+--------------------------------------------------------------------+
 |                                                                    |
 |                 INITIAL STEADY STATE: I_SS,ISSMOD                  |
 |                                                                    |
 +--------------------------------------------------------------------+

 MEANING: PREDPP-PK global variables
 CONTEXT: PK routine

 USAGE:
      USE PRMOD_INT, ONLY: I_SS,ISSMOD

 GLOBAL DECLARATION:
      INTEGER(KIND=ISIZE) :: I_SS,ISSMOD

 DISCUSSION:

 Used  for the Initial Steady State feature of PREDPP, with the general
 non-linear models (ADVAN6, ADVAN8, ADVAN9, ADVAN13, ADVAN14, ADVAN15).

 By default, the initial conditions  (i.e.,  compartment  amounts)  are
 zero at the start of each individual record.  Different initial condi-
 tions may be computed using the I_SS (Initial Steady State) feature of
 MODEL and/or PK.

 ISSMOD may be set by MODEL.
 (Note that the option of the $MODEL record is I_SS, not ISSMOD.)
 Default: -1 (MODEL does not request the I_SS feature)
 Values  0,  1, 2 and 3 are permitted.  Value 0 requests that no steady
 state be computed.  ISSMOD values 1, 2, or 3 requests that PREDPP com-
 pute  an  initial  steady  state  for the model before the first event
 record of an individual record, or after a reset event.   The  results
 are  identical  to those that would be computed by a steady-state dose
 event record with SS=ISSMOD and AMT=0 and RATE=0.  If endogenous  drug
 is  specified  in  the differential equations, non-zero initial condi-
 tions will be computed.

 I_SS may be set by PK.
 Default: -1 (PK does not request the I_SS feature)
 Values are the same as ISSMOD, with the same effect.  This allows I_SS
 to  be  set  conditionally, e.g., if some subjects are at steady-state
 and others are not.  If both ISSMOD and I_SS are set, then  the  value
 of I_SS overrides that of ISSMOD.

 There is no difference between values 1, 2 and 3 of I_SS unless the PK
 routine also uses the compartment  initialization  feature  A_0.   The
 I_SS  feature  behaves exactly like a steady state dose record in this
 regard.  Specifically,

      With I_SS=1 ("reset"), values of A_0 are ignored.
      With I_SS=2 ("sum"), values of A_0 are added to the SS values.
      With I_SS=3 ("initial ests"), values of A_0 are used  as  initial
      estimates when computing the SS values.

 (and similarly for ISSMOD).

 If  ISSMOD  is  set, or I_SS is set at ICALL=1, PREDPP will acknowlege
 the fact in the NONMEM output.

 When I_SS or ISSMOD is set, even if only to 0, then SSS and the appro-
 priate SS routine are included in the NONMEM executable.

 When  I_SS  or  ISSMOD  is set, even if only to 0, or verbatim code is
 present in the $PK or $INFN block, then NM-TRAN  includes  I_SS,ISSMOD
 in the PK and/or INFN routine.

 Example: $PK may include code such as
   IF (TYPE.EQ.1) THEN
   I_SS=1
   ELSE
   I_SS=0
   ENDIF

 (See $model, $pk, model, pk).

 Location prior to NONMEM 7: prdpk4

 REFERENCES: None.


  
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