that time there were only small variations. In the supine position and with normal breathing there was a spontaneous variation of the P-wave amplitude of +/- 12%. The P-wave amplitude was influenced by body position and maximal breathing movements to a minor extent. The threshold of stimulation was 0.9 +/- 0.4 V after one week. Later there was a small decrease in the threshold which, however, still remained significantly higher than at the time of insertion. The total resistance of the electrode system was about 700 ohms and P/QRS-ratio about 4 +/- 3. During an observation time ranging from 4 to 11 months there were no electrode dislocations. The electrodes were connected to the intended pacemakers without complications. In conclusion, the transvenous endocardial atrial lead, Medtronic 6961, shows attractive and promising qualities. The electrophysiological data recorded are suitable for the pacemakers in use. The electrode definitely deserves further evaluation.

SUMMARY Despite some reports on the use of the atrial transvenous endocardial lead, Medtronic 6991, there is still very little information on the electrophysiological properties of the lead. A lead extension wire was connected to the atrial lead, passed subcutaneously, and brought through the abdominal skin. P wave amplitude, threshold of stimulation, and electrode resistance were repeatedly determined through the atrial lead during four weeks after the electrode insertion in 17 patients. Mean P wave amplitude at insertion was 3-7 ± 1 0 (mean ± SD) mV. It decreased significantly to a lowest mean level of 2-3 ±09 mV after one week. From that time there were only small variations. In the supine position and with normal breathing there was a spontaneous variation in the P wave amplitude of ± 15 per cent. P wave amplitude was only to a minor extent influenced by body position and maximum breathing movements. The threshold of stimulation was 1-7 ±0-7 V (impulse duration 0 5 mi) at the time of electrode insertion. It increased to 2 7 ± 1 0 V after four weeks. The electrode resistance varied about 600 ohms.A total of 28 patients received an atrial transvenous endocardial lead. Three electrode dislocations occurred, all during the first 24 hours; apart from these three, the electrodes were connected easily to the intended pacemakers and have functioned well during the observation period of between two and 24 months. In conclusion, the transvenous endocardial atrial lead Medtronic 6991 has shown attractive qualities. The electrophysiological data recorded are suitable for the pacemakers in use. So far this atrial lead seems promising and deserves further evaluation.Despite the increasing number of patients with pacemakers, there has not been a corresponding increase in the use of atrial synchronous and atrial inhibited pacing. In Sweden not more than 1 per cent of initially implanted pacemakers are atrial synchronous (Karlof and Lagergren, 1973; Edhag, 1977). Many patients would, however, improve haemodynamically with an atrial inhibited or atrial synchronous pacemaker (Lagergren et al., 1966;DeSanctis, 1971;Furman, 1973). The reason for the limited use of atrial pacing is probably the lack of an electrode which combines a simple technique of introduction with long-term reliability. Recently promising results have been reported by the use of a transvenous J-shaped atrial tined lead (Smyth et al., 1976;Kleinert et al., 1977). These reports deal with the technique of introduction and electrophysiological characteristics of the lead at the time of electrode insertion.The present investigation was undertaken to study the electrophysiological characteristics of the Received for publication 17 April 1979 atrial electrode by means of repeated measurements throughout a period of four weeks after the introduction of the electrode. The report also contains information about our present clinical experience with the electrode. Subjects and methods PATIENTSThe subjects consisted of 28 patients; pertinent data from these are pres...