Date: 201910
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Illness / Injury
Detector: Person Flight Attendant
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Contributing factors: Equipment / Tooling
Primary problem: Equipment / Tooling

Narrative: We departed ZZZ. Reaching our final altitude of 10;000 ft. we gave the FA (Flight Attendant) three chimes and turned off the sterile light. Immediately after this the FA called us using the emergency call button and notified us that one of the passengers could not breathe and she was going to administer him oxygen. We said okay and to call us back after. We then [requested priority handling] with Center and told them our situation and that we wanted to return to ZZZ. At this point we were right about 25 NM from ZZZ. ATC immediately gave us a heading and a lower altitude to bring us back to ZZZ. The FA called us back and informed us that when she went to administer oxygen to the passenger; the oxygen cylinder was not working; and was not providing him any oxygen. After the FA informed the passenger that we would be returning to ZZZ; he did not want to and did not want to 'cause a scene.' We were on the ground in about 8 minutes in ZZZ after [requesting priority handling] with ATC. Paramedics met the aircraft at the gate and escorted the man off to give him medical attention.FA heard passenger say that he could not breathe. Passenger was having trouble breathing. FA called cockpit using the emergency call button. We [requested priority handling] and returned back to ZZZ where paramedics were waiting at the gate. Oxygen cylinder should be tested and verified to make sure it works before a flight is conducted.


Second narrative: Upon leveling at cruise altitude; FA (Flight Attendant) contacted flight deck via EMRG (Emergency) channel and informed the Captain that a passenger was having trouble breathing. The FA attempted to provide the passenger oxygen via the portable oxygen bottle; but the bottle was not working. The Captain [requested priority handling] and informed ATC of the issue and requested a return to ZZZ. ATC issued a clearance to return to ZZZ. Flight crew coordinated with Dispatch. Aircraft landed with no further issue. Medics met the aircraft at the gate and helped the passenger; who continued to have breathing problems after landing.Flight Attendant notified flight crew of situation. Passenger was having trouble breathing after departure. Also of note is that the aircraft was operating under MEL XX-XX-X and was unpressurized at the time. The cabin altitude at the time of the event was 9;200 ft. This may or may not have been a contributing causal factor.Captain [requested priority handling] and returned to ZZZ in order to provide the passenger with medical assistance in the most timely fashion. FA continued to provide assistance as appropriate to the passenger until medical assistance was available. [I suggest] continued adherence to policy and procedures.


Synopsis: EMB-145 flight crew reported returning to departure airport due to an ill passenger.