Echocardiography in Patients With Atrial Fibrillation and Flutter

Prevalence and Impact of Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, with an estimated prevalence of more than 3 million Americans in 2012. Although the true prevalence of atrial flutter (AFL) is not known, it is suspected that a large proportion of patients with cavotricuspid isthmus–dependent AFL, also known as typical flutter, have AF. Other forms of AFL, referred to as atypical, are encountered after cardiac surgery, in congenital heart disease, during treatment with class I antiarrhythmic agents, or after catheter ablation of AF. From an echocardiography perspective, management of AFL is largely similar to that of AF.

The prevalence of AF is expected to increase over the coming decades, in part because of increasing longevity of the population, a greater prevalence of comorbidities, and improved mortality rates for ischemic heart disease and congestive heart failure. Advanced age is independently associated with the development of AF. In the United States, it is estimated that approximately 15% of the population older than 65 years is affected with AF. This percentage increases to about 25% among those older than 80 years. Other cardiovascular conditions are also associated with an increased prevalence of AF, including ischemic heart disease, valvular heart disease, hypertension, hypertrophic cardiomyopathy, and congestive heart failure. Heart failure with preserved systolic function is associated with a higher prevalence of AF compared with heart failure with reduced systolic function. AF is also associated with obesity and obstructive sleep apnea, two conditions that are increasingly prevalent.

The Framingham Heart Study showed AF to be independently associated with increased mortality rates. It is also associated with significant morbidity, most notably from ischemic stroke, and with increased office and emergency room visits and inpatient hospitalizations. All of these factors make AF a major contributor to health care expenditure in the United States.

Atrial Remodeling Associated With Atrial Fibrillation

AF is associated with electrical and structural remodeling of the atria. Electrically, AF is associated with shortening of the atrial myocyte action potential duration and a reduced rate adaptation of calcium release. Structurally, AF is associated with atrial myocyte loss, increased interstitial fibrosis, and collagen deposition.

Autopsy studies have demonstrated that patients with a history of AF have higher rates of atrial fibrosis compared with patients without a history of AF, independent of age. Other studies, which included patients undergoing cardiac surgery, have demonstrated that biopsies from the atria of patients with AF have a higher collagen content compared with those from patients without a history of the arrhythmia. Animal studies have shown that AF leads to progressive adverse atrial remodeling and worsening of the atrial substrate, a process often referred to as AF begets AF . These ultrastructural changes of the atrial tissue are often accompanied by gross morphologic changes that can be seen on imaging, including increased atrial size measured by atrial diameter, surface area, and volume and reduced atrial contractile function.

Echocardiography in the Workup of Patients with Atrial Fibrillation and Flutter

The clinical presentation of patients with AF and AFL varies greatly. Some patients are minimally symptomatic, and the arrhythmia is discovered incidentally on physical examination or electrocardiographic evaluation. Clinical symptoms range from subtle effects, such as decreased stamina and exercise capacity; to palpitations, chest pain, and shortness of breath; to more serious symptoms such as syncope, stroke, or transient ischemic attack.

After the electrical abnormality is identified, echocardiographic evaluation plays a key role in the workup ( Table 39.1 ). Initial questions are usually addressed with a transthoracic echocardiographic (TTE) study. Although AF and AFL are atrial arrhythmias, a comprehensive echocardiographic evaluation of the whole heart and hemodynamics is always warranted. This includes qualitative and quantitative assessment of atrial and ventricular sizes, valvular stenosis or regurgitation, and ventricular systolic function, diastolic filling pattern, and intravascular volume status. Some patients go on to more invasive echocardiographic examinations such as transesophageal echocardiography (TEE) and intracardiac echocardiography (ICE) during the course of their AF treatment.

TABLE 39.1

Use of Echocardiography in Patients With Atrial Fibrillation as Recommended by Guidelines.

Study	When	Reason
TTE	As part of the initial evaluation of all patients with AF	To assess for structural heart disease, ventricular function, valvular disease, and LA size
TEE ^,	If a cardioversion or ablation is planned when the duration of AF has been > 48 hours or is unknown and there has not been therapeutic anticoagulation for > 3 weeks. Some electrophysiologists perform a TEE on all patients before AF ablation regardless of anticoagulation status or presenting rhythm.	To rule out intracardiac thrombus with a focus on the LAA
ICE ^47,96,97	During AF ablation or possibly LAA closure	To aid in transseptal puncture, assess for complications, image the catheters, and visualize cardiac structures, including the pulmonary veins and LAA

AF , Atrial fibrillation; ICE , intracardiac echocardiography, LAA , left atrial appendage.

Atrial Size

M-mode and two-dimensional (2D) TTE are commonly used to measure the left atrial (LA) diameter in the parasternal long-axis view. This correlates with the atrial surface area measured in the apical long-axis view and with atrial volume measured using computed tomography (CT) or magnetic resonance angiography. ^, However, the LA diameter can underestimate atrial dilation, especially when dilation occurs in an asymmetric fashion. The American Society of Echocardiography (ASE) advocates the use of a 2D-derived LA volume (cm ³ ) measured by the Simpson or the biplane method from the apical 4-chamber (4C) and 2-chamber (2C) views using the following formula:

LA volume = 1.7 (A4C × A2C) / 0 . 5 (L4C + L2C)

$LA volume = 1.7 (A4C × A2C) / 0 . 5 (L4C + L2C)$

where L is length (cm) and A is surface area (cm ² ) ( Fig. 39.1 ). A normal LA volume derived from echocardiographic measurement ranges from 22 to 52 mL in women and from 18 to 58 mL in men, or 22 ± 6 mL/m ² when normalized to body surface area ( Table 39.2 ).

TABLE 39.2

Reference Ranges for LA Size.

From Lang RM, Badano LP, Mor-Avi V, et al. Recommendations for cardiac chamber quantification by echocardiography in adults: an update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr . 2015;28:1–39.

Parameter	Women	Men
RA minor axis dimension (cm/m ² )	1.9 ± 0.3	1.9 ± 0.3
RA major axis dimension (cm/m ² )	2.5 ± 0.3	2.4 ± 0.3
2D echocardiographic RA volume (mL/m ² ) ^a	21 ± 6	25 ± 7
AP dimension (cm)	2.7–3.8	3.0–4.0
AP dimension index (cm/m ² )	1.5–2.3	1.5–2.3
A4C area index (cm ² /m ² )	9.3 ± 1.7	8.9 ± 1.5
A2C area index (cm ² /m ² )	9.6 ± 1.4	9.3 ± 1.6
A4C volume index MOD (mL/m ² )	25.1 ± 7.2	24.5 ± 6.4
A4C volume index AL (mL/m ² )	27.3 ± 7.9	27.0 ± 7.0
A2C volume index MOD (mL/m ² )	26.1 ± 6.7	27.1 ± 7.9
A2C volume index AL (mL/m ² )	28.0 ± 7.3	28.9 ± 8.5
LA volume index (mL/m ² ) ^a	Normal: 16–34 Mildly enlarged: 35–41 Moderately enlarged: 42–48 Severely enlarged: >48	Normal: 16–34 Mildly enlarged: 35–41 Moderately enlarged: 42–48 Severely enlarged: >48

A2C, Apical 2-chamber; A4C, apical 4-chamber; AL, area length; AP, anteroposterior; MOD, method of disks.

a Recommended measurement to determine chamber size.

Atrial Function

Atrial function includes two components: (1) a reservoir or conduit for blood returning from the vena cava and the pulmonary veins and (2) a contractile chamber that augments ventricular filling, similar to a supercharger on an internal combustion engine.

Assessment of atrial mechanical function includes complementary information derived from M-mode and transmitral Doppler echocardiography. M-mode enables measurement of the excursion of mitral valve leaflets, even in the setting of arrhythmia. In the absence of mitral valve stenosis, normal mitral valve excursion is about 13 mm from a closed to a fully open position ( Fig 39.2 ). Anterior leaflet excursion is sometimes used to assess LA function and is reduced in low cardiac output states, with elevated left ventricular (LV) filling pressure, and with atrial mechanical dysfunction.

Fig. 39.2, Normal anterior mitral valve leaflet excursion.

Transmitral Doppler is an accurate method to assess flow across the mitral valve during ventricular filling. Early diastolic passive ventricular filling is captured by the E wave, whereas active late filling, which includes the contribution from atrial contraction, is captured by the A wave. The size, envelope, and relationships of the E and A waves are influenced by heart rate, loading conditions, and the sample volume position on pulsed-wave (PW) Doppler interrogation. The most common pulse volume position is between the tips of the mitral valve leaflets ( Fig. 39.3 ).

Fig. 39.3, TTE 2D–guided spectral display of transmitral pulsed-wave Doppler echocardiographic flow velocity.

Common measures used to assess LA function include the height of the A wave, which can be used to calculate atrial contraction force, and the atrial emptying fraction, which is derived from the calculated minimum (end of ventricular diastole) and maximum (end of ventricular systole) atrial volumes. The magnitude of the A wave depends on the loading conditions and is commonly absent when the echocardiogram is done during AF, resulting in significant limitations to this method of assessing atrial function.

Another echocardiographic method to estimate atrial function uses speckle tracking echocardiography to assess wall deformation. Longitudinal deformation of the atrial wall is assessed in the mid-septal and mid-lateral walls in the apical 4-chamber view. Wall strain and strain rate versus time curves are generated from these areas of interest ( Fig. 39.4 ). Global peak atrial longitudinal strain of 42.2% ± 6.1% and global time to peak longitudinal strain of 368 ± 30 ms have been reported as reference indices of normal atrial myocardial deformation.

Fig. 39.4, 2D LA speckle tracking echocardiography.

Patients with AF imaged during sinus rhythm have a reduced LA emptying fraction and reduced lateral atrial wall strain compared with control subjects without AF. Patients with AF and high levels of atrial fibrosis measured by magnetic resonance imaging (MRI) with late gadolinium enhancement also have a reduced atrial strain, indicating that atrial fibrosis is associated with reduced mechanical atrial function. ^,

Echocardiographic parameters, including atrial size and function and ventricular size and relaxation pattern, can predict incident AF. Data from the Framingham Heart Study showed that an increase in the E/A ratio was independently associated with incident AF. ^, Impaired LV relaxation has also been associated with incident AF in patients older than 65 years. TTE evidence of LA enlargement by surface area and volume measurements also predicts incident AF, independent of parameters such as age, diabetes, hypertension, congestive heart failure, coronary artery disease, and stroke ( Table 39.3 ).

TABLE 39.3

Association of Measured Echocardiographic Parameters and Risk of Incident Atrial Fibrillation or Recurrence After Catheter Ablation.

Echocardiographic Measure	Associated Risk
Increased E/A ratio on tissue Doppler imaging	Increased risk of AF
Increased A-wave velocity	U-shaped association with risk of AF
Increased atrial diameter	Increased risk of AF
Increased LV wall thickness	Increased risk of AF
Decreased LV fractional shortening	Increased risk of AF
Impaired ventricular relaxation, pseudonormal ventricular filling, and restrictive ventricular filling pattern	Increased risk of AF
Increased LA volume	Increased risk of recurrence after catheter ablation
Reduced LA wall strain	Increased risk of recurrence after catheter ablation

AF , Atrial fibrillation.

Right atrial (RA) size is increased in AF and AFL, but RA size and function have been less well studied than left-sided parameters. Conditions such as chronic lung disease, pulmonary emboli, and pulmonary hypertension preferentially affect the right heart and have been associated with AF and AFL. An apical 4-chamber view is preferred for the assessment of RA size on TTE ( Fig. 39.5 ). From this view, the RA diameter, surface area, and volume can be measured.

Fig. 39.5, TTE imaging of the RA from an apical 4-chamber view.

RA reservoir and contractile function are less well studied and understood. Under normal hemodynamic conditions, the right heart pressures are significantly lower than the left, and early passive right ventricular (RV) filling is the norm. Pulmonary vascular resistance is also low, and a mild amount of tricuspid valve regurgitation is not uncommon. For the right heart, reduced tricuspid annular plane systolic excursion (TAPSE) has been associated with AF in patients with systolic heart failure.

Atrial size and function can be assessed using TEE. TEE is not routinely used solely for this evaluation because it is more invasive and less comfortable for the patient than TTE. It does offer a distinct advantage in evaluating the left and right atrial appendages for function and existence of thrombus. TEE is most commonly performed in AF to answer the important question of whether a thrombus is present; the answer dictates the management of AF patients from the perspective of reducing stroke. The following sections address the relationship of AF and thromboembolization in more detail.

Stroke and Atrial Fibrillation

The association between AF and ischemic stroke was established through multiple cohort studies. AF is associated with a fivefold increase in the risk of stroke, and AF-related thromboembolization is implicated in about 20% of ischemic strokes. This may be an underestimation because some strokes of uncertain origin (i.e., cryptogenic) are suspected to be related to AF. AF-associated strokes are more devastating and lead to more disability and death than other ischemic strokes. Stroke is therefore a major cause of mortality, morbidity, and health care expenditure in AF, and one of the initial steps in managing patients with AF is to estimate the thromboembolic risk and determine the best strategy to reduce it.

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