New page describing APACHE IV

[alistairewj]

It's probably worthwhile to have a high level page describing APACHE IV.

Some Information covered: which stays are eligible, how mortality is calculated, how certain flags are calculated. For example, APACHE IV's "ACTUALUNITMORTALITY" captures mortality even if the patient was transferred out of the ICU to another ICU, where as the patient discharge location will just state that the patient left the unit alive (to another unit). Said another way, transfer stays are combined and the transfer stay’s unit discharge location is credited to the first ICU stay. So, If patient died in last transfer stay then the first ICU stay will get “Death” as discharge location. Example table:

patientUnitStayID	Discharge location in Patient table	Unit Admit Type	apacheStayID	patientUnitStayID credited with the combined data	Discharge location for apache
1	Other ICU	admit	89	1	Death
2	Other ICU	transfer	89	1	Death
3	Death	transfer	89	1	Death