instruction: what is the difference between patient 016-18575's sao2 second measured on the current icu visit compared to the first value measured on the current icu visit?
response: SELECT (SELECT vitalperiodic.sao2 FROM vitalperiodic WHERE vitalperiodic.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '016-18575') AND patient.unitdischargetime IS NULL) AND NOT vitalperiodic.sao2 IS NULL ORDER BY vitalperiodic.observationtime LIMIT 1 OFFSET 1) - (SELECT vitalperiodic.sao2 FROM vitalperiodic WHERE vitalperiodic.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '016-18575') AND patient.unitdischargetime IS NULL) AND NOT vitalperiodic.sao2 IS NULL ORDER BY vitalperiodic.observationtime LIMIT 1)