what was the dose of sevelamer - that was first prescribed to patient 6170 on their current hospital encounter?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 6170 and admissions.dischtime is null ) and prescriptions.drug = 'sevelamer' order by prescriptions.startdate asc limit 1