The Science Journal of the American Association for Respiratory Care

2002 OPEN FORUM Abstracts

The Effect of Ventilator Circuit Length on Delivered Volume during High Frequency Oscillatory Ventilation.

J. Davies BS MA RRT, M. Leonard RRT, G. Ahearn MD, M. Gentile RRT, N. MacIntyre MD.

Introduction: Piston displacement during high frequency oscillatory ventilation (HFOV) determines the pressure and volume changes in the ventilator circuitry. However, circuit compliance may have significant effects on the transmission of these pressures and volumes into the lungs of patients. One factor affecting circuit compliance is circuit length.

Hypothesis: We hypothesized that the use of a longer ventilator circuit would lower the tidal volume delivery to the patient during HFOV.

Methods: A Sensormedics 3100b oscillator was attached to an Ingmar PMG 3000 test lung (Ingmar Medical, Ltd, Pittsburgh, Pa.). Compliance of the test lung was set at 20 ml/cm H2O, resistance to mimic a typical patient with acute lung injury. We used the following settings: FiO2 = 0.21, mean pressure = 15 cm H2O, delta pressure = 70 cm H2O, and I time % = 33%. Two ventilator circuits were used: the first was 51 inches long with a circuit compliance of 0.5 ml/cm H2O (long circuit); the second was 38 inches long with a compliance of 0.3 cm H2O (short circuit). Delivered tidal volume to the test lung was determined with oscillator bias flows of 20, 30 40 and 50 lpm, and breathing frequencies of 3, 4, 5 and 6 Hz. Data were collected over a 3-minute time period for each combination of bias flow and frequency. For each combination of bias flow and frequency, tidal volume delivery with each circuit was compared by two-tailed paired t-tests. Significance was defined as p<0.05.

Results: Data analysis showed a statistical significance (p<. 0001) in ml of tidal volume delivery with differing lengths of oscillator tubing under all test conditions (Table).

Bias Flow – 20 Long Short Bias Flow – 30 Long Short
3 Hz 125 134 3 Hz 111 119
4 Hz 106 111 4 Hz 96 97
5 Hz 93 98 5 Hz 85 90
6 Hz 81 92 6 Hz 75 80
Bias Flow – 40 Long Short Bias Flow – 50 Long Short
3 Hz 102 114 3 Hz 102 111
4 Hz 87 91 4 Hz 83 90
5 Hz 76 79 5 Hz 74 78
6 Hz 69 73 6 Hz 66 69

Conclusion: Longer oscillator circuits with higher circuit compliances serve to dampen tidal volume delivery during HFO. This has the potential to affect gas transport. While the use of a longer oscillator circuit may aid in convenience around the bedside (since the oscillator can be positioned farther away from the bedside), clinicians must be aware of the potential loss of volume to the lungs as a consequence of longer circuitry.


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