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C HAPTER 1 8 / Cardiac Electrophysiology Procedures 405
events when compared with the signal-averaged ECG. 31,32 How- during the second part. Most centers combine the diagnostic and
ever, to date, no prospective clinical trials have demonstrated therapeutic segments of the study into one procedure. 44
enough evidence to support the widespread use of any of these tests
for high-risk screening and further research is still needed. 30,33 Radiofrequency Catheter Ablation
The head-upright tilt test is a noninvasive, provocative test used
to reproduce and diagnose NMS otherwise known as vasodepres- Catheter ablation techniques have been used for more than
sor, neurocardiogenic, or vasovagal syncope. NMS is manifested by 20 years. Originally, high-energy, direct-current shocks were de-
a combination of vasodilation and bradycardia, which occurs when livered through catheters, using a standard defibrillator, to the en-
the feedback mechanisms between the parasympathetic nervous docardial ablation site. 45,46 The technique was not widely used,
system and sympathetic nervous system break down. Both systems however, because of the high complication rate, including cardiac
are thought to activate alternately or simultaneously. Normal cir- tamponade and immediate and late sudden death. 47,48 As a result,
culatory function is interrupted when both systems discharge rap- efforts to find a safer energy source were pursued. In 1986, ra-
idly. Vagal stimulation becomes exaggerated and causes bradycar- diofrequency (RF) energy was applied through catheters to create
dia, vasodilation, or both in the presence of sympathetic nervous endocardial lesions. 49,50 RF energy is a form of electrical energy
system stimulation. 34,35 During the head-upright tilt test, the pa- that is produced by high-frequency alternating current. As the
tient is positioned on a tilt table with a footboard. There are vari- current passes through tissue, heat is generated. RF current is used
ous protocols for inducing NMS. Basically, an upright tilt at 60 to in the operating room to coagulate blood vessels and to ablate ab-
70 degrees for 20 to 45 minutes is performed. If syncope is not in- normal tissue during neurosurgery. RF current used during endo-
duced, isoproterenol is administered in increasing doses for 15 to cardial catheter ablation is alternating current with a 500,000 to
20 minutes or until a positive result occurs. A positive response re- 750,000-Hz frequency range. The current passes from the elec-
produces the patient’s syncope along with documentation of trode tip to a large-surface-area skin patch. The current is typically
bradycardia, hypotension, or both. 36–38 Indiscriminate use of tilt applied for 10 to 60 seconds at a time using 45 to 55 W. Catheter
table testing in patients with clear-cut vasovagal syncope should be delivery of RF energy causes tissue heating in a small area around
48
avoided as 25% to 30% of these patients will have a “false nega- the electrode. The typical lesion is 3 mm 4 mm 5 mm. Al-
tive” result, which may confuse the diagnosis. The tilt test is most ternate forms of energy for lesion generation are currently under
appropriately used in patients with histories suggestive of vasovagal development, including cryoablation, ultrasound, laser, and mi-
syncope, but when the diagnosis is uncertain. 23,30 crowave energy sources.
Invasive EP studies are indicated when a noninvasive evaluation
for syncope is negative and the suspicion for a cardiac cause re- Techniques
mains high. 39–41 Sinus node function is evaluated by measuring
the sinus node recovery time. Overdrive pacing is performed in the The first part of the procedure, the diagnostic phase, was de-
high right atrium for 30 to 60 seconds. 42 A prolonged sinus node scribed previously. After a diagnosis is made, an ablating catheter
recovery time may be an indication of sick sinus syndrome. The is positioned at the targeted area. The ablating catheter can be
His–Purkinje system is evaluated by measuring the HV interval steered and has four to six electrodes 2 to 5 mm apart. The
during sinus rhythm, and during incremental atrial pacing and catheter tip is 4 to 8 mm long and serves as the electrode through
atrial refractory period determinations. A prolonged HV interval is which RF current is applied. The targeted area is located using flu-
5
an indication of infrahisian disease. AV node function is also eval- oroscopy and by observing the electrogram patterns recorded by
uated by incremental atrial pacing and refractory period determi- the distal mapping electrode pair. Recently developed three-
nations. The Wenckebach point is recorded during incremental dimensional (3-D) mapping systems have vastly improved the
pacing, whereas the effective refractory periods of the atrium and precision and efficiency of mapping. 51
AV node are recorded with the introduction of atrial extrastimuli.
The atrium is refractory when the atrial extrastimuli fail to capture Complications
the atrium. The AV node is refractory when the atrial extrastimuli
capture the atrium but fail to result in a His bundle depolarization Two of the most common complications associated with catheter
(AV block). Permanent pacing may be indicated if abnormalities ablation are inadvertent complete heart block when ablating in
are found. Attempts are also made to induce ventricular and close proximity to the conduction system and cardiac perforation
supraventricular tachycardia during EP testing for syncope. with tamponade when ablating within the atria, coronary sinus or
In all cases, the findings of the EP study along with reproduc- other cardiac veins, or right ventricle. Fewer than 1% to 2% of the
tion of the patient’s symptoms and other findings in the work-up occurrences of these complications have been reported. Rare
must be evaluated carefully to determine the appropriate course of complications include creating inadvertent arrhythmogenic foci,
therapy. The mechanism for syncope remains unexplained in ap- producing mitral or tricuspid regurgitation when ablating at or
proximately 40% of episodes. 43 The prognosis for this latter near valves, systemic embolization and stroke (particularly when
group of patients is good. 4 ablating in the left heart), and the creation of fixed lesions in coro-
nary arteries when RF is applied in an adjacent area. 5,52 Specific
complications related to ablation within the left atrium are ad-
INTERVENTIONAL EP AND dressed under the section on atrial fibrillation.
CATHETER ABLATION
Indications
This interventional procedure includes a diagnostic EP study and
catheter ablation. The mechanism of the arrhythmia is confirmed Combination EP study and catheter ablation procedures are in-
during the first part of the procedure, and the ablation takes place dicated for patients with supraventricular tachycardias caused