Monitoring of electrograms (EGMs) sensed by pacemakers (PMs) during their lifetime is the utmost importance, since changes of these signals, due to modifications of the electrode-myocardial interface may occur compromising the proper functioning of the PM. Since after implantation these intracardiac signals are accessible only by means of a telemetry system, it is necessary to ascertain how faithfully the telemetric signals reproduce the direct ones. To this aim we devised an accurate method, based on linear system theory, which allows the estimation of the transfer function of the PM-telemetry system and the comparison of a set of time and frequency domain parameters (wave amplitude, slew rate, frequency at the spectrum peak and bandwidth) computed on both direct and telemetric signals. To overcome the problem that these two signals cannot be recorded simultaneously, an appropriate model of the input and output signals of the PM-telemetry system and of their interrelationship was identified. This model relies on a set of reasonable assumptions about the nature of the direct signal (supposed to have a fixed morphology within a predefined analysis window) and of noise sources corrupting both the direct and telemetric signals. Owing to the involvement of deterministic and stochastic signals, the theory of quasi-stationary signals was employed to obtain the system transfer function. To test our method, direct and telemetric recordings were performed on 12 patients with atrial sensing carrying Medtronic Pacemakers (Mod Elite 7077/7086, programmer Mod 9760) with unipolar steroid eluting leads (Medtronic Capsure 4523). The results we obtained demonstrate that the specific system considered acts as a low-pass filter with a -3 dB frequency at about 90 Hz this ensures that the telemetric signal contains almost all the information of the direct one and can be used to monitor the changes of the signals sensed by the PM.