isEscalated standard field id for Custom button

I am trying to create a custom button case that would result in a child case. I would like to carry over the isEscalated flag from the Parent Case over to the Child Case but I do not know what the field id is for the isEscalated field. Here is a snippet of that custom button.

/500/e?nooverride=1
&RecordType=01234000000M5NX
&cas28_lkid={!Case.Id}
&cas28={!Case.CaseNumber}
&cas7={!Case.Status}
&cas8={!Case.Priority}
&cas11={!Case.Origin}
&cas14={!Case.Subject}
&?????={!Case.IsEscalated}
&saveURL=/{!Case.Id}
&retURL=/{!Case.Id}