Continuous Ambulatory Electrocardiography
Abnormal heart rhythms and inadequate blood flow to the heart muscle may occur only briefly or unpredictably. To detect such problems, doctors may use continuous ambulatory electrocardiography (ECG), in which the ECG is recorded continuously for 24 hours while the person engages in normal daily activities.
For this procedure, the person wears a small battery-powered device (Holter monitor) held on with a shoulder strap. The monitor detects the heart's electrical activity through electrodes attached to the chest and records the ECG. While wearing the monitor, the person notes in a diary the time and type of any symptoms. Subsequently, the ECG is run through a computer, which analyzes the rate and rhythm of the heart, looks for changes in electrical activity that could indicate inadequate blood flow to the heart muscle, and produces a record of every heartbeat during the 24 hours. Symptoms recorded in the diary can then be correlated with changes in the ECG.
If necessary, the ECG can be transmitted by telephone to a computer at the hospital or doctor's office for an immediate reading as soon as symptoms occur.
An event monitor is used when a person must be monitored longer than 24 hours. It is similar to a Holter monitor, but it records only when the user activates it—that is, when symptoms occur. If symptoms occur so rarely that they cannot be captured during 24-hour monitoring, an event monitor may be placed under the skin for up to a year. A small magnet is used to activate this monitor.
Continuous Ambulatory Blood Pressure Monitoring
If the diagnosis of high blood pressure is in doubt (for example, if the measurements taken in the office vary too much), a 24-hour blood pressure monitor may be used. The monitor is a portable battery-operated device, worn on the hip, connected to a blood pressure cuff, worn on the arm. This monitor repeatedly records blood pressure throughout the day and night over a 24- or 48-hour period. The readings determine not only whether high blood pressure is present but also how severe it is.
Electrophysiologic testing is used to evaluate serious abnormalities in heart rhythm or electrical conduction. Testing is done in the hospital. After injecting a local anesthetic, a doctor inserts a catheter with tiny electrodes at its tip through an incision, usually in the groin, into a vein or sometimes an artery. The catheter is threaded through the major blood vessels into the heart chambers, using fluoroscopy (a continuous x-ray procedure) for guidance. The catheter is used to record the ECG from within the heart and to identify the precise location of the electrical conduction pathways.
Usually, a doctor intentionally provokes an abnormal heart rhythm during testing to find out whether a particular drug can stop the disturbance or whether an operation will help by eliminating abnormal electrical connections within the heart. If necessary, a doctor can quickly restore a normal rhythm with a brief electrical shock to the heart (cardioversion). Although electrophysiologic testing is an invasive procedure and an anesthetic is required, the procedure is very safe: The risk of death is 1 in 5,000. This procedure usually takes 1 to 2 hours.
Tilt Table Testing
Tilt table testing is usually recommended for people who experience fainting (syncope) for an unknown reason and who do not have a structural heart disorder (such as aortic valve stenosis). Typically, people are strapped to a motorized table and remain lying flat for 15 minutes. Then they are tilted head up at a 60° to 80° angle for 45 minutes to see whether they feel faint or their blood pressure and heart rate decrease. If blood pressure does not decrease, the person is given isoproterenol (a drug that stimulates the heart) intravenously in a dose large enough to accelerate the heart rate by 20 beats per minute, and the test is repeated. The procedure often appears to indicate a heart disorder when none is present (a false-positive result).
Anyone thought to have a heart disorder has chest x-rays taken from the front and the side (see Plain X-Rays). The x-rays show the shape and size of the heart and outline blood vessels in the lungs and chest. Abnormal heart shape or size and abnormalities such as calcium deposits within heart tissue are readily seen. Chest x-rays also can detect information about the condition of the lungs, particularly whether blood vessels in the lungs are abnormal and whether there is fluid in or around the lungs.
X-rays can detect enlargement of the heart, which is often due to heart failure or a heart valve disorder. The heart does not enlarge when heart failure results from constrictive pericarditis, in which scar tissue forms throughout the sac that envelops the heart (pericardium).
The appearance of blood vessels in the lungs is often more useful in making a diagnosis than the appearance of the heart itself. For instance, enlargement of the pulmonary arteries (the arteries that carry blood from the heart to the lungs) and narrowing of the arteries within the lung tissue suggest high blood pressure in the pulmonary arteries, which may lead to thickening of the muscle of the right ventricle (the lower heart chamber that pumps blood through the pulmonary arteries to the lungs). X-rays of other parts of the body may be taken to detect blockages in other blood vessels.
Spiral (helical) computed tomography (CT) may be used to detect structural abnormalities of the heart, the sac that envelops the heart (pericardium), major blood vessels, lungs, and supporting structures in the chest. Some very rapid CT scanners (multidetector CT) can take a picture during a single heartbeat. Such rapid CT scanning (called CT coronary angiography) is sometimes used to evaluate the arteries that supply blood to the heart (coronary arteries). Typically, a dye that can be seen on x-rays (radiopaque dye or contrast agent) is injected into the person's vein. The person is asked not to breathe during a scan so that the image will not be blurred.
Electron beam CT, previously called ultrafast or cine CT, is used mainly to detect calcium deposits in the coronary arteries, an early sign of coronary artery disease.
Computed tomography angiography (CTA) is a type of CT that is used to produce three-dimensional images of the major arteries of the body, except the coronary arteries. The images are similar in quality to those produced by conventional angiography (see Computed Tomography). CTA can be used to detect narrowing of the arteries supplying organs and aneurysms and tears in major arteries. CTA can also detect clots that have broken off within a vein, traveled through the venous bloodstream, and lodged in the small arteries of the lungs (pulmonary emboli).
Unlike conventional angiography, CTA is not an invasive procedure. The radiopaque dye is injected into a vein rather than into an artery as in angiography. CTA usually takes less than 1 to 2 minutes.
Magnetic Resonance Imaging
With magnetic resonance imaging (MRI—see Magnetic Resonance Imaging), a powerful magnetic field and radio waves are used to produce detailed images of the heart and chest. This expensive and sophisticated procedure is used predominantly for the diagnosis of complex heart disorders that are present at birth (congenital) and to differentiate between normal and abnormal tissue.
MRI has some disadvantages. It takes longer to produce MRI images than computed tomography (CT) images. Because of the movement of the heart, the images obtained with MRI are fuzzier than those obtained with CT. However, newer MRI scans that are timed to match specific parts of the ECG (called gated MRI) are much clearer than conventional MRI scans.
Magnetic resonance angiography (MRA) is a type of MRI that focuses on blood vessels rather than organs. MRA produces images of blood vessels and blood flow similar in quality to those produced by conventional angiography but is not an invasive procedure (see see Angiography of Peripheral Blood Vessels). MRA can be used to detect bulges (aneurysms) in the aorta, narrowing of the arteries supplying the kidneys (renal stenosis), and a narrowing or blockage in the arteries that supply blood to the heart (coronary arteries) or the arms and legs (peripheral arteries).
In radionuclide imaging (see Radionuclide Scanning), a tiny amount of a radioactive substance (radionuclide), called a tracer, is injected into any vein. The amount of radiation the person receives is tiny—less than that produced by most x-rays. The tracer emits gamma rays, which are detected by a gamma camera. A computer analyzes this information and constructs an image to show the different amounts of tracer taken up by tissues.
Radionuclide imaging of the heart is particularly useful in the diagnosis of chest pain when the cause is unknown. If the coronary arteries are narrowed, radionuclide imaging is used to learn how the narrowing is affecting the heart's blood supply and function. Radionuclide imaging is also used to assess improvement in blood supply to the heart muscle after bypass surgery or similar procedures and may be used to help determine a person's prognosis after a heart attack.
Different tracers are used depending on which disorder is suspected. For evaluating blood flow through heart muscle, the tracers typically used are technetium-99 sestamibi or thallium-201, and images are obtained after the person has an exercise stress test (see see Stress Testing). The amount of tracer absorbed by the heart muscle cells depends on the blood flow. At peak exercise, an area of heart muscle that has an inadequate blood supply (ischemia) absorbs less tracer—and produces a fainter image—than neighboring muscle with a normal supply. In people unable to exercise, an intravenous injection of a drug, such as dipyridamole, dobutamine, or adenosine, may be used to simulate the effects of exercise on blood flow. These drugs divert the blood supply from abnormal to normal blood vessels, depriving the area with inadequate blood flow even further.
After the person rests for a few hours, a second scan is done, and the resulting image is compared with that obtained during exercise. Doctors can then distinguish areas of the heart where inadequate blood flow is reversible (usually caused by narrowing of the coronary arteries) from areas where it is irreversible (usually caused by scarring due to a previous heart attack).
If a heart attack may have occurred very recently, technetium-99m is used instead of thallium-201. With technetium, damage due to a heart attack can be detected after 12 to 24 hours and up to about 1 week. Unlike thallium, which accumulates primarily in normal tissue, technetium accumulates primarily in abnormal tissue. However, because technetium also accumulates in bone, the ribs somewhat obscure the image of the heart.
Radionuclide imaging exposes people to less radiation than comparable x-rays. However, the radioactive material does remain in the person's body for a few days, so people may trigger radiation alarms in airports for a few days after the procedure.
A specialized type of radionuclide imaging called single-photon emission computed tomography (SPECT) can produce a series of computer-enhanced cross-sectional images. A three-dimensional image can also be produced. SPECT provides more information about function, blood flow, and abnormalities than does conventional radionuclide imaging.
Positron Emission Tomography
In positron emission tomography (PET—see Positron Emission Tomography), a substance necessary for heart cell function (such as oxygen or sugar) is labeled with a radioactive substance (radionuclide) that gives off positrons (electrons with a positive charge). The labeled nutrient is injected into a vein and reaches the heart in a few minutes. PET is used to determine how much blood is reaching different parts of the heart muscle and how different parts of the heart muscle process (metabolize) various substances. For example, when labeled sugar is injected, doctors can determine which parts of the heart muscle have an inadequate blood supply because those parts use more sugar than normal.
PET scans produce clearer images than do other radionuclide procedures. However, the procedure is very expensive and not widely available. It is used in research and in cases in which simpler, less expensive procedures are inconclusive.
Pulmonary Artery Catheterization
Pulmonary artery catheterization is sometimes a useful measure of overall heart function in people who are critically ill, particularly when fluids are being given intravenously. Such people include those who have severe heart or lung disorders (such as heart failure, heart attack, abnormal heart rhythms, or pulmonary embolism when these disorders are accompanied by complications), those who have just undergone heart surgery, those who are in shock (see see Shock), and those who have severe burns.
Pulmonary artery catheterization is also done to measure pressure in the right heart chambers and to estimate pressure in the left heart chambers, the amount of blood the heart pumps per minute (cardiac output), resistance to blood flow in the arteries that carry blood from the heart (peripheral resistance), and the volume of blood. This procedure can provide useful information about cardiac tamponade (see see Cardiac Tamponade: The Most Serious Complication of Pericarditis) and pulmonary embolism (see see Pulmonary Embolism).
As in right heart catheterization, a catheter with a balloon at its tip is inserted into a vein, usually in the neck (under the collarbone) or an arm, and is threaded toward the heart. The tip of the catheter may be passed through the superior vena cava or inferior vena cava (the large veins that return blood to the heart from the upper and lower parts of the body) and through the right atrium and right ventricle to the pulmonary artery. The balloon at the catheter's tip is lodged in the pulmonary artery. A chest x-ray is taken or fluoroscopy (a continuous x-ray procedure) may be used to make sure the tip is placed correctly.
The balloon is inflated to temporarily block the pulmonary artery, so that pressure in the capillaries of the lungs (pulmonary capillary wedge pressure) can be measured. This measurement is an indirect way to determine pressure in the left atrium. Blood samples can be taken through the catheter, so that the oxygen and carbon dioxide levels in the blood can be measured.
The procedure may cause many complications, but they are usually rare. They include an air pocket between the layers of membranes covering the lungs (pneumothorax), abnormal heart rhythms (arrhythmias), infection, damage or clotting in the pulmonary artery, and injury to an artery or vein.
Central Venous Catheterization
In central venous catheterization, a catheter is inserted into one of the large veins of the neck, upper chest, or groin. This procedure is most often used to give intravenous fluids or drugs when a catheter cannot be inserted into an arm or a leg vein (peripheral intravenous catheter). Central venous catheterization is occasionally used to monitor central venous pressure (pressure in the superior vena cava, the large vein that returns blood to the heart from the upper part of the body). Central venous pressure reflects the pressure in the right atrium when it is filled with blood. This measurement helps doctors estimate whether the person is dehydrated and how well the heart is functioning. But it has largely been replaced by pulmonary artery catheterization.
Angiography of Peripheral Blood Vessels
Angiography of the peripheral arteries (those of the arms, legs, and trunk—except those supplying the heart) is similar to coronary angiography, except the catheter is threaded to the artery being investigated (see Angiography). Angiography may be done to detect narrowing or blockage of an artery, a bulge (aneurysm) in an artery, or an abnormal channel between an artery and a vein (arteriovenous fistula). Angiography is often done to determine whether angioplasty (see Understanding Percutaneous Coronary Intervention (PCI)) or coronary artery bypass grafting (see Coronary Artery Bypass Grafting) is needed.
Angiography of the aorta (aortography) can be used to detect abnormalities (such as an aneurysm or a dissection) in the aorta. It can also be used to detect leakage of the valve between the left ventricle and the aorta (aortic regurgitation).
Digital subtraction angiography may be done before selective angiography to detect and visualize problems such as narrowing or blockage of an artery. However, this type of angiography is seldom adequate to determine whether surgery (with or without angioplasty) is needed. Digital subtraction angiography is not used for coronary arteries because it is unnecessary. Clear images of these arteries can be obtained when a radiopaque dye is injected directly into a coronary artery.
In digital subtraction angiography, images of arteries are obtained before and after a radiopaque dye is injected, and a computer subtracts one image from the other. Images of tissues other than the arteries (such as bones) are thus eliminated. As a result, the arteries can be seen more clearly, much less dye is required, and the procedure may be safer than standard angiography.
Last full review/revision February 2013 by Michael J. Shea, MD