1999 OPEN FORUM Abstracts

THE EFFECT OF ACE® SPACER ON CO₂ ACCUMULATION IN A NEONATAL VENTILATOR-LUNG MODEL

Jim Keenan, BS, RRT, Ralph A. Lugo, PharmD, John W. Salyer RRT, BS, MBA, FAARC. University of Utah College of Pharmacy and School of Medicine, Primary Children's Medical Center, Salt Lake City, Utah.

Background: Aerosolized albuterol (ALB) is commonly administered to mechanically ventilated neonates via MDI and spacer. Most manufacturers recommend placing spacers in the inspiratory limb, immediately proximal to the circuit wye. This location may be impractical during neonatal ventilation due to continuous flow and difficulty in adapting spacers to the circuit. Thus, practitioners often place spacers between the wye and endotracheal tube (ETT). However, large-volume spacers introduced at this location may result in CO₂ retention due to mechanical dead space. The objective of this bench study was to determine the relationship between CO₂ accumulation in an ACE® spacer and the time that a spacer remains in-line between the wye and the ETT. In addition, we sought to determine the effect of different tidal volumes on CO₂ accumulation.

Methods: The model consisted of a Bird VIP ventilator in a time cycled (rate 20), pressure-limited, continuous flow (7L/min) mode. The circuit wye was attached to an ACEÒ spacer and a neonatal test lung. The model was constructed so that inspiratory tidal volume was vented through the test lung into the environment while maintaining a ventilating pressure of 20/1 mm Hg. Expiratory tidal volume was manually returned to the model through a one-way valve by a resuscitation bag attached to a source of 5% CO₂, thus simulating in vivo end-tidal CO₂ (35 mm Hg). Inspiratory and expiratory tidal volumes were measured continuously by computerized pneumotachography (Ventrak, Novametrix). For each ventilator breath, expiratory tidal volume (5% CO₂) was manually matched to the inspiratory tidal volume. The accumulation of CO₂ in the spacer was measured by placing a calibrated end-tidal CO₂ monitor (Novametrix) on the ventilator-end of the spacer. Instantaneous CO₂ measurements were obtained from gas displaced from the spacer during exhalation. Three replicate experiments were conducted for each of the three tidal volumes tested: 9 mL, 15 mL, and 25 mL. For each experiment, end-tidal CO₂ was measured for a total of 3 minutes.

Results: Data are represented as mean (SD) end-tidal CO₂ (mm Hg) for each 30 second interval (n=30).

CONCLUSION: Accumulation of CO₂ within the spacer increased with time and reached a maximum of 6-7 mm Hg in this model. Due to the short duration of spacer use when administering drugs via MDI, this level of CO₂ exposure is likely clinically insignificant for the majority of ventilated newborns.

OF-99-205