Right Atrial Anatomy

The right atrial (RA) endocardial surface is composed of many orifices and embryonic remnants, accounting for an irregular, complex surface. The RA endocardium is architecturally divided into three anatomically distinct regions; each is a remnant of embryological development. The posterior smooth-walled RA, derived from the embryonic sinus venosus, receives the superior vena cava (SVC), the inferior vena cava (IVC), and the coronary sinus (CS). It also contains the fossa ovalis, the sinus node, and the atrioventricular (AV) node (AVN). The anterolateral trabeculated “true” RA, derived from the true embryonic RA, is lined by horizontal, parallel ridges of muscle bundles that resemble the teeth of a comb (the pectinate muscle). It contains the RA appendage and free wall. The atrial septum is primarily derived from the embryonic septum primum and septum secundum.

The posterior smooth-walled RA and the anterolateral trabeculated RA are separated by the crista terminalis on the lateral wall and the eustachian ridge in the inferior aspect. The sulcus terminalis, where the sinus node is located, is a subtle groove on the epicardial surface of the heart corresponding to the crista terminalis. The crista terminalis is a C-shaped, convex, thick muscular ridge that runs from the high septum, anterior to the orifice of the SVC superiorly, and courses caudally along the posterolateral aspect of the RA. In its inferior extent, it courses anteriorly to the orifice of the IVC. As the crista reaches the region of the IVC, it is extended by the eustachian valve ridge. The eustachian valve is the remnant of the embryonic sinus venosus valve, which manifests as a flap of variable thickness and mobility along the orifice of the IVC; this valve can continue as a ridge superiorly along the floor of the RA to the ostium of the CS (CS os), to join the valve of the CS, form the tendon of Todaro, and then continue onto the interatrial septum as the inferior limbus of the fossa ovalis.¹

The SVC enters the roof of the RA between the base of the RA appendage and the superior margin of the interatrial septum. The IVC enters the posterolateral portion of the floor of the RA along the inferior margin of the interatrial septum. The CS enters the inferior aspect of the RA adjacent to the inferior margin of the interatrial septum, slightly more anterior and medial relative to the orifice of the IVC, and closer to the tricuspid annulus. On the lower third of the interatrial septum lies the fossa ovalis.

The tricuspid annulus lies anterior to the body of the RA, and its inferior portion lies a short distance (approximately 1 to 4 cm) anterior to the eustachian ridge, although its course varies among individuals.² The cavotricuspid isthmus (CTI) is the part of the RA between the ostium of the IVC and the eustachian ridge (posteriorly) and the tricuspid annulus (anteriorly). The CTI runs in an anterolateral-to-posteromedial direction, from the low anterior RA to the low septal RA (see Fig. 17-1). Its width and muscle thickness are variable, from a few millimeters to more than 3 cm in width and more than 1 cm in depth. The CTI becomes wider in a medial-to-lateral direction, and it is thinnest in its central portion. A thick eustachian ridge (greater than 4 mm) is seen in 24% of patients. At mid-diastole, the central isthmus is straight in 8% of patients, concave in 47% of patients, and pouch-like (more than 5 mm) in 45% of patients.^3,4 The eustachian ridge (often composed of partly or largely fibrous tissue) occurs as an elevation on the CTI. The area between the tricuspid annulus and the eustachian ridge is referred to as the subeustachian isthmus, whereas the downslope of the eustachian ridge leads to the junction of the RA and IVC. The pectinates, as they fan out from the crista terminalis or other muscle bundles on the CTI, typically spare the myocardium just atrial to the tricuspid valve. This smooth portion of the cavotricuspid annulus is referred to as the vestibular portion.¹ In the normal heart, CTI anatomy can be flat, “hilly” (from prominent eustachian ridge or pectinate muscles, or both), concave, or have a pouch-like recess.

Typical Atrial Flutter Circuit

Typical atrial flutter (AFL) is a type of macroreentrant atrial tachycardia (AT) that uses the CTI as an essential part of its circuit. The circuit boundaries are the tricuspid annulus, crista terminalis, IVC orifice, eustachian ridge, CS os, and probably fossa ovalis. These barriers (lines of conduction block) can be functional or anatomical and are necessary to provide adequate path length for the flutter reentry circuit. Whereas the anterior boundary of the tachycardia circuit has been well established as being the tricuspid ring, the posterior boundaries are more complex and not as well defined, and they occur at a variable distance from the anterior border; the posterior borders are narrowest in the region of the eustachian ridge and widest in the anterior part of the RA.^2,5–8

The CTI provides the protected zone of slow conduction necessary for the flutter reentry circuit. The area of slowest conduction is probably localized in the lateral aspect of the CTI in younger patients and in the medial aspect in older patients.⁹ Conduction velocity in the CTI during pacing in sinus rhythm is slower in patients with typical AFL compared with those without any history of AFL.^3,10 The mechanism of the slower conduction velocity in the CTI, relative to the interatrial septum and RA free wall, is uncertain but can be related to the anisotropic fiber orientation. With aging or atrial dilation, intercellular fibrosis can change the density of gap junctions and produce nonuniform anisotropic conduction through the trabeculations of the CTI. Additionally, the CTI and RA in patients with typical AFL are significantly larger than those in a control population.⁶

The crista terminalis plays an important role as a functional barrier during typical AFL. Conduction delay and rate-related transverse block across the crista terminalis has been consistently observed in sinus rhythm and during pacing in humans. A line of transverse conduction block along the crista terminalis serving as a lateral boundary can be determined by the presence of double and split potentials recorded during AFL or rapid pacing from either side of the crista terminalis during electrophysiological (EP) testing. Structural characteristics of the crista terminalis influence transverse conduction; steep slope and arborization of the crista terminalis have been implicated as geometric factors in its transverse conduction block. Typical AFL is more likely to occur in the setting of a thicker and continuous crista terminalis, and these patients are more likely to exhibit transverse crista terminalis conduction block at longer cycle lengths (CLs) as opposed to controls. Similarly, the region posterior to the crista terminalis (the posterior smooth-walled RA) also has been shown to demonstrate functional transverse conduction block during AFL or rapid pacing.^11,12

Typical AFL is of two types: counterclockwise and clockwise.⁵ In counterclockwise AFL, the activation wavefront propagates caudocephalically up the septal side of the tricuspid annulus toward the crista terminalis and advances cephalocaudally along the lateral wall of the RA to reach the lateral tricuspid annulus, after which it propagates through the CTI (“counterclockwise” as viewed in the left anterior oblique [LAO] view from the ventricular side of the tricuspid annulus). The width of the activation wavefront in typical AFL varies considerably, determined by the distance between the anterior and posterior boundaries at any given part of the circuit. It is very narrow inferiorly at the CTI and substantially wider moving upward. The substantial distance between anterior and posterior borders as well as the anatomical barriers superiorly, combined with variability in the completeness of the posterior border, creates conditions for substantial variability in the upper part of the circuit. Despite a relatively similar activation sequence, the active circuit (as determined by entrainment mapping) is variable. Most commonly, the reentrant wavefront courses not around the tricuspid annulus but obliquely between anterior and posterior borders away from the tricuspid annulus along any available, more rapidly conducting segments. Consequently, significant portions of the RA, including areas around the tricuspid annulus, can often be passively activated. In many subjects, the upper portions of the circuit pass behind the RA appendage and lie near or at the posterior circuit border, or they bifurcate around the SVC or RA appendage, or both. The posterior border can extend completely or partially between the IVC and the SVC.^7,8

The flutter circuit is entirely confined within the RA. Left atrial (LA) activation occurs as a bystander and follows transseptal conduction across the inferior CS-LA connection, Bachmann bundle, or fossa ovalis.^2,6

In clockwise (reverse typical) AFL, activation propagates in the direction opposite to that in counterclockwise typical AFL (Fig. 12-1). Clockwise AFL is observed in only 10% of clinical cases, despite the fact that it is easily inducible in the EP laboratory with programmed electrical stimulation. Clockwise AFL can be induced in the EP laboratory in approximately 50% of patients who clinically present with only counterclockwise AFL. The 9:1 clinical predominance of counterclockwise AFL can be related to the localization of an area with a low safety factor for conduction in the CTI, close to the atrial septum. Additionally, counterclockwise AFL is more likely to be induced with rapid atrial pacing from the CS os. Conversely, clockwise AFL is more likely to be induced with pacing from the low lateral RA pacing. These observations may be related to the anisotropic properties of the CTI and the development of rate-dependent conduction delays and unidirectional block necessary for tachycardia induction, which may be affected by the site of stimulation.^2,6 Spontaneous AFL initiation may be related to rapid bursts of pulmonary vein discharges (atrial fibrillation [AF]).

FIGURE 12-1 Endocardial activation during counterclockwise (upper panel) and clockwise (lower panel) typical atrial flutter (AFL) in the same patient. Catheter position and wavefront activation during the tachycardia are illustrated in a left anterior oblique fluoroscopic view (right side). The ablation catheter (Abl) is positioned at the cavotricuspid isthmus (CTI), and the Halo catheter is positioned around the tricuspid annulus, with the distal end at the lateral aspect of the CTI. ABL = ablation site; CS = coronary sinus; CS_dist = distal coronary sinus; CS_prox = proximal coronary sinus; TA_dist = distal tricuspid annulus; TA_prox = proximal tricuspid annulus.

Double-Wave Reentry

A typical AFL circuit with a large excitable gap may allow a second excitation wave to be introduced into the flutter circuit by a critically timed atrial extrastimulus (AES), so that two wavefronts occupy the same circuit simultaneously. This type of AFL is designated double-wave reentry.

Double-wave reentry is manifest by acceleration of the tachycardia rate but with identical surface and intracardiac electrogram morphology. It can be recognized by the simultaneous activation of the superior and inferior regions of the tricuspid annulus, with all activation being sequential. This rhythm rarely lasts for more than a few beats and can serve as a trigger for AF. Because the CTI is still a necessary part of the circuit, double-wave reentry is amenable to CTI ablation.^2,6

Epidemiology

It is estimated that the overall incidence of AFL in the United States is 88 per 100,000 person-years. AFL accounts for approximately 15% of supraventricular arrhythmias and frequently coexists with or precedes AF. Although in clinical practice AFL appears to be less common than paroxysmal supraventricular tachycardia, population-based data show that in the general population, AFL is diagnosed for the first time more than twice as often. Adjusted for age, the incidence of AFL in men is more than 2.5 times that of women. Paroxysmal AFL can occur in patients with no apparent structural heart disease, whereas chronic AFL is usually associated with underlying heart disease, such as valvular or ischemic heart disease or cardiomyopathy. At highest risk of developing AFL are men, older adults, and individuals with preexisting heart failure or chronic obstructive lung disease. In approximately 60% of patients, AFL occurs as part of an acute disease process, such as exacerbation of pulmonary disease, following cardiac or pulmonary surgery, or during acute myocardial infarction.¹³

Clinical Presentation

Patients with AFL may be completely asymptomatic, or they may present with a spectrum of symptoms ranging from palpitations, lightheadedness, fatigue, reduced activity tolerance, or dyspnea to acute pulmonary edema or acute coronary syndrome in susceptible patients. The severity of symptoms usually depends on the ventricular rate during the AFL, the presence of structural heart disease, and baseline left ventricular function. Some patients remain asymptomatic until they present with a thromboembolic event or with decompensated heart failure secondary to tachycardia-induced cardiomyopathy. AFL occurs in approximately 25% to 35% of patients with AF, in which case AFL may be associated with more intense symptoms because of more rapid ventricular rates.

Initial Evaluation

ECG diagnosis of typical AFL is frequently accurate, but it can occasionally be misleading (see later). Cardiac evaluation with echocardiography is required to assess for structural heart disease. Other tests may be required to evaluate for potential substrates or triggers of AFL.

Principles of Management

P Waves

Flutter waves appear as atrial complexes of constant morphology, polarity, and CL. Typically, flutter waves are most prominent in the inferior leads (II, III, aVF) and lead V₁. In the inferior leads, they resemble a picket fence (sawtooth) because the leads are primarily negative. This consists of a downsloping segment, followed by a sharper negative deflection, and then a sharp positive deflection, with a positive overshoot leading to the next downsloping plateau. The relative size of each component can vary markedly.²

Counterclockwise AFL (Fig. 12-2) can be characterized by pure negative deflections in the inferior leads, negative and then positive deflections that are equal in size, or a small negative and then a larger positive deflection. Those three varieties coexist with tall positive, small positive, or biphasic P waves in lead V₁, respectively. With progression across the precordium, the initial component rapidly becomes inverted and the second component isoelectric usually by lead V₂ to V₃. This produces the overall impression of an upright flutter wave in lead V₁, which becomes inverted by lead V₆. A negative deflection always precedes the positive deflection in the inferior leads in counterclockwise AFL, and the degree of positivity in the inferior leads appears to be related to the coexistence of heart disease and LA enlargement. Lead I is low-amplitude isoelectric, and lead aVL is usually upright.¹⁵

FIGURE 12-2 Surface 12-lead ECG of counterclockwise typical atrial flutter (AFL) with 2:1 atrioventricular (AV) conduction (left), counterclockwise typical AFL with variable AV conduction (middle), and clockwise typical AFL with 4:1 AV conduction (right).

The surface ECG appearance of clockwise typical AFL is more variable than that of counterclockwise typical AFL, but in many respects, clockwise AFL presents an inversion of the appearance in counterclockwise AFL. Clockwise AFL generally has broad positive deflections in the inferior leads, with characteristic notching (see Fig. 12-2).² However, there is an inverted component preceding the upright notched component. Depending on the amplitude of this component, the appearance can be of continuous undulation without an obviously predominant upright or inverted component. On other occasions, it may appear that the inverted component is dominant, thus superficially mimicking counterclockwise AFL. Lead V₁ is characterized by a wide negative and usually notched deflection. There is transition across the precordium to an upright deflection in lead V₆. Lead I is usually upright, and lead aVL is low-amplitude negative and notched.¹⁵

Typical AFL usually has an atrial rate of 240 to 340 beats/min. However, AFL can be slower in patients receiving antiarrhythmic agents or after incomplete CTI ablation (Fig. 12-3), whereby flutter CLs as long as 450 milliseconds have been observed. If the ventricular response is half the atrial rate, it can be difficult to identify flutter waves “buried” within the QRS or T waves (Fig. 12-4). Close inspection of the QRS and T waves, and comparisons with ECGs obtained in normal sinus rhythm, can help identify buried flutter waves. Furthermore, vagal maneuvers and AVN blockers can slow AV conduction and unmask the flutter waves.

FIGURE 12-3 Surface 12-lead ECGs during counterclockwise (left) and clockwise (right) typical atrial flutter (AFL) in a patient on flecainide therapy. Note the slow flutter cycle length (approximately 350 milliseconds) secondary to the effects of flecainide.

FIGURE 12-4 Surface 12-lead ECG of counterclockwise typical atrial flutter (AFL) with 2:1 atrioventricular (AV) conduction (A) and with 4:1 AV conduction (B). Note that when the ventricular response is half the atrial rate, it is difficult to identify flutter waves “buried” within the QRS or T waves (A). However, when the ventricular rate is slowed, flutter wave morphology becomes more easily visualized. C shows atrial fibrillation that developed in the same patient after ablation of the cavotricuspid isthmus. Note that fibrillation waves are coarse and can mimic flutter waves; however, close inspection reveals changes in rate and morphology of atrial activity that are inconsistent with AFL.

In patients who have undergone extensive LA ablation for treatment of AF, the P wave morphology during CTI-dependent AFL can be very different from the foregoing discussion because of alteration of intraatrial and interatrial wavefront propagation. Similarly, non–CTI-dependent macroreentrant ATs can mimic typical CTI-dependent AFL on the surface ECG. Thus, arrhythmias that appear to be typical AFL may not be, whereas others that are actually typical AFL may not appear to be.

Atrioventricular Conduction

Most commonly, 2:1 AV conduction is present during AFL. Variable AV conduction and larger multiples (e.g., 4:1 or 6:1) are not uncommon. Slowing the atrial rate during AFL caused by antiarrhythmic drugs or following a prior incomplete CTI ablation can result in a paradoxical increase in the ventricular rate caused by better AVN conduction of the slower flutter beats (Fig. 12-5). Rapid 1:1 AV conduction is most commonly seen in patients with anterogradely conducting bypass tracts (Fig. 12-6), but it may also be present in cases of enhanced AVN conduction secondary to high sympathetic tone (e.g., exercise, sympathomimetic drugs).²

FIGURE 12-5 Effects of antiarrhythmic drugs on typical atrial flutter (AFL). Baseline surface ECG in a patient with AFL and variable atrioventricular (AV) conduction (A). Treatment with propafenone results in slowing of the atrial rate during AFL (B) and a paradoxical increase in the ventricular rate caused by better AV nodal conduction of the slower flutter beats (C). Right bundle branch block aberrancy is observed during 1:1 AV conduction.

FIGURE 12-6 Surface 12-lead ECGs during clockwise typical atrial flutter in a patient with a left posteroseptal bypass tract (BT). A, 2:1 atrioventricular (AV) conduction with QRS fusion (secondary to conduction over both the BT and the AV node). B, 1:1 AV conduction over the BT with fully preexcited QRS morphology.

QRS Morphology

The QRS complex during AFL is often identical to that during sinus rhythm. However, flutter beats can be aberrantly conducted because of functional bundle branch block, most frequently right bundle branch block (see Fig. 12-5). Even with normal ventricular conduction, the QRS complex may be slightly distorted by temporal superimposition of flutter waves on the QRS complex. Thus, the QRS complex can appear to acquire a new or larger R, S, or Q wave.²

Induction of Tachycardia

Programmed electrical stimulation protocol typically involves atrial burst pacing from the high RA and CS (down to the pacing CL at which 2:1 atrial capture occurs) and single and double AESs (down to the atrial effective refractory period [ERP]) at multiple CLs (600 to 200 milliseconds) from the high RA and CS. Administration of an isoproterenol infusion (0.5 to 4 µg/min) may be required to facilitate tachycardia induction.

AFL can be induced readily with programmed electrical stimulation in most patients with a clinical history of AFL. Reproducible initiation of counterclockwise AFL is possible in more than 95% of patients.¹⁶ Rapid atrial pacing is more likely to induce AFL than a single AES, but as likely as introducing two AESs. On the other hand, the frequency of single or double AESs initiating AFL is low in patients without a history of AFL (less than 10%). Counterclockwise AFL is more likely to be induced by stimulation from the CS os; conversely, clockwise AFL is more likely to be induced with low lateral RA pacing. Induction of AFL usually occurs once unidirectional CTI block develops during pacing (Fig. 12-7). The faster the pacing rate and the shorter the AES coupling intervals, the more likely it will be that AF is induced, which is usually self-terminating but can be sustained in less than 10% of patients with no clinical history of AF. The significance of induction of AF in these patients is uncertain.

FIGURE 12-7 Surface ECG and intracardiac recordings of initiation of counterclockwise typical AFL. Burst pacing from the coronary sinus (CS) results in wavefronts proceeding in both directions around the tricuspid annulus (TA), colliding around TA 5-6 (arrows); on the third cycle, one wavefront blocks before arriving at TA 1-2 but proceeds around the TA in the opposite direction. When pacing stops, this unopposed wavefront continues around the TA as counterclockwise AFL. dist = distal; prox = proximal.

Tachycardia Features

Typical AFL is characterized by a constant CL, polarity, morphology, and amplitude of the recorded bipolar electrograms and by the presence of a single constant macroreentrant circuit with a constant atrial activation sequence. The atrial rate is very regular, with cycle-to-cycle variation of less than 2%; it is rare for AFL to display considerable variation in the CL. The atrial CL is usually between 190 and 250 milliseconds, although the atrial rate can be slower in patients receiving antiarrhythmic agents or following a prior unsuccessful ablation of the CTI. It is not uncommon for clockwise and counterclockwise AFLs to occur in the same patient, and they often have similar rates, although clockwise AFL can have a slower rate.

As noted, AFL is usually associated with 2:1 AV conduction, but variable AV conduction and larger multiples are not uncommon. Variable AV block is the result of multilevel block; for example, proximal 2:1 AV block and more distal 3:2 Wenckebach block result in 5:2 AV Wenckebach block. It is likely that the proximal 2:1 block occurs in the upper part of the AVN, whereas Wenckebach block occurs in the lower part of the AVN. Distal Wenckebach behavior in the His bundle (HB) would result in a similar AV conduction pattern but is unlikely to occur. In most cases, the nonconducted flutter impulses block in the AVN. However, infranodal AV block can occur, especially in the presence of prolonged His-Purkinje system refractoriness caused by antiarrhythmic agents or during Wenckebach cycles in the AVN that leads to long-short cycle activation of the His-Purkinje system.

The presence of anterogradely conducting bypass tracts with a short refractory period can result in preexcited AFL with rapid 1:1 AV conduction. Infusion of isoproterenol can enhance AVN function and occasionally facilitate 1:1 AV conduction, especially when the atrial rate is relatively slow.² Adenosine increases the degree of AV block, but it also shortens atrial refractoriness and can result in the degeneration of AFL into AF.

Diagnostic Maneuvers during Tachycardia