when was the first time that on the first hospital encounter patient 71192 was prescribed a medication via lock?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 71192 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and prescriptions.route = 'lock' order by prescriptions.startdate asc limit 1