X-rays (also called radiographs) of the chest frequently help diagnose heart disease in pets. Finding generalized enlargement of the heart or enlargement of specific heart chambers makes the presence of heart disease more likely. The images may also provide clues as to the specific disease present. For example, fluid in the lungs is a common finding in congestive heart failure. Although chest x-rays are useful in evaluating patients with heart disease, they have certain limitations. The presence of fluid in the lungs does not definitively confirm a disease originating from the heart or exclude another origin, such as pulmonary (lung) disease. Also, assessment of overall heart size and the size of specific heart chambers is typically far less accurate than testing by echocardiography (ultrasonography).
Electrocardiography is the recording of the heart’s electrical activity from the body surface with the use of electrodes. It can be used to identify heart arrhythmias, such as bradycardia (slower than expected rhythm), tachycardia (faster than expected rhythm), or other abnormalities of rhythm (such as sinus arrhythmia or sinus arrest).
Electrocardiography can also detect conduction disturbances, or failures of the electrical signals that cause the heart to contract to pass through the heart tissue. These include first-, second-, and third-degree atrioventricular block.
Finally, electrocardiography can identify chamber enlargement, which is indicated by waveform abnormalities shown on the electrocardiogram recording. Different readings suggest enlargement of the different chambers. While the electrocardiogram may suggest chamber enlargement, chest x-rays and echocardiography (ultrasonography) are more effective.
Echocardiography is a type of ultrasonography used to evaluate the heart, the aorta, and the pulmonary artery. Echocardiography complements other diagnostic procedures by examining and displaying the working heart and moving images of its action. Heart chamber and wall dimensions can be determined; the physical structure and motion of valves can be seen; and pressure differences, blood flow volumes, and several measurements of heart function can be calculated. There are 3 main types of echocardiography: 2-dimensional, m-mode, and Doppler. Two-dimensional echocardiography provides a wedge-shaped, 2-dimensional video of the heart as it moves. M-mode echocardiography is produced by a 1-dimensional beam of ultrasound that penetrates the heart, providing an “ice-pick view.” Differences in tissue density that are detected by the beam are then plotted on a screen. This is typically used to measure chamber size, wall thickness, valve motion, and the size of the aorta and pulmonary artery. Doppler echocardiography detects changing frequencies of the ultrasonic beam as it contacts moving red blood cells to measure the speed of blood flow and thus identify turbulent or high-speed flow. This can locate heart murmurs.
Cardiac catheterization involves the placement of specialized catheters (thin, flexible tubes) into the heart, aorta, or pulmonary artery. Cardiac catheterization may be performed when other diagnostic tests are insufficient to identify specific heart abnormalities or are unable to identify the severity of a lesion. It may also be used for presurgical evaluation, treatment, and in clinical research. Diagnostic and presurgical cardiac catheterization, however, have largely been replaced by echocardiography (see above). Most of the time now, cardiac catheterization is done to surgically repair defects in the heart.