Plasma enzymes in myocardial infarction Application of a two-compartment model in assessing myocardial release of enzyme

Early diagnosis of acute myocardial infarction with a rapid latex agglutination test for semi-quantitative estimation of serum myoglobin

A rapid latex agglutination test for the detection of elevated levels of myoglobin in serum was evaluated in a prospective study of 236 patients consecutively admitted to a Coronary Care Unit on suspicion of acute myocardial infarction (AMI). The final diagnosis was made according to the WHO criteria. The prevalence of AMI was 0.45 with a male to female ratio of 2:1. In all patients at least two blood samples were collected with 4 hours interval 4-12 hours after the onset of symptoms. All sera were analysed for myoglobin by a latex agglutination test and by a radioimmunoassay (RIA). The latex test was performed twice, first as an emergency test by the technical assistant on duty and later by another well-trained technical assistant as her daily routine work. If the latex test was carried out each day by the same well-trained technical assistant, the test results agreed well with the RIA test results, and the false-negative fraction for patients with AMI constituted 0.06 and the false-positive fraction for patients without AMI 0.46. However, when the latex test was performed by the occasional technical assistant on duty, a relatively high degree of discrepancy was observed between the latex test results and the RIA test results, thus giving a false-negative fraction of 0.11 and a false-positive one of 0.36. In conclusion, performed under optimal laboratory conditions, the latex test can be used as a reliable method to estimate elevated levels of serum myoglobin. However, used as a bedside emergency examination, the test results correlated rather poorly to the RIA test results, and consequently the latex myoglobin test seems to be of minor clinical importance in the early evaluation of patients with suspected AMI.

Serum phosphate increase during short-term beta-adrenoceptor blockade in thyrotoxicosis

We studied the changes in blood plasma electrolytes during acute and short-term treatment with four different beta-adrenergic-blocking agents in 31 patients with hyperthyroidism. Serum phosphate increased during the first four hours and remained elevated after one week both after cardioselective (acebutolol) and non-cardioselective (oxprenolol, pindolol and timolol) beta-blockade (p less than 0.05). Albumin decreased after four hours but no change was found after one week. Serum creatinine increased after one week in the acebutolol-treated group, while sodium, potassium and albumin-corrected calcium in serum did not change. The hyperphosphatemic effect may be due to blockade of beta 1-adrenoceptors, since the effect of the cardioselective acebutolol was no less than that of the non-cardioselective beta-adrenoceptor blockers.

Acute beta-blockade changes the extracellular distribution of thyroid hormones

The acute (within hours) changes in the concentrations of T4, T3, reverse-T3 (rT3) and T3 resin uptake test (T3RU) were studied in 31 hyperthyroid patients for 4 h after po treatment with either acebutolol, oxprenolol, pindolol or timolol. In 21 of the patients, the changes were compared to changes in the serum concentrations of alpha 2-macroglobulin (a macromolecule) and two middle-sized molecules; thyroid hormone-binding globulin (TBG) and albumin in order to calculate the changes in extracellular distribution of the thyroid hormones and to distinguish between changes due to a changed metabolism and changes due to a changed distribution of the thyroid hormones. Acebutolol, oxprenolol and timolol caused a decrease in serum T3 after 1/2 h, and acebutolol and oxprenolol also a decrease in rT3 after 1/2 - 1 h, the changes reversed within 2 h. A concomitant decrease in serum albumin and TBG suggests a change in the extracellular distribution of middle-sized molecules to which thyroid hormones are attached, as an explanation of the acute decrease (1 h) of the thyroid hormones. The small and insignificant change in alpha 2-macroglobulin indicates that the changes are mainly extravascular, but the difference of alpha 2-macroglobulin changes between the drugs (acebutolol/timolol vs pindolol/oxprenolol) could depend on the intrinsic sympathomimetic activity of the drugs.

Limitation of enzymatic models for predicting myocardial infarct size

The possibility of predicting myocardial infarct size from early enzyme measurements was studied using a physiological two compartment distribution model. Based on this the time dependent appearance function in plasma was calculated for creatine kinase, aspartate aminotransferase, and lactate dehydrogenase in 29 patients suffering from acute myocardial infarction. On average, the appearance function of the three enzymes started four hours after the onset of symptoms, and the maximum was reached after 12 hours for creatine kinase, 13 hours for aspartate aminotransferase, and 22 hours for lactate dehydrogenase. The cumulated appearance function was used as an acceptable estimate of infarct size. The prediction of infarct size from defined points of the appearance function curve for each of the three enzymes was attempted according to a set schedule during the first 25 hours after the onset of myocardial infarction. The prediction using creatine kinase was superior to the other enzymes. Even so, a reliable prediction could only be established at the very earliest from nine hours and this is too late, as irreversible loss of myocardium occurs rapidly after the onset of symptoms. This, together with the fact that other models have unacceptable variability of the prediction, lead to the conclusion that enzymatic predictive models are of no practical value in clinical intervention studies to reduce infarct size.