Cardiovascular Investigations
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).
Imaging
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.
