2007 OPEN FORUM Abstracts
EFFECT OF LUNG MECHANICS ON PRESSURE AMPLITUDE ATTENUATION BY THE ENDOTRACHEAL TUBE IN A NEONATAL HIGH FREQUENCY VENTILATOR TEST LUNG MODEL
R. Gillette1, S. Messier1
Background: It is widely taught that amplitude of pressure oscillation in high frequency ventilation is sharply attenuated by the endotracheal tube (ETT). However, theoretical models as well as in vivo and bench studies have suggested the degree of attenuation depends on lung compliance (C) and other distal impedance variables, e.g. airway resistance (R).
Objective: To investigate the dependence of ETT attenuation of pressure amplitude on lung compliance and respiratory system resistance, ETT size, frequency (F), and proximal pressure amplitude (PA), thus flow, as well as on ventilator type.
Methods: A SensorMedics 3100A (Viasys) or Bronchotron (Percussionaire) ventilator was connected to a 1.5 L calibration syringe (Hans Rudolph) used as a test lung with compliance of 0.05-1.1 ml/cmH2O set by varying its volume of compressible air, via a 2.5 or 3.5 mm ETT and calibrated airway resistor of 0-250 cmH2O/L/s, simulating sick newborn lungs. Flow and pressure were measured with a Florian hot wire anemometer and pressure transducer (Acutronic). The ratio (OPR) of post- to pre-ETT oscillatory pressure amplitude was studied at various combinations of other variables on each ventilator.
Results: OPR ranged from 0.1 - 1.1, depended highly on lung impedance (R and C), and was always lower for the 2.5 than 3.5 ETT at equivalent settings. For high R (~200) and/or low C (~ 0.1) there was little dependence on F, but that nearly-fixed level of OPR did differ with R, C, PA (flow), and ETT size. With low R (~20), OPR for any F and both ETTs dropped from 1.0 to 0.1-0.3 as C rose from 0.1 to 1. At low R (~20) and high C (~ 1.0) OPR dropped rapidly from 0.2-0.4 to 0.05-0.2 as F rose from 5 to 10 Hz. Lower OPR was seen with higher proximal PA (flow) if R was low, and the reverse if R was high, suggesting positive flow dependence of R. This was more pronounced on the Bronchotron at the same PA.
Conclusion: OPR is markedly dependent on lung mechanics. It is highly attenuated in healthy lungs but minimally with high R or low C as in severe disease. Results were consistent with calculated ratios of impedance (based on R, C, frequency, and inertance). Both ventilators were similar, with Bronchotron results more complex and less linear, perhaps due to changing waveform as PA changes. We speculate that high OPR may exacerbate effects of regional variation in lung mechanics on tidal volume and pressure distribution, injury, and gas exchange.