*2.3.1 High right atrium*

A diagnostic catheter is positioned from the femoral vein and contacted with the lateral wall of the right atrium at right atrium—superior vena cava junction.

## *2.3.2 Coronary sinus*

The coronary sinus runs transversely in the left atrioventricular groove on the posterior side of the heart. A multielectrode catheter is inserted into the coronary sinus

#### **Figure 3.**

*Standard catheter positions in left anterior oblique (LAO), anteroposterior (AP), and right anterior oblique (RAO) projections. Abbreviations: CS—coronary sinus decapolar catheter; HRA—high right atrium; RVa—right ventricle apex.*

from femoral, jugular internal, or subclavian vein. For femoral approach, steerable catheters are used. CS catheter allows to record IEGMs coming from the left atrium and ventricle. Moreover, this position is easily reproducible and serves as a reference point during the EPS. Thus, CS catheters play an important role in EP labs.

#### *2.3.3 His bundle*

For recording His bundle electrogram, a catheter is inserted via femoral vein to the high septal part of the right ventricle and pulled back slowly with clockwise torquing till characteristic His bundle electrogram appears.

#### *2.3.4 Right ventricular apex*

A diagnostic catheter is advanced from femoral vein to apical right ventricle, which allows to record local ventricular IEGMs.

#### **2.4 Introduction to evaluate intracardiac electrocardiograms, basic intervals**

Generally, IEGMs mean the electrical activity between two electrodes at the tip of the catheter (bipolar recording) [4]. The main difference between surface ECG and IEGMs is that the surface ECG records a summation of the electrical activity of the heart, while in contrast, IEGMs show only the electrical activity of a localized area, i.e., IEGMs are local intracardiac electrograms. Importantly, these are displayed together on the monitor system facilitating accurate interpretations of the electrical signals (**Figure 4**).

After catheter placement, a routine EPS starts with the measurement of basic intervals [4]. Ideally basic intervals should be measured during sinus rhythm.

#### *2.4.1 PA interval*

The PA interval represents the interval between the earliest atrial activation (recording in any channel) in the region of the sinus node and at the region of the atrioventricular node. Usually, the earliest atrial activation is represented by the P wave onset on the surface ECG. Normal value is 25–55 milliseconds (ms).

#### *2.4.2 AH interval*

AH interval represents the conduction time from the low-right atrium at the interatrial septum through the atrioventricular (AV) node to the His bundle. It is measured between the atrial electrogram recorded by the His bundle catheter and the beginning of the His electrogram itself. The normal range is 55–150 ms [4]. The AH interval is sensitive to autonomic tone. A prolonged AH interval may indicate AV nodal disease or high vagal tone, whereas a shorter than normal AH can occur during sympathetic activation.

#### *2.4.3 HV interval*

HV interval reflects conduction through the His-Purkinje system and is measured on the His bundle electrogram from the beginning of the His deflection to the earliest identified ventricular activity on the surface ECG. An HV interval of 35–55 ms is

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

#### **Figure 4.**

*Snapshot from an electrophysiology study. The upper four channels represent lead I, II, V1, and V6 of surface ECG. The paper speed is 200 mm/s. on the distal His (His d) channel, we can recognize three different wavefront characteristics of the His bundle: the first one is the A (atrial) wave (synchronous to P wave on surface ECG), the last is called V (ventricular) wave (synchronous to QRS complex on the surface ECG). In the middle, a sharp signal represents His bundle electrogram. AH interval could be measured from the beginning of A to the sharp His signal. HV interval is measured on the His bundle electrogram from the beginning of the His deflection to the earliest identified ventricular activity on the surface ECG. CS electrograms show atrial activation (synchronous to P wave on surface ECG again). Note that first activation occurs on CS 9,10 which is the proximal pair of electrodes. CS 9,10 is at the ostium of the coronary sinus, thus these electrodes are the closest to the sinus node. In the case of a correctly positioned CS catheter, CS 9,10 should be activated first during normal sinus rhythm. Finally, a local ventricular electrogram can be easily identified on the catheter at RVa position. Asterix represents PA interval.*

considered normal. In the presence of anterograde conducting accessory pathway, the HV interval may be shorter. Prolonged HV interval represents infrahisian conduction disturbances.

### **2.5 Time measurement and pacing**

### *2.5.1 Cycle length*

Time measurements are reported in milliseconds in the case of EP procedures. To characterize the heart rate, the cycle length (CL) is used instead of the frequency. CL represents the length of time between each atrial or ventricular beats. For example, a tachycardia with a heart rate of 150 beats per minute has a CL of 400 ms (**Figure 5**). The faster the heart rate the shorter the CL.

#### *2.5.2 Pacing*

Besides IEGM recordings, electrode catheters previously inserted in the heart are also used for pacing. An external stimulator is connected to the catheters. When pacing starts, electrical current is passed by catheters resulting in cardiac cells'

#### **Figure 5.**

*Atrial pacing at a cycle length of 400 ms, which means a rate of 150 beats per minute. Sharp pacing artifacts are present before P waves. P waves are negative in the inferior leads (pacing from coronary sinus ostium). Paper speed is 100 mm/s.*

depolarization near the catheter's electrode. The depolarization of these cells generates an electrical wavefront, spreading over the heart as the impulse originating from the sinus node. As a result, stimulator pacing generates cardiac impulse artificially. Carefully positioned catheters can impulse the heart from almost any position. During the EPS, pacing is used to introduce electrical impulses in predetermined patterns and at precise time intervals. Such pacing is called programmed stimulation [1]. Programmed stimulation consists of the main type of pacing technique: burst and extrastimulus pacing.

### *2.5.2.1 Burst pacing*

Burst pacing consists of implementing a series of electrical impulses (so-called drive train) at a fixed cycle length. By definition, each impulse is called S1 and the difference between the impulses is the same (**Figure 6**).
