Cardiac dyssynchrony

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. Dysynchrony can be subdivided into electrical and mechanical and depending on where it occurs into one of the following ^1-3:

• atrioventricular dyssynchrony
• interventricular dyssynchrony
• intraventricular dyssynchrony

Epidemiology

Mechanical dyssynchrony is common in patients with heart failure ¹.

Associations

Cardiac dyssynchrony can be associated with the following conditions ^1-3:

• left bundle branch block
• atrioventricular block
• myocardial fibrosis
• hypertrophic cardiomyopathy
• pulmonary hypertension
• congenital heart disease
• arrhythmogenic right ventricular cardiomyopathy

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:

• worsening heart failure
• mitral regurgitation
• cardiac arrest
• sudden cardiac death

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.

Classification

Cardiac dyssynchrony has been classified into the following subtypes ^4-7:

• 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
• 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:

• cardiac volumes and measurements
• cardiac wall motion abnormalities
• 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