So you are doing an open thoractomy and patient starts desating... Surgeon was unable to do a VATS for the possible wedge resection due to the patient's current status. He is a 55 y/o male with a hx of lupus who came in with a resp. infection. He is currently in ARDS, septic, renal failure (dialyzed today), and just came off Levo. Patient is currently on a ventilator in the ICU with the following settings:
Vt: 440
RR: 36
FiO2: 100%

Transported patient directly to OR with the ICU ventilator and the patient was kept on the current settings per the ARDS protocol the ICU was using. The only thing hooked up to the anesthesia machine is the ETCO2 and monitors. No gases were used since the patient was being ventilated from the ICU vent. He is on Versed, Fent, and Propofol gtts.

Patient is right side down and surgeon is doing a resection and biopsy of the left lung. BP is stable, ETCO2 has been kept around 17, and sats were mid 90s and trending down. Have been in the low to mid 80s for the last 10min and have reached as low as 76%.

Students first, but what could be the cause of this and what are some things you would want to know/do to remedy this obvious problem.