#' Concentration predictions for 2-compartmental PK model, single or multiple infusions
#'
#' @param t vector of time
#' @param dose dose
#' @param tau dosing interval
#' @param t_inf infusion time
#' @param CL clearance
#' @param V volume of central compartment
#' @param Q inter-compartimental clearance
#' @param V2 volume of peripheral compartment
#' @param ruv residual error (list)
#' @export
pk_2cmt_inf <- function(
t = c(0:24),
dose = 100,
tau = 12,
t_inf = 1,
CL = 3,
V = 30,
Q = 2,
V2 = 20,
ruv = NULL
) {
k <- CL / V
tmp <- c()
dat <- data.frame(t = t, dv = 0)
t_dos <- t %% tau
n_dos <- floor(t/tau)
unq_dos <- unique(n_dos) + 1
# reparametrization:
terms <- (Q/V) + (Q/V2) + (CL/V)
beta <- 0.5 * (terms - sqrt(terms^2 - 4*(Q/V2)*(CL/V)))
alpha <- ((Q/V2)*(CL/V))/beta
A <- (1/V) * (alpha - (Q/V2))/(alpha-beta)
B <- (1/V) * ((beta - Q/V2)/(beta-alpha))
for(i in seq(unq_dos)) {
sel <- n_dos >= i-1
tmp <- dat[sel,]
tmp$t <- tmp$t - (i-1)*tau
## equation:
tmp[tmp$t <= t_inf,]$dv <-
(dose/t_inf) * (
(A/alpha) * (1-exp(-alpha*tmp$t[tmp$t <= t_inf])) +
(B/beta) * (1-exp(-beta *tmp$t[tmp$t <= t_inf]))
)
tmp[tmp$t > t_inf,]$dv <-
(dose/t_inf) * (
(A/alpha) * (1-exp(-alpha*t_inf)) * exp(-alpha * (tmp$t[tmp$t > t_inf] - t_inf)) +
(B/beta) * (1-exp(-beta *t_inf)) * exp(-beta * (tmp$t[tmp$t > t_inf] - t_inf))
)
dat[sel,]$dv <- dat[sel,]$dv + tmp$dv
}
if(!is.null(ruv)) {
dat$dv <- add_ruv (dat$dv, ruv)
}
return(dat)
}
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