Knowledge of how arrhythmias are initiated and perpetuated is fundamental to understanding the techniques described in this chapter.

The electrical wavefront

If a nidus of myocardial cells depolarizes faster (rapid phase 4 of the action potential) than the SAN, they will act as the source of wavefronts that conduct through the heart. They may be atrial or ventricular, but if they occur in the atria they will override the SAN. As they occur from a single site they are often termed focal. Common sites are where myocardial cells suddenly change shape/size or those under abnormal pressures such as; the junction of veins/atrium (superior vena vava (SVC), pulmonary veins (PVs)), crista terminalis, coronary sinus (CS), AV node area, mitral/tricuspid ring, ventricular outflow tracts.

Re-entry

The electrophysiology (EP) study is most useful for the diagnosis of tachycardia. When this has already been documented or is strongly suspected, then it is usually the first part of a combined procedure with catheter ablation to cure the arrhythmia. Note that in EP it is usual to discuss cycle lengths (in ms) rather than heart rates e.g. 60/min = 1000 ms, 100/min = 600 ms, 150/min = 400 ms.

Mapping electrical activity in the heart

The corrected sinus node recovery time and sinoatrial conduction time are both measures of SAN function. Unfortunately, however, they are unreliable tests as SAN function is greatly influenced by autonomic tone, drugs and observer error. SAN dysfunction is best assessed with ambulatory monitoring and exercise testing. It is very rare that invasive EP testing would contribute to the decision to give a patient a permanent pacemaker and therefore it is not part of our routine procedure.

AV conduction

In normal hearts there is only one connection between the atrium and ventricle, the AV node. Activation of the atrium (during V pacing) or ventricle (during A pacing or SR) should therefore start at the AV node. Accessory pathways (APs) do not decrement. Their presence can be identified by both abnormal activation patterns and conduction physiology using incremental and extrastimulus pacing.

Atrial pacing: As the AV node decrements, a greater proportion of ventricular activation will occur via the AP. Thus, non-decremental AV conduction times and broadening of the QRS complex will be observed as the pacing interval shortens. Note: If the ERP of the AP is shorter than the ERP of the AV node, then the QRS will suddenly narrow and the AV time suddenly prolong when the AP blocks.

Ventricular pacing: The normal order of atrial activation is His then CS (proximal to distal) and finally the HRA, termed concentric activation. If the atrium is activating via an AP, an eccentric activation pattern is observed. The site of the earliest atrial activation will localize the AP. Non-decremental VA conduction is also seen.

Induction of arrhythmia

An EP study focusing on induction of ventricular arrhythmias (the VT stimulation study) has previously been used to risk stratify for SCD, and to decide on the effectiveness of antiarrhythmic drugs in suppressing VT, and the need for an ICD. Evidence has now accumulated, however, that it is of little predictive value, and that decisions regarding ICD prescription should be based on other risk factors, particularly LV function. The EP study can be useful prior to ICD implant for other reasons:

Programmed ventricular stimulation is performed using the protocol devised by Wellen, or a modification thereof (see below).

Thus, the pacing protocol becomes gradually more aggressive. The more aggressive the induction protocol, the less specific the result. The most useful result is induction of a sustained monomorphic VT with one or two extrastimuli. This indicates a potential substrate for ventricular arrhythmias. Non-sustained VT, polymorphic VT, and VF are all non-specific results.

EP procedures have become increasingly complex (e.g. ablation of AF or atrial tachycardia in congenital heart disease), leading to longer procedures with greater X-ray exposure. Non-fluoroscopic electroanatomical mapping systems help overcome these challenges by generating a three-dimensional (3D) image as the ablation catheter is moved around the cardiac chamber being mapped. Two systems are commonly used; Carto (Biosense Webster) and Navx (St Jude Medical), which locate the catheters using magnetic and electrical fields respectively.

These maps can be used as anatomical guides, and can display the timings (activation maps) or voltages of ICegrams represented by colours. The maps can be viewed in any orientation or from internally. Points of interest or labels, e.g. ablation lesions, can be marked, which gives memory of where the catheter has been. This is particularly useful in ablation of AF where a series of ablation points need to be delivered to from continuous circular or linear ablation lines. The identification of areas of scar (voltage mapping) is essential in VT ablation, as these are critical to its re-entrant mechanism.

Non-contact mapping (Ensite Array, St Jude Medical), allows simultaneous collection of ICegrams from the entire endocardial surface of the chamber in which the catheter is placed. An array of 64 unipolar electrodes, mounted on a balloon, records the signals from the endocardium conducted through the blood. The ‘virtual’ ICegrams are reconstructed and displayed on a map of the chamber. The mechanism of a regular tachycardia can be determined from a single ectopic beat.