CARDIAC ENZYMES TEST
Cardiac enzyme studies measure the levels of enzymes and proteins that are linked with injury of the heart muscle. These include the enzyme creatine kinase (CK), and the proteins troponin I (TnI) and troponin T (TnT). Low levels of these enzymes and proteins are normally found in your blood, but if your heart muscle is injured, such as from a heart attack, the enzymes and proteins leak out of damaged heart muscle cells, and their levels in the bloodstream rise.
Because some of these enzymes and proteins are also found in other body tissues, their levels in the blood may rise when those other tissues are damaged. Cardiac enzyme studies must always be compared with your symptoms, your physical examination findings, and electrocardiogram (EKG, ECG) results.
Enzymes are proteins in the body and they act as catalysts. Catalysts are substances which change chemical reactions and rates of these reactions in the body. With their presence, reactions are either slowed or speeded.
Enzymes are found in all body cells and in other places in the body. When limiting our discussion to the cardiac enzymes, we are referring to the enzymes released into the bloodstream during myocardial damage. These enzymes can be used in the diagnosis of an MI. These blood tests are considered blood chemistry tests. However, we include them here as a separate chapter because they are so unique.
The term isoenzyme will also be used in this section. An isoenzyme (also known as Isozyme) is an enzyme that may appear in multiple forms, with slightly different chemical or other characteristics, and be produced in different organs, although each enzyme performs essentially the same function. The various forms are distinguishable in analysis of blood samples, which aids in the diagnosis of disease. Isoenzymes that catalyze the same physiologic reaction may also appear in different forms in different animal species.
To summarize, a protein enzyme is composed of (one or more) “isoenzyme.” These isoenzymes are very similar to each other in chemical composition, but have diferences that can be measured by certain lab tests. For example, the CPK enzyme has three distinct isoenzymes. These isoenzymes are:
- CK-BB (CK1) Isoenzyme #1
- CK-MB (CK2) Isoenzyme #2
- CK-MM (CK3) Isoenzyme #3
All three of these isoenzymes make up the main enzyme CPK (creatine phosphokinase) (also called CK–creatine kinase).
3 CARDIAC ENZYMES
Following are the main cardiac enzymes:
- LDH (also called LD)
- CPK (also called CK)
Serum Glutamic Oxaloacetic Transaminase, called: AST, (Aspartate Aminotransferase) A blood chemistry test for the level of SGOT in blood (is released with tissue necrosis).
Normal Values: 5-40 U/ml (Frankel) 4-36 IU/L; or 16-60 (Karmen) U/ml U/L at 30 degrees C; or 8-33 (SI units) at 37 degrees C.
This enzyme shows an elevation 8-12 hours after infarction. Peak levels are reached 24-48 hours after the MI. This enzyme is not particularly indicative of an MI. Other conditions can also cause a rise in the levels. High levels of SGOT may be obtained with trauma to the skeletal muscles, in liver disease, pancreatitis and others. SGOT is found in: heart muscle, liver, some also in skeletal muscle, kidneys and the pancreas. Demerol and morphine may elevate the levels temporarily. This enzyme then, is used with other enzyme results to more definitely diagnose the MI. AST levels elevate in 6-10 hours following acute MI. They peak in 24 to 48 hours.
- Please note that decreased levels of enzyme are found in pregnancy, diabetic ketoacidosis, beriberi. Elevations can be caused by hepatitis, trauma, musculoskeletal disease, IM injection, pancreatitis, liver cancer, and strenuous exercise.
- Explain purpose of test to patient
- do not give IM injections before the blood tests; and if serial specimens are taken, still give no IM injections, remember that very few meds can be given that do not affect the AST levels.
Test: LDH, Lactic Dehydrogenase ( also called LD)
An intracellular enzyme present in nearly all metabolizing cells in the body. The highest concentration of enzyme is located in the heart, skeletal muscle, liver, kidney, brain, and erythrocytes. There are 5 isoenzymes of LDH. This is a blood chemistry test to measure the amount of enzyme in the blood.
LDH catalyzes the reversible conversion of muscle lactic acid into pyruvic acid, an essential step in the metabolic process that ultimately produce cellular energy. Because LDH is present in almost all body tissues, cellular damage increases total serum LDH, limiting the diagnostic usefulness of this test.
Isoenzymes LD1 and LD2 appear primarily in the heart, red blood cells and kidneys. LD3 is primarily in the lungs. LD4 and LD5 are located in the liver, skin, and the skeletal muscles.
Total LDH: 150-450 U/ml (Wroblewski-LaDue method), 60-120 U/ml (Wacker method) 70-200 IU/L–results are different according to method used. Always check your own hospital for results used. These values have a wide range of normal and abnormal results.
Newborn: 300-1500IU/L Child: 50-150 IU/L
- LD1—17.5% to 28.3% of total
- LD2—30.4% to 36.4% of total
- LD3—19.2% to 24.8% of total
- LD4—-9.6% to 15.6% of total
- LD5—-5.5% to 12.7% of total
Because many common diseases increase total LDH (LD) levels, isoenzyme electrophoresis is usually necessary for diagnosis. In some disorders, total LDH may be within normal limits, but abnormal proportions of each enzyme indicate specific organ tissue damage. For example, in acute MI, the LD1 and LD2 isoenzyme ratio is typically greater than 1 within 12 to 48 hours after onset of symptoms (known as flipped LD). Midzone fractions (LD2, LD3, LD4) can be increased in granulocytic leukemia, lymphomas, and platelet disorders.
HOW IT IS DONE?
The health professional drawing your blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
Cardiac enzyme studies are often repeated over several hours for comparison.
CARDIAC LAB VALUES AND HEART DISEASE INTERPRETATIONS
Creatine Kinase aka Creatine Phosphokinase (CK) – Levels rise 4 to 8 hours after an acute MI (Myocardial Infarction), peaking at 16 to 30 hours and returning to baseline within 4 days. Creatine Kinase is muscle enzyme found in the heart and muscles. Increased CK-MB is seen with heart muscle damage. Increased CK-MM is seen with skeletal muscle injury. Strenuous exercise, weight lifting, surgical procedures, high doses of aspirin and other medications can elevate CK. It may also increase when using cholesterol-lowering medications.
- Creatine Kinase (male) 25 – 90 U/L
- Creatine Kinase (female) 10 – 70 U/L
CK-MB CK isoenzyme – It begins to increase 6 to 10 hours after an acute MI, peaks in 24 hours, and remains elevated for up to 72 hours.
- Creatine Kinase-MB 0 – 5 U/L
(LDH) Lactate dehydrogenase – Total LDH will begin to rise 2 to 5 days after an MI; the elevation can last 10 days. LDH is an enzyme released in the blood with cell injury. It is used as a late marker to detect a heart attack. It is also elevated with liver and kidney disease, pernicious and megaloblastic anemias, malignancy, progressive muscular dystrophy and pulmonary emboli.
- LDH 45 – 90 U/L
LDH-1 and LDH-2 LDH isoenzymes – Compare LDH 1 and LDH 2 levels. Normally, the LDH-1 value will be less than the LDH-2. In the acute MI, however, the LDH 2 remains constant, while LDH 1 rises. When the LDH 1 is higher than LDH 2, the LDH is said to be flipped, which is highly suggestive of an MI. A flipped pattern appears 12-24 hours post MI and persists for 48 hours.
- LDH-1: 18 – 33 percent
- LDH-2: 28 – 40 percent
Alanine Aminotransferase (ALT; also called SGPT)– It is a liver enzyme and may increase when using cholesterol-lowering medications.
- ALT 5 – 50 U/L
SGOT – Serum glutamic oxaloacetic transaminase is an enzyme that is normally present in liver and heart cells. SGOT is released into blood when the liver or heart is damaged. The blood SGOT levels are elevated with liver damage (hepatitis) or with an insult to the heart (heart attack). There are some medications that can also raise SGOT levels. SGOT is also called aspartate aminotransferase (AST). SGOT will begin to rise in 8-12 hours and peak in 18-30 hours. SGOT is a liver enzyme that is released into the bloodstream following injury or death of cells. Increased SGOT or AST is seen with liver disease, myocardial infarction (MI) and some medications. May increase when using cholesterol-lowering medications.
- SGOT aka AST 7 – 40 U/L
Myoglobin – early and sensitive diagnosis of myocardial infarction in the emergency department This small heme protein becomes abnormal within 1 to 2 hours of necrosis, peaks in 4-8 hours, and drops to normal in about 12 hours. Myoglobin is a protein found in certain types of muscle. Elevated myoglobin may indicate muscle injury or inflammation.
- Myoglobin, serum (male): 30 – 90 µg/ml
- Myoglobin, serum (female): 30 – 90 µg/ml
Troponin Complex – Peaks in 10-24 hours, begins to fall off after 1-2 weeks. Troponin T is a protein found in the blood and is related to contraction of the heart muscle. Troponin T is valuable for detecting heart muscle damage and risk.
- Tropinin I: < 0.0 – 0.10 µg/mL
- Tropinin T: < 0.0 – 0.10 µg/mL