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Letters to the Editor will be published, if suitable, and as space permits. They shoula not exceed 1,000 words (typed double spaced) in length, and may be subject to editing or abridgment. To the Editor: In 1971, Shell and co-workers' presented a mathematical model for estimating the extent of myocardial infarction from serial serum CPK measurements. These calculated estimates were compared to estimates of infarct size based on depletion of enzyme activity from myocardium. These depletion estimates of the extent of injury were considered to represent an equivalent of anatomic injury. There was no pathological confirmation of this assumption. The model, therefore, compares estimates of damage from enzyme activity measurements in serum to estimates based on disappearance of enzyme activity from myocardium. During the intervening five years several modifications have been introduced.2 3 Each of these methods involves the following basic formula Serum Enzyme Estimate of foT f(t)dt Kw BW Myocardial Injury CPKD KR In this formula, f(t) represents the enzyme appearance function which is the amount of CPK in ml of the body distribution volume dE at time, t, and is defined as f(t) =-+ KdE. E(t) is the enzyme value at time t. Kd is the proportional disappearance rate constant which by convention is negative and expresses the percent per minute decay rate of serum enzyme levels. Kw = the proportion of body weight (BW) in which released enzyme is distributed. The numerator then represents the total accumulated enzyme. In the denominator, CPK0 represents the estimate of the amount of enzyme (IU) depleted per gram of infarcted rnyocardium and KR represents the proportion of myocardial enzyme which escapes from the infarcted myocardium to the body distribution space. Norris 2 suggested that the use of an individualized disappearance function (kdl) would improve the original method although complete calculation of the serum enzyme estimation of the extent of in-farction in terms of gram-equivalents was not recommended. More recently, the K., concept was adopted by Roberts and co-workers who also recommended new values for Kw and KR. The sensitivity of the mathematical model to variations in parameter values has been presented.4 However, it has not been shown that newer modifications2 3 for estimating extent of myo-cardial injury actually improve correlation with myocardial CPK depletion estimates of damage. To test the hypothesis that these recent recommendations improve the relationship between serum enzyme estimates and myocardial depletion estimates of the extent of myocardial …

Mathematical modifications fail to improve CPK estimates of extent of infarct.