### Description

The final size of an acute myocardial infarct can be estimated based on the initial ST-segment elevations.

Patient selection: Applies to patients with anterior or inferior myocardial infarctions.

Measurement of ST segment changes:

(1) The TP segment is used as the isoelectric line.

(2) The increase in the ST segment is measured to the nearest 0.5 mm (where 0.1 mV = 1 mm) at the J point, except for lead aVR.

Equations of Aldrich et al

size in percent  for an anterior myocardial infarct =

= (3 * ((1.5 * (number of leads with increased ST segments >= 1 mm)) - 0.4)) =

= (4.5 * (number of leads with increased ST segments >= 1 mm)) - 1.2

size in percent for an inferior myocardial infarct =

= (3 * ((0.6 * ((increase in ST segment for lead II) + (increase in ST segment for lead III) + (increase in ST segments for lead avF))) + 2.0)) =

= (1.8 * ((increase in ST segment for lead II) + (increase in ST segment for lead III) + (increase in ST segments for lead avF))) + 6

Modification of Clemmensen et al

Clemmensen et al found that the equations of Aldrich et al predicted anterior infarct size performed acceptably, but that the equation for inferior infarction did not. They suggested the following equation (a mixture of Aldrich's anterior and inferior equations) for inferior infarcts.

size in percent for an inferior myocardial infarct =

= (3 * ((0.6 * ((increase in ST segment for lead II) + (increase in ST segment for lead III) + (increase in ST segments for lead avF))) + 2.0)) + (3 * ((1.5 * (number of other leads with increased ST segments >= 1 mm)) - 0.4)) =

= (1.8 * ((increase in ST segment for lead II) + (increase in ST segment for lead III) + (increase in ST segments for lead avF))) + (4.5 * (number of other leads with increased ST segments >= 1 mm))+ 4.8

Limitations:

• The size prediction performs best when no thrombolytic therapy is given.