Anterior mitral leaflet has a classical M shaped motion. Infrequently , M mode echo will record a triphasic pattern .
The exact answer is not known . I guess it is a normal variant.
Often it is recorded when there is a long and redundant AML , especially if the M-mode cut is too close to the tips.
Though it is not common , I have seen in few the triphasic gets converted into classical M shaped pattern if the cursor is moved slightly away from the tip of AML.
Relationship to Heart rate
Some times it appears in slow heart rate and tends to disappear with tachycardia .
Triphasic Doppler filling vs Triphasic M-Mode
We do not know yet , how the triphasic AML motion correlate with triphasic Doppler filling pattern which is considered a fairly good evidence for LV dysfunction.