Transthoracic Echocardiography
What is the role of echocardiography in the diagnosis of cardiac arrhythmias?
Ventricular arrhythmias are the main cause of sudden cardiac death in western countries, responsible for about 1-2 cases per 1000 inhabitants per year. Although 50% of sudden cardiac deaths due to complex ventricular arrhythmia occur in hearts without overt structural heart disease, in the remaining 50% of cases there is a predisposing condition that can be analyzed by imaging techniques which are both diagnostic and prognostic.
Among these, echocardiography represents the first screening test in patients with known or suspected cardiac arrhythmia, both in the case of supraventricular arrhythmias (atrial fibrillation, supraventricular tachycardias) and ventricular arrhythmias (including repetitive ventricular extrasystolia and ventricular tachycardia), both in patients who survived cardiac arrest due to ventricular fibrillation and in patients with a family history of sudden death.
Echocardiography enables an accurate assessment of the myocardium and valve systems, from a morphological and functional point of view, and indirectly of myocardial perfusion, through the study of the coronary reserve with a physical or pharmacological stress test. These objectives are obtained without the need for administration of ionizing radiation and at a low cost compared to cardiac magnetic resonance imaging (MRI), computed tomography (CT) and coronary angiography.
What is the role of echocardiography in the diagnosis and risk stratification of sudden death?
The most recent cardiological guidelines for the prevention of sudden death recommend performing an echocardiogram in all patients with suspected or known complex ventricular arrhythmia (level of evidence IB).
The main pathologies responsible for sudden cardiac death that can be investigated using the echocardiographic method include ischemic, acute and chronic heart disease, acute and chronic heart failure, congenital heart disease and arrhythmogenic cardiomyopathies, including dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmogenic right ventricular dysplasia, and noncompaction cardiomyopathy, infiltrative diseases of the myocardium, of which the most common is amyloidosis, and Chagas disease. To these are added the arrhythmogenic diseases of the valves, as in the case of mitral valve prolapse and inflammatory cardiopathies (endocarditis, myocarditis and pericarditis). In all the aforementioned pathological conditions, the echocardiographic examination is able to provide important information useful both for diagnosis and, through the development of particular prognostic indices, for the prevention of sudden cardiac death.
For example, in patients with known heart disease, regardless of the etiology of the same, the severe reduction of the ejection fraction of the left ventricle below 35%, assessed echocardiographically with the 2D or 3D method, has been associated with a significant increase in risk of sudden arrhythmic death, and current guidelines almost entirely base the indication to implant a defibrillator system in primary prevention on this parameter.
However, the ability of echocardiography to predict the risk of complex ventricular arrhythmias is relatively limited, depending on the action of several triggering and transient factors that together contribute to activating the arrhythmogenic substrate (ischemia, hydro-electrolyte disorders, pro-arrhythmic effect of drugs etc).
The severe reduction of the ejection fraction, therefore, does not represent a sufficiently sensitive and specific parameter in assessing the risk of sudden death. There are, in fact, several conditions in which even with a conserved left ventricular ejection fraction, malignant ventricular arrhythmias can occur.
In order to better predict this risk, important indexes with prognostic value have been developed, such as the Global Wall Motion Score Index (WMSI), validated as a prognostic index for a composite discharge endpoint appropriate by the ICD and mortality from all causes.
Similarly, Relative Wall Thickness (RWT), which reflects the geometric characteristics of the left ventricle, can be used as a marker of adverse events in patients with left ventricular dysfunction, being the state associated with a risk of complex ventricular arrhythmias and arrhythmic death in patients with ischemic / non-ischemic heart disease, heart failure, idiopathic dilated heart disease and left bundle branch block.
Another particularly important index is represented by the Global Longitudinal Strain (GLS). The quantification of GLS, an expression of myocardial function as a result of electro-mechanical coupling, has proven to be able to predict the risk of sudden death, complex ventricular arrhythmias, and shock from the ICD in patients with heart failure with reduced ejection fraction.
A strength of the echocardiographic method is the possibility of associating pharmacological or physical stress with the examination, thus enabling the evaluation of the coronary reserve through any alterations in the ventricular segmental kinetics. In particular, the pharmacological stress echocardiogram is particularly useful in cases where ventricular arrhythmias are suspected for ischemic genesis, but there are conditions such as the use of digitalis, left ventricular hypertrophy, left bundle branch block, ST segment depression -T> 1 mm at rest or Wolff-Parkinson-White syndrome, which make the electrocardiographic trace uninterpretable during stress.
A further example of how echocardiography plays a fundamental role in both diagnosis and prognostic evaluation of various cardiac pathologies comes from the study of the so-called arrhythmogenic cardiomyopathies.
Hypertrophic cardiomyopathy, for example, is a significant cause of sudden death in all age groups and, in particular, in young athletes. International guidelines recommend the use of an algorithm for defining a risk score for sudden death that includes echocardiographic elements, such as the maximum thickness of the cardiac wall, obstruction to the left ventricular outflow, atrial dimensions and GLS. The latter index, in a study in patients undergoing defibrillator implantation, was shown to be able to predict appropriate shock therapy.
The echocardiographic examination also plays a crucial role in the diagnosis and prognosis of Arrhythmogenic Right Ventricular Dysplasia (ARVD). Diagnostic parameters such as regional akinesia, dyskinesia, right ventricular dilation, global or regional, can easily be evaluated by 2D and 3D echocardiography. In a particular study, the combination of these echocardiographic findings with specific electrocardiographic parameters enabled the identification of patients at risk of arrhythmias even in the early stage of disease (13).
Finally, in Noncompaction cardiomyopathy, characterized by an intra-myocardial hypertrabeculation of the left or biventricular ventricle, with a thinned epicardial component, whose clinical manifestations can vary from completely asymptomatic to heart failure, thromboembolic events, and malignant ventricular arrhythmias / sudden death, the ventricular end-diastolic diameter and the reduction of the ejection fraction evaluated by echocardiography were able to predict the risk of ventricular arrhythmias and sudden death.
What is the role of echocardiography in the selection of patients to undergo a resynchronizer implant (CRT)?
The numerous echocardiographic indices for the selection of patients to undergo a resynchronizer implant (CRT) proposed in the literature are based on the use of conventional techniques (M-mode, Doppler) and advanced techniques (tissue Doppler imaging, strain, strain rate, tissue tracking). One strategy is based on research and quantification of the delay of mechanical activation of the lateral wall of the left ventricle, given as the assumption that in the presence of left bundle branch block (LBBB) the lateral wall of the left ventricle is activated last, and, for this reason, it represents the ideal site for implanting the left catheter. Another strategy is to quantify the degree of global contractile dyssynchrony of the left ventricle, given that, in the presence of LBBB, the propagation of the electrical impulse is fragmented, especially in the case of ischemic heart disease, where the wall with the greatest delay seems to be the antero- septal rather than lateral.
The results of the large and multicenter PROSPECT study show that echocardiography has a modest added value compared to the duration of the QRS in the selection of patients to be candidates for CRT. Strain and strain rate are the only echocardiographic techniques capable of distinguishing between active and passive movement of a ventricular wall. However, the data in the literature on the role of these techniques in CRT are insufficient and contradictory, so much so that reliability and utility have yet to be defined. In patients with post-ischemic dilated heart disease, the combined use of Tissue Doppler (evaluation of dyssynchrony) and MRI (evaluation of any scars) could reduce the percentage of “non responders” patients by up to 5%.
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