what was the first dose of ipratropium bromide neb on the current hospital visit, prescribed to patient 18866?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18866 and admissions.dischtime is null ) and prescriptions.drug = 'ipratropium bromide neb' order by prescriptions.startdate asc limit 1