what was the isotonic sodium chloride dose that was prescribed to patient 9566 in this hospital encounter for the first time?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 9566 and admissions.dischtime is null ) and prescriptions.drug = 'isotonic sodium chloride' order by prescriptions.startdate asc limit 1