how much heparin sodium was prescribed on the last hospital visit for the last time to patient 14621?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 14621 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'heparin sodium' order by prescriptions.startdate desc limit 1