what is the dose of sodium bicarbonate that is first prescribed to patient 10811 since 05/2103?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 10811 ) and prescriptions.drug = 'sodium bicarbonate' and strftime('%y-%m',prescriptions.startdate) >= '2103-05' order by prescriptions.startdate asc limit 1