During anesthesia, a scavenging system reservoir bag distends during inspiration. What is the most likely cause?

During anesthesia, a scavenging system reservoir bag distends during inspiration. What is the most likely cause?

• A. An incompetent pressure-relief valve in the mechanical ventilator
• B. An incompetent pressure-relief valve in the patient breathing circuit
• C. An incompetent inspiratory unidirectional valve in the patient breathing circuit
• D. An incompetent expiratory unidirectional valve in the patient breathing circuit

Answer: (A)

Reservoir bag distension in the scavenging system during inspiration points to a failure of the machine’s pressure-relief path. When the ventilator delivers gas, any excess pressure in the scavenging line should be vented to atmosphere by the pressure-relief valve. If that valve is incompetent, there’s no safe outlet for the incoming gas, so the waste gas back-presses into the scavenger bag and makes it swell during inspiration. If a patient-side valve were faulty, the pattern would relate more to rebreathing or backflow within the breathing circuit itself and would not explain the bag inflating specifically during inspiration. An inspiratory or expiratory valve problem would alter flow direction in the circuit and produce different signs, whereas the key clue here is the relief path failing to vent excess pressure from the scavenging system.

Reservoir bag distension in the scavenging system during inspiration points to a failure of the machine’s pressure-relief path. When the ventilator delivers gas, any excess pressure in the scavenging line should be vented to atmosphere by the pressure-relief valve. If that valve is incompetent, there’s no safe outlet for the incoming gas, so the waste gas back-presses into the scavenger bag and makes it swell during inspiration.

If a patient-side valve were faulty, the pattern would relate more to rebreathing or backflow within the breathing circuit itself and would not explain the bag inflating specifically during inspiration. An inspiratory or expiratory valve problem would alter flow direction in the circuit and produce different signs, whereas the key clue here is the relief path failing to vent excess pressure from the scavenging system.