instruction: what was the first time patient 10855 had their minimum arterial bp mean on the current intensive care unit visit?
response: SELECT chartevents.charttime FROM chartevents WHERE chartevents.icustay_id IN (SELECT icustays.icustay_id FROM icustays WHERE icustays.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 10855) AND icustays.outtime IS NULL) AND chartevents.itemid IN (SELECT d_items.itemid FROM d_items WHERE d_items.label = 'arterial bp mean' AND d_items.linksto = 'chartevents') ORDER BY chartevents.valuenum, chartevents.charttime LIMIT 1