Background: Asymmetric waveforms with shorter inspiration than expiration have been preferred recently in HFO. We have investigated how the asymmetry affects the efficacy of CO2-removal.

Subjects: 4 female domestic pigs, age 1 day, weight 1.6 - 1.9(mean 1.76) kg

Interventions: Pigs were anaesthetized and intubated (tubeØ 3.5mm). HFO was applied with an HFV-Infantstar (software version 48) or a Sensor Medics 3100A, which both generate square-wave pulses. The animals were ventilated with fractional inspiratory times (FIT) of 18%, 30% or 50%(I:E = 1:4.6, 1:2 or 1:1) in random order. Oscillation amplitude was adjusted to result in paCO2-values between 35 and 45 mmHg and measured with a ventilator-independent pressure gauge connected between the y-piece and the endotracheal tube. Mean airway pressure was 5.5 - 12 cmH2O, oscillation frequency was 10 Hz throughout

Results: Mean amplitudes needed to achieve the desired paCO2-values were 48.6 cmH2O at 18% FIT, 39.5 (30% FIT) and 27.5(50% FIT) (p<0.05).

Conclusion: Efficacy of asymmetric high frequency oscillation with shorter inspiration than expiration is proportional to the fractional inspiratory time and the I:E-ratio, with symmetric oscillation being most effective.