The Science Journal of the American Association for Respiratory Care

2002 OPEN FORUM Abstracts

A Comparison of Three Nasal Continuous Positive Airway Pressure Systems, (NCPAP), Employed in an in-vitro Lung Model Simulating a Low Birth Weight Premature Neonate:

Laurene Eckbold1, Melanie Hickey1, Thomas Blackson, RRT1, 2, Timothy Cox, RRT3, Thomas H. Shaffer, PhD4, Suzanne Touch, MD5. 1Delaware Tech & Community College, Wilm., DE, 2Christiana Care Health System, Newark, DE, 3A.I. duPont Hospital for Children, Wilm., DE, 4The Lung Center, Nemours Foundation, Wilm., DE, 5Thomas Jefferson University, Phil., PA

Purpose: The purpose of this study was to evaluate the effect of three different NCPAP systems on ventilatory parameters measured in a lung model designed to mimic a premature neonate.

Background: Currently, there is renewed interest in utilizing an underwater seal, ?bubble NCPAP?, to implement NCPAP in very low birth weight, premature neonates. Numerous theories have been proposed to account for the improved outcomes experienced by neonates treated with bubble NCPAP. There is literature to suggest that NCPAP implemented via this system may mimic high frequency oscillatory ventilation, (HFOV).

Methods: A bench model was developed using a drive ventilator to power one side of a Michigan test lung in order to simulate the spontaneous tidal volumes of a 700-gram infant. A 2.5 mm ID endotracheal tube was used to simulate airway resistance. The lung compliance was set at 0.001 L/cm H2O. A ?nose? was fashioned around a 15 mm ID adapter to allow connection of nasal prongs to the breathing simulator. A fixed resistance, differential flow sensor was placed in line between the ?nose? and the endotracheal tube to measure pressure, volume, and flow based parameters during the trials. Positive end expiratory pressure, (PEEP), was established using an underwater seal, a spring-loaded PEEP valve, and a Bear Cub ventilator in the CPAP mode. The same breathing circuit, oxygen blender, flow meter, and nasal prongs were used to evaluate bubble CPAP and the spring-loaded PEEP valve. A conventional infant ventilator circuit was used on the Cub, in conjunction with the ventilator?s flow meter and integrated PEEP valve. In a separate test with bubble CPAP exclusively, the flow transducer was replaced with a hot-wire anemometer designed to measure HFOV tidal volumes and frequency. The lung simulator mimicked an infant breathing at a respiratory rate of 40 breaths per minute, tidal volume of 4.1 ml, and minute volume of 0.166 liters per minute under control conditions with no PEEP applied. Simulator settings were held constant for all test conditions. Each system was evaluated at 7, 8, 9 and 10 LPM of source gas flow and NCPAP set at 5 cm H2O.

Results:

Flow Rate Bubble MV (LPM) Spring-load MV (LPM) Cub MV (LPM) Bubble PEEP Spring-load PEEP Cub PEEP
7 LPM 0.21 0.24 0.169 5.69 5.14 4.28
8 LPM 0.21 0.24 0.17 5.9 5.64 3.92
9 LPM 0.22 0.24 0.16 6.13 5.93 3.17
10 LPM 0.25 0.24 0.155 6.43 5.87 3.31

The hot-wire anemometer was unable to detect tidal volumes in the HFOV monitoring mode under flow conditions of 7, 8, and 9 LPM. When flow was increased to 10 LPM, the monitor registered a 5 Hz high frequency component but no detectable tidal volumes.

Conclusion: Tidal volume, minute volume, and PEEP levels were all influenced by both the gas flow rate powering the NCPAP system as well as the type of expiratory valve used to sustain PEEP in this model. Both the bubble NCPAP and the spring-loaded NCPAP systems augmented tidal volume and minute volume, by as much as 50%, when compared to the control. The Cub NCPAP had no significant effect on simulator tidal volume or minute volume. Bubble NCPAP and the spring-loaded NCPAP system exceeded the desired PEEP setting by as much as 28%. The measured PEEP increased with increasing flow rate. The Cub produced a PEEP level lower than reflected by both the manometer and mean airway pressure LCD of the ventilator by as much as 36%. Measured PEEP decreased with increasing flow rate. An HFOV tidal volume effect with bubble NCPAP was not measurable in this lung model.

OF-02-133

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