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A simple test commonly will yield invaluable information about complex disorders. Specific blood tests may yield useful information regarding the status of the cardiovascular system. The information may be nonspecific or insensitive but nonetheless may be useful in helping to confirm or exclude a critical diagnosis. The common blood tests that assist in cardiovascular diagnosis are the cardiac enzyme profile, blood lipid profile, serum electrolytes, arterial blood gases, complete blood count (CBC), erythrocyte sedimentation rate (ESR), and coagulation studies. These will be discussed individually in the following pages.
The cardiac enzymes are a group of intracellular enzymes (cellular proteins that cause or expedite biochemical reactions) that are normally present in blood in fairly constant amounts. They are released into the blood in greater amounts when myocardial injury occurs. Increased levels of these enzymes are used to confirm the clinical or electrocardiographic diagnosis of acute myocardial infarction (AMI). The cardiac enzyme panel is made up of three enzymes: (1) creatine kinase and its three isoenzyme fractions; (2) lactic de-hydrogenase and its isoenzyme fractions; and (3) serum glutamic oxalacetic transaminase. The cardiac enzyme profile is drawn on admission and every 8 hours for the next 24 hours and then daily for 2 days in patients suspected of having sustained AMI.
Creatine kinase (CK or CPK) is normally found in the brain, heart, and skeletal muscle. Isoenzymes or subtractions of CK are present and are specific for the source tissue. The brain contains mainly CK type 1 (CK-BB); the heart muscle contains both CK type 2 (CK-MB) and type 3 (CK-MM); and the skeletal muscle contains mainly CK-MM and trace amounts of CK-MB (<3%). With damage to the heart, total CK and the myocardial specific fraction CK-MB are released in amounts roughly proportional to the amount of heart tissue that is damaged. The release occurs soon after AMI, peaks within 36 hours, and returns to normal within 2-4 days. If the patient presents in the first hours of AMI, total CK and CK-MB may be within normal limits. Total CK will also be elevated with skeletal muscle damage, in which case, CK-MB will allow the differentiation between skeletal and heart muscle damage. The absolute normal values of total CK and CK-MB vary with the measurement technique used, so one must always know the reference normal values in order to interpret a given value of total CK or CK-MB.
Lactic dehydrogenase (LDH) is found in many body tissues and, like CK, has isoenzymes that are specific for various tissues. The LDH isoenzyme that is found in highest concentration in heart muscle is LDH 1. It is normally detected in the blood but in lesser amounts that LDH 2. With AMI, total LDH is elevated and LDH 1 is found in greater amounts than LDH 2. The rise of LDH is slower than that of CK, starting 12-18 hours after AMI, peaking at about 72 hours, and remaining elevated for 7-10 days. LDH and its isoenzymes are most useful in patients who present 2-3 days after AMI, at which time total CK and CK-MB may have returned to normal levels. The range of normal values for LDH also varies with the method by which it is determined, and the reference normal range is needed to interpret a given value.
Serum glutamic oxalacetic transferase (SCOT) is also present in many tissues, and elevations of it are the least specific for myocardial injury. Elevations of SCOT occur with injury to the heart, liver, skeletal muscle, lysis of red blood cells, and injury to other tissues. After AMI, SCOT elevations are detected in 8-12 hours, peak in 18-24 hours, and return to normal in 4-5 days. Because of the lack of specificity of SCOT and the availability of CK and LDH, SCOT is infrequently used as a part of the routine evaluation of patients suspected of AMI.
Extension of AMI or reinfarction will cause a secondary rise of all of the above enzymes, and such a secondary rise is diagnostic of an extension.
Blood lipids, consisting of cholesterol, phospholipids, and triglycerides, circulate bound to plasma proteins and are called lipoproteins. The lipoproteins can be separated and classified by their electrophoretic mobility (movement when placed in an electrical field). They are classified by density and fall into three categories: (1) high density lipoproteins (HDL), which are mainly cholesterol and phospholipid and are responsible for the transport of cholesterol from the tissues to the liver for metabolism; (2) low density lipoproteins (LDL), which contain nearly equal amounts of cholesterol and triglycerides and are responsible for the deposition of cholesterol in the tissues; and (3) very low density lipoproteins (VLDL), which are primarily triglyceride.
The level of HDL is inversely related to the risk of developing coronary artery disease (CAD). High HDL levels are associated with the female sex, exercise, and moderate alcohol consumption. The data for the association with alcohol consumption is conflicting. Overall mortality is increased in those who drink. Low levels of HDL and increased risk for CAD are associated with the male sex, smoking, sedentary life-style, poor control of diabetes, and progesterone containing oral contraceptives.
The level of LDL is directly related to the risk of developing CAD. Low density lipoproteins have been implicated in the genesis of atheroma, and high levels are associated with increased and premature atherosclerosis.
Blood lipid profiles are, therefore, helpful prognostically in assessing a given patient's risk of developing CAD. Acute stress situations, such as AMI, acutely alter the lipid profile and invalidate its use as a prognostic tool. A lipid profile should not be used for risk stratification until 4-6 weeks after AMI and should be drawn after a 12-hour fast.
The established population norms for serum cholesterol increase with age. However, a total cholesterol greater than 250 mg/dl, at any age, is abnormal and is associated with increased risk of CAD. It has been established that reducing a patient's total cholesterol will decrease that patient's cardiovascular risk.
Serum triglycerides are elevated in association with many of the disease processes that increase the risk of CAD but do not appear to be independent risk factors in and of themselves. Elevations of triglycerides are associated with noncardiac diseases that may require therapy (e.g., diabetes, alcoholism); therefore, the level of triglycerides should be measured to fully characterize a patient's lipid status.
Abnormalities of serum electrolytes may be caused by many factors, including medications, congestive heart failure, renal insufficiency, diabetes, dehydration, and AMI. Electrolyte imbalances can cause dysrhythmias (low serum potassium), altered mental status (low serum sodium), enhancement of drug toxicity, weakness, renal insufficiency, and death. It is, therefore, essential to carefully monitor serum electrolytes in patients with acute and chronic cardiovascular disorders both on and off medications. Unfortunately, the serum level may not reflect the true intracellular stores of a given electrolyte. The serum level is maintained at near-normal levels at the expense of the intracellular stores. Serious complications
may be avoided if the serum level is maintained in the mid-normal range. The most important levels to measure are sodium (Na+), potassium (K+), chloride (C1-), magnesium (Mg++), calcium (Ca+), and phosphorus (P-). The normal values are given in Table 16.1.
| Table 16.1 Serum Electrolytes: Normal Values | |||
| Na+ | 136 - 142 | mEq/L | |
| K+ | 3.5 - 5.0 | mEq/L | |
| Mg++ | 1.5 - 2.5 | mEq/L | |
|
or |
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| 1.8 - 3.0 | mg/dl | ||
| Ca++ | 4.5 - 5.3 | mEq/L | |
|
or |
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| 9.0 - 10.6 | mg/dl | ||
| P- | 1.8 - 2.6 | mEq/L | |
|
or |
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| 2.5 - 4.8 | mg/dl | ||
| Cl- | 95 - 105 | mEq/L | |
Arterial blood gases (ABGs) are frequently used to manage critically ill patients with circulatory, pulmonary, or metabolic disorders; to establish the adequacy of oxygenation or ventilation; and to assess the acid-base balance. Peripheral or central venous blood gas analysis yields limited information regarding cardiac or pulmonary function, whereas arterial blood gas analysis is quite informative. The arterial partial pressure of oxygen (p02) is a measure of the efficacy of oxygenation. The partial pressure of carbon dioxide (pO2) is a measure of the efficacy of ventilation or gas exchange in the lungs. The arterial pH is a measure of the hydrogen ion content of the blood and reflects acid-base balance. Arterial pH is decreased in academia and increased in alkalemia. PCO2and pH are interrelated, and an acute change in one will result in a change in the other. The percent saturation (% Sat) is a measurement of the saturation of the hemoglobin molecules and reflects the availability of the oxygen to the tissues. The normal values for ABG (at sea level) are given in Table 16.2. Altitude affects the values of the p02 and percent saturation and must be taken into consideration when evaluating ABG results.
| Table 16.2 ABGs (at Sea Level): Normal Values | ||
| pO2 | 90 - 100 | mm/Hg |
| pCO2 | 35 - 45 | mm/Hg |
| pH | 7.35 - 7.45 | (arterial) |
| 7.31 - 7.41 | (venous) | |
| % Sat | 90% - 100% | |
The complete blood count (CBC) measures the number and morphology of the circulating red blood cells (RBCs) and white blood cells (WBCs). It will detect anemia (decreased number of RBCs), polycythemia (increased number of RBCs), leukocytopenia (decreased number of WBCs), leukocytosis (increased number of WBCs), and alterations of the morphology of either red or white blood cells. Marked anemia may cause tissue hypoxia and angina in patients with CAD by decreasing the number of RBCs that are available to carry oxygen to the tissues. Polycythemia may be due to chronic hypoxia, cyanotic heart disease, or chronic pulmonary disease. Leukocytosis accompanied by an increase in immature WBCs indicates the presence of systemic infection, which should be diagnosed and treated because it will increase the body's oxygen requirements and will put an additional strain on the heart. In a patient with CAD, the additional strain may be sufficient to cause cardiac ischemia, angina, or AMI. Leukocytosis without immature WBCs is frequently seen with AMI and represents a response to stress; it must be differentiated from leukocytosis due to an infection. The white blood cell morphology is determined on the peripheral blood smear. The peripheral smear may also yield important information in patients with prosthetic heart valves. Patients with prosthetic valve dysfunction will frequently demonstrate signs of red cell destruction (hemolysis). Normal values for the CBC are given in Table 16.3.
| Table 16.3 CBC: Normal Values | ||
| Hemoglobin (Hgb) | 12 - 16 gm/100 ml | (Female) |
| 14 - 18 gm/100 ml | (Male) | |
| Hematocrit (Hct) | 37% - 47% | (Female) |
| 42% - 52% | (Male) | |
| WBC | 4300 - 10,000/mm3 | |
The erythrocyte sedimentation rate (ESR or Sed rate) is a measure of how fast RBCs settle when left to stand in a heparinized tube and reflects the levels of various plasma proteins. Inflammation causes an increase in these proteins and, therefore, in the ESR. Examples of conditions that cause an increase in the ESR are AMI, bacterial endocarditis (SBE), Dressler's syndrome (post-myocardial infarction syndrome), and pericarditis. The ESR is reduced in congestive heart failure. Normal values are given in Table 16.4.
| Table 16.4 Erythrocyte Sedimentation Rate (Westergrin Method): Normal Values | ||
| Males under age 50 | <15 mm/hr | |
| Males over age 50 | <20 mm/hr | |
| Females under age 50 | <20 mm/hr | |
| Females over age 50 | <30 mm/hr | |
Cardiovascular disease usually does not cause abnormalities of the blood coagulation system by itself.
However, oral and systemic anticoagulants are frequently used in the treatment of cardiovascular disorders, and their efficacy is measured by the alterations that they cause in the blood coagulation profile.
Oral anticoagulants of the warfarin type cause a prolongation of the prothrombin time (PT or Pro time). They are vitamin K analogs that inhibit the production of clotting factors, resulting in a decrease in the ability of the body to form a fibrin clot. This type of anticoagulant is used in patients with artificial heart valves to reduce the inherent thromboembolic risk. Many medications alter the effect of a given dose of warfarin, so the PT must be checked regularly and conflicting medications should be altered.
The effect of heparin anticoagulation is measured by its effect on the partial thromboplastin time (PTT). Normal values and the range for therapeutic anti-coagulation are given in Table 16.5.
| Table 16.5 Coagulation Profile: Normal Values and Range | ||
| Normal | Therapeutic Anticoagulation | |
| Pro Time | 12 - 14 seconds | 22 - 26 seconds |
| PTT | 30 - 45 seconds | 60 - 90 seconds |
The volume of daily urine flow, in relation to fluid intake, is essential in the assessment of the cardiovascular system. The intake and output (I&O), if carefully recorded, is one of the best methods of assessing a patient's ongoing fluid balance. It is helpful in patients with congestive heart failure, renal failure or insufficiency, in shock states, and in the periopera-tive period. Together with daily weights, the I&O is the best way to evaluate the response to and the need for diuretic therapy.
Evaluation of the urine sediment and chemical composition is also clinically useful. The presence of RBCs in the urine is always abnormal and may indicate infection, bacterial endocarditis, renal disease, or trauma. The presence of WBCs is also always abnormal and denotes an inflammatory process some-Table 16.5. Coagulation Profile: Normal Values and Range where in the urinary tract. Protein in the urine (pro-teinuria) is frequently detected in patients with congestive heart failure but may also be seen in a number of other noncardiac conditions. The measurement of spot (single void) urinary sodium can help differentiate between renal and cardiovascular causes of decreased urine output.
The standard 12-lead electrocardiogram (scalar ECG) is discussed elsewhere and is a simple and extremely useful diagnostic tool (see Chapter 5).
The exercise test (treadmill, exercise tolerance test, ETT stress test) is a graded exercise tolerance test designed to evaluate a person's ability to exercise in a monitored and standardized fashion. It is usually performed on a treadmill or stationary bike. The ETT is used to evaluate patients with chest pain, to assess aerobic work capacity and overall cardiovascular fitness, and to evaluate the prognosis of a patient with a recent myocardial infarction. The patient is monitored electrocardiographically during exercise and recovery in order to detect ECG changes suggesting ischemia or the presence of cardiac dysrhythmias. The blood pressure is taken every few minutes, and the heart rate is monitored constantly. The ETT is stopped when the patient reaches a predetermined heart rate, the blood pressure falls, severe chest pain occurs, signs of marked myocardial ischemia are noted, serious dysrhythmias occur, or the patient can go no further.
The vector cardiogram (VCG) is a specialized ECG that records the instantaneous sum of the electrical forces generated by the heart in three dimensions and displays them according to direction and magnitude. The standard ECG displays the electrical forces in only one plane at a time. The VCG is more sensitive than the scalar ECG and can differentiate variations of normal from truly abnormal ECGs. It takes a great deal more experience to read and interpret the VCG than the scalar ECG.
The ambulatory ECG, frequently called a Holter Monitor record, is a recording of a patient's ECG continuously for 12-24 hours. Electrodes are taped to the patient's chest and connected to a recorder that is carried on a belt or shoulder strap. The ECG is recorded on magnetic tape, which is later scanned for abnormal heart rhythms. The time of any abnormalities is noted and can be compared with a diary in which the patient notes any symptoms. The ambulatory ECG can be used to evaluate patients with symptoms of palpitations, dizziness, or syncope and to evaluate the need for and efficacy of antiarrhythmic therapy. In several recent research studies, it has been used to detect periods of asymptomatic or silent ischemia.
The chest x-ray yields information about the size, position, and contour of the heart and pulmonary vessels. Pulmonary venous congestion and cardiac enlargement suggest congestive heart failure. Pulmonary infiltrates, rib fractures, or chest masses, which may cause chest pain, dyspnea, or hemoptysis, can also be identified. It should be remembered that the chest x-ray may be totally normal in the early hours of acute myocardial infarction, despite the presence of mild congestive heart failure.
Cardiac fluroscopy, or real-time viewing of the heart and great vessels with x-ray, is used to screen young (<55-year-old) patients for coronary or valvular calcification and to evaluate radiopaque prosthetic heart valves. In these young individuals, the presence of coronary or valvular calcification suggests the presence of significant stenosis of the coronaries or heart valve.
Ultrasound is high frequency sound that is above the audible range. The sound waves are directed into the body by a transducer, which also acts as a receiver. The sound waves are reflected by the underlying tissues, and returning sound waves or echoes are received by the transducer and can be processed in several ways to yield images of the heart and other organs. The technique is totally noninvasive and does not harm the patient or alter the process being studied. The several types of cardiac ultrasound or echo-cardiography are discussed in the following section.
A-mode scan (amplitude mode) is rarely used alone for cardiac imaging. The returning echoes are displayed as a function of depth and echo intensity. This one-dimensional display gives information on the depth and linear dimension of a structure.
B-mode scan (brightness mode) is used for abdominal or pelvic scanning and yields a static or still frame two-dimensional image of the underlying structures.
M-mode scan (motion mode) is used exclusively for cardiac imaging and is recorded on moving paper or video tape. A narrow beam of ultrasound is aimed at the heart, and the size and motion of the small portion of the underlying heart can be recorded over time. Valve motion, chamber wall motion, and chamber sizes directly under the transducer can easily be recorded and measured. M-mode is quite sensitive in detecting pericardial effusion. Because of its very narrow field of view, M-mode echo does not give information regarding the lateral size or spatial orientation of structures.
Two-dimensional, or real-time imaging, is used to image the heart (2-D echo) or fetus in utero. It is a B-scan that is repeated between 30-60 times/second, yielding an image similar to a TV image. It provides excellent spatial orientation with a 60-90 degree field of view. Two-dimensional echo allows the visualization of ventricular wall and valve motion, cardiac chamber size and shape, identification of pericardial effusion, intracardiac masses, or valvular vegetations.
The Doppler effect is the change in the frequency of a sound due to motion between the object emitting the sound and the person or device receiving the sound. An example is the change in pitch of a train whistle as the train passes an observer. The pitch is higher as the train approaches and lower as the train goes away. The change in the pitch is proportional to the difference in relative direction and speed between the two objects. This effect is used to determine the direction and speed (velocity) of blood flowing through the heart and blood vessels. It can detect valvular regurgitation and stenosis, estimate pressure gradients across heart valves, and measure cardiac output. Color Doppler provides a two-dimensional color display of Doppler velocities and looks like a noninvasive color angiogram. It gives spectral orientation to Doppler flow images.
Phonocardiography (Phono) is a technique that records the heart sounds by means of microphones applied to the chest wall. It is used to time and analyze the sounds of the cardiac cycle and to make a permanent record of the sounds made by prosthetic heart valves for later comparison. A simultaneous ECG is recorded for timing purposes.
The skin pulsations produced by the carotid pulse and jugular venous waves can be recorded by pressure sensitive transducers placed over the maximal pulsations. The carotid pulse waveform provides information about left ventricular ejection and aids in the diagnosis of aortic stenosis and in the assessment of left ventricular function. The jugular venous pulse provides information about right ventricular filling, cardiac rhythm, pericardial disease, and tricuspid re-gurgitation.
The apical impulse of the heart can be palpated and recorded on the precordium. The pattern of motion and the displacement are used to time cardiac events and assess apical wall motion. The position and motion can help diagnose cardiac enlargement and left ventricular hypertrophy.
Nuclear cardiac imaging is a rapidly developing diagnostic discipline in which an intravenous bolus of a radiopharmaceutical is combined with scintigraphic imaging. The most commonly performed diagnostic tests are thallium perfusion myocardial scintigraphy, with and without exercise; radionuclide cineventricu-lography (MUGA or first-pass scanning); and techne-tium myocardial infarct scanning. These tests are processed and analyzed with highly sophisticated computer techniques.
Thallium 201 (TI201) is a potassium analog taken up by normally perfused and functioning myocardial cells. TI201 is most commonly used in conjunction with an exercise test (treadmill). The patient is exercised to just below peak exercise capacity, the TI201 is injected intravenously, and the patient is exercised for an additional minute. After allowing the patient to recover for 5 minutes, cardiac imaging is done in three or four views. The same views are repeated 3-4 hours later and compared. TI201 accumulates in the myocardium in direct proportion to the blood flow reaching each region of the heart and the functional integrity of the myocardial tissue. If there is an area of the heart that is underperfused (myocardial ischemia), that area will take up less TI201 and will appear as a cold spot on the scan. With rest, the ischemia resolves, and the TI201 will be taken up in that area (redistribution) and will appear similar to the rest of the heart. An area of dead tissue (scar) will not be able to take up the TI201 and will not change with rest (irreversible defect); therefore, it is a good noninvasive technique for diagnosing the presence of coronary artery disease. It is particularly useful when the resting ECG is abnormal because the stress ECG will be uninterpretable.
Radionuclide cineventriculography (first-pass or MUGA scanning) is done after intravenous injection of the patient's own red blood cells, to which technetium radioisotope has been attached. The material stays in the bloodstream and can be followed as it passes through the heart and blood vessels.
The first-pass technique follows the bolus of tagged material on its first transit through the heart. It is useful in detecting intracardiac shunting and assessing right and left ventricular function. It requires a repeat injection of tagged material for each measurement and is therefore limited by the total number of radioactivity that may be safely given and by a buildup of background radioactivity in the bloodstream, which decreases its accuracy.
Equilibration or MUGA scanning is done after the bolus has distributed throughout the entire blood pool. A computer is gated to the ECG and the cardiac cycle (the interval between R waves on the ECG) is divided up into multiple segments. Images are created during each segment and are repeated each cardiac cycle. All the images collected during each segment are added together to form a composite image for that segment. The segments can then be played, in sequence, as a continuous movie. This technique is used to evaluate left ventricular wall motion and global function. It major advantage over the first pass technique is that the scanning can be repeated as often as needed over a few hours without requiring additional isotope. It can also be combined with exercise to assess the heart's functional and wall motion response to exercise.
Technetium 99m pyrophosphate (T99m pyp) binds to areas of high calcium content. Calcium accumulates in an area of muscle damage and will be detected as an accumulation of the isotope or a hot spot. Acute injury to the heart or AMI can be detected by this technique. The scan should be done at least 12-18 hours after a suspected heart attack but before 7-10 days. T99m pyp is injected intravenously, and the patient's heart area is scanned 3 hours later in three views. Because bone contains calcium and therefore takes up T99m pyp, much experience is required in reading these scans. The sensitivity of the test is reduced with very small infarcts and after 6-7 days. T99m pyp scanning is very useful in making the diagnosis of recent MI if the patient presents with equivocal ECG findings and enzyme changes outside of the time frame during which the enzymes would be positive. It is also useful in patients with baseline ECG abnormalities that preclude the electrocardiographic diagnosis of acute MI (left bundle branch block or reinfarction in the area of prior extensive MI).
Cardiac catheterization of the right or left heart is the standard method used to evaluate cardiac anatomy and functional status. Right heart catheterization is accomplished by the insertion of a catheter, percutaneously or via a cutdown, into a vein and advancement of the catheter into the central venous circulation, the right heart chambers, and the pulmonary artery. This can be done with a simple flexible catheter (for central venous pressure measurement or fluid infusion) or with a variety of specialized catheters. The most common approach is with the triple lumen balloon flotation catheter. This catheter is most commonly placed percutaneously via the internal jugular or subclavian venous system and is advanced by watching the characteristic pressure waveforms obtained from the tip of the catheter as it is passed progressively through the veins, right atrium, right ventricle, and pulmonary artery. These pressures are recorded and give information regarding the state of hydration (central venous pressure) and the function and compliance of the right and left ventricles. The presence of intracardiac shunting can be assessed by determining the amount of oxygen contained in the blood withdrawn from the cardiac chambers. Cardiac output can be measured by the thermodilution technique, which is a variation of the indicator dilution technique. Room temperature or iced saline is injected into the right atrium where it mixes with and lowers the temperature of the blood. As the blood flows by, this temperature drop is measured by a temperature sensing device placed near the tip of the catheter in the pulmonary artery, and cardiac output is calculated. Radiopaque dye (contrast) can be injected through angiographic catheters into the right atrium, right ventricle, or pulmonary artery to outline the anatomy of the right heart chambers and pulmonary vasculature.
Left heart catheterization is performed by one of two techniques: the Sones' technique or the Judkins' technique. The Sones' technique introduces the catheter via a cutdown on the brachial artery in the anti-cubital fossa, and the Judkins' technique introduces the catheters via the percutaneous approach to the femoral artery in the groin. After the catheters are passed retrograde up the brachial or femoral artery under fluoroscopic guidance, pressures are measured in the aorta and left ventricle. Gradients across the aortic valve can be measured, and the filling, function, and compliance of the left ventricle can be assessed. Dye can be injected into the left ventricle (ventriculogram), the aortic root (aortogram) and selectively into the individual coronary arteries (coronary angiography). Such injections outline the anatomy of the left heart chambers, aortic root, and coronary arteries. Left ventricular function can be assessed by calculation of the ejection fraction (percent of the blood ejected from the left ventricle with each heart beat).
In the presence of severe aortic stenosis, it may be impossible to advance the catheter across the aortic valve. Because it is critical that the pressure gradient across the aortic valve be measured, the transsep-tal approach may be used. A long needle is placed via the femoral vein with its tip positioned in the foramen ovale of the interatrial septum. The interatrial septum is then punctured, and a catheter is advanced over the needle into the left atrium and through the mitral valve into the left ventricle. The gradient can then be measured between the catheter in the left ventricle and the one in the aortic root.
Left and right heart catheterization are essential in the preoperative evaluation of congenital and acquired heart disease and in the assessment of the extent and distribution of coronary artery disease. The risk of left heart catheterization is small but significantly greater than that of right heart catheterization. The risk of transseptal catheterization is still greater but well worth the risk if standard left heart catheterization is unsuccessful.
The study of the electrical system of the heart and the evaluation of disorders of cardiac rhythm (dys-rhythmia) is called electrophysiologic testing (EP studies) and is a reasonably new diagnostic technique when used in clinical practice. EP studies are used to evaluate the etiology of syncope and palpitations, the presence of bypass tracts (abnormal electrical connections between the heart chambers), and the therapy of complex supraventricular and ventricular dys-rhythmias. Several catheters are placed in the right heart via the venous system and in the left heart via the arterial system. The heart is then stimulated, and its electrical activity is recorded from multiple intracardiac sites. The sequence of electrical activation allows the localization of abnormalities of the conducting system. Trials of drug therapy that suppress the induction of complex dysrhythmias have a higher likelihood of being successful than those that are empirically determined (see Chapter 14, Antiarrhy-thmics).
The anatomy of the arteries can be studied by a combination of Doppler and echocardiography. The Doppler echocardiogram yields a real-time image of the blood vessel and a display representing blood flow. The Doppler technique may be used to gather data about occlusive disease of the arteries and veins. The moving red blood cells will change the frequency of an ultrasound beam. Arterial and venous blood flow create different sounds because of different flow velocities. Characteristic flow changes occur with vascular obstructions, which can be identified by Doppler. Blood flow velocities increase in the region of stenosis (narrowing). This and other characteristic changes can be used to semiquantify the degree of stenosis. The amount of obstruction can be characterized by these changes as mild, moderate, or severe. A Doppler examination requires 20-30 minutes to complete. Doppler flow studies are used most often to evaluate the carotid bifurcation, the proximal internal carotid artery, and the peripheral arteries. Doppler echo is valuable in the diagnosing internal carotid artery occlusion and evaluating asymptomatic bruits and lower extremity claudication.
A majority of the arteries and veins in the body can be imaged by ultrasound. Segmental narrowing and occlusion can be documented and evaluated. The most common clinical application of ultrasonic vascular imaging is for the evaluation of the abdominal aorta. The aorta readily lends itself to ultrasonic imaging along its entire course. The thoracic aorta can be evaluated by echocardiography, and the abdominal aorta, from the xyphoid to the aortic bifurcation, can be evaluated by abdominal ultrasound. Aortic ultrasound is the primary technique for the diagnosis and serial evaluation of abdominal aortic aneurysm. Serial studies can detect changes in size and help in the timing of surgical intervention.
As with nuclear studies of the heart, the radionu-clide 99mTc is injected into a vein, and its flow is followed along the vein's course. It is primarily used to evaluate possible lower extremity venous occlusion.
125I Fibrinogen uptake test Iodine (125I) is used rather than technetium for the purpose of diagnosing a developing thrombus. Fresh thrombus takes up, and is composed of, fibrinogen and, therefore, concentrates 125I. After injection, a delayed scan is made over the suspected area of thrombosis, and any recent clots will light up.
Contrast venography is the standard test for the diagnosis of venous thrombosis. A contrast agent (dye) is injected into a superficial vein on the ankle or foot and serial x-rays are taken as the dye flows toward the central veins. If a thrombosis is present, it may be seen partially or totally occluding the vein.
Nursing ManagementPriorities when preparing a patient for a diagnostic test or procedure are:
Nursing Diagnosis Most Frequently Associated with Preparing a Patient for a Test or ProcedureAnxiety related to knowledge deficit about rationale, outcome of testing. |