Echocardiography

INVESTIGATIONS

Echocardiography

What’s new ?

John Chambers

• Stress echocardiography still under-used but accurate for diagnosis of coronary disease and risk stratification • Contrast greatly improves image quality for stress studies and is expected to image myocardial perfusion • 3D likely to become routine and already offering improved volume calculations and spatial information • Portable echocardiography is likely to extend the use of echocardiography to non-cardiologists in all clinical situations, potentially even the home

What is echocardiography? The echocardiography machine is a powerful computer that images the heart using ultrasound. It reveals cardiac anatomy and physiology, allowing diagnosis of diseases of the myocardium, valves, pericardium and aorta. The functional significance of coronary disease can be determined using stress echocardiography. Types of echocardiography and their uses are shown in Figure 1. Two-dimensional (2D) imaging − ultrasound is sound at a frequency of more than 20,000 Hz. In the body, frequencies of 1.9–10 MHz are typically used. Most ultrasound entering the body is scattered or absorbed, but some is reflected back to the transducer at interfaces where the acoustic density of tissue changes. The two-dimensional echocardiographic image is therefore a map of the acoustic density of the heart, but resembles a pathological section. Second harmonic imaging is available with all modern machines; reception is at twice the frequency of the transmitted wave, improving penetration and reducing artefact. M-mode (motion mode) is produced by transmitting and receiving down only one scan line of a sector, as guided by 2D echocardiography. Doppler echocardiography uses similar pulses of ultrasound but analyses them differently. If ultrasound is reflected from blood moving towards the probe, its wavelength is shortened. Conversely, if the blood is moving away, the wavelength is prolonged. The computer analyses the change between the transmitted and returning ultrasound wavelengths and resolves velocities to produce a graph of velocity against time (Figure 2). • Continuous-wave Doppler is the oldest Doppler modality and involves two crystals, one transmitting and the other receiving continuously. This technique is sensitive for short-lived or localized flow abnormalities and can resolve high velocities accurately. • Pulsed Doppler (Figure 2a) was introduced to record flow at a relatively focused region within the heart by incorporating a delay between transmission and recording; this allows for the time taken for the ultrasound to travel to and from the desired level within the heart. The disadvantage of this technique is that there is a potential sampling error, which limits the maximum velocity that can be resolved accurately.

Uses of echocardiography Two-dimensional imaging • Reveals anatomy and motion (e.g. whether an aortic valve has two instead of three cusps and how well the valve opens, whether the left ventricular shape is normal and how well each part of the wall thickens) • Measurement of left ventricular cavity size and wall thickness, if the M-mode cursor cannot be placed perpendicular to the septum and posterior wall • Measurement of left ventricular outflow tract diameter for the calculation of stroke volume • Estimation of left ventricular volumes and ejection fraction • Planimetry of (tracing around) the orifice area of the mitral valve in mitral stenosis M-mode • Measurement of left ventricular cavity size and wall thickness • Estimation of left ventricular mass • Timing of events within the heart (e.g. Q wave to mitral valve opening as a measure of diastolic function) • In combination with colour Doppler to aid timing of flow Colour flow mapping • Screening for abnormal flow (e.g. valve regurgitation, shunt) • Semi-quantitative estimation of the grade of valve regurgitation Pulsed Doppler • Assessment of diastolic left ventricular function from recording at the mitral valve and in a pulmonary vein • Estimation of the orifice area of the aortic valve (in combination with 2D and continuous-wave Doppler) • Calculation of stroke volume and cardiac output • Estimation of shunt size Continuous-wave Doppler • Calculation of the grade of stenotic valve lesions • Estimation of pulmonary artery pressure • Semi-quantitative assessment of the grade of valve regurgitation Doppler tissue imaging • Assessment of left ventricular diastolic function • Detection of early left ventricular systolic dysfunction • Determining suitability for biventricular pacing

John Chambers is Consultant and Reader in Cardiology and Head of Non-invasive Cardiology at Guy’s and St Thomas’ Hospitals, London, UK. Conflicts of interest: none.

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Colour Doppler. This is a four-chamber view of the heart recorded with the probe placed at the apex. There is a mechanical mitral valve replacement, and a broad jet of regurgitation through the tricuspid valve. 3

What is involved for the patient? Patient information leaflets for all types of echocardiography are available on the British Society of Echocardiograpy website (www. bsecho.org). Transthoracic echocardiography is performed in a darkened room that should be private, temperature-controlled and large enough for comfort. The skin of the chest is exposed and electrodes are attached to allow timing of echocardiographic events with the cardiac cycle. The patient then lies semi-recumbent on his or her left side, usually with the left arm behind the neck to enlarge the rib spaces. The echocardiographer sits beside the patient. Ultrasound gel acts as a couplant, enabling the ultrasound to pass between the probe and the body. Most images are taken from the left of the sternum and around the left breast, but some are obtained in the epigastrium and at the suprasternal notch. An out-patient study takes 20–30 minutes to perform and 10 minutes to report. Transoesophageal echocardiography1 is semi-invasive and requires adequate clinical facilities and a team usually comprising a nurse and a clinician. The procedure is associated with low morbidity (bradyarrhythmia or tachyarrhythmia, bleeding and haematoma, oesophageal perforation, inhalation pneumonitis), and deaths are occasionally reported, usually in patients who are already critically ill or in those with unsuspected oesophageal malignancy. The patient must be prepared as in Figure 4 and informed consent obtained. Oxygen must be given, usually by nasal cannulae, and the oxygen saturation is checked by pulse oximetry. Blood pressure must be checked before, if necessary during, and after the procedure. An oropharyngeal xylocaine spray induces local anaesthesia, and most centres administer intravenous sedation (diazemuls or midazolam). During the procedure, the patient lies on the left with a mouth-guard in place. The probe is usually inside the patient for about 10−15 minutes. The entire study, including a preliminary transthoracic study and other preparations, takes about 1 hour.

Doppler modalities. a shows pulsed Doppler recorded in the left ventricular outflow tract just below the aortic valve. The signal represents a graph of velocity against time and the area of the waveform is the velocity−time integral, which is the distance travelled on each cycle by an average packet of blood (in this case 22.21 cm). This becomes the stroke volume when multiplied by the cross-sectional area of the left ventricular outflow tract (4.46 cm2, not shown, giving 99 ml) and cardiac output when further multiplied by heart rate (53 beats/minute in this example, giving a cardiac output 5.3 l/minute). b shows continuous-wave Doppler recorded through the aortic valve with forward flow below and regurgitant flow above the line. 2

• Colour flow mapping is an automated version of pulsed Doppler and is used to screen for flow abnormalities. It calculates mean blood velocity and direction of flow at multiple points down the scan lines. Using colour coding, the velocity information is then superimposed on the image (Figure 3). • Doppler tissue imaging records low-velocity signals from the myocardium rather than from the blood, as in the other Doppler modalities. This technique is used for assessing left ventricular (LV) diastolic function, detecting early signs of systolic dysfunction, and determining whether biventricular pacing is indicated in patients with heart failure.

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Afterwards, the patient is allowed to recover, and once fully awake and coordinated is asked to swallow a little cold water to check for pain and aspiration. The patient can then return to bed or can go home with an escort. He or she must be warned not to drive, sign documents or make important decisions for the rest of the day.

Checklist before transoesophageal echocardiography • Has the reason for the investigation been thought through? • Does the patient understand why the procedure is necessary and what is involved? • Is the patient awake, with a cough reflex? • Can the patient lie flat on the left side? • Does the patient have dysphagia, a history of oesophageal surgery (including dilatation) or a history of haematemesis (upper gastrointestinal investigation necessary before transoesophageal echocardiography)? • Is the patient an infective risk (HIV, hepatitis B or C, methicillinresistant Staphylococcus aureus)? • Has the patient been nil-by-mouth for at least 4–6 hours? • Is there intravenous access? • Has a nurse escort been arranged?

Indications2 A basic minimum data set is collected in all studies, but additional views, measurements and manoeuvres may be required, depending on the clinical situation. Requests for echocardiography should therefore include adequate clinical information and a focused reason for the study. The writing of the request form should not be left to the most junior member of the team, who often has no idea of the thought process behind the request. Echocardiography is most unambiguously necessary when the results are likely to change management; for example, when a pericardial effusion is suspected. It may be indicated for academic reasons, but then tends to be given a lower priority, because of

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Indications for transthoracic echocardiography • Family history of transmissible disease (e.g. tuberous sclerosis, hypertrophic cardiomyopathy)

Suspected heart failure • Symptoms or signs of heart failure, particularly with abnormal ECG or chest radiograph • Unexplained hypotension

Stroke, transient ischaemic attack and peripheral emboli • Any patient with abrupt occlusion of a major peripheral or visceral artery • Clinical evidence of relevant structural heart disease (e.g. mitral stenosis) • Clinical suggestion of endocarditis or myxoma • Strong suggestion of cardiac emboli (e.g. both peripheral and cerebral events)

Pulmonary disease • Suspected pulmonary hypertension and evaluation of response to treatment • Suspected cor pulmonale Valve disease • Murmur suggesting at least moderate likelihood of organic disease Ejection systolic murmur filling most of systole, or any pan-systolic murmur Any diastolic murmur Abnormal second heart sound Wide pulse pressure and displaced apex beat or enlarged cardiac shadow

Chest pain • Suspected aortic dissection • Suspicion of post-infarction complication (pan-systolic murmur, cardiogenic shock) • Post-infarction to assess left ventricular function • Diagnosis of ischaemia, if not possible using standard investigations and when echocardiography can be performed soon after pain

• Re-evaluation when symptoms change, or severe valve disease even with no symptoms • Evidence of endocarditis • Known significant valve disease in a pregnant woman • Routinely soon after valve replacement, or when symptoms or signs suggest dysfunction, or routinely after 5 years in those with a biological replacement valve

Diseases of the aorta • Suspected aortic dissection or rupture • Widened mediastinum on chest radiography • Serial studies, if the ascending aorta is dilated Screening • Family history of genetically transmitted cardiovascular disease (e.g. Marfan’s syndrome, hypertrophic cardiomyopathy, dilated cardiomyopathy) • Before and after potentially cardiotoxic chemotherapy, especially herceptin

Arrhythmias and syncope • Clinical suspicion of structural disease (e.g. ventricular tachycardia, atrial fibrillation) in patient aged < 60 years, or abnormal cardiovascular examination • Atrial fibrillation, when the decision to start warfarin will be influenced by left atrial size and left ventricular function 5

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the scarcity of resources. Repeat studies are usually indicated only when a change in clinical condition has occurred or is expected. An echocardiographer can perform about 1500−2000 studies per year and each machine about 2000−2500 studies. The indications for transthoracic echocardiography are listed in Figure 5. There are some situations in which echocardiography is often requested, but may not be appropriate. • Echocardiography usually shows normal LV systolic function when ECG and chest radiography are normal. • Echocardiography cannot make the diagnosis in pericarditis, and is indicated only when an effusion or associated myocarditis is suspected. • Echocardiography must not be used as part of a ‘stroke screen’. It is sometimes requested even before imaging has confirmed an infarct. The diagnostic yield of transthoracic echocardiography in stroke is very low unless there are relevant abnormalities on ECG or clinical examination. • Echocardiography must never be part of a ‘fever screen’. Minor thickening of valves is so common that serious confusion can arise if these are allowed to suggest endocarditis. There must be clinical suspicion of endocarditis before echocardiography is requested. • Echocardiography is not indicated for palpitations or isolated ventricular ectopic beats in the absence of evidence of structural disease. • Echocardiography is unnecessary in patients with a soft ejection systolic murmur and a normal second heart sound in the absence of symptoms. Almost any individual can be found to have a murmur if auscultated carefully enough. • Echocardiography is not indicated routinely after uncomplicated valve replacement with a mechanical prosthesis, beyond the initial postoperative study. Transoesophageal vs transthoracic echocardiography − the two approaches are complementary. Transthoracic echocardiography is better for imaging the apex of the left ventricle, the anterior aortic root and the inferior vena cava, and often the upper part of the ascending thoracic aorta. The transoesophageal approach is better

Indications for transoesophageal echocardiography (with preparatory transthoracic study) Suspected endocarditis • In all cases of prosthetic valve endocarditis • Transthoracic study is non-diagnostic • Consider in pacemaker-related endocarditis Cerebral infarction, transient ischaemic attack, peripheral embolism • Patients aged < 50 years with cerebral infarction • Patients aged > 50 years without evidence of cerebrovascular disease or other obvious cause in whom the findings of echocardiography will change management (e.g. to start warfarin if a patent foramen ovale is found) Before cardioversion • Previous cardioembolic event • Anticoagulation contraindicated • Atrial fibrillation of < 48 hours’ duration in the presence of structural heart disease Prosthetic valve • Patient is unwell, even when transthoracic study is normal • To improve quantification of mitral regurgitation • Suspected endocarditis Native valve disease • To determine feasibility and safety of balloon mitral valvotomy • To determine whether mitral regurgitation is repairable Atrial septal defect • To determine whether percutaneous closure is possible Aorta • To diagnose dissection, intramural haematoma or transection • To determine the size of the aorta Poor transthoracic window • Rare indication Perioperative • To confirm preoperative diagnosis • To assess mitral valve repair • To detect myocardial ischaemia • To confirm de-airing after bypass • To assess haemodynamically unstable patients on ITU 7

for the atria (Figure 6), pulmonary veins, posterior aortic root and descending thoracic aorta. Intraoperative transoesophageal echocardiography is used to assess LV function, de-airing of the heart and the competence of valve repair. It is also used in highrisk patients undergoing non-cardiac surgery, to detect myocardial ischaemia. Indications for transoesophageal echocardiography are listed in Figure 7. Stress echocardiography − the indications and contraindications are given in Figure 8.3 A study is performed at rest and then after

Transoesophageal echocardiogram showing mitral regurgitation. This is a two-chamber view from a low oesophageal position with a broad jet in the left atrium. 6

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exercise or during pharmacological stress, which is usually provided by an intravenous infusion of dobutamine. Myocardial ischaemia causes a reduction in myocardial thickening before and sometimes in the absence of either ST segment depression or chest pain. Stress echocardiography is therefore more sensitive than conventional exercise testing for predicting coronary disease, with a sensitivity of 80% and a specificity of 90%. Furthermore, the extent and site of the affected myocardium correlates with the functional significance and site of the coronary lesion, and also stratifies the risk of cardiac events in both the presence and the absence of previous myocardial infarction. Patients with a normal stress echocardiogram do not usually need to undergo coronary angiography. In patients known to have coronary disease, stress echocardiography is used to determine the functional significance of an equivocal lesion, to decide whether revascularization is necessary. The sensitivity and specificity of low-dose dobutamine echocardiography for predicting recovery of regional function following revascularization are 84% and 81%, which is comparable to positron emission tomography and gadolinium-enhanced MRI. Stress echocardiography is also used in valve disease, mainly for detection of contractile reserve in patients with aortic stenosis and low ejection fraction.

New technologies and applications Real-time three-dimensional echocardiography is rapidly becoming established. It is likely to improve the assessment of wall motion abnormalities at rest and during stress, and will improve quantification of intracavitary volumes and myocardial mass. It is useful for defining the anatomy in congenital heart disease and the morphology of mitral valves. Portable echocardiography – small machines the size of laptop computers are now being used for portable studies.4 In criticallyill patients on the ICU or A&E department, they assess loading conditions, ventricular function and the presence of pericardial fluid or valve defects. They are also used to complement clinical examination on ward rounds and in general practice surgeries. Their relatively low cost has enabled their purchase by non-cardiac departments, and they may be useful for physicians in acute medicine. Contrast echocardiography − in this technique, an agent producing microbubbles approximately the size of RBCs is injected intravenously, then crosses the pulmonary capillaries and opacifies the left heart. This is used to improve endocardial delineation for the diagnosis of wall motion abnormalities both at rest and during stress, and to aid diagnosis of thrombus and apical hypertrophy. New imaging techniques, including pulse inversion and multiple harmonic imaging, improve the detection of microbubbles in the myocardium, allowing imaging of myocardial perfusion. This remains a research technique, but is expected to improve the accuracy of wall motion analysis alone for the diagnosis of coronary disease and myocardial hibernation. 

Indications for and contraindications to stress echocardiography Indications • Prediction of coronary disease in patients unsuitable for exercise testing (e.g. resting ECG changes, unable to walk) or likely to have a false positive ECG (e.g. women) • Risk stratification in known coronary disease (e.g. after myocardial infarction) • After coronary angiography, to assess functional significance of an equivocal lesion • To assess adequacy of myocardial perfusion (e.g. before noncardiac surgery) • To determine the presence of viability in apparently infarcted myocardium • To assess valve disease (e.g. aortic stenosis with impaired left ventricle, moderate aortic stenosis and nonspecific symptoms, moderate mitral regurgitation but severe breathlessness)

REFERENCES 1 Flachskampf F A, Decoodt P, Fraser A G et al. Recommendations for performing transesophageal echocardiography. Eur J Echocardiography 2001; 2: 8–21. 2 Cheitlin M D, Alpert J S, Armstrong W F et al. ACC/AHA guidelines for the clinical application of echocardiography: executive summary. J Am Coll Cardiol 1997; 29: 862–79. 3 Senior R, Monaghan M, Becher H, Mayet J, Nihoyannopoulos P. Stress echocardiography for the diagnosis and risk stratification of patients with suspected or known coronary artery disease: a critical appraisal. Supported by the British Society of Echocardiography. Heart 2005; 91: 427–36. 4 Chambers J, Senior R. Portable echocardiography: a review. Br J Cardiol (in press).

Contraindications These are similar to the contraindications to exercise stress testing and are relative rather than absolute • Early after admission with acute coronary syndrome • Severe aortic stenosis (except for low-gradient, low-flow aortic stenosis) • Hypertrophic obstructive cardiomyopathy • Significant dysrhythmia (e.g. ventricular tachycardia, uncontrolled atrial fibrillation) • History of ventricular tachycardia • Acute intercurrent illness, including hypokalaemia, uncontrolled hypertension, deep vein thrombosis and pulmonary embolism

FURTHER READING Chambers J. Echocardiography in clinical practice. London: Parthenon, 2002. (A short introductory book aimed at physicians in hospital or the community who are in primary control of the patient and who need to request an echocardiogram.) Kerut E K, McIlwain E F, Plotnick G D. Handbook of echo-doppler interpretation. Oxford: Blackwell Science, 1996. (A good integration of clinical and more technical echocardiography.) Oh J K, Seward J B, Tajik A J. The echo manual. 2nd ed. Philadelphia: Lippincott-Raven, 1999. (A well-presented practical guide.)

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