Specific investigations may be required to confirm a diagnosis of cardiac disease. Basic tests, such as electrocardiography, chest X-ray and echocardiography, can be performed in an outpatient clinic or at the bedside. Procedures such as cardiac catheterisation, radionuclide imaging, computed tomography (CT) and magnetic resonance imaging (MRI) require specialised facilities.
The electrocardiogram (ECG) is used to assess cardiac rhythm and conduction. It provides information about
The electrocardiograph. The components correspond to depolarisation and repolarisation, as depicted. The upper limit of the normal range for each interval is given in brackets.
chamber size and is the main test used to assess for myocardial ischaemia and infarction. The basis of an ECG recording is that the electrical depolarisation of myocardial tissue produces a small dipole current which can be detected by electrode pairs on the body surface. These signals are amplified and either printed or displayed on a monitor (Fig.).
During sinus rhythm, the SA node triggers atrial depolarisation, producing a P wave. Depolarisation proceeds slowly through the AV node, which is too small to produce a depolarisation wave detectable from the body surface. The bundle of His, bundle branches and Purkinje system are then activated, initiating ventricular myocardial depolarisation, which produces the QRS complex. The muscle mass of the ventricles is much larger than that of the atria, so the QRS complex is larger than the P wave. The interval between the onset of the P wave and the onset of the QRS complex is termed the ‘PR interval’ and largely reflects the duration of AV nodal conduction. Injury to the left or right bundle branch delays ventricular depolarisation, widening the QRS complex. Selective injury of one of the left fascicles (hemiblock) affects the electrical axis. Repolarisation is slower and spreads from the epicardium to the endocardium. Atrial repolarisation does not cause a detectable signal but ventricular repolarisation produces the T wave. The QT interval represents the total duration of ventricular depolarisation and repolarisation.
The standard 12–lead ECG
The 12-lead ECG (Box) is generated from ten physical electrodes that are attached to the skin. One electrode is attached to each limb and six electrodes are attached to the chest. In addition, the left arm, right arm and left leg electrodes are attached to a central terminal acting as an additional virtual electrode in the centre of the chest (the right leg electrode acts as an earthing electrode).
The twelve ‘leads’ of the ECG refer to recordings made from pairs or sets of these electrodes. They comprise three groups: three dipole limb leads, three augmented voltage limb leads and six unipole chest leads. Leads I, II and III are the dipole limb leads and refer to recordings obtained from pairs of limb electrodes.
Lead I records the signal between the right (negative) and left (positive) arms. Lead II records the signal between the right arm (negative) and left leg (positive). Lead III records the signal between the left arm (negative) and left leg (positive). These three leads thus record electrical activity along three different axes in the frontal plane. Leads aVR, aVL and aVF are the augmented voltage limb leads. These record electrical activity between a limb electrode and a modified central terminal. For example, lead aVL records the signal between the left arm (positive) and a central (negative) terminal, formed by connecting the right arm and left leg electrodes (Fig. ). Similarly augmented signals are obtained from the right arm (aVR) and left leg (aVF). These leads also record electrical activity in the frontal plane, with each lead 120° apart. Lead aVF thus examines activity along the axis +90°, and lead aVL along the axis −30°, and so on. When depolarisation moves towards a positive electrode, it produces a positive deflection in the ECG;
The appearance of the ECG from different leads in the frontal plane. A Normal. B Left axis deviation, with negative
deflection in lead II and positive in lead I. C Right axis deviation, with negative deflection in lead I and positive in lead II.
depolarisation in the opposite direction produces a negative deflection. The average vector of ventricular depolarisation is known as the frontal cardiac axis. When the vector is at right angles to a lead, the depolarisation in that lead is equally negative and positive (isoelectric). In Figure A, the QRS complex is isoelectric in aVL, negative in aVR and most strongly positive in lead II; the main vector or axis of depolarisation is therefore 60°. The normal cardiac axis lies between −30° and +90°. Examples of left and right axis deviation are shown in Figures B and C. There are six chest leads, V1–V6, derived from electrodes placed on the anterior and lateral left side of the chest, over the heart. Each lead records the signal between the corresponding chest electrode (positive) and the central terminal (negative). Leads V1 and V2 lie approximately over the RV, V3 and V4 over the
The sequence of activation of the ventricles. A Activation of the septum occurs first (red arrows), followed by spreading of the impulse through the LV (blue arrows) and then the RV (green arrows). B Normal electrocardiographic complexes from leads V1 and V6.
interventricular septum, and V5 and V6 over the LV (Fig.above). The LV has the greater muscle mass and contributes the major component of the QRS complex. The shape of the QRS complex varies across the chest leads. Depolarisation of the interventricular septum occurs first and moves from left to right; this generates a small initial negative deflection in lead V6 (Q wave) and an initial positive deflection in lead V1 (R wave). The second phase of depolarisation is activation of the body of the LV, which creates a large positive deflection or R wave in V6 (with reciprocal changes in V1). The third and final phase involves the RV and produces a small negative deflection or S wave in V6.
The ECG in ischaemia and infarction
When an area of the myocardium is ischaemic or undergoing infarction, repolarisation and depolarisation become abnormal relative to the surrounding myocardium. In transmural infarction, there is initial ST segment elevation (the current of injury) in the leads facing or overlying the infarct; Q waves (negative deflections) will then appear as the entire thickness of the myocardial wall becomes electrically neutral relative to the adjacent myocardium. In myocardial ischaemia, the ECG typically shows ST segment depression and/or T-wave inversion; it is usually the subendocardium that most readily becomes ischaemic. Other conditions, such as left ventricular hypertrophy and electrolyte disturbances, can cause similar ST and T-wave changes.
Exercise (stress) ECG
Exercise electrocardiography is used to detect myocardial ischaemia during physical stress and is helpful in the diagnosis of coronary artery disease. A 12-lead ECG is recorded during exercise on a treadmill or bicycle ergometer. The limb electrodes are placed on the shoulders and hips rather than the wrists and ankles. The
A positive exercise test (chest leads only). The resting 12-lead ECG shows some minor T-wave changes in the inferolateral leads
but is otherwise normal. After 3 minutes’ exercise on a treadmill, there is marked planar ST depression in leads V4 and V5 (right offset). Subsequent coronary angiography revealed critical three-vessel coronary artery disease.
Bruce Protocol is the most commonly used for testing. BP is recorded and symptoms assessed throughout the test. Common indications for exercise testing are shown in Box . A test is ‘positive’ if anginal pain occurs, BP falls or fails to increase, or if there are ST segment shifts of more than 1 mm (see Fig.). Exercise testing is useful in confirming the diagnosis in patients with suspected angina, and in such patients has good sensitivity and specificity (see Box ). False-negative results can occur in patients with coronary artery disease, and some patients with a positive test will not have coronary disease (false-positive). It is an unreliable population
screening tool because, in low-risk individuals (e.g. asymptomatic young or middle-aged women), an abnormal response is more likely to represent a falsepositive than a true positive test. Stress testing is contraindicated in the presence of acute coronary syndrome, decompensated heart failure and severe hypertension.
Sinoatrial disease (sick sinus syndrome). A continuous rhythm strip from a 24-hour ECG tape recording illustrating periods of sinus
rhythm, atrial ectopics, junctional beats, sinus bradycardia, sinus arrest and paroxysmal atrial fibrillation.
Continuous (ambulatory) ECG recordings can be obtained using a portable digital recorder. These devices usually provide limb lead ECG recordings only, and can record for between 1 and 7 days. Ambulatory ECG recording is principally used in the investigation of
patients with suspected arrhythmia, such as those with intermittent palpitation, dizziness or syncope. For these patients, a 12-lead ECG provides only a snapshot of the cardiac rhythm and is unlikely to detect an intermittent arrhythmia, so a longer period of recording is useful (see Fig. ). These devices can also be used to assess rate control in patients with atrial fibrillation, and are sometimes used to detect transient myocardial ischaemia using ST segment analysis. For patients with more infrequent symptoms, small, patient-activated ECG recorders can be issued for several weeks until a symptom episode occurs. The patient places the device on the chest to record the rhythm during the episode. With some devices, the recording can be transmitted to hospital via telephone. Implantable ‘loop recorders’ resemble a leadless pacemaker and are implanted subcutaneously. They have a lifespan of 1–3 years and are used to investigate patients with infrequent but potentially serious symptoms, such as syncope.
Plasma or serum biomarkers can be measured to assess myocardial dysfunction and ischaemia. Brain natriuretic peptide
This is a 32-amino acid peptide and is secreted by the LV along with an inactive 76-amino acid N-terminal fragment (NT-proBNP). The latter is diagnostically more useful, as it has a longer half-life. It is elevated principally in conditions associated with left ventricular systolic dysfunction, and may aid the diagnosis and assess prognosis and response to therapy in patients with heart failure
Troponin I and troponin T are structural cardiac muscle proteins that are released during myocyte damage and necrosis, and represent the cornerstone of the diagnosis of acute myocardial infarction. However, modern assays are extremely sensitive
and some have a normal reference range and can detect very low levels of myocardial damage, so that elevated plasma troponin concentrations are seen in other acute conditions, such as pulmonary embolus, septic shock and acute pulmonary oedema. The diagnosis of MI therefore relies on the patient’s clinical presentation.
This is useful for determining the size and shape of the heart, and the state of the pulmonary blood vessels and lung fields. Most information is given by a posteroanterior (PA) projection taken in full inspiration. Anteroposterior (AP) projections are convenient when patient movement is restricted but result in magnification of the cardiac shadow. An estimate of overall heart size can be made by comparing the maximum width of the cardiac outline with the maximum internal transverse diameter of the thoracic cavity. ‘Cardiomegaly’ is the term used to describe an enlarged cardiac silhouette where the ‘cardiothoracic ratio’ is greater than 0.5. It can be caused by chamber dilatation, especially left ventricular dilatation, or by a pericardial effusion. Artefactual cardiomegaly
may be due to a mediastinal mass or pectus excavatum , and cannot be reliably assessed from an AP film. Cardiomegaly is not a sensitive indicator of left ventricular systolic dysfunction since the cardiothoracic ratio is normal in many affected patients (false-negative) and also lacks specificity with many patients with apparent cardiomegaly having normal echocardiograms (false-positive).
Dilatation of individual cardiac chambers can be recognised by the characteristic alterations to the cardiac silhouette:
• Left atrial dilatation results in prominence of the left atrial appendage, creating the appearance of a straight left heart border, a double cardiac shadow to the right of the sternum, and widening of the angle of the carina (bifurcation of the trachea) as the left main bronchus is pushed upwards (Fig. ).
Chest X-ray of a patient with mitral stenosis and regurgitation indicating enlargement of the LA and prominence of
the pulmonary artery trunk.
• Right atrial enlargement projects from the right heart border towards the right lower lung field.
• Left ventricular dilatation causes prominence of the left heart border and enlargement of the cardiac silhouette. Left ventricular hypertrophy produces rounding of the left heart border (Fig. ).
Chest X-ray of a patient with aortic regurgitation, left ventricular enlargement and dilatation of the ascending aorta.
• Right ventricular dilatation increases heart size, displaces the apex upwards and straightens the left heart border. Lateral or oblique projections may be useful for detecting pericardial calcification in patients with constrictive pericarditis or a calcified thoracic aortic aneurysm, as these abnormalities may be obscured by the spine on the PA view. The lung fields on the chest X-ray may show congestion and oedema in patients with heart failure (see Fig.), and an increase in pulmonary blood flow
Radiological features of heart failure. A Chest X-ray of a patient with pulmonary oedema. B Enlargement of lung base showing
septal or ‘Kerley B’ lines (arrow).
(‘pulmonary plethora’) in those with left-to-right shunt. Pleural effusions may also occur in heart failure.
Two-dimensional echocardiography Echocardiography, or cardiac ultrasound, is obtained by placing an ultrasound transducer on the chest wall to image the heart structures as a real-time, twodimensional ‘slice’. This permits the rapid assessment of cardiac structure and function. Left ventricular wall thickness and ejection fraction can be estimated. Common indications for echocardiography are shown in Box.
This depends on the Doppler principle that sound waves reflected from moving objects, such as intracardiac red blood cells, undergo a frequency shift. The speed and direction of the red cells, and thus of blood, can be detected in the heart chambers and great vessels. The greater the frequency shift, the faster the blood is moving. The derived information can be presented either as a plot
of blood velocity against time for a particular point in the heart (Fig.) or as a colour overlay on a twodimensional real-time echo picture (colour-flow Doppler, Fig.). Doppler echocardiography can be used to detect valvular regurgitation, where the direction of
Doppler echocardiography in aortic stenosis. A The aortic valve is imaged and a Doppler beam passed directly through the left ventricular outflow tract and the aorta into the turbulent flow beyond the stenosed valve. B The velocity of the blood cells is recorded to determine the maximum velocity and hence the pressure gradient across the valve. In this example, the peak velocity is approximately 450 cm/sec (4.5 m/sec), indicating severe aortic stenosis (peak gradient of 81 mmHg).
Echocardiographic illustration of the principal cardiac structures in the ‘four-chamber’ view. A The major chambers and valves. B Colour-flow Doppler has been used to demonstrate mitral regurgitation: a flame-shaped (yellow/blue) turbulent jet into the left atrium.
blood flow is reversed and turbulence is seen, and is also used to detect high pressure gradients associated with stenosed valves. For example, the normal resting systolic flow velocity across the aortic valve is approximately 1 m/sec; in the presence of aortic stenosis, this is increased as blood accelerates through the narrow orifice. In severe aortic stenosis, the peak aortic velocity may be increased to 5 m/sec (see Fig. ). An estimate of the pressure gradient across a valve or lesion is given by the modified Bernoulli equation:
Pressure gradient (mmHg)
= 4 × ( peak velocity in m/sec ) square
Advanced techniques include three-dimensional echocardiography, intravascular ultrasound (defines vessel wall abnormalities and guides coronary intervention), intracardiac ultrasound (provides high-resolution images) and tissue Doppler imaging (quantifies myocardial contractility and diastolic function).
Transthoracic echocardiography sometimes produces poor images, especially if the patient is overweight or has obstructive airways disease. Some structures are difficult to visualise in transthoracic views, such as the left atrial appendage, pulmonary veins, thoracic aorta and interatrial septum. Transoesophageal echocardiography (TOE) uses an endoscope-like ultrasound probe which is passed into the oesophagus under light sedation and positioned behind the LA. This produces high-resolution images, which makes the technique particularly valuable for investigating patients with prosthetic (especially mitral) valve dysfunction, congenital abnormalities (e.g. atrial septal defect), aortic dissection, infective endocarditis (vegetations that are too small to be detected by transthoracic echocardiography) and systemic embolism (intracardiac thrombus or masses).
Stress echocardiography is used to investigate patients with suspected coronary artery disease who are unsuitable for exercise stress testing, such as those with mobility problems or pre-existing bundle branch block. A two-dimensional echo is performed before and after infusion of a moderate to high dose of an inotrope, such as dobutamine. Myocardial segments with poor perfusion become ischaemic and contract poorly under stress, showing as a wall motion abnormality on the scan. Stress echocardiography is sometimes used to examine myocardial viability in patients with impaired left ventricular function. Low-dose dobutamine can induce contraction in ‘hibernating’ myocardium; such patients may benefit from bypass surgery or percutaneous coronary intervention.
Computed tomographic imaging
Computed tomography (CT) is useful for imaging the cardiac chambers, great vessels, pericardium, and mediastinal structures and masses. Multidetector scanners can acquire up to 320 slices per rotation, allowing very high-resolution imaging. CT is often performed using a timed injection of X-ray contrast to produce clear images of blood vessels and associated pathologies. Contrast scans are very useful for imaging the aorta in suspected aortic dissection (see Fig.), and the pulmonary arteries and branches in suspected pulmonary embolism .
Images from a patient with an acute type B aortic dissection that had ruptured into the left pleural space and was repaired by deploying an endoluminal stent graft. A CT scan illustrating an intimal flap (arrow) in the descending aorta and a large pleural effusion. B Aortogram illustrating aneurysmal dilatation; a stent graft has been introduced from the right femoral artery and is about to be deployed. C CT scan after endoluminal repair. The pleural effusion has been drained but there is a haematoma around the descending aorta. D Aortogram illustrating the stent graft. E Three-dimensional reconstruction of aortic stent graft.
Some centres use cardiac CT scans for quantification of coronary artery calcification, which may serve as an index of cardiovascular risk. However, modern multidetector scanning allows non-invasive coronary angiography (Fig. ) with a spatial resolution approaching
Computed tomography coronary angiography demonstrating normal coronary arteries (arrows).
that of conventional coronary arteriography and at a lower radiation dose. CT coronary angiography is particularly useful in the initial elective assessment of patients with chest pain and a low or intermediate likelihood of disease, since its negative predictive value is very high: that is, excluding the presence of coronary artery disease. Modern volume scanners are also able to assess myocardial perfusion, often at the same sitting.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) requires no ionising radiation and can be used to generate cross-sectional images of the heart, lungs and mediastinal structures. It provides better differentiation of soft tissue structures than CT but is poor at demonstrating calcification. MRI scans need to be ‘gated’ to the ECG, allowing the scanner to produce moving images of the heart and mediastinal
structures throughout the cardiac cycle. MRI is very
Sagittal view of an MRI scan from a patient with longstanding aortic dissection, illustrating a biluminal aorta. There is sluggish flow in the false lumen (FL), accounting for its grey appearance. (TL = true lumen)
useful for imaging the aorta, including suspected dissection (see Fig. , and can define the anatomy of the heart and great vessels in patients with congenital heart disease. It is also useful for detecting infiltrative conditions affecting the heart. Physiological data can be obtained from the signal returned from moving blood, which allows quantification of blood flow across regurgitant or stenotic valves. It is also possible to analyse regional wall motion in patients with suspected coronary disease or cardiomyopathy. The RV is difficult to assess using echocardiography because of its retrosternal position but is readily visualised with MRI. MRI can also be employed to assess myocardial perfusion and viability. When a contrast agent, such as gadolinium, is injected, areas of myocardial hypoperfusion can be identified with better spatial resolution than nuclear medicine techniques. Later redistribution of this contrast, so-called delayed enhancement, can be used to identify myocardial scarring and fibrosis (Fig. ). This can help in selecting patients for
Cardiac magnetic resonance imaging. A Recent inferior myocardial infarction with black area of microvascular obstruction (arrow).
B Old anterior myocardial infarction with large area of subendocardial delayed gadolinium enhancement (white area, arrows).
revascularisation procedures, or in identifying those with myocardial infiltration such as that seen with sarcoid heart disease and right ventricular dysplasia.
This involves passage of a preshaped catheter via a vein or artery into the heart under X-ray guidance, which allows the measurement of pressure and oxygen saturation in the cardiac chambers and great vessels, and the performance of angiograms by injecting contrast media into a chamber or blood vessel. Left heart catheterisation involves accessing the arterial circulation, usually via the radial artery, to allow catheterisation of the aorta, LV and coronary arteries. Coronary angiography is the most widely performed
procedure, in which the left and right coronary arteries are selectively cannulated and imaged, providing information about the extent and severity of coronary stenoses, thrombus and calcification (Fig.). This permits
The left anterior descending and circumflex coronary arteries with a stenosis in the left anterior descending vessel.
A Coronary artery angiogram. B Schematic of the vessels and branches.
planning of percutaneous coronary intervention and coronary artery bypass graft surgery. Left ventriculography can be performed during the procedure to determine the size and function of the LV and to demonstrate mitral regurgitation. Aortography defines the size of the aortic root and thoracic aorta, and can help quantify aortic regurgitation. Left heart catheterisation is a daycase procedure and is relatively safe, with serious complications occurring in fewer than 1 in 1000 cases. Right heart catheterisation is used to assess right heart and pulmonary artery pressures, and to detect intracardiac shunts by measuring oxygen saturations in different chambers. For example, a step up in oxygen saturation from 65% in the RA to 80% in the pulmonary artery is indicative of a large left-to-right shunt that might be due to a ventricular septal defect. Cardiac output can also be measured using thermodilution techniques.
Left atrial pressure can be measured directly by puncturing the interatrial septum from the RA with a special catheter. For most purposes, however, a satisfactory approximation to left atrial pressure can be obtained by ‘wedging’ an end-hole or balloon catheter in a branch of the pulmonary artery. Swan–Ganz balloon catheters are often used to monitor pulmonary ‘wedge’ pressure as a guide to left heart filling pressure in critically ill patients).
Patients with known or suspected arrhythmia are investigated by percutaneous placement of electrode catheters into the heart via the femoral and neck veins. Electrophysiology study (EPS) is most commonly performed to evaluate patients for catheter ablation, normally done during the same procedure. It is occasionally used for risk stratification of patients suspected of being at risk of ventricular arrhythmias.
The availability of gamma-emitting radionuclides with a short half-life has made it possible to study cardiac function non-invasively. Two techniques are available, although their use is declining due to the availability of equivalent or superior imaging techniques that have lower or no exposure to ionising radiation.
Blood pool imaging
The isotope is injected intravenously and mixes with the circulating blood. A gamma camera detects the amount of radiation-emitting blood in the heart at different phases of the cardiac cycle, thereby permitting the calculation of ventricular ejection fractions. It also
allows the assessment of the size and ‘shape’ of the cardiac chambers. Myocardial perfusion imaging
This technique involves obtaining scintiscans of the myocardium at rest and during stress after the administration
of an intravenous radioactive isotope, such as 99technetium tetrofosmin (see Fig. ). More sophisticated quantitative information is obtained with positron emission tomography (PET), which can also be
A myocardial perfusion scan showing reversible anterior myocardial ischaemia. The images are cross-sectional tomograms of the LV. The resting scans (left) show even uptake of the 99technetium-labelled tetrofosmin and look like doughnuts. During stress
(e.g. a dobutamine infusion), there is reduced uptake of technetium, particularly along the anterior wall (arrows), and the scans look like crescents (right).
used to assess myocardial metabolism, but this is only available in a few centres.