9 Electrophysiology Studies
9.1 Basics
An electrophysiological examination study (EPS) is an invasive procedure in which intracardiac potentials are recorded using catheter electrodes. The electrodes are inserted into the heart using the Seldinger technique, similar to cardiac catheterization. EPS makes it possible to obtain detailed images of the spread of electrical excitation in the heart and to measure conduction and refraction times precisely. In addition, by inducing arrhythmia during the examination, the underlying pathological mechanism of the rhythm disorder (e.g., an accessory conduction pathway) can be determined.
In addition to a purely diagnostic purpose, it is also possible to destroy or modify the morphological correlate of a rhythm disorder using an ablation catheter and thus effectively treat the arrhythmia.
9.2 Indication
The indication for an EPS must be made on a case-by-case basis. Among other things, it depends on the underlying arrhythmia, the age of the child, the symptoms, the treatment options, and any potential underlying structural heart disease.
In childhood, an EPS is most frequently performed to investigate supraventricular tachycardia—usually with the option of catheter ablation. Other indications are cardiovascular arrest of unclear cause or syncopes for which no conclusive explanation could be found using non-invasive diagnostic tests.
Note
When determining the indication for EPS in childhood, it must be taken into consideration that in children weighing less than 15 kg, the rate of complications of the examination and ablation is considerably higher than for larger children.
9.3 Procedure
The examination is generally performed under deep sedation in children, but longer procedures when ablation is planned are usually performed under general anesthesia. After consulting with the electrophysiologists who will perform the examination, anti-arrhythmic agents should be discontinued five half-lives before the examination if clinically tolerable.
During the examination, electrode catheters are inserted into the femoral vein after it is punctured using the Seldinger technique. It may be difficult to probe the coronary venous sinus, which is particularly necessary to investigate supraventricular tachycardia (see Chapter 9.3.1), via an inguinal access, so that an access from above may have to be selected (left basilic vein, right internal jugular vein, or left subclavian vein). If the left ventricle or the area of the mitral valve annulus need to be examined more closely (e.g., for left accessory pathways), the femoral artery is also punctured and the catheter is advanced by retrograde insertion into the left ventricle. For an examination of the left atrium—if the foramen ovale is closed and there is no atrial septal defect—the atrial septum must be punctured in order to access the left atrium from the right atrium (transseptal puncture).
The catheter electrodes are generally positioned under fluoroscopy guidance. Multipolar catheters are used. The electrodes at the tips of the catheters are spaced at 2 to 5 mm.
9.3.1 Catheter Positions
For an EPS, four standard electrode positions are usually used (Fig. 9.1):
High right atrium (HRA)
Bundle of His (HB)
Right ventricular apex (RVA)
Coronary sinus (CS)
High right atrium (HRA)
Positioning the electrode catheter at the junction between the superior vena cava and the right atrium allows potentials from the area of the sinus node to be measured. Sometimes the catheter is also advanced into the right atrial appendage. The HRA potential is the earliest potential recorded in the intracardiac lead. The right atrium can also be stimulated via the catheter.
Bundle of His (HB)
In order to position the catheter in the region of the bundle of His, it is first advanced into the right ventricle. Then it is withdrawn toward the right atrium until an atrial signal appears in addition to the ventricular signal. After rotating the catheter slightly in a dorsal direction, the His potential appears between the atrial and the ventricular signal.
The bundle of His ECG is important for investigating atrioventricular (AV) conduction disorders and diagnosing arrhythmia. Signals that appear before the bundle of His potential have a supraventricular origin. Signals recorded after the bundle of His potential stem from the ventricle.
Right ventricular apex (RVA)
The right ventricular catheter is generally positioned in the right ventricular apex, less frequently in the right ventricular outflow tract to answer a specific clinical question.
Coronary venous sinus (CS)
The coronary sinus catheter records signals from the left atrium as well as from the left ventricle. A coronary sinus catheter is used primarily to evaluate supraventricular tachycardias—especially if an accessory pathway is suspected.
9.3.2 Evaluation of an Intracardiac ECG
Atrial signals
Normally, the first signal of the intracardiac ECG that appears is the signal from the HRA catheter positioned near the sinus node (Fig. 9.2). This is followed by the signal from the lower right atrium recorded from the bundle of His catheter. The last atrial signal is the left lateral atrial signal recorded from the coronary venous sinus. If there are ectopic atrial foci or supraventricular tachycardias, the order of the signals changes depending on the origin of the arrhythmia and speed of excitation.
AV and His transition
The AV transition is determined on the basis of the AH interval. The AH interval extends from the start of atrial depolarization (A) to the start of the His bundle potential (H). The AH interval in children is normally 50 to 120 ms (Table 9.1).
Parameter | Normal value |
AH interval | 50–120 ms |
HV interval | 25–50 ms |
RVA activation | 5–35 ms |
Corrected sinus node recovery period | < 275 ms |
Sinus node recovery period in percent | < 166% |
Wenckebach point | < 380 ms |
Sinoatrial transition period | < 200 ms |
Atrial effective refractory period | 170–250 ms |
AV node effective refractory period | 220–350 ms |
Ventricular effective refractory period | 200–300 ms |
In the His–Purkinje system, transition is reflected by the HV interval recorded from the His bundle catheter. The normal values for children are 25 to 50 ms (Table 9.1). A shortened HV interval can occur with accessory conduction pathways, for example.
Ventricular signals
The spread of excitation to the ventricles is described based on three catheter locations:
Right ventricular apex (RVA catheter)
Right ventricular inflow tract (ventricular signal of the His bundle catheter)
Left ventricular base (ventricular signal of the coronary sinus catheter)
Generally, the signal of the RVA catheter is the first potential to appear shortly after the appearance of the QRS complex in the ECG. A right bundle branch block can delay the RVA signal. If there are accessory conduction pathways, the first signal to appear may be the ventricular potential from the His bundle catheter or the coronary sinus catheter, depending on the location of the pathway.
Sinus node function
Sinus node function is generally assessed by means of the sinus node recovery period. To measure this, the right atrium is stimulated by the HRA catheter for 30 to 60 s at a rate higher than the patient’s resting rate. When the stimulation is abruptly ended, there is a brief pause before the sinus node itself jumps into action again as a pacemaker. An excessively long pause is a sign of sinus node dysfunction. The sinus node recovery period is usually indicated as the corrected sinus node recovery period. To determine this, the normal interval between two normal sinus beats at rest is subtracted from the interval between the last stimulus and the first sinus node activity.
Alternately, the sinus node recovery period can also be indicated as a percentage of the normal interval between two sinus beats at rest.
AV node function
AV node function is assessed using the Wenckebach point. When the Wenckebach point is exceeded, not every atrial action is transmitted to the ventricle. The Wenckebach point is determined by first stimulating the atrium at a fixed frequency and then reducing the time between two stimuli in 10-ms increments, that is, continuously increasing the stimulation rate. Normally, the interval between two stimuli before Wenckebach symptoms occur should be less than 380 ms in adolescents and adults, equivalent to a frequency of over 185/min.
The effective refractory period of the AV node is determined by first conducting basis stimulation in the atrium, usually with eight stimuli. The stimulation is applied at a rate somewhat exceeding the patient’s own rate and ensures “electrophysiological stability.” Subsequently, an extra stimulus is coupled 10 ms prematurely. This extra stimulus is delivered progressively earlier in 10-ms increments. Due to the decremental (delayed) conduction of the AV node, the interval between atrial stimulation and ventricular response (AH interval) is increasingly lengthened. When the coupling interval is sufficiently short, conduction in the AV node ceases. This interval between the last stimulus conducted and the first stimulus that is not conducted is described as the effective refractory period of the AV node. In children, it is between 220 and 350 ms.
The effective refractory period of the atrial myocardium can be determined by analogy.
Dual AV node physiology
Different AH conduction periods may be noted using programmed atrial stimulation. The different conduction periods are signs of two or more AV node pathways with varying refractory periods (dual AV nodes).
A patient with a dual AV node is predisposed to AV nodal re-entrant tachycardia. By definition, a dual AV node is present if a “jump” can be detected in the intracardiac ECG. An AH interval (or VA interval) that increases by more than 50 ms after the extra stimulus is shortened by 10 ms is proof of a dual AV node.
Ventricular effective refractory period
The ventricular effective refractory period is determined in a manner similar to the effective refractory period of the atrial myocardium and the AV node. Using the RVA catheter, several (usually eight) ventricular stimuli are first delivered at a base cycle length before shortening the cycle length in increments of 10 ms. The cycle length that is so short that it can no longer trigger a ventricular response is equivalent to the effective ventricular refractory period. In addition, measuring the ventricular refractory period with ventricular stimuli should be used to investigate whether retrograde conduction from the ventricle to the atria is also present.
Programmed atrial stimulation
Programmed atrial stimulation can be used to determine not only the conduction and refractory periods discussed above, but supraventricular tachycardia can also be induced using special stimulation methods. Arrhythmia may also be triggered by pharmaceutical provocation (e.g., with orciprenaline).
When supraventricular arrhythmia is induced, the type of arrhythmia, the pathological mechanism, and possibly how to end it can be more precisely determined. Furthermore, there is the option of ablating the morphological correlate of the arrhythmia (see Chapter 9.3.3).
Programmed ventricular stimulation
Different sites of the ventricles can be stimulated. Stimulation is routinely delivered to the apex of the right ventricle, but it can also be delivered in the right ventricular outflow tract or in the left ventricle if needed. In addition to determining the ventricular refractory period described above and investigating retrograde conduction from the ventricles to the atria, an attempt can be made to induce ventricular arrhythmia using stimuli with different cycle lengths, possibly at different locations. The origin of spontaneous ventricular arrhythmia can be detected in this way and it may also be possible to ablate the morphological correlate of the arrhythmia.
Pace mapping
The principle of pace mapping is to use atrial or ventricular stimulation to find the location at which the rhythm induced by stimulation has the same morphology as spontaneously occurring tachycardia. A 12-channel ECG that was recorded during spontaneous tachycardia must be available in order to compare the induced rhythm and spontaneous tachycardia.
Catheter mapping
In catheter mapping, an intracardiac ECG is recorded to see where the earliest excitation during tachycardia occurs, for which it must be possible to induce tachycardia using EPE. Computer-assisted navigation systems and visual methods (e.g., LocaLisa, CARTO map) are usually used for precise mapping. These methods reduce fluoroscopy time and increase patient safety.
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