what was the monthly maximum amount of vp shunt drainage given to patient 46764 since 179 months ago?
select max(outputevents.value) from outputevents where outputevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 46764 ) ) and outputevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'vp shunt drainage' and d_items.linksto = 'outputevents' ) and datetime(outputevents.charttime) >= datetime(current_time,'-179 month') group by strftime('%y-%m',outputevents.charttime)