what is the total bag dose that until 51 months ago patient 27591 has been prescribed?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 27591 ) and prescriptions.drug = 'bag' and datetime(prescriptions.startdate) <= datetime(current_time,'-51 month')