Chapter 28 Electrophysiological Evaluation of Atrial Fibrillation
Introduction
Atrial fibrillation (AF), one of the earliest arrhythmias described, is the most common cardiac arrhythmia in humans and is particularly common in older adults. The Framingham Heart Study noted that the incidence of AF in individuals age 60 years was 4% and as high as 15% in those older than 70 years.1 Effective evaluation and management of AF will have substantial clinical and public health impact in an aging population. The mechanisms of AF have been widely debated and are often extrapolated from experimental data. These are discussed in Chapter 4. Human studies have reported data regarding clinical mechanisms that are at significant variance with some experimental concepts. Clinical investigations have centered on the relative role of triggers and substrate, focal automaticity and organized versus random re-entry, and the applicability of experimental and mathematical models to human AF. The clinical electrophysiological study (EPS) remains the gold standard for the investigation and evaluation of human AF.
The major objectives in a clinical EPS of a patient with AF are presented in Box 28-1.
Box 28-1 Goals for Electrophysiological Evaluation of Atrial Fibrillation
Electrocardiographic Definition and Clinical Classification
AF is characterized by the absence of P waves and the presence of small, irregular oscillations (fibrillatory, or f, waves) on electrocardiographic recordings. Fibrillation has been defined as either fine or coarse, based on the ability to discern well-defined f waves, without evidence of organized activity in the surface electrocardiogram (ECG). More recently, organized tachycardias have been documented at the onset of AF events, in intracardiac recordings at surgery, or during clinical EPSs performed in patients with AF.2–4 Recent reports that used advanced mapping techniques have documented multiple organized atrial tachyarrhythmias in a given patient with AF.5 Careful evaluation of ECGs with high gain or body surface mapping reveals evidence of varying f-wave morphology and periods of atrial flutter or atrial tachycardia (AT). (Figure 28-1, A).6,7
The current clinical classification of AF defines new-onset AF (first episode) or recurrent AF (≥2 episodes) with three types of presentation.8,9 Paroxysmal AF is defined as recurrent AF that terminates spontaneously within 7 days, whereas persistent AF is AF that is sustained beyond 7 days, or lasts fewer than 7 days but requires pharmacologic or electrical cardioversion (see Figure 28-1, B). The difference between the two clinical AF presentations may lie in the probability of AF termination within a defined period, which, in turn, may be based on the complexity of the AF mechanisms initiating or maintaining AF. When cardioversion fails or is not attempted, the patient has been defined as having permanent AF. More recently, aggressive cardioversion with high-energy, repeated shocks or intracardiac shocks has demonstrated effective termination of even permanent AF, leading to decreasing use of this term. Thus, it is more appropriate to define persistent AF as being new-onset or established (duration ≥1 year) persistent AF. A particular patient may have AF episodes that fall into one or more of these categories. The relationship of clinical classes of AF to EPS findings in each class is examined later.
Intracardiac Electrophysiological Studies for the Evaluation of Atrial Fibrillation
Early Electrophysiological Observations in Atrial Fibrillation
Early EPSs in patients with AF used limited recording and stimulation methods. The focus of the studies was largely on measurement of atrial electrophysiological properties, assessment of atrial conduction in the right atrium (RA) and the left atrium (LA), and the atrioventricular (AV) propagation of paced atrial beats or spontaneous or induced AF or other concomitant tachyarrhythmias. Two or three multi-polar catheters were typically positioned in the high RA and the mid-RA, His bundle region, and the coronary sinus.10 Abbreviation of atrial effective and functional refractory periods during programmed atrial stimulation is seen in patients with AF, with concomitant loss of rate adaptation atrial refractoriness. In addition, an increase in the dispersion of atrial refractoriness in the atria occurs in these patients compared with control subjects.11 Intra-atrial conduction times in humans show increased conduction delay in the RA, manifest as increased P-A intervals, and prolonged inter-atrial conduction time measured as P–distal coronary sinus interval. These delays account for the prolonged global P-wave duration seen on the ECG (Figure 28-2). More recently, other markers of conduction delay during sinus rhythm have been identified, including fragmented or multi-phasic intracardiac atrial potentials and late potentials on P-wave signal-averaged ECG recordings. Regional intracardiac atrial electrogram recordings show a propensity for split potentials in certain locations (e.g., crista terminalis, coronary sinus, eustachian ridge at the cavo-tricuspid isthmus, posterior LA), suggesting that anatomic structure can alter atrial electrical propagation patterns. Attuel emphasized the importance of increased dispersion of atrial refractoriness in these patients and its relationship to inducible AF.11,12
Programmed Stimulation for Induction of Atrial Fibrillation
Programmed atrial stimulation was first introduced in patients with AF by Haft more than 30 years ago.13 Single-paced atrial premature beats during atrial pacing or sinus rhythm–induced episodes of AF or flutter in these patients. This was accepted as a marker of atrial vulnerability to AF. Subsequently, Bauernfeind et al studied patients with Wolff-Parkinson-White (WPW) syndrome and AF and performed tests to select an effective drug therapy.14 Effectiveness in these studies was defined by prolongation of atrial refractoriness, abolition of accessory pathway conduction, and suppression of inducible AF. A standardized protocol for programmed stimulation in AF was first proposed by our group in 1999 and is described in detail later in the chapter.15
Atrial Fibrillation Mapping and Anatomic-Physiological Correlations
More recently, the focus of electrophysiological evaluation has shifted to mapping of either induced or spontaneous AF, detailed activation mapping of the RA and LA for triggers, organized tachycardias, identification of an abnormal atrial substrate, or all of these findings. All of these are demonstrable with atrial tissue voltage mapping or atrial electrogram morphologic abnormalities in sinus rhythm or AF (Figure 28-3). Such mapping is accomplished by placement of multiple electrode catheters in the RA and LA and is complemented by three-dimensional contact mapping or noncontact mapping (NCM) of the atrium. In this chapter, the multi-electrode catheter approach is discussed first. Three-dimensional mapping techniques are discussed in detail elsewhere. We have combined these two methods in a single procedure designed to achieve a complete electrophysiological evaluation, with simultaneous bi-atrial mapping complemented by high-resolution NCM mapping of the atrium of interest. This approach permits beat-to-beat real-time activation mapping of AF from onset to termination.
Clinical Electrophysiological Techniques for the Evaluation of Atrial Fibrillation
Contact Electrode Catheter Techniques
Clinical EPS in a patient with AF should consist of a systematic approach to the analysis of arrhythmias by recording and measuring a variety of electrophysiological parameters and events with the patient in sinus rhythm, AF, or both and by evaluating his or her response to programmed electrical stimulation. The study includes the measurement of conduction intervals (if necessary after cardioversion), the use of programmed atrial stimulation, and responses to a variety of interventions. Electrograms are recorded at paper speeds of 100 to 200 mm/s. At a minimum, multi-electrode catheters are placed in the high lateral RA, across the bundle of His, and in the coronary sinus to record left atrial electrograms and activation. Right atrial recordings can be obtained from the free wall, the septum, and the tricuspid isthmus for regional right atrial activation patterns (Figure 28-4). A duo-decapolar catheter is widely used for this purpose. This catheter is typically placed with its distal electrode in the low lateral right atrial free wall with the proximal set lying along the inter-atrial septum (see Figure 28-4, C). This allows simultaneous recordings of the anterior free wall of the RA and the inter-atrial septum.
Left atrial recordings can be indirectly obtained epicardially from the left pulmonary artery or directly endocardially after a trans-septal puncture. In the former, a decapolar catheter is placed in the left lower pulmonary artery in a branch typically encircling the left atrial appendage (LAA) (see Figure 28-4, C). In this fashion, recordings from the left atrial lateral wall, left superior pulmonary vein (PV), roof of the LA and PVs, and right superior PV can be indirectly obtained to evaluate conduction in the superior LA. Trans-septal left atrial mapping requires inter-atrial septal puncture at the fossa ovalis by using a Brockenborough needle mounted within a trans-septal sheath and dilator assembly, usually guided by intracardiac echocardiography. Although detailed technical steps in this procedure are beyond the scope of this chapter, the important elements are illustrated in Videos 28-1 through 28-6 on the Expert Consult web site that accompanies this textbook. With the trans-septal approach, the distal electrode can be placed in the left superior PV, across the left atrial roof and the fossa ovalis. The catheter can also be manipulated into each of the PVs for mapping (Figure 28-5; see Videos 28-7 to 28-10). Note the proximity of the left pulmonary artery electrode catheter to the trans-septal electrode catheter along the superior LA (see Figure 28-5, B).
The ablation catheter is repositioned in the left inferior pulmonary vein region for mapping.
Alternatively, preformed electrode catheters can be used to map the atrium or PVs. A circular preformed multi-electrode catheter (e.g., Lasso catheter, Biosense Webster, Diamond Bar, CA) or small linear catheters (e.g., Cardima Pathfinder, Cardima Inc., Fremont, CA) can also be used for mapping within tubular structures—such as the PVs, the superior vena cava (SVC) or the inferior vena cava (IVC), the coronary sinus, or atrial appendages—or at their ostia in the atrium. The Lasso catheter comes in different configurations (10 or 20 electrode poles) and variable spacing (8 mm, 2-6-2 mm) and curve diameters (see Figure 28-5, D). It allows circular mapping at the PV ostia, which can have varying anatomic configurations, and within the veins for segmental mapping. The Pathfinder catheter permits placement well inside the vein and its branches. Because ablation within the PV is being undertaken with decreasing frequency, this is most useful in careful assessment of gaps in isolation lines. Activation within the veins during sinus rhythm, paced atrial stimuli, and spontaneous premature atrial beats can be recorded and localized. Figure 28-6 shows recordings during atrial pacing in a patient with persistent AF. The ablation catheter records a delayed potential within the left superior PV, and a spontaneous atrial premature beat originates from this site with electrogram sequence reversal confirming the PV potential (PVP) as the second component of the local electrogram.
FIGURE 28-6 Pulmonary vein potential shown by arrow during atrial pacing as a second deflection after atrial electrogram and as trigger of atrial premature beat preceding the surface P wave and atrial electrograms in the coronary sinus and bundle of His regions. See legend to Figure 28-1 for abbreviations.
A multi-electrode basket catheter has been designed for global right atrial and left atrial mapping.16 Recently, the use of the High Density Mesh Mapper–Ablator catheter (Bard Electrophysiology, Lowell, MA) was described.17 It allows multiple simultaneous recordings along vertical splines in a circumferential array when the basket or mesh catheter is opened to achieve contact with the endocardium in the atrium of interest (Figure 28-7). A computer contour of atrial activation is developed on the basis of electrogram timing. Severe atrial dilation or anatomic abnormalities may impede complete recording sequences. One prospective study of PV isolation in patients with AF who have this device demonstrated that the method was safe and yielded good primary success rates and a favorable clinical outcome at 6 months.17 A more recent report showed a substantially higher AF recurrence rate at 18 months.18
Pacing Techniques During Electrophysiological Evaluation
One standard programmed stimulation protocol based on prior studies conducted by our group is to use one to three atrial extrastimuli using two drive trains.15
