A device to select the optimal time to replace the pacemaker activated by a lithium-iodine battery: results of 8 years of experience

The discharge curve of lithium-iodine batteries does not havefeatures which make the clinical decision to replace the pacemakereasy. The output voltage fall corresponding to the end of theirlife is evidenced by variations of the output variables (frequencyand/or length of pulse) and the time of elective replacementtime (ER) is arbitrarily established by the manufacturer, ERmay be too early or too late in relation to the threshold valueof myocardial stimulation. Furthermore, the variables indicatingend of life may be changed by troubles in the circuit components,irrespective of the state of battery. A device to detect theoptimal time for replacement (OR) of pacemaker was studied in1692 subjects with implanted LEM VVl electro-stimulators. Thisdevice consists of a resistance of 110 connected in serieswith the catheter through a magnetic switch, which is activatedby placing a magnet over the skin. The transient connectionof the resistor reduces the output voltage by an amount dependingon the resistive load between the electrodes. In a follow-up of 35 000 months the OR device revealed a verygood reliability. Pacemaker replacement, established when stimulation becomesineffective during the application of the magnet or when thespike amplitude in the standard oscilloscope rapidly declined,was necessary in 77 patients (six of them were excluded dueto a circuit blackout). The OR device was useful in 53 patients(74.6%) with a chronic myocardial threshold of more than 2.5V, useless in 18 patients with a myocardial threshold lowerthan 2.5 V; to obviate this limitation a new OR device is describedfeigning a temporary increment of battery impedance. The employmentof the OR device rather than using ER enabled the patients togain 1333 months of stimulation. The gain of pacing varied froma few months to 3 years, with an average of 18.7 months.