Echocardiography: Investigations and procedures

Echocardiography

Echocardiography essentially means ultrasonography of the heart. It is a very useful, noninvasive method by which the heart and surrounding structures can be imaged. It requires technical expertise to obtain images and clinical expertise to interpret the results appropriately. The following features are typically assessed:

·        Anatomical features such as cardiac chamber size, myocardial wall thickness and valve structure or lesions. Ventricular aneurysms or defects such as atrial or ventricular septal defects can be seen.

·        Functional features including wall motion (any localised wall motion abnormality as well as a general assessment of the overall contractility of the ventricles, often measured as fractional shortening or ejection fraction) and valve motion. Doppler ultrasound is used to measure the velocity of jets of blood, e.g. to assess severity in valve stenosis.

·        The aortic root may be examined for dilatation or dissection.

·        Pericardial fluid appears as an echo-free space between the myocardium and the parietal pericardium.

·        Mass lesions such as thrombus or tumour may be seen within the heart.

The principles of echocardiography are the same as those of ultrasound. A transducer is used to generate ultrasound waves that are directed at the heart. When a wave encounters an interface of differing echogenicity, some of it is reflected and some absorbed. Any reflected waves (echoes) that reach the transducer are sensed and processed into an image. The time taken for the wave to bounce back measures the distance of the interface. Tissues or interfaces that reflect the waves strongly such as bone/tissue or air/tissue will appear very white (echogenic) and also prevent any tissues underneath from being imaged well. Fluid is anechoic, so appears black. The ribs and lungs limit the ability to visualise the heart because they cast acoustic shadows. Transoesophageal echocardiography (TOE) is a more invasive method used when poor views are obtained on transthoracic echocardiography, or when more detailed views are required particularly of structures near the oesophagus such as the atria and great vessels, the mitral valve or prosthetic valves. A transducer probe is mounted on the tip of a flexible tube that is passed into the oesophagus. The patient needs to be nil by mouth prior to the procedure, local anaesthetic spray is used on the pharynx, and intravenous sedation may be required for the procedure to be tolerated.

There are three types of echocardiography: two dimensional, M-mode and Doppler.

Two dimensional is useful for evaluating the anatomical features. Standard views are obtained.

·        Left parasternal: With the transducer rotated appropriately through a window in the third or fourth intercostal space, long and short axis views can be obtained.

·        Apical: This is a view upwards from the position of the apex beat and gives a long axis view of the heart, where all four chambers can be seen simultaneously.

M-mode is a way by which the motion of individual structures along a narrow path can be carefully studied. It is a one-dimensional view (depth) with time as the second dimension on the image produced. Structures that do not move appear as a horizontal line, whereas structures that move, e.g. valves, are seen as zigzag lines, which move in time with the cardiac cycle. The distances between structures, or the thickness of structures, can therefore be carefully measured at different times of the cardiac cycle.

Doppler allows the analysis of the direction and velocity of blood flow, and therefore is particularly useful in the evaluation of valve lesions. It is used to calculate pressure gradients, e.g. in aortic stenosis. It can also be used to generate 2-D images with simultaneous imaging of flow direction and velocity.

Common indications for echocardiography:

·        Suspected valvular heart disease, including infective endocarditis.

·        Heart failure, to assess left ventricular function and look for any valve lesions or regurgitation, and any evidence of a cardiomyopathy.

·        Postmyocardial infarction for suspected complications, such as ventricular septal rupture or papillary muscle rupture. It will also identify areas of ischaemic myocardium or previous myocardial infarction as areas of hypokinetic or akinetic myocardium, as well as an overall assessment of left ventricular function.