ABSTRACT
INTRACARDIAC ELECTROGRAMS Intracardiac electrograms (IEGMs) and surface electrocardiograms (ECGs) both record cardiac electric activity. However, there are some differences that one must understand for appropriate interpretation of intracardiac electrograms. The surface electrocardiogram, which is recorded outside of the body on the surface, provides information about the electrical activity of the entire heart. Intracardiac electrograms are recorded inside the heart by intracardiac multi-polar catheters and display local electrical activity near the recording electrodes of the catheter (near field) as well as more remote significant cardiac electrical activity (far field) (Figure 19.1). The IEGMs are usually filtered differently from the ECG to minimize noise and interference. The paper recording speed is also generally faster than the standard 25 mm/s 12-lead ECG speed (100 or 200 mm/s more frequently). The intervals measured during an electrophysiology study are generally expressed in milliseconds (ms) rather than beats per minute (bpm) (Figure 19.2a and b). An exception to this rule is the term ‘incremental atrial or ventricular pacing which refers to a rate in bpm instead of ms (see chapter 20). To obtain the cycle length in ms, the following formula is used:
Cycle length in ms 60 000/rate in bpm
Conversely, the rate is obtained as the following:
Rate in bpm 60 000/cycle length in ms
Thus, a rate of 60 bpm corresponds to a cycle length of 1000 ms, 100 bpm corresponds to 600 ms, 120 bpm to 500 ms, 150 bpm to 400 ms, 200 bpm to 300 ms, etc. As a result of this formula, heart rates in bpm and cycle length in ms have an inverse relationship. For instance, as the heart rate increases from 125 bpm to 150 bpm, the cycle length decreases from 500 to 400 ms.
