The influence of plasma volume changes on enzymatic estimation of infarct size

In 18 patients, myocardial injury was estimated by mathematical analysis of the rise in plasma activity of alpha-hydroxybutyrate (alpha-HBDH) dehydrogenase as observed by multiple sampling after admission. This enzyme represents the lactate dehydrogenase isoenzymes LDH1 and LDH2. Changes in plasma volume were assessed by determining hematocrit values at the same time as the enzyme activities. Infarct size was expressed in IU/liter of plasma and in grams of heart muscle per liter of plasma (g/liter). Significant changes in plasma volume, reflected in hematocrit changes, occurred (average 12%): 10 patients without pulmonary edema showed an average change of 7%; for the group of eight patients with pulmonary edema, the change was 17%. When calculated infarct size was not correlated for plasma volume changes, a significant overestimation occurred (11.7%, range 0.7-29.7%). In the subgroup of patients with pulmonary edema the mean overestimation was 15.8%, and in patients without pulmonary edema, the mean overestimation was 8.5%. It is concluded that plasma volume changes after an acute myocardial infarction have to be taken into account when infarct size is calculated on the basis of plasma enzyme levels.