Subclavian and Internal Jugular Approaches

Both these approaches have their advocates. The internal jugular approach has a lower risk of pneumothorax, but it may be somewhat more difficult to maneuver the wire and to maintain sterility during a longer procedure or with a retained wire because of proximity to the hair. The subclavian approach, particularly on the left, permits the wires to follow a smooth arc toward the heart. In the authors’ lab, the preference is the subclavian approach, in part because sometimes the wires are retained overnight after certain ablation procedures for confirmation testing before discharge the following morning.

Femoral Approach

The femonal approach is the most common approach for all EPSs; some electrophysiologists use it almost exclusively. Multiple-polar catheters can help keep the number of punctures to the minimum. For example, octopolar and decopolar catheters with appropriately spaced poles can be positioned so that a single wire can perform pacing and recording from the right atrium and from both the right ventricular septum and the apex. Such a wire together with another for recording the His bundle potential is all that is needed for most comprehensive diagnostic EPSs.

Trans-septal Puncture and Catheterization

This technique dates back to the l950s but has become important to the electrophysiologist for its role in the ablation of arrhythmias on the left side of the heart. The classic procedure uses a Brockenbrough needle, which is advanced through a special long sheath. For electrophysiology purposes, the classic Mullins sheath with a distal curve (i.e., like an umbrella handle) has largely been supplanted by sheaths with special shapes to facilitate placement in the desired portions of the left atrium. The more current steerable sheaths provide greater flexibility when performing left atrial catheter ablation.

The biggest danger associated with the trans-septal technique is tamponade caused by perforation of the left atrium, usually posteriorly. Several techniques have been used to reduce this risk. The literature suggests a comparably low incidence of complications with most techniques.^4–6 Approaches to the pulmonary veins, presently a matter of interest in atrial fibrillation ablation, are almost exclusively via the trans-septal route, often with several punctures. It must be emphasized that the trans-septal technique is fraught with many subtleties and potential complications. The details are far beyond the scope of this section, and no one should attempt the technique only on the basis of general experience and a reading of this part of the text.

Retrograde Approach

For certain types of ablations, the retrograde (femoral artery to aortic valve to left ventricle) approach may be used. Ablation of left-sided Wolff-Parkinson-White (WPW) syndrome can be accomplished using either the trans-septal approach or the retrograde approach. Endocardial ablation of ventricular tachycardia (VT) may also be performed using either of the aforementioned methods.

Epicardial Catheterization

Patients with tachycardia originating from the epicardial surface of the heart require mapping within the pericardial space. This technique was initially described by Sosa et al.⁷ When performing epicardial ablation, patients typically require general anesthesia. A coronary angiogram is required to avoid ablation of areas adjacent to the coronary arteries. The operator must also be cognizant of the course of the phrenic nerves so as to avoid diaphragmatic paresis.

Rate and Frequency

Beats per minute is commonly described as rate. Frequency usually refers to (1) how often an arrhythmia occurs spontaneously, or (2) the signal filtering settings in hertz. In other languages, frequency often means “rate in beats/min,” which can introduce confusion in some communications.

Straight Pacing

Pacing stimuli are delivered at a constant rate or CL. The duration may be indefinite as with temporary pacing, but for EPSs, it is usually for a specified number of stimuli or seconds. As indicated in the previous paragraph, progressively faster pacing can be described as “incremental” (beats/min) or “decremental” (CL).

Ramps

With pacing, a series of stimuli is delivered, with each interstimulus interval successively differing from its predecessor. Most often, the interval decreases, resulting in progressively faster pacing during ramps.⁵ For example, stimulation could begin at a CL of 400 ms (150 beats/min), with each of 10 successive intervals shortening by 10 ms so that at the end of the ramp, the CL would be 300 ms (200 beats/min). Clinically, ramps are used for the assessment of conduction and for the initiation and termination of tachycardias. Ramps that start fast and then slow down are occasionally used in the treatment of tachycardias. The various uses of ramp pacing will be detailed in the sections below.

Extrastimulus Technique

After a series of spontaneous or paced beats at a constant CL (a type of straight pacing, designated S1-S1), an extrastimulus is introduced at a somewhat shorter CL that is designated S1-S2. After observing the response, the process is repeated, with the S1-S2 intervals progressively shortened. Sometimes, double or triple extrastimuli (S3-S4) are introduced. The extrastimulus technique is used for the assessment of refractory periods (see below), for initiation and termination of tachycardias, and as a diagnostic tool during ongoing tachycardias.

Stimulus Amplitude and Pulse Duration

These are particularly important during extrastimulus testing. Higher amplitudes and longer pulse durations permit more closely coupled stimuli to “capture” (depolarize) the heart.^8,9 Excessively large stimuli may cause fibrillation. For these reasons, most EPSs involve stimuli at two to four times the diastolic threshold (in milliamperes or volts) and 1- to 2-ms pulse duration.¹⁰

Ultra-Rapid Train Stimulation

A series of stimuli are delivered at such a rapid rate (typical CLs are 10 to 60 ms) that it is not expected that each of these will capture the tissue being stimulated. The method helps assess vulnerability to inducible ventricular fibrillation (VF) by using progressively higher stimulus amplitudes with successive trains. With stimulus amplitudes in conventional pacing ranges, trains have been used for both initiation and termination of regular monomorphic tachycardias. At very low (subthreshold) amplitudes, trains have been used as a tool to alter local tissue responses, which, in turn, can affect certain tachycardias.^11,12

Stimulation Protocols

Protocols did not come fully formed right at the beginning of clinical EP. Different centers around the world developed their own EP protocols. In general terms, broad differences are present in what constitutes a “complete” EPS. In more specific terms, differences exist in the sequence in which extrastimuli are delivered and in the stimulus amplitudes and pulse durations used. These differences can affect the sensitivity and the specificity of protocols and also the time required to complete a protocol. Over the years, demands for a “universal protocol” have been made, but such a protocol is not likely to come any time soon. Guidelines suggest that a given stimulation protocol must be able to reproduce clinical arrhythmia in at least 90% of cases.¹⁰

Effect of Signal Filtration and Interelectrode Spacing

During EPSs, the scale of interest ranges from macro-events (When does the QRS complex start, or what is the QRS duration?) to micro-events (How many milliseconds does it take for a wavefront to move from the His bundle to the right bundle branch?) (Figure 20-2). The ability to distinguish between macro-events and micro-events is based on the effects of signal filtration and interelectrode spacing.

FIGURE 20-2 Beat-to-beat variation during normal sinus rhythm. A, Electrocardiogram (ECG) lead V1 is displayed together with the His bundle electrogram (HBE) lead filtered at 40 to 500 Hz and recorded simultaneously at three gain settings. Paper speed is 100 mm/s. Beat-to-beat variation can materially affect the slope of the “first rapid deflection.” In the 5 beats shown, these variations resulted in a 20-second spread in traditionally measured A-H intervals. A_L (local atrial activity) is designated by an L connected by a line to a dot at the point in the upper low-gain HBE tracing. A_L was easy to measure throughout the changes in amplitude and remained in constant relationship to the H deflection. B, Atrial electrograms during normal sinus rhythm at paper speed 250 mm/s. The onset of the first rapid deflection changes from beat to beat. Measurements of A_L-H is constant at 128 ms. Local atrial activity L and L* on the HBE at 30 to 500 Hz was determined by using a simultaneously unfiltered tracing at 1 to 1250 Hz. A, onset of atrial activity as conventionally measured and designated by an arrow from the letter A to the site measured; L is the timing of local activity on electrograms filtered at 30 to 40 to 500 Hz. The letters L, L* are connected by a line to a dot at the designated site of the tracings; L* is the timing of the electrogram that corresponds most closely to the local electrogram recorded on the unfiltered lead. On the tracings, the timing of the local deflection is indicated by an arrow.

(From Fisher JD, Baker J, Ferrick KJ, et al: The atrial electrogram during clinical electrophysiologic studies: Onset versus the local/intrinsic deflection, J Cardiovasc Electrophysiol 2:398–407, 1991.)

Low and Wide

Macro-events such as the QRS duration are best measured with electrograms filtered at about 0.5 to 100 Hz. Most of the electrical energy of the heartbeat occurs in these low-frequency ranges, and low-frequency signals also propagate over greater distances than do high-frequency signals. The archetype of these recordings is the surface ECG. The recommended filtration is 0.5 to 100 Hz, but very little difference is noted in the appearance of the QRS complex when filtration is set as low as 0.5 to 20 Hz. Interelectrode distances are also relatively great, for example, arm to arm, or arm or chest to leg. With intracardiac catheter electrodes, inclusion of the lower frequencies (starting at 0.5 Hz) and relatively wide interelectrode spacing will maximize inclusion of “far-field” signals from parts of the heart that are not close to the recording electrodes.

For example, with filtering at 0.5 to 100 Hz, the lead in the ventricle will record a relatively broad electrogram, possibly with a low-amplitude atrial wave followed by a higher-amplitude ventricular signal, followed, in turn, by a broad, gently rounded T wave. The ventricular deflection will usually show several components. As the wavefront approaches the electrodes from afar, a progressively steeper deflection will culminate in a point. At this point, as the wavefront passes by the recording electrodes, a very rapid reversal in slope occurs, making a near-vertical deflection that has the highest dV/dt of the entire electrogram, which is known as the intrinsic deflection. This culminates in another reversal of direction as the wavefront continues to move away from the recording electrodes and inscribes a deflection that is a mirror image of the curve inscribed by the approaching wavefront.

Higher and Closer

“Local” events such as the precise timing of a His bundle deflection are best accomplished by filtration settings beginning at 30 or 40 Hz and going up to 500 Hz or higher. This takes advantage of the fact that higher frequencies do not propagate as well and that many of the structures of interest (such as the His bundle) contain relatively few cardiac fibers and, thus, relatively low amounts of energy. Close interelectrode spacing (2 to 10 mm) and filtering at 30 to 500 Hz combine to maximize recording of local “micro-signals” and exclude far-field or macro-signals. It is possible to focus even more closely on local events by filtering at 100 to 1000 Hz. However, so little energy is present at these frequencies that the recorder-amplifier system must be used at high gains, where the signal-to-noise ratio tends to eliminate any potential benefit.

In the case of signal filtration, the terminology again becomes somewhat confusing, rather like the alternative meanings for incremental and decremental pacing. With filtration, the “high-pass” level is not the higher end of the frequency range but, rather, the hurdle above which frequencies are recorded. Similarly, the “low-pass” level is the frequency below which a signal will be recorded. Thus, interestingly, for a typical 30- to 500-Hz filtration range, the high pass is 30 Hz, and the low pass is 500 Hz.

Clipping and Limiting

In some instances, very large gains are necessary to focus on an area of interest. For example, recordings in the His bundle region typically include the low medial right atrium, the His bundle deflection, and the right ventricular inflow region. If only a very small His bundle deflection can be recorded, the gain may be increased on the recording system. This may make the atrial and right ventricular inflow deflections so large that they would cover much of the screen, overlapping with other simultaneous recordings. In such instances, clippers or limiters that electrically chop the maximum amplitude of signals are applied so that they remain within designated bounds.

Notch Filters

During the construction of an EP laboratory, it is important to have significant bioengineering input to ensure that electromagnetic interference with the recording apparatus is minimized. Nevertheless, some interference from alternate current is virtually unavoidable, creating the telltale 50- to 60-Hz pattern that can render recordings almost unreadable. Notch filters that are optimized for 50 or 60 Hz can go far to mitigate the interference problem. Although the notch is optimized at 50 or 60 Hz, some attenuation of signals at nearby frequencies is present. In most instances, this does not have a clinically important effect on the recorded electrograms.

Monophasic Action Potential Recordings

Monophasic action potential (MAP) recordings provide information about local transmembrane depolarization and repolarization. An MAP catheter has 5-mm–spaced, silver-silver chloride matrix electrodes that may be positioned into the atrium or the ventricle. Standard EP recording systems should be set to 0.05 to 1000 Hz to capture lower-frequency components of the action potential. Currently, MAP recordings provide insight into cellular EP and are largely used in clinical research (Figure 20-3).¹⁴

FIGURE 20-3 Monophasic action potential recording from the right ventricle. Leads from top to bottom: Electrocardiogram surface leads I, II, aVF, and V1, bipolar endocardial right ventricular (RVA) electrogram, and monophasic action potential (MAP).

Extrastimulus Technique

The extrastimulus technique (briefly introduced above) is the heart of EPSs used primarily for assessing refractoriness and tachycardia induction. Typically, the baseline drive comprises 8 beats, which may be sinus rhythm but are usually delivered at a constant CL (S1-S1) by a stimulator. The drive is then followed successively by single, double, and triple extrastimuli, which are designated S2, S3, and S4. Generally, the extrastimuli are initiated at 80% to 90% of the drive CL. All these stimuli are delivered with uniform specifications, usually 1- to 2-ms pulse duration and an amplitude two to four times the diastolic threshold. Of the various methods for shortening (decrementing) S1-S2 intervals and subsequently S2-S3, and S3-S4, most are variations of either the tandem method or the simple sequential method (Figure 20-4).^15,16

FIGURE 20-4 Method of introducing extrastimuli. A, The tandem method; after the first extrastimulus (S2) reaches the effective refractory period (ERP), it is moved out 40 to 50 ms. The second extrastimulus (S3) is introduced and the S2-S3 interval is decremented until S3 fails to capture. The S1-S2 interval is then decremented until S3 again captures. Decrements of S2 and S3 continue in this tandem fashion until the shortest intervals where S2 and S3 capture are reached. For the third extrastimulus (S4), S3 is moved out 40 to 50 ms, and the process is repeated for S3 and S4. B, Simple sequential method; the S1-S2 interval is decremented until the ERP of S2 is reached. S2 is then placed 10 to 20 ms later to ensure capture, and the process is repeated with S3 and then S4. Clinical results are comparable with the two methods (P = NS).

(From Fischer JD, Kim SG, Ferrick KJ, Roth JA: Programmed ventricular stimulation using tandem versus simple sequential protocols, Pacing Clin Electrophysiol 17(3 Pt 1):286–294, 1994.)

Tandem Method

The S1-S2 is decremented in 10-ms steps until S2 does not capture; the S1-S2 interval is then increased by 40 to 50 ms. A late S3 is then introduced and the S2-S3 interval decremented until S3 fails to capture. Then, the S1-S2 and S2-S3 intervals are alternated (tandem) until S3 fails to capture. S3 is then moved out by 40 to 50 ms and the process repeated with S4. Some laboratories do use an S5 or S6 as well, but stopping with triple extrastimuli (S4) is more typical.

Simple Sequential Method

The S1-S2 interval is decremented in 10-ms steps until S2 fails to capture. It is then moved 10 ms later or until reliable capture is attained. At that point, the second extrastimulus (S3) is decremented similarly and finally a third (S4).

A prospective randomized trial has demonstrated that the tandem method and the simple sequential method produce comparable results in terms of inducibility of the clinical arrhythmia, inducibility of “nonclinical” arrhythmias, and noninducibility.¹⁵ The simple sequential method takes significantly less time to perform.¹⁵

Refractory Periods

The classic technique is to deliver eight drive stimuli, all designated as S1. After the last S1, an extrastimulus (S2) is delivered at an interval somewhat shorter than the S1-S1 interval. The process is repeated with decrements in the S1-S2 interval, usually until S2 reaches refractoriness, that is, fails to capture. The S1-S2 interval is usually decremented in 10-ms steps, although 20-ms or even 30-ms steps may be used in clinical laboratories for very long S1-S2 intervals. Refractory periods tend to be shorter with shorter S1-S1 intervals. An exception is sometimes seen in the AV node, where refractory periods may be prolonged as the S1-S1 is decreased from 1000 ms to 600 ms but thereafter usually tend to shorten as the S1-S1 is further decreased (Figure 20-5). Depending on the objective of the EPS, refractory period testing may be carried out with stimulation at several atrial or ventricular sites and at two or more drive (S1-S1) CLs. Several different refractory periods warrant further discussion (see below).^17–25

FIGURE 20-5 Effective refractory period (ERP) variability in different parts of the atrioventricular conduction system. In the case of the His-Purkinje system (HPS), atrium (A), and ventricle (V), the ERP consistently shortens with shorter paced cycle lengths (CLs). The atrioventricular nodal (AVN) ERP may initially prolong with shorter paced CLs, and this trend can continue (dashed line) or the AVN ERP can shorten as the paced CL is further shortened.

(From Fisher JD: Primer of cardiac electrophysiology, Armonk, NY, Futura, In press.)

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