The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Tom Blackson1, Joseph Ciarlo1, David H. Horney1, Tim Cox2, Thomas H. Shaffer3

Background: One ventilation strategy that is espoused for use with the 3100B high frequency oscillator (HFO), is to adjust the pressure amplitude (ΔP) based upon "wiggle factor". The assumption is that ventilation increases with increasing ΔP. Purpose: We evaluated the effect of increasing ΔP on tidal volume (VT) delivery during ventilation of a test lung model of ARDS with the 3100B HFO.

Materials & Method: A bench model of ARDS was created with a Michigan test lung, (Michigan Instruments, Grand Rapids, MI) set with a compliance of 0.02 L/cmH2O. All trials were conducted with a mean airway pressure: 30 cm H2O, bias flow: 30 L/min., and inspiratory time: 33%. We evaluated frequencies of 3 through 6 HZ in combination with ΔPs between 30 and 90 cm H2O adjusted in 10 cm H2O pressure increments. Airway resistance (Raw), was varied using parabolic resistors (PR) #5 and #20 for each test condition. A hot-wire anemometer flow sensor, (Florian, Acutronics), was placed between the ventilator "Y" and the PR. Twelve consecutive volume measurements at each test condition were recorded and used for analysis. In addition, the change in power setting needed to effect each 10 cm H2O change in ΔP was also documented.

Results: Incremental changes in VT were inconsistent across the ΔP ranges tested under all test conditions. VT changes ranged from a minimum of -0.08 mL for a 10 cm H2O ΔP change at a f of 6 HZ with a PR #5 to a maximum of 58.75 mL at a f of 3 HZ and a PR #5. Incremental changes in VT were greatest at ΔP between 50 - 80 cm H2O and were smaller at both lower and higher ΔPs regardless of frequency or Raw. VT reaches a plateau prior to achieving maximum ΔP under certain combinations of frequency and Raw. A power change of greater than 1 needed to accomplish a ΔP change of 10 cm H2O was associated with a VT plateau.

Conclusion: Fixed increases in ΔP produce inconsistent changes in VT delivered from the 3100B in this test lung model. Exhaled VT reaches a plateau despite further increases in the ΔP under certain combinations of f and Raw. In the absence of bedside VT monitoring, VT plateau can not be identified via ΔP monitoring feedback provided by the 3100B HFO, but may be identified by trending the relationship between power setting and ΔP. Clinical Implications: Clinical management of ventilation and acid/base balance may be delayed if effective increases in VT delivery are presumed to result from increased ΔP adjustments. Clinical study is warranted.