Invasive Tests

The heart is a pump that drives blood into the two great vessels and maintains adequate pressure to ensure proper blood supply throughout the body. The relationship of flow and pressure in the two circulations and their respective changes are called hemodynamics . At one time hemodynamic information could only be inferred from certain observations or extrapolated from animal measurements. A new era in cardiology arrived when hemodynamic measurements became possible in humans by means of cardiac catheterization . The technique

of introducing a catheter, or thin tube, into the heart was demonstrated in Germany in 1929, but cardiac catheterization was first put to practical use in 1941 by the French-American physiologist André Cournand, who won the Nobel Prize in 1956. Cardiac catheterization remains the basic technique of many invasive procedures for the diagnosis and treatment of heart disease.

The term "cardiac catheterization" in its early days was used to mean a specific, rather limited test consisting of the introduction of a catheter into the circulation, either through a vein into the right side of the heart and the pulmonary artery or through an artery into the aorta and the left side of the heart. The catheter permitted collecting blood samples from various parts of the circulation and measuring pressures in the heart and arteries. The earliest application of cardiac catheterization was in the diagnosis of some congenital heart diseases by showing abnormal communication between the two sides of the heart and abnormal pressures in the right side of the circulation. The original method involved making an incision in the skin and directly introducing the catheter into a vein under the skin. A simplified technique, called the percutaneous method, was first introduced in Sweden. It consists of puncturing a vein with a needle through the skin and introducing a wire through the needle. The needle is then withdrawn, and the catheter is slid into the vessel over the wire. This technique is particularly valuable in catheterizing an artery, a procedure that otherwise would require minor surgery (fig. 17). Percutaneous introduction of catheters into the blood vessels is now the basis for almost all invasive procedures, not only for diagnostic purposes but also for a variety of treatments of cardiovascular conditions. The most important application of this technique is in contrast cardiovascular radiology, particularly angiocardiography.

Cardiac catheterization as a general term now applies to a combined invasive diagnostic study involving several components. The goals of cardiac catheterization as an isolated procedure include

gathering basic hemodynamic data (cardiac output, intracardiac pressures) to evaluate the function of the two ventricles with the patient at rest and during mild exercise (pedaling a stationary bicycle during the procedure)

diagnosing pulmonary hypertension

determining pressure gradients between chambers (see chap. 7), which may indicate the severity of stenosis of cardiac valves

detecting abnormal communications between the two sides of the heart

Cardiac catheterization is almost always combined with angiocardiography, considered indispensable in reaching a diagnosis.

A special application of cardiac catheterization technique is hemodynamic monitoring of seriously ill patients in intensive-care or coronary-care units. This involves introducing into a vein a small catheter with an inflatable balloon near its tip. The balloon is then inflated, and the catheter is carried by the flow of venous blood into the right atrium, the right ventricle, and the pulmonary artery. This procedure does not require a cardiac laboratory or fluoroscopic facilities and can be performed in the intensive-care unit of a

Figure 17. (a) The pathway of venous (right-sided) catheterization of the heart.
The catheter enters the body via the femoral vein in the groin and is
conducted through the inferior vena cava into the right side of the heart and
hence into the pulmonary artery. (b) The pathway of arterial (retrograde)
catheterization of the heart. The catheter is introduced into a femoral artery in
the groin and is advanced (against the flow of blood) into the aorta and through the
aortic valve into the left ventricle. The catheter can also be redirected,
above the aortic valve, to enter the origin of each coronary artery in order to
perform a coronary arteriogram.

hospital. The catheter can be left in place for several days, permitting continuous display of pressures inside the heart or pulmonary artery and intermittent determination of cardiac output.

Angiocardiography , or contrast radiography of the heart and blood vessels, is the most frequently performed study of the circulatory system and is used for examining other parts of the body as well (brain, kidney, lower extremities). It consists of introducing into the blood a liquid containing a radiopaque substance that shows up when photographed. Most tests require that the X-ray image be recorded on 35-mm film to appreciate the motion, although some tests require only still photographs. The contrast material used to be injected into the bloodstream through a vein, but it was found that the material became too dilute by the time it reached the parts of the circulation to be investigated. The standard technique now is to introduce the contrast material directly at the point of investigation by means of a catheter (selective angiocardiography ).

Coronary arteriography is the most frequently performed invasive cardiac test. Special catheters are introduced into the large artery in the groin and guided into the aorta. The performed catheter tips are conducted to the opening of each coronary artery as it leaves the aorta. A small amount of contrast material is injected directly into each, and the distribution of the opacified blood is recorded on 35-mm X-ray film. An alternate route for introducing the catheter into the arterial system is through the brachial artery (at the inside of the elbow).

Left ventriculography is the standard procedure for evaluating contractions of the left ventricle as well as calculating the ejection fraction (fig. 18) (see the discussion of nuclear ventriculography above, pp. 39–40). In patients undergoing coronary arteriography, a ventriculography is usually performed at the same time, since the data obtained by this method are more reliable than those obtained by the noninvasive technique.

Angiocardiography plays an important role in detecting congenital heart disease. In conjunction with cardiac catheterization, it allows the physician to diagnose some of the most complex lesions. It is also essential in evaluating disease of the cardiac valves by displaying valvular incompetence.

A refinement of angiographic technique is digital subtraction angiography which enhances the faint image of opacified blood

Figure 18. Two frames from a film showing the left
ventriculogram of a healthy person, performed by
injecting a contrast dye through a cardiac catheter into
the left ventricle. The upper frame (outline retouched)
shows the volume of the left ventricle at the end of
systole (lowest volume), the lower frame at the end of
diastole (highest volume). The difference between the two
volumes represents the amount of blood ejected into the
aorta by the recorded heartbeat. The volume of ejected
blood (stroke volume) divided by the highest volume in
diastole represents the percentage of blood pumped by a
heartbeat (the ejection fraction, in this example 8₄
percent), an important index of cardiac performance.

when the contrast substance is too dilute to be seen under ordinary radiography. This technique permits visualization of various parts of the vascular system after the contrast material is injected into a peripheral vein. This type of angiocardiography thus becomes a noninvasive procedure (intravenous injections are considered noninvasive). It provides clear pictures of the aorta and great vessels as well as somewhat limited images of the heart. Detailed viewing, however, still requires selective angiocardiography, particularly in visualizing the coronary arteries.

Another invasive diagnostic procedure indicated in special conditions is the electrophysiological study . Wires are introduced into various areas of the heart and record electrical potential directly from the inside of the heart. Such studies may also employ stimulation of the heart by an electronic pacemaker to observe responses of the heart and induce certain arrhythmias that reproduce those experienced previously by the patient.