The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Tom Blackson1, Joseph Ciarlo1, David H. Horney1, Tim Cox1, Thomas H. Shaffer1

Background: According to the manufacturer the primary ventilation strategy espoused for use with the 3100B high frequency oscillator (HFO), is to adjust the pressure amplitude (ΔP) upward to counter high PaCO2's followed by reduction of the frequency (f), as a secondary option for reducing PaCO2. Recently, an expert panel has recommended an alternate strategy suggesting that f adjustments may be the most expedient approach for lowering PaCO2 during HFO with increases in ΔP being a secondary consideration. Purpose: We evaluated both f and ΔP changes on a test lung model of ARDS with the 3100B HFO to compare tidal volume delivery using the two published approaches for ventilation management.

Materials & Methods: A bench model of ARDS was created with a Michigan test lung set with a compliance of 0.02 L/cmH2O. The test lung was ventilated during all trials with a mean airway pressure: 30 cm H2O, bias flow: 30 L/min., and inspiratory time: 33%. We evaluated frequencies of 3, 4, 5, and 6 HZ in combination with pressure amplitudes between 30 and 90 cm H2O adjusted in 10 cm H2O pressure increments. Airway resistance was varied using parabolic resistors #5 and #20 for each test condition. A hot-wire anemometer flow sensor capable of measuring HFO tidal volumes, was placed between the ventilator "Y" and the airway resistor. Twelve consecutive volume measurements at each test condition were recorded and used for analysis.

Results: Incremental changes in tidal volume were not linear with either approach. Tidal volume changes using ΔP adjustments ranged from a minimum of 0 mL to a maximum of 58.75 mL for a 10 cm H2O ΔP change. Incremental changes in tidal volume using f adjustments ranged from a minimum of 0.9 mL to a maximum of 54.58 mL for a 1 Hz f change.

Conclusion: Neither ventilation strategy consistently resulted in the greatest increase in tidal volume delivery under all test conditions. Although there appears to be an optimal ΔP range within which f adjustments produce the greatest incremental change in tidal volume, this ΔP range varied with changes in airway resistance. Clinical application of both published approaches to ventilation adjustment would benefit from tidal volume monitoring. Clinical Implications: Effective clinical management of ventilation and acid/base balance may be delayed if effective increases in tidal volume delivery are presumed to result from either increased ΔP or reduced f adjustments. Clinical study is warranted.