7 lines (6 with data), 252 Bytes

m258a 0 128 0 13:10:00 17/03/1989
# Start time=13:10
# First beat=13:18:23.312
# <age-range>: 50-54, <sex>: Male, <medication>: None
# <RR-low-limit(sec)>: 0.531,  <RR-high-limit(sec)>: 1.164
# Note: recording with a timing signal, flutter-compensated