Factores socio-ambientales

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There are 3 basic B-mode echocardiographic modalities:

- M-mode

- 2-dimensional (2-D) and - Doppler echocardiography

Others, are basically modifications of these three.

They are :

Stress echocardiography Contrast echocardiography

Foetal echocardiography

Intravascular echocardiography Intra-operative echocardiography

Echo monitoring of invasive procedures Colour Doppler echocardiography 3-Dimensional echocardiography 4-Dimensional echocardiography

(1) M-MODE

M-mode echocardiography equipments use a transducer containing one crystal which emits a single ultrasound beam of 1000 to 2000 pulses per second. This produces a narrow “iced-picked” view of the cardiac structures.

The depth of the echo is displayed on the vertical axis, and the time on the

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horizontal axis. The recording thus appears as a continuous graph of the depth of the structures with respect to time. The spatial resolution (the ability to differentiate and recognize structures that are close together) is very high, about 1 to 2cm along the axis of the sound beam. The temporal resolution is also very high. This makes it possible to obtain high resolution images of rapidly moving structures such as valve opening, closing, fluttering and subtle wall abnormalities⁸⁶.

M-mode echocardiography is useful in measuring left ventricular wall

thickness and internal dimensions. It is superior to electrocardiography in detecting left ventricular hypertrophy.⁸⁷ Left ventricular internal dimensions can be used to assess the systolic function of the left ventricle by calculating the fractional

shortening. Left ventricular mass is also derived from M-mode measurements. The main disadvantages of M-mode echocardiography are that: it provides only a one dimensional view of the heart; the cardiac

structures are displayed in an unfamiliar format that bears no resemblance to the cardiac anatomy and it is limited in its ability to provide information regarding the spatial orientation of the cardiac structure. It therefore needs

specialized training for interpretation. M-mode is no longer used alone, but only as an adjunct to 2-D or Doppler. Stand alone M-mode machines are no longer

produced. Two-dimensional echocardiography was introduced to overcome these disadvantages.

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2. TWO-DIMENSIONAL ECHOCARDIOGRAPHY

Two-dimensional echocardiographic equipments use a transducer containing one or two crystals that are mechanically rotated or electronically fired in a sequential manner. The transducer transmits and receives 120 discrete ultrasound beams through a 60 to 90^o sector in order to produce a fan-shaped

image of the heart in a cross section. Since it utilizes multiple ultrasound beams, processing of two dimensional image takes a longer time than an

M-mode image. The sampling rate is thus slower (30 to 60 times per second as against 1000 times per second by M-mode). There is therefore a substantial decrease in resolution.

A cross sectional image of the heart is depicted with two-dimensional echocardiography. Thus direct and accurate visualization of the entire heart,

intracardiac structures and great vessels is possible. Global left ventricular function can be assessed by this technique. Left ventricular systolic and

diastolic volumes are calculated using the Simpson’s rule.⁸⁸ Stroke volume and cardiac output can be derived from these values. The major limitations

of two-dimensional echocardiography is its inability to image blood cells and provide data about velocity, direction, timing and spatial profile of blood flow. To correct this limitation, Doppler echocardiography was introduced.

3. DOPPLER ECHOCARDIOGRAPHY

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Doppler echocardiography is based on a physical principle known as the “Doppler effect”. This was first described in 1842 by an Austrian mathematician and

physicist, Johann Christian Doppler.⁸⁹ Doppler effect is the change in the

frequency of sound waves when the source of sound is moving in relation to the receiver. If the source of sound is moving towards

the receiver, the frequency would be increasing but if the source of sound is moving away from the receiver, the frequency would be decreasing. Doppler echocardiography is, therefore, based on the frequency shift between the

transmitted and the returning ultrasound. When a transmitted ultrasound meets a moving target such as a column of blood, the frequency of the returning echo is different, higher if the target is moving towards the transducer. This difference in frequency–the Doppler shift –is within the

audible range and can be displayed as audible signals or as visual signals called spectral trace, on an oscilloscope.

There are several types of Doppler studies, all of which can be performed using a single probe:

(a) PULSED WAVE DOPPLER (PWD)

PWD uses a single crystal to study the patterns of blood flow, including the detection of abnormal flow, shunts and cardiac output. It transmits ultrasound in pulses and the waiting period in between the pulses is used to receive the returning echoes. By timing the transmitted and returning echoes, and by calculating the

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velocity of ultrasound in body tissues, one can determine the location or depth of the returning echoes. However, the maximum velocity recordable by the PWD is limited. Once this is exceeded,

the tracing above this limit is cut off and placed in the opposite channel, the so-called frequency aliasing.

(b) CONTINUOUS WAVE DOPPLER (CWD)

CWD uses two separate adjacent crystals, one that continuously transmits sound and the other, which continuously receives reflected sound. The velocity recorded by CWD is very high but the range gating is poor. It records all echoes along its path. One cannot ascertain the depth or the tissue

producing the recorded echoes. This limitation is partly overcome by superimposing CWD on a 2-D image.

Both PWD and CWD record flow from only a single ultrasonic beam. This limitation has been overcome by the use of Colour Doppler. This adds

the possibility of studying pressure gradients across valves from flow and valve area observations. There is a recent development in Doppler

echocardiography, known as

(c) COLOUR FLOW MAPPING

This technique allows the visualization of intracardiac blood flow

superimposed on a two dimensional echocardiographic display. Flow towards the

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transducer is depicted as red; flow away from the transducer is blue. Green is added in mosaic pattern to represent turbulent flow. Colour-coded

Doppler is very useful in the detection and mapping of regurgitant and shunt lesions, while facilitating the evaluation of congenital heart disease.

4. NEWER DEVELOPMENT IN ECHOCARDIOGRAPHY:

Other echocardiography modalities have come into use to a varying extent in the recent past, especially within the last decade. These include:

(a) Transoesophageal echocardiography: this is of great use in patients whom the examination from the usual transthoracic approach is technically

difficult or impossible. It allows examination of structures such as the atria,

assessing prosthetic valves, aortic dissection, vegetations and intracardiac masses.

It has a major application now in cardiac surgery, both during and after surgery.

(b) Intravascular ultrasound: the ultrasonic transducer is placed in a small catheter so that a vessel can be imaged through the lumen. This can evaluate atherosclerosis from within the arteries, and the heart from within the cardiac chambers, using a rotation transducer mirror, or phased array multi-element systems.

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(c) Contrast echocardiography: this makes use of the fact that ultrasound is an extremely sensitive detector of intravascular bubbles. The injection of almost any liquid into the intravascular spaces will introduce many micro

bubbles that appear as a cloud of echoes on the echocardiogram. Thus the injection into the blood stream of a marker such as saline, agitated or sonicated angiographic contrast agent, sonicated albumin , indocyanine or some of the patients own blood, may be used as a substitute for the Doppler examination for certain types of flow and shunt visualization. This technique has potential for numerous clinical uses.

(d) Stress echocardiography: this aids the overall management of patients with suspected coronary artery disease and acute myocardial infarction since

stunned or hibernating myocardium can be unmasked, even before patients report chest pain or ST segment changes are seen on the electrocardiogram. Global

changes in left ventricular function can also be assessed. Dobutamine is commonly used as the biochemical stress agent.

(e) Digital echocardiography: this is the digital acquisition, formatting,

analysis, storage and review of ultrasound data. The technique was initially developed to reduce some of the practical difficulties encountered during the performance of stress echocardiography. The most common format used the observation of cardiac wall motion in ‘quad screen’ format where four synchronized image loops are displayed simultaneously on the screen.

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(f) Doppler Tissue Imaging and Colour Kinesis: These are new technologies which directly examine myocardial motion and encode

movement of the myocardium or endocardium in colour. Hypokinetic, akinetic or dyskinetic segments of cardiac walls are thus demonstrated. Regional myocardial thickening and ventricular synchrony are readily

apparent, and chamber volume measurements may be more accurately defined.

These new techniques need additional studies before they can be applied clinically.

(g) Automated Boundary Detection: This is a recently developed technique which uses ultrasonic backscatter technology to characterize

tissue properties. It incorporates a border detection algorithm for delineating the endocardial blood interface. The system automatically detects the blood and tissue borders, which can be displayed on a two

dimensional sector image. It is used in calculating blood area changes in the cardiac cycle, and subsequently diastolic function indices.

(h) Three-Dimensional Echocardiography: Reconstructed three- dimensional images of the heart using multiple two- dimensional image are now being proposed and actually put into use. A technique orients a two-dimensional transducer in a three-dimensional space using spark gap sensors. Another technique creates 3-D images of the heart using gated, reconstructed 2-D

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examinations. Chamber dimensions can, with this technique be estimated with greater accuracy than is possible using cross-

sectional methods. Valves can be seen and assessed with great accuracy.^85,90 This allows better definition of septal defects as the image resembles what the surgeon would see intra-operatively. However, 3-D

images take an exceedingly long time to produce, about 5 seconds for one image.

Attempts were then made to produce 3-D images in real-time. The result is now 4-Dimensional echocardiography (4-D).

(IV) ECHOCARDIOGRAPHY IN NIGERIA

Echocardiography came into Nigeria about three decades ago.⁹¹ Since its debut, physicians have found it most useful both in diagnosis of various forms of heart disease and clinical research.⁹² The M-mode echocardiography was the first form of echocardiography to be introduced in Nigeria. Lawal and Falase, had used this to assess left ventricular

function in a study of hypertensive patients at University College Hospital, Ibadan.⁹³

M-mode echocardiography became available at the University of Nigeria Teaching Hospital in 1978. By 1988, Two-dimensional

echocardiography with PWD and CWD became available;⁹⁴ and colour flow Doppler in 2003. Many other health institutions, including private

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hospitals, have since then acquired this facility, both for diagnosis and medical research.^95,96.97

(V) ECHOCARDIOGRAPHY AS A TOOL FOR INVESTIGATING CHRONIC RHEUMATIC MITRAL VALVE DISEASE

The pathological changes in the valve apparatus produce dramatic alterations to the echocardiographic images. On M-mode recording,

thickening of the leaflets and reduction in their mobility can be appreciated.

Commissural fusion alters the motion of the posterior leaflet; instead of moving in the posterior direction to that of the anterior, it is pulled forward when the valve opens at the beginning of diastole. In normal valve, rapid early diastolic filling of the left ventricle means that the valve can partially close in mid diastole, and it then reopens during atrial systole. This sequence

produces the characteristic ‘M’ shape of the recording of the anterior mitral valve, with the mid-diastolic down slope of the ‘M’ designated as the ‘E-F slope’. With mitral stenosis, however, rapid filling is not possible, so the valve leaflets have to remain as widely separated as they can throughout diastole, as a result of persistent pressure gradient between the left atrium and left ventricle. The

‘M’ shape of the anterior mitral valve disappears. This is replaced by a reduced ‘E-F slope’.⁹⁸ The normal posterior leaflet is usually a

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mirror image of the anterior leaflet. However, in mitral stenosis, the most striking qualitative change is that, with early diastole, the posterior leaflet now moves upward in the same direction as the anterior leaflet⁹⁹.

These changes are also apparent on two-dimensional echocardiographic recordings which have superceded M-mode technique for making this judgment.

Commissural fusion causes the valve leaflets to

assume a dome in diastole, with the anterior leaflet forming a characteristic

‘elbow’ due to its tethering to the posterior leaflet. Doming indicates that the valve cannot accommodate all the blood available for delivery into the left ventricle. Thus, the body of the leaflets separates more widely than the edges.

Doming is one of the main two-dimensional features of any stenotic valve. The 2-D echo hallmark of mitral valve stenosis is the restriction of motion of anterior mitral valve leaflet tip.

Echocardiography is a very sensitive method of detecting mitral stenosis and simple inspection of the recordings can often differentiate mild and severe stenosis. Quantification of stenosis has however, proved more difficult by M-mode. Early attempt measured ‘E-F slope’, which indicates the difference between early and mid diastolic leaflet position, but this proved unsatisfactory because the ‘E-F slope’ is influenced by factors such as cardiac output and ventricular stiffness.^100,101

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Two-dimensional echocardiography is able to visualize the mitral orifice directly in diastole, thus, providing information previously available only to the surgeons. Planimetry of a still- frame image allows the mitral valve area orifice to be measured.¹⁰² Numerous studies have shown that, the mitral valve area measured with two-dimensional echocardiography

correlates well with the severity of mitral stenosis.^101-107

Doppler echocardiography methods have also been shown to be useful for estimation of mean mitral gradient, trans-mitral velocity and mitral

pressure half time, which are, sensitive indicators of mitral valve stenosis.

One of the Doppler features of mitral stenosis is that the rate of decrease in diastolic flow after the E-point is reduced. The peak velocity is also higher than normal. Such methods are particularly useful in a patient with previous commissurotomy and also serve to double-check the direct planimetric approaches.

M-mode can also show abnormal mitral leaflet closure patterns in mitral valve regurgitation. This consists of incomplete closure, mitral valve prolapse, ruptured chordae tendinae or flail mitral valve; in addition to interventricular septum systolic motion and other indirect evidences of mitral valve regurgitation such as dilated left atrium and left ventricle.

Two-dimensional echocardiography shows the above signs ascribed for M-mode echocardiography. This study may reveal the aetiology of the

regurgitation by showing mitral valve prolapse, thickened leaflet with fused

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commissures and decreased motion in rheumatic disease.

The detection and assessment of mitral regurgitation is primarily by Doppler echocardiography.^108,109 Pulse waved, continuous wave and colour Doppler echocardiography are extremely sensitive for detecting mitral regurgitation which appears as turbulent systolic signals within the left atrium directed away from the transducer. The extent of the penetration and the area of the regurgitant jet can be used to estimate severity. Colour flow Doppler provides a nearly real-time flow map of the origin and direction of mitral regurgitation. Large colour jets that occupy more than half of the left atrium, extend to the posterior portion of the atrium or into the appendage or pulmonary veins indicates significant regurgitation.^110,111

Echocardiography can be performed repeatedly, lending itself to

follow up studies and does a comprehensive examination in a relatively short period of 15-30 minutes. It is safe for the patient, there being no appreciable side effect associated with it.^112. It is thus a handy tool for assessing the

severity of mitral valve disease. Data so obtained could be used for evaluating cardiac conditions and therapeutic outcome. Given its degree of accuracy, it is as yet comparatively cheap, and this increases its attractiveness for patient care, medical diagnosis, prognosis and follow up and hence for medical research.^113,114

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