2001 OPEN FORUM Abstracts
EFFECTSOF AUTO-TUBE COMPENSATION (ATC) ON TIDAL VOLUME (VT) DELIVERY DURINGCONTROLLED MECHANICAL VENTILATION
M Siobal BS RRT,RH Kallet MS RRT, RW Kraemer CRTT, J Tang MD Respiratory Care Services, Departmentof Anesthesia, San Francisco General Hospital, University of California SanFrancisco.
Background:ATC augments ventilator inspiratory flow rate [I to achieve the proximalairway pressure target at the distal tip of the endotracheal tube (ETT), thuscounterbalancing the imposed WOB of the artificial airway. ATC can be activatedin all pressure-regulated modes of ventilation. We inquired how ATC may impactVT and lung pressures in the absence of patient triggering duringsimulated lung protective ventilation.
Methods: A MichiganInstruments TTL lung simulator was set at a compliance of 27 mL/cm H2Oand attached to a Dräger E-2 ventilator. Pressure control (PC) and volume controlwith Auto-Flow (VC-AF) modes of ventilation were studied. Also, pressure support(PS) was used as a reference point to compare the performance of ATC. A VTof 400 mL at a respiratory rate = 20 was used. During PC and PS, a VTof 400 mL was obtained with a set pressure = 16-17 cm H2O. No end-expiratorypressure was used. During PS, breaths were ?auto-triggered? by lightly compressingthe model bellows, creating a small flow spike that captured the flow triggerthreshold (5 L/m). Experiments were done with ATC turned off and at an ATC of100% compensation set to the ETT size used in the model (7.0 mm ID). Measurementswere made with a Ventrak pulmonary mechanics monitor; 10 breaths were used foranalysis. The end-inspiratory pressure in the compartment (PCOMP)was used as an analog for lung pressure. Pressure also was measured at the distaltip of the ETT at the point of peak airway pressure in the circuit (PAP). Datawas reported as mean ± standard deviation and analyzed using repeated-measuresanalysis of variance and Student Newman Keul tests. Alpha was set at 0.05.
Results: DuringPS, ATC increased peak [I and PAP to achieve the preset PS levelat distal tip of the ETT. VT delivery was increased (despite an absenceof post-trigger inspiratory effort), thus PCOMP also increased. However,during PC, flow augmentation during ATC caused an overshoot in the distal ETTpressure (20 vs. 16 cm H2O). During VC-AF, ATC caused a transientincrease in [I, PAP and VT that was corrected by the ventilatorwithin 3 breaths. As a result, only the comparisons of VT [Iand PCOMP (with vs. without ATC) during VCAF were not statisticallysignificant (*p > 0.05).
|Mode||VT(mL)||[I (L/min)||PCOMP(cmH2O)||PAP (cm H2O)|
|PSV||428 ± 7||67 ± 0.8||16 ± 0||17 ± 0|
|PSV + ATC||744 ± 19||88 ± 1.2||25.8 ± 0.8||30 ± 1|
|PCV||433 ± 2||64 ± 0.7||16 ± 0||16 ± 0|
|PCV + ATC||699 ± 9||84 ± 1||25 ± 0||29 ± 0.5|
|VC-AF||425 ± 3*||63 ± 0.4*||16 ± 0*||16 ± 0|
|VC-AF + ATC||438 ± 84*||61 ± 8*||16 ± 3*||18 ± 4|
Conclusion:Our results indicate that ATC increases PAP above the operator-set pressurelevel. Both VT and [I delivery are increased (particularlyduring PCV), even in the absence of inspiratory effort. Therefore, ATC shouldnot be used when strict control of VT and lung pressure is intended.