To induce arrhythmias, a technique known as programmed electrical stimulation is performed. Programmed electrical stimulation uses standard pacing of the atrium or ventricle at a predetermined heart rate or cycle length with the placement of premature extrastimuli. The premature extrastimuli start at a preset interval and get progressively more premature.
Generally, programmed stimulation starts at a drive cycle length faster than the sinus cycle length. Standard pacing typically occurs at a drive cycle length of 8 beats to 10 beats at 600 milliseconds (ms), 500 ms, or 400 ms. This drive cycle length is followed by a single premature stimulus that is often 100 ms to 200 ms shorter than the drive cycle length.
The interval between the last drive cycle length and the first premature beat is called the coupling interval. This coupling interval is shortened by intervals of 10 ms to 20 ms after each drive train until the paced stimulus fails to capture the myocardium. Subsequently, a second stimulus can be added. The cycle length of the first stimulus should be increased by 30 ms or 40 ms from the refractory period, and the second stimulus could be the same as or slightly less than the coupling interval of the first stimulus. There may be stylistic differences amongst the various EP operators with respect to how programmed electrical stimulation is performed.
For nomenclature, the stimulus recorded for the drive cycle length is called S1. Each subsequent premature stimulus is labeled sequentially: S2, S3, S4, etc. In other words, S2 is the first extrastimulus, S3 is the second extrastimulus, and S4 is the third extrastimulus.