Cardiac dyssynchrony refers to deviations in timing and/or disturbance of the normal sequence of activation and contraction between the atria and ventricles of the heart, the right and left ventricle or among the ventricular wall segments.
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Epidemiology
Mechanical dyssynchrony is common in patients with heart failure 1.
Associations
Cardiac dyssynchrony can be associated with the following conditions 1-3:
atrioventricular block
Clinical presentation
Clinically cardiac dyssynchrony presents with heart failure, that is symptoms include fatigue, dyspnea on exertion or at rest, orthopnea, paroxysmal nocturnal dyspnea, nocturia etc. 2.
Abnormal findings on electrocardiogram include left bundle branch block morphology and a prolonged QRS duration of ≥150 ms 2-4.
Atrioventricular dyssynchrony might present with a prolonged PR interval.
Complications
If left untreated cardiac dyssynchrony can lead to the following 2,3:
Pathology
An adequate cardiac function requires synchronous electrical activation with impulses propagated from the atrioventricular node through the His bundle, the Purkinje fibers and the myocardium 2.
Electrical dyssynchrony refers to a prolonged QRS duration, whereas mechanical dysynchrony is distinct from the latter and characterized by abnormal mechanical contractions with different regional timing and shortening patterns due to abnormal electrical pulse conduction 3,4 resulting in an overall inefficient pump function. In this process some of the cardiac work is lost due to the following 3,4:
early activation and contraction with no ejection resulting
paradoxical rebound stretching of the early activated regions
paradoxical prestretching and prolonged contraction of the remote regions
In addition, an early tension-build up leads to higher stress levels in the late activated remote myocardium.
Overall this leads to a decline in systolic function increased end-systolic volume and wall stress, as well as delayed relaxation and reduction in cardiac output resulting in an inefficient pump function 3-5.
In the long term, this leads to remodeling on an organic, cellular and molecular basis resulting in structural contractile and electrical alterations of the myocardium such as asymmetric hypertrophy and myocardial fibrosis prolonged activation and repolarization 3.
Types
Dysynchrony can be subdivided into electrical and mechanical and depending on where it occurs into one of the following 1-7:
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atrioventricular dyssynchrony
the delay between atrial and ventricular contraction
reduced ventricular filling due to shortened ventricular filling time and early atrial contraction
mitral valve insufficiency with late diastolic regurgitation
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interventricular dyssynchrony
delay between right and left ventricular activation and contraction 4
paradoxical septal motion with a displacement of the septum towards the left ventricle
decreased left ventricular ejection fraction
-
intraventricular dyssynchrony
disruption of the normal sequence of activation and contraction between wall segments
discoordination of contraction between ventricular wall segments
early contracting segments do not contribute to ejection and stretch later
higher wall stress in late contracting segments
worsening mitral regurgitation
Radiographic features
Mechanical dyssynchrony can be demonstrated on either on echocardiography or MR imaging 3,6-8:
late diastolic mitral regurgitation might indicate atrioventricular dyssynchrony
paradoxical septal motion might indicate interventricular dyssynchrony
septal flash and/or apical rocking pattern indicate intraventricular dyssynchrony
Cardiac strain imaging techniques are able to demonstrate dyssynchronous cardiac strain patterns including early contraction and rebound stretching with other areas displaying pre-stretching and prolonged shortening patterns 1.
Both methods are also used to assess cardiac function.
Echocardiography
Interventricular dyssynchrony can be assessed by measuring the interventricular mechanical delay measured as the time difference between the closure of the aortic and pulmonary valves 7.
Intraventricular dyssynchrony can be depicted and assessed with tissue Doppler imaging (TDI) or speckle tracking echocardiography 3,7. Historically it has been evaluated on M-mode by comparing the posterior wall contraction with the timing of the rapid filling phase 3.
MRI
A paradoxical septal movement with premature right ventricular contraction might be visualized on cine imaging. MR tagging or MR feature tracking can be used to assess intraventricular dyssynchrony 3,6,9.
In addition, MRI might demonstrate diffuse myocardial fibrosis, scar tissue or mitral valve incompetence.
Nuclear medicine
Phase analysis with phase polar maps on gated single-photon emission computed tomography (GSPECT) myocardial perfusion imaging (MPI) can be used to assess ventricular dyssynchrony 1.
Radiology report
The radiology report should include a description of the following:
evidence of mechanical dyssynchrony if obvious or present
asymmetric hypertrophy
evidence of diffuse myocardial fibrosis or myocardial scar
mitral valve incompetence
Treatment and prognosis
The treatment of cardiac dyssynchrony is cardiac resynchronisation therapy 1-3. Selection criteria are based on clinical criteria or rather electrocardiographic findings and left ventricular ejection fraction 10.
Practical points
no additional prognostic information by demonstrating mechanical dyssynchrony was found on top of a wide QRS complex in the selection of patients for cardiac resynchronisation therapy 3
imaging-based findings e.g. myocardial scar tissue, mitral regurgitation or right ventricular dysfunction are important factors that might require additional or alternative treatment 10