Intracardiac electrogram

FIGURE 6.1 Initiation of atrial fibrillation (AF) with ectopic beats arising from the left superior pulmonary vein (PV). Shown are three surface leads (I, aVF, and VI), five intracardiac electrograms form the coronary sinus (CS prox to CS distal) and five electrograms from a catheter in the left superior PV (PV1 through PV5). After a sinus beat, a premature PV potential appears on PV3, which initiates AF (arrow). 

FIGURE 6.3 Pulmonary vein (PV) isolation. Left panel shows three sinus beats with surface leads aVF, VI, and V6, and ten intracardiac electrograms from the left superior PV (PV-1 through PV-10) before PV isolation. Low amplitude potentials represent atrial activity (small arrow) followed by high amplitude PV potentials (large arrow). Right panels shows elimination of PV potentials after encircling radiofrequency ablation. 

FIGURE 11.4 The surface ECG (ECG LEAD II) represents the sum total of the electrical potentials of all depolarizing tissue. The intracardiac electrogram (V EGM) shows only the potentials measured between the lead electrodes. This allows the evaluation of signals that may be hidden within the surface ECG. 

The cardiac pacemaker lead is a relatively fragile cable of insulated conductor wire implanted into the hostile environment of the human body. Its function is to interface the power source and sophisticated electronics of the pulse generator with the heart. The pacemaker lead plays a critical role in delivering both the output pulse from the pulse generator to the myocardium and the intracardiac electrogram from the myocardium to the sensing circuit of the pulse generator. 

Sensing of intracardiac electrograms. With its broad inter-electrode distance, the unipolar system sees more of the heart in which to detect a spontaneous intracardiac electrical event and thus it was assumed that unipolar was superior to bipolar sensing. In reality, the modem unipolar and bipolar pacing systems show comparable and usually excellent atrial and ventricular electrogram amplitudes and slew rates which usually exceed the standard limits of the sensing circuit by a comfortable margin. 

A diagnostic tool that has achieved popularity is the ability to transmit an intracardiac electrogram, or lEGM, which enables the clinician to assess what the pacemaker is seeing. Much of this information is analyzed and transmitted in digital format. lEGMs can be transmitted in real time or stored and... 

Fig. 3.14 Undersensing of a premature ventricular contraction. In the example, surface ECG, refractory periods and intracardiac electrograms (EGM) are shown. The electrograms generated by native QRS complexes are 7 to 8 mV and are appropriately sensed (because the sensitivity is set to 5 mV). The electrogram from the premature ventricular contraction is only 4 mV and is not sensed by the pacemaker. The problem can be corrected by reducing the sensitivity value to 3 mV (making the pacemaker more sensitive).

The pacing calheter is advanced with the balloon inflated while continuously recording the surface ECG and intracardiac electrogram. A large atrial signal... 

Feuer J, Florio J, Shandhng AH, Alternate methods for the determination of atrial capture thresholds utihzing the telemetered intracardiac electrogram, PACE 1990 13 1254-1260. 

Sarmiento JJ, Clinical utility of telemetered intracardiac electrograms in diagnosing a design dependent lead malfunction, PACE 1990 13 188-195. 

Nalos PC, Nyitray W, Benefits of intracardiac electrograms and programmable sensing polarity in preventing pacemaker inhibition due to spurious screw-in lead signals, PACE 1990 13 1101-1104. 

---