*2.5.2.2 Extrastimulus testing*

Extrastimulus testing means introduction of a drive train (usually 8 beats, S1) followed by one or more extrastimuli with shorter coupling interval than the cycle length of the drive. S2 means the first programmed extrastimulus, S3 is used for the second, and so on (**Figure 7**).

### **2.6 Measurement of refractory periods**

During EPS, refractory periods of cardiac tissues can be characterized by measuring effective, functional, and relative refractory periods [5].

*Electrophysiology Study: Interpretation of Intracardiac Electrocardiograms DOI: http://dx.doi.org/10.5772/intechopen.102079*

#### **Figure 6.**

*Ventricular burst pacing from RVa position. Note the pacing artifact on RVa channel and before the QRS complexes. Pacing cycle length is fixed (500 ms). Paper speed is 50 mm/s.*

#### **Figure 7.**

*Atrial extrastimulus testing from coronary sinus catheter (CS). The drive train consists of 8 beats at a cycle length of 400 ms is followed by an extrastimulus with a shorter coupling interval (300 ms). Ablation catheter (ABL) is in the His region showing His bundle electrogram. Paper speed is 50 mm/s.*

### *2.6.1 Effective refractory period*

Most commonly used as it is part of a routine EPS. It represents the longest coupling interval that fails to capture the tissue or be conducted over the structure (**Figure 8**).

#### *2.6.2 Functional and relative refractory period*

A routine EPS does not include the measurements of the functional and relative refractory period. Functional refractory period means the lower limit shortest "output" coupling interval that can be produced by any "input" interval. Relative

### **Figure 8.**

*Programmed atrial stimulation from CS 9,10. Drive cycle length is 400 ms. Panel A shows ventricular contraction (i.e., QRS complex) after S2 extrastimulus at a coupling interval of 240 ms. However, Panel B represents atrioventricular block after S2 extrastimulus at a coupling interval of 230 ms. In this case, effective refractory period of the AV node (AVNERP) is 230 ms.*

refractory periods represent the point at which latency begins to occur. RRP means "input" interval to a tissue at which the "output" interval just begins to differ from the "input" interval.
