Pacing threshold interval with decreasing and increasing output

The study was made in 44 patients, of which 23 received a new endocardial electrode and 21 were investigated during pulse-generator replacement. Indication for pacemaker therapy was sino-atrial bradycardia in 12 patients and atrio-ventricular block in 32 patients. When the pacing threshold was determined with decreasing output it was found to be 0.6V/0.7 mA for new implantations and 2.0 V/2.0 mA with pulse generator replacement. When determined with increasing output the threshold was found to be an average 0.20 V/0.20 mA higher than when determined with decreasing output (threshold interval). The maximal difference observed was 0.70 V/0.55 mA. This threshold interval was of the same size irrespective of pacemaker indication and spontaneous activity and whether the electrode was newly implanted or not. Pacemaker ventricular block of Mobitz type I with Wenckebach periodicity was never observed.