Proper sensing of atrial and ventricular electric activity is mandatory to ensure reliable “demand” stimulation (i.e.: the heart is paced only when the intrinsic activity is absent or too low). Ineffective sensing can imply competitive pacing, which would affect the hemodynamic function and possibly trigger a tachycardia.
The great relevance of a good sensing led to the development of highly selective techniques of intracavitary electrogram detection, aimed at isolating atrial and ventricular local signals from the overall electric activity. The information gained with this approach, however, is restricted to the presence and timing of the intrinsic electrogram, which is an essential piece of information for pacemaker regulation, but does not allow the assessment of the origin and nature of the related cardiac events.
In contrast, the surface ECG is a complex signal, not suitable to the regulation of a cardiac pacemaker. On the other hand, it allows to distinguish different cardiac activation modalities based on the tracing morphological features. Such information of great diagnostic value is not normally available in an implantable stimulator
Medico S.p.a. has developed a technology which provides a signal with properties similar to the surface ECG, by using the electrodes of a dual-chamber pacing system. This innovative multipolar electrogram is referred to as intracardiac ECG (iECG).
The iECG complements and does not replace at all the conventional atrial and ventricular electrograms, which are applied in the pacemaker sensing function. The iECG tracing shows an atrial and a ventricular deflection in correspondence with the P wave and the QRS complex, respectively. The morphology of the atrial component depends on the form of ongoing atrial activity (sinus rhythm, retroconduction, atrio-ventricular re-entry). Similarly, the morphology and duration of the ventricular component reflect the type of ventricular activity (intrinsic conduction, pacing, ectopic beats, ventricular tachycardia).
Therefore, cardiac activity characterization can be achieved by evaluating the modifications of the iECG signal. This principle has several applications. Currently, the most important is the discrimination of ventricular and supraventricular tachycardias in case of a tachyarrhythmia episode. If the iQRS features the same morphology as in sinus rhythm, a ventricular tachycardia can be excluded.
Modification of the ventricular component of the iECG (iQRS) in the presence of a ventricular ectopic beat (PVC).
Modification of the ventricular component of the iECG (iQRS) in the presence of a ventricular tachycardia (VT).
In the presence of a supraventricular tachycardia (SVT) the iQRS is unchanged, while the iP waveform is modified with respect to sinus rhythm (arrows).