B737-800
Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Workload; Situational Awareness
Aircraft Equipment Problem Critical
Flight Crew Became Reoriented; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft
While at cruise at FL370; during 2nd half of second bathroom break; PNF became aware of a lack of O2 pressure from his mask. Gauge on overhead indicated near zero. Conferred with Dispatch (weather and fuel burn); Maintenance and FOM and descended to FL250. We positioned a therapeutic O2 tank on flight deck with 2 masks unwrapped and ready to use and continued to our destination without further problems. Did not hear the result from Maintenance but assume an oxygen leak.
On second Biffy break; I noticed a lack of 02 in my mask and on the overhead gauge. We set up a therapeutic bottle with two masks at the ready; checked destination weather; reviewed FOM guidance; verified time and burn at FL250; requested descent clearance and advised Dispatch\Maintenance.
During cruise flight the B737-800 Captain noticed no oxygen in his mask while the First Officer was on a physiological break; and; assuming a leak; had a cabin supplemental bottle and two masks brought to the flight station and descended to FL250.