al	patient alarms (237* only)
all	monitor status and patient alarms (237* only)
ari	corrected machine-derived beats (x_mgh* only)
atr	reference beats, ST, etc. (x_edb only)
ecg	corrected machine-derived beats (x_slpdb only)
log	timeline annotations (237 only)
st	reference sleep stages (x_slpdb only)