2001 OPEN FORUM Abstracts
EFFECTOF TUBE COMPENSATION (TC) VS PRESSURE SUPPORT (PS) ONTIDAL VOLUME (VT) & AIRWAY PRESSURES: A BENCH STUDY
CarlHaas, MLS, RRT and Paul Loik, RRT University of MichiganHealth System, Ann Arbor MI
BACKGROUND:PS provides a constant pressure assist, regardless of patient effort, to overcomeresistance of an artificial airway while TC provides a variable pressure assistlevel in direct proportion to patient effort. Clinically, it is unclear whetherVT is augmented by high airway pressure seen using TC with small artificialairways.
STUDY QUESTIONS:1) What impact does TC have on VT, proximal, distal, and end-inspiratory pressures,compared to PS; and 2) What influence do artificial airways have on PS and TC?
Methods: Lungmodel: A computerized lung simulator (ASL-5000, Igmar Medical) was set toa single lung model with C=50 mL/cmH2O, R=10 cmH2O/L/s,RR=20, an inspiratory effort of ?15 cm H2O and settings resultingin an I:E of ~1:3.0. Ventilator settings: A PB-840 ventilator (CarlsbadCA) was set to PEEP=0 cmH2O, FIO2=0.21, and trigger= ?1cmH2O. For PS breaths, flow acceleration=50% and expiratory sensitivity=10%.The ventilator was tested at PS=0, 5, and 10 cmH2O; and TC=50 and100%. Airways: Endotracheal tubes (ETT) of an ID of 6, 7, 8, and 9 mmwere studied. The distal end of the ETT was attached to the lung simulator andthe proximal end to the circuit wye with identical adapters. Measurements:Exhaled VT, peak pressure (Pprox) and end-inspiratory pressure (Pei)from the ventilator were recorded. The simulator measured flow, volume, andpressure. These measurements were taken immediately distal to the airway tested,therefore airway pressure is considered to reflect ?carinal? pressure (Pdistal).During pressure verification, Pdistal read 1 cmH2O >Pprox and Pei. Data: Data from the ventilator andsimulator for 10 breaths were analyzed for each data-set.
Results: As areference, PS=0 with no airway attached resulted in a VT=435 mL. Table 1 showsthe result on VT [in mL] and the percent boost in VT compared to PS=0 with thesame ETT.
|ETT=6||278||326 (17%)||398 (43%)||303 (9%)||386 (39%)|
|ETT=7||329||386 (17%)||471 (43%)||350 (6%)||432 (31%)|
|ETT=8||367||435 (19%)||527 (44%)||373 (2%)||422 (15%)|
|ETT=9||381||453 (19%)||563 (48%)||387 (2%)||409 (7%)|
Table 2 shows the effecton three pressures: Pprox, Pdistal and Pei(in cmH2O).
Conclusions:1) For a given inspiratory effort, VT is reduced as ETT size is reduced; 2)for a given ETT size, PS boosts VT in an incremental manner (similar %, regardlessof ETT) and TC augments VT in a variable manner (less % boost in VT as ETT increases);3) TC=100% augments VT > TC=50%, especially with small ETTs; 4) the highairway pressure noted at the ventilator (Pprox) using TC=100% withsmall ETTs is not present at the carina (Pdistal) and therefore doesnot translate into a larger VT; and 5) with the PB-840, Pei may betterreflect Pdistal. The clinical relevance of these findings remainsto be determined.