Macruz Index is a proportion between the P wave duration and PQ segment (not interval) duration (P/PQ). The idea comes from observation, that there's an inverse proportionality between those two values.
With reference range between <1;1,6>, Macruz index >1,6 indicates P mitrale, while <1 indicates P pulmonale (but diagnostic value of the latter is less significant).
See also Morris index.
G. P. Human and H. W. Snyman, Value of the Macruz Index in the Diagnosis of Atrial Enlargement, Circulation 1963, 27:935-938.