Appendix B: Investigations in cardiac surgery
ECG
A resting EGG may provide valuable information but
a cardiological textbook should be consulted for EGG
changes associated with all of the conditions discussed in the
text.
Chest radiography
Many patients with ischaemic heart disease are ex-smokers so pulmonary lesions may be noted in addition to the aortic and cardiac dimensions, although the chest radiograph is usually normal. Chamber hypertrophy, valve calcification and abnormalities of the great vessels may point to congenital or valvular heart disease but the chest radiograph may again be normal (Fig. 48.60).
Exercise ECG
To obtain objective evidence of myocardial ischaemia, an exercise EGG may be performed in patients with suspected ischaemic heart disease. Under carefully controlled conditions with full resuscitation equipment available, the patient is asked to perform a graded series of treadmill exercises. The work rate is increased every 2 minutes and continuous EGG recording is performed. The test is terminated when the patient complains of pain or dyspnoea, or there are significant ST segment changes or arrhythmia on the EGG. This test is potentially dangerous in patients with valvular lesions and other tests (e.g. isotope scanning) may be required to demonstrate ischaemic myocardium in these patients.
Isotope scanning
The radioisotope thallium is taken up by perfused myocardium and therefore ischaemic myocardium is shown up as a cold spot'. A myocardial uptake gated acquisition (MUGA) scan uses labelled erythrocytes to demonstrate left ventricular function and the ejection fraction (Fig. 48.61).
Coronary angiography (Fig. 48.62)Cardiac catheterisation
Cardiac catheterisation involves the introduction of a balloon flotation catheter (Swann-Ganz) into the right atrium, across the tricuspid valve and into the pulmonary artery. The pressure changes as the catheter passes through the cardiac chambers are characteristic. An estimate of the left atrial pressure can be obtained by wedging the balloon in a peripheral pulmonary artery. The cardiac output and other parameters can be calculated using a thermodilution technique.
Echocardiography
This is an extremely valuable examination because it can demonstrate valve thickness, calcification and motion, in addition to ventricular wall thickness, motion and chamber dilatation. Ultrasonic waves are transmitted through the heart until they reach an interface. They are reflected back to a probe and an image of the cardiac chambers, wall movement and valve motion can be made. The gradient across the stenotic valve can be estimated by a Doppler probe and regurgitation may also be demonstrated. Coronary angiography is used in patients suspected of having coexistent ischaemic heart disease and the actual gradient across the aortic valve may be shown during this procedure.
Other investigations
CT and MRI are not widely used for myocardial investigations but are useful in visualising the great vessels (aneurysms and dissection) and for demonstrating mediastinal and thoracic pathology.