In cases of right atrial enlargement the duration of the P wave hardly changes, but the P-R interval increases, so that the P--R segment ratio falls below the normal range.Left atrial enlargement, on the other hand,does not affect the P-R interval, but the P wave lengthens at the expense of the P-R segment.The result is a- ratio above P-R segment the normal maximal limit of 1.6.In combined atrial enlargement, both P-R interval and P wave are prolonged. It follows that in such cases the ratio may P-R segment be normal.

1. Macruz index
Left atrial enlargement

2. CRHD,SEVRE MS,LAE,RVH,PAH:Chest X-Ray
LAE:Straightned left heart border

3. CRHD,SEVRE MS,LAE,RVH,PAH:12 lead ECG

4. Morris index
A to B=0.12sec
B to C=2mm
ABXBC=0.12X2=0.24mmxsec≥0.0
4mmxsec
Multiply the length x breadth of the terminal negative portion of the P wave in lead V1 (the P terminal
force). This gives the Morris index. The area exceeds 0.04 mm-sec in left atrial enlargement. (one small
square on the ECG taken at 25 mm / sec paper speed)

5. Macruz index
A to B=widened P width=1.6mm
B to C=narrowed PR segment=0.4mm
P/PR segment ≥1.6

6. http://circ.ahajournals.org/content/27/5/935.full.pdf
In 1958, Macruz, Perloff, and Case1 proposed a simple formula for electrocardiographic
recognition of right, left, and combined atrial enlargement. The Macruz index has been
widely used in recent years. It is based on the measurement of the P-R interval,P-wave
duration, and P-R segment (time between end of electric atrial systole and onset of electric
ventricular systole). According to the above authors, the ratio of the P-wave duration to the
P-R segment as measured in standard lead II in normal persons, is relatively constant within
the limits of 1.0 and 1.6, with an average value of 1.2.