library(reshape2)
library(tidyr)

# example data frame
x = data.frame(
  id   = c(1, 1, 2, 2),
  time = c(1, 2, 3, 4),
  SBP = c(1, 0, 1, 0),
  HR  = c(0, 1, 0, 1)
)

# collapse the data frame
x_melt <- melt(data = x, id.vars = "id", measure.vars = c("SBP", "HR"))


# solution using gather from tidyr
x_long <- gather(x, "id", "value", 3:4)