what was a total dose of bupivacaine 0.75% that patient 60142 had been prescribed since 05/2101?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 60142 ) and prescriptions.drug = 'bupivacaine 0.75%' and strftime('%y-%m',prescriptions.startdate) >= '2101-05'