Initial conditions in the Population PKPD model of human physiology data

How can I create a Population PKPD Model with an Epinephrine Infusion that has Initial Conditions as a Covariate or set to the values at time=0. This is due to the body already has endogenous epinephrine levels.

Also, I would like to have the PD portion, which would be Cardiac Output (would increase during infusion) or Systemic Vascular Resistance (would decrease during infusion).

Then I would like to add a categorical covariate as a gender or genotype.

Thank you for help with creating this model.

Sincerely, Andy