During volume-controlled ventilation through a circle absorber system, the agent analyser repeatedly shows delivered sevoflurane concentrations well above the vapourizer dial setting. The discrepancy is greatest when peak inspiratory pressure is high and disappears when the patient is switched to spontaneous ventilation. What is the mechanism responsible?
- A Compression of gas in the vapourizer bypass channel by the descending ventilator bellows
- B Desiccation of soda lime generating heat that increases vapourization within the circuit
- C Calibration drift of the infrared agent analyser caused by water vapour in the sampled gas
- D Intermittent retrograde flow of carrier gas into the vapourizing chamber during each inspiration, forcing out extra vapour ✓
Explanation
The pumping effect occurs with positive pressure ventilation: the intermittent rise in circuit pressure pushes gas backward into the variable bypass vapourizer, compressing the vapour-rich chamber gas and delivering a bolus of additional agent into the bypass stream, so output exceeds the dial setting. It is most marked at high peak pressures and low fresh gas flows, and is absent during spontaneous ventilation. Modern vapourizers add baffles and a long spiral inlet to minimize it. Calibration drift would not correlate with the ventilatory cycle, which excludes option C.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.