instruction: when was the last measurement of patient 022-199074's sao2?
response: SELECT vitalperiodic.observationtime FROM vitalperiodic WHERE vitalperiodic.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '022-199074')) AND NOT vitalperiodic.sao2 IS NULL ORDER BY vitalperiodic.observationtime DESC LIMIT 1