The index of the eventEntry that is used when a falling
threshold is crossed. The eventEntry identified by a
particular value of this index is the same as identified by
the same value of the eventIndex object. If there is no
corresponding entry in the eventTable, then no association
exists. In particular, if this value is zero, no associated
event will be generated, as zero is not a valid event index.
This object may not be modified if the associated
cHcAlarmStatus object is equal to active(1).