Volume-Guarantee Ventilation: Pressure May Decrease During Obstructed Flow

Background: Two unexpected observations were made during ventilation with the Drager Babylog 8000+ in volume-guarantee mode: (a) during complete obstruction to gas flow down the endotracheal tube (ETT), positive inspiratory pressure (PIP) was reduced to half way between the maximum inflating pressure and the positive end expiratory pressure (PEEP) even though the set expired tidal volume had not been achieved; (b) an external Drager waveform monitor may stop displaying real-time waveforms when a tube-obstructed alarm is activated.Objective: To investigate these phenomena using a test lung.Method: A 50 ml Drager test lung was attached to the ventilation circuit of a Drager Babylog 8000+. Partial obstruction to ETT flow was induced by compressing the tubing leading to the test lung, and complete obstruction was achieved by clamping. Recordings were made from the digital Output of the ventilator at 125 Hz.Results: When the ETT flow was completely obstructed during VG ventilation, a constant PIP was set midway between the set maximum and PEEP, This did not happen during partial obstruction. The external waveform monitor display froze when ETT flow was completely obstructed.Conclusions: During complete ETT obstruction, the PIP is set to a pressure midway between maximum PIP and PEEP even if this is less than the PIP used before the obstruction. Further research is needed to evaluate whether this reduction in PIP is associated with prolongation of precipitating events.