Intracardiac electrogram parameters, electrode surface area and pacer input impedance: their correlations.
Small surface area electrodes are accused of sensing defects which were related to alterations that they induce in the endocardiac electrograms. Since several factors affect the cardiac signal coming from electrode to sensing circuit, i.e. electrode surface area, electrode-tissue interface, pacemaker input impedance and sensing amplifier pass-band, Authors present their studies performed on 252 implanted electrodes of various type. Study was carried out by connecting in parallel to the recorder a variable resistor in order to simulate different pacer input impedances. The results showed a significant reduction in RS amplitude when recorder was paralleled with resistor values lower than 40 K. Slew rates showed a similar behaviours since RS steep tract did not change his duration with load, while total QRS duration is reduced. High speed analysis has shown that the RS segment is not linear in about 40% of cases: the main tract is used for calculations. The most significant attenuations and distortions of endocardial electrogram were observed with smallest electrodes and lowest resistances parallel connected: in these cases the sensing impedance at the electrode-tissue interface appears to be between 3 to 5 K ohms. The results suggest that the most of the alledged sensing faults experienced in the past were probably due to small tip electrodes connected to low input impedance generators or to impending failure situations. The AA. conclude that the main question does not concerne a true electrode inefficiency but a wrongly chosen pacemaker-electrode combination, i.e. small tip electrode connected with old generator models. To avoid the evaluation error, it would be instrumental that the pacemaker manufacturers would specify input characteristics of their generators. So, the implanting clinician becomes able to exactly evaluate the true signal arriving to the sensing circuit by connecting in parallel with the recorder input a resistor whose value approximates the input resistance of the generator to be implanted.