Key investigations for cardiovascular disease are the electrocardiogram (ECG; see Chapter 14), chest X-ray and echocardiogram. Others include exercise ECG testing, ambulatory blood pressure monitoring, lipid profile, cardiac enzyme assays and catheterization with coronary or pulmonary angiography.
The chest X-ray (CXR) is an essential diagnostic tool. The initial CXR is taken in the postero-anterior (PA) direction, with the patient upright and at full inspiration. Figure 33a shows the major structures in which gross abnormalities can be detected, such as enlargement of the heart chambers and major vessels, and a normal PA CXR. Heart size and cardiothoracic ratio (size of heart relative to thoracic cavity) can also be estimated. This ratio is normally <50%, except in neonates, infants and athletes, but may be greatly increased in heart failure (see Chapter 46). Calcification due to tissue damage and necrosis may be detected by CXR if significant (Figure 33c). Enlargement of the main pulmonary arteries coupled with pruning of the peripheral arteries suggests pulmonary hypertension, whereas haziness of the lung fields is indicative of pulmonary venous hypertension and fluid accumulation in the tissues.
Echocardiography and Doppler ultrasound Echocardiography can be used to detect enlarged hearts and abnormal cardiac movement, and to estimate the ejection fraction. An ultrasound pulse of ∼2.5 MHz is generated by a piezoelectric transmitter–receiver on the chest wall, and is reflected back by internal structures. As sound travels through fluid at a known velocity, the time taken between transmission and reception is a measure of distance. This allows a picture of internal structure to be built up. In an M-mode echocardiogram the transmitter remains static, and the trace shows changes in reflections with time. In two-dimensional (2D) echocardiograms the transmitter scans backwards and forwards, so that a 2D picture is built up. Echocardiography is non-invasive and quick. However, when imaging the heart it is restricted by the presence of the rib cage and air in the lungs, which reflect or absorb the ultrasound. This interference can be minimized by using specific locations on the chest. Alternatively, the probe can be placed in the oesophagus (transoesophageal echocardiography, TOE). Although more invasive, this provides greater resolution (Figure 33b) and improved access to pulmonary artery, aorta and atria.
Sound reflected back from a moving target shows a shift in frequency; for example, if the target is moving towards the source, the frequency is increased. This Doppler effect can be used to calculate the velocity of blood movement from the frequency shift in the ultrasound pulse caused by reflection from red cells, and the pressure gradient across obstructions from the Bernoulli equation: P = 4 × (velocity)2. Blood flow can be calculated if the cross-sectional area of the vessel is estimated using echocardiography.
Radiopaque catheters (opaque to X-rays) are introduced into the heart or blood vessels via peripheral veins or arteries. Catheters with small balloons at the tip (Swan–Ganz catheters) assist placement from the venous side as the tip moves with the flow. Placement can be ascertained from the pressure wave-form and X-rays. Catheters are used for measurement of pressures or cardiac output, for angiography, or to take samples for estimating metabolites and PO2. Left atrial pressure cannot be measured directly as it requires access via the mitral valve. Instead, a Swan–Ganz catheter is passed through the right heart, and is wedged in a distal pulmonary artery. As there is thus no flow through that artery, the pressure is the same throughout the capillaries to the pulmonary vein. This pulmonary wedge pressure is an estimate of left atrial pressure.
Angiography A radiopaque contrast medium is introduced into the lumen of cardiac chambers, and coronary (Figure 33d), pulmonary or other blood vessels. This allows direct visualization of the blood and vessels with X-rays, and can be used to examine cardiac pumping function and to locate blockages (e.g. emboli) in the vasculature (Figure 33d).
Advances in medical imaging techniques have provided several powerful diagnostic aids of particular use in cardiac disease.
Nuclear imaging Radiopharmaceuticals introduced into the heart or circulation are detected by a gamma camera, and their distribution (depending on type) can be used to measure or detect cardiac muscle perfusion, damage and function. Three-dimensional information can be obtained in a similar fashion using single photon emission computed tomography (SPECT). The most common tracers used are thallium-201 (201Tl), and technetium-99m (99mTc) labelled sestamibi (a large synthetic molecule of the isonitrile family), which are distributed according to blood flow and taken up by living cardiac muscle cells. These therefore show up brightly immediately after infusion; ischaemic and infarcted areas remain dark because of poor perfusion. Whereas over time 201Tl will redistribute into ischaemic areas as well, 99mTcsestamibi will not, so a delayed 201Tl image will show infarcted areas only. This is useful for determining savable areas of the heart prior to angioplasty or coronary bypass. However, 99mTc has a higher photon energy and shorter half-life, allowing lower radionucleotide doses with better images. It is therefore better for SPECT, and the higher energy allows gated acquisition (sequential images taken during a cardiac cycle), and evaluation of resting left and right ventricular function in combination with either resting or exercise myocardial perfusion.
Magnetic resonance imaging (MRI) Radiofrequency stimulation of hydrogen atoms held in a high magnetic field emits energy, which can be used to generate a high-fidelity image that reflects tissue density, MRI is useful for the location of masses and mal- formations, including aneurysms. It is entirely non-invasive and uses no damaging radiations.