The Science Journal of the American Association for Respiratory Care

2002 OPEN FORUM Abstracts

PEDIATRIC CIRCUITS VS. CORRECTION FOR COMPRESSIBLE VOLUME DURING LOW TIDAL VOLUME STRATEGIES – DOES IT MATTER?

Terri Clark, RCP, RRT, Tom Malinowski, RCP RRT, FAARC, James Lamberti MD. Inova Fairfax Hospital, Falls Church, VA.

BACKGROUND: Correcting for ventilator circuit compressible volume (CV) is important during low tidal volume ventilation. Two methods used to correct CV include substitution of a pediatric circuit for a standard adult volume ventilator circuit, or the correction for CV via formula. We sought to determine if either method was superior as determined by FECO2% and/or auto-PEEP using a bench model.

METHOD: One chamber of a dual chambered test lung (VentAid? Training Test Lung) was connected to a mechanical ventilator set to specific parameters (table 1). 320 ml/minute of 100% CO2 was continuously bled into the test lung to simulate CO2 production. A carbon dioxide sampling line was placed between the lung chamber and the ventilator circuit wye. Three deadspace volumes (100, 150, 200 mL) were added between the wye and test lung to mimic physiologic deadspace. CV was determined by a previously described standard process; occlude the ventilator wye, observe corresponding volume and peak pressure during ventilator cycling, dividing the observed volume by the peak inspiratory pressure minus PEEP. The CV was then added to the set tidal volume. The FECO2 was allowed to stabilize between settings and prior to recording values (>1 minute).

Group Imposed VD (mL) Frequency (bpm) Vt (mL) Vt corrected (mL) I:E ratio Ve (L/min)
A 100 – 200 30 250 300 1:2 7.5
B 100 – 200 25 350 412 1:2 8.75
C 100 – 200 20 450 525 1:2 9.0

Test lung compliance 0.02 L/cm H2O

 

Group

VD (mL) Baseline FECO2 % Peds Circuit FECO2 %

Comp. Vol. Corr. FECO2 %

A

“0” 1.9 1.6 1.4
A 100 2.6 2.8 1.8
A 150 3.15 3.35 2.9
A 200 5.94 5.4 4.2
B “0” 1.7 1.2 1.2
B 100 1.8 1.6 1.4
B 150 2.4 2.3 1.7
B 200 3.7 2.9 2.2
C “0” 1.0 .9 .9
C 100 1.4 1.6 1.4
C 150 1.7 1.8 1.7
C 200 2.4 2.0 1.9

Results: In this bench model, CO2 clearance beyond baseline levels was enhanced most by correction for CV, rather than by substitution with a pediatric circuit. Auto-PEEP was not observed in any of the groups.

DISCUSSION: Correction for CV was not identified in the initial ARDSnet trial publication, but has subsequently been identified as a factor worthy of consideration. Correcting for CV enhances CO2 clearance and may reduce dyspnea associated with low tidal volume strategies.

OF-02-153

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