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).