1-18
1-18. BLOOD STUDIES
a. Electrolytes. Serum electrolyte studies are frequently performed on cardiac patients. Of particular significance are calcium, sodium, and potassium.
(1) Calcium has a role in cell permeability, formation of bones and teeth, blood cell coagulation, nerve impulse conduction, and normal muscle contraction. Elevated calcium levels (hypocalcaemia) may cause HTN and cardiac arrest. Decreased calcium levels (hypocalcaemia) may cause tetany, convulsions, hypotension, and cardiac arrhythmias.
(2) Sodium functions in maintaining the concentration of extra cellular fluid, acid-base balance, water balance, and nerve conduction. Elevated sodium levels (hypernatremia) may cause weight gain, pitting edema, HTN, and tachycardia. Decreased sodium levels (hyponatremia) may cause hypotension and tachycardia. If depletion is severe, vaso-motor collapse may occur.
(3) Potassium is the dominant cellular electrolyte. It facilitates contraction of skeletal and smooth muscle to include myocardial contraction. Potassium is also concerned in acid-base balance, nerve impulse conduction, and cell membrane function. Both decreased potassium levels (hypokalemia) and increased potassium levels (hyperkalemia) diminish the excitability and conduction rate of the cardiac muscle. This may lead to bradycardia, tachycardia, cardiac arrhythmias, and cardiac arrest.
b. Cultures. Blood cultures are performed to detect the presence of bacteria in the blood. This test is useful in the diagnosis of bacterial endocarditis.
c. Enzymes. Enzyme studies are done to detect damage to the myocardium. The enzymes creatine phosphokinase (CPK) and lactic dehydrogenase (LDH) are found in increased levels after myocardial tissue
damage. However, these enzymes are also present in other tissue, and blood levels may be elevated as a result of damage to skeletal muscles, the liver, the kidneys, and other organs. This results in a false positive.
d. Isoenzymes. Isoenzymes are forms of enzymes that can be differentiated from one another.
(1) One isoenzyme of the enzyme CPK is present in significant amounts only in myocardial tissue. This isoenzyme is identified as CPK-MB.
(2) Lactic dehydrogenase has five isoenzymes, and cardiac muscle is associated with large amounts of the isoenzymes LDH1.
(3) The determination of the isoenzymes (CPK-MB and LDH is more specific in evaluation myocardial damage than simple enzyme determinations.
