However, other factors such as age, sex, and exercise also affect CPK levels. In fact, elevated CPK is common in adults who exercise regularly. Normal levels of serum CPK vary from 0 to approximately 188 micrograms per liter. Different laboratories may report slightly different upper limits as within the normal range because of differences in test procedures and population differences.
During exercise, especially of the highintensity burst type, considerable amounts of CPK leak into the extracellular fluid and plasma as a result of changes in membrane activity. The type of exercise and the duration affect the elevation of serum CPK. Non-weight-bearing activities such as rowing and swimming cause less pronounced CPK elevations than comparable weightbearing events. CPK elevations have been found to be linear with the duration of exercise up to 5.5 hours. Longer durations of exercise are associated with an accelerated increase in CPK.
Exercise-induced increases in the level of serum CPK are temporary. Resting levels of CPK changes generally become elevated during exercise days and then return to normal levels during rest days. Peaks in serum CPK have been reported to occur at 5 minutes, 11 hours, 24 hours, 25 hours, and even 4-6 days following exercise. With continued physical activity, CPK levels remained elevated. Resting levels of serum CPK are higher in exercised trained individuals than in untrained ones.
CPK elevations vary with age and sex. Older individuals are less likely to have elevations after exercise than are middle-aged or young adults. Women are more likely to have exercise induced elevations than are men.
Some researchers suggest that muscle cell permeability increases during exercise and others think that muscle degeneration and necrosis occurs. Muscular soreness has been reported to accompany elevated CPK levels. While increases in serum CPK are common in adults who exercise, there are also pathological states, such as myocardial infarction or heart attack, which may be accompanied by an increase in serum levels of CPK.
There are two isoenzymes, i.e., chemically different but functionally identical forms of CPK. The isoenzyme produced in the brain is identified as CPK B and the isoenzyme produced in muscle is identified as CPK M. As a rule of thumb, cardiac muscle stress produces CPK MB, and skeletal muscle stress produces CPK MM. CPK BB is an indicator of stress of brain tissue and is not usually examined in routine physical examinations unless there is a suspicion of stroke, tumor, etc.
There are divergent opinions about the proportion of CPK MB in skeletal muscle as well as about the proportion of total CPK in association with acute myocardial infarction. Studies have shown large elevations in CPK MB in some women after step exercise and in highly trained female endurance runners. In the absence of symptoms and other positive physical examination information, elevated CPK MB is not usually considered sufficient evidence of a heart attack.
Identification of these isoenzymes requires laboratory testing procedures not routinely completed for LSAH.
Approximately 16 percent of the 211 CPK tests completed for the LSAH participants during 1992-94 reported elevated serum levels. During this time period, about half of that 16 percent of laboratory tests were also examined for the isoenzymes. The isoenzymes identified in these tests have consistently been CPK MM. These data suggest that study participants are exercising prior to physical examinations. Questions regarding type and frequency of exercise routinely asked during the examination aid the physician in assessing the relationship between elevated CPK values and recent exercise.
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