Discussion:
To our knowledge, ours is the first case report demonstrating the mechanical phenomenon of atrial alternans by TEE Doppler echocardiography in both atria in a patient with atrial flutter. An earlier study in ten patients with mechanical pulsus alternans suggested an atrial mechanism in two patients evidenced by lack of an ”a” preceding each strong beat on apexcardiogram and echo M mode (1) and in an animal model of rapid atrial pacing (2). Clinical data on atrial alternans is sparse since atrial repolarization occurs during electrical activity of ventricles in the conventional ECG and hence measurements of atrial repolarization require invasive methods. Atrial alternans manifested by beat to beat alterations of atrial action potentials has been proposed as a precursor of atrial fibrillation (3).
In clinical practice, mechanical cardiac alternans is due to ventricular alternans and mechanisms of its development have been well studied along with its role in development of ventricular arrhythmias, however, alternans in atrial tissue remains much less investigated. Excitation-contraction coupling and intracellular Ca2+ regulation in ventricular and atrial tissue vary suggesting differences in alternans generation and regulation (4).
Cardiac involvement in light chain amyloidosis is associated with a high burden of cardiac arrhythmia including atrial fibrillation. In a small thick ventricle in cardiac amyloid, atrial contraction causes a significant contribution to diastolic filling, since early diastolic filling period is short due to rapid rise in LV filling pressure. Occurrence of atrial fibrillation or flutter in cardiac amyloid therefore reduces LV diastolic filling, reduces cardiac output and increases LA filling pressure which leads to new or worsening heart failure as in our patient who had diffuse cardiac amyloid infiltration and had slow atrial flutter with fixed 2:1 block. LA mechanical function was poor during atrial flutter, however LV diastolic filling improved post cardioversion from mechanical atrial contraction as shown on mitral inflow Doppler. Right atrial function improved significantly post cardioversion. LA appendage Doppler velocities however did not improve significantly post cardioversion likely due to post cardioversion left atrial appendage stunning. We have described this phenomenon of differential atrial stunning, left greater than right earlier (5), which in another patient led to a new LA appendage thrombus formation (6).
Our patient had undergone ablation procedures for atrial fibrillation as well as for atrial flutter in the past. In addition, our patient had cardiac amyloidosis confirmed by MRI with delayed gadolinium enhancement of both the ventricles as well as both the atria. Atrial fibrillation and atrial flutter have been traditionally associated with left atrial enlargement, however, our patient had normal left atrial volumes on cardiac MRI. The atria were likely stiff and non compliant due to amyloid infiltration. Both scar related abnormality from prior ablations as well as significant extracellular atrial amyloid infiltration with increased atrial stiffness and progressive loss of atrial function may have contributed to atrial alternans (7). Resolution of alternans during sinus rhythm suggests that increased atrial contraction rate in impaired atria in our patient likely caused the atrial alternans due to variable repolarization after alternate beat as has been shown in an animal model of atrial pacing.2 One MRI study found that atrial reservoir function was significantly worse in patients with TTR compared to light chain cardiac amyloid, despite similar maximum left atrial volume and sphericity index, indicating worse left atrial function in TTR compared to light chain amyloid (8).
Heart failure can induce atrial remodeling, including electrical remodeling atrial fibrosis, stretch and dilatation, that may contribute to atrial action potential alternans and mechanical atrial alternans (9). Our patient had normal LA volumes and did not have significant mitral or tricuspid regurgitation to cause atrial stretch but marked LV diastolic dysfunction from amyloid infiltration caused elevated LA pressure and likely caused atrial stretch. Mechanical stretch, and hypertrophy have been shown to be associated with arrhythmogenic atrial alternans (10,11).
Evaluation of right atrial appendage along with LA appendage, as well as the tissue Doppler imaging of mitral and tricuspid annuli pre cardioversion and in situ TEE during cardioversion allowed us to evaluate LV, atrial and appendage function post cardioversion in our patient and diagnose atrial alternans. This approach has many advantages as reported in our earlier studies (5,6,12,13).