The Science Journal of the American Association for Respiratory Care

2009 OPEN FORUM Abstracts

HIGH FREQUENCY VENTILATOR (HFV) TIDAL VOLUME (TV) DISTRIBUTION TO INHOMOGENEOUS LUNGS IN A NEONATAL TEST LUNG MODEL

Robert Gillette; Neonatology, Wilford Hall Medical Center, San Antonio, TX

BACKGROUND: Gas exchange with HFV is believed to depend on frequency (F) x TV2. Distribution of TV between lung regions with inhomogeneous compliance (C) and resistance (R), common in severe disease, and its variation with F and pressure amplitude (ΔP), are not well studied. METHOD: A Sensormedics 3100A or Percussionaire Bronchotron HFV was connected to two 3.0 L calibration syringes used as rigid-container-model test lungs with C of 0.15-2.1 ml/cmH2O set by varying their volumes, via a 4 mm endotracheal tube, a 15 mm Y, and parabolic airway R’s of 0-250 cmH2O/L/s in line with each lung. TV to each lung was measured with a Florian hot wire anemometer (Acutronic), varying F (keeping either ΔP or FxTV2 constant) or varying ΔP (with F constant), at various C and R of each lung. Mean airway pressure (MAP) and ΔP were measured proximally and in each lung. RESULTS: With bilateral very low R, distribution of TV reflected the ratio of C between lungs over a wide range and all F’s. TV distribution between lungs with equal high R (>200) was nearly equal, more so at higher F and more-equal C. With very unequal R (e.g. 5 vs 200) TV was more equally distributed at lower F if C was poor to moderate/equal bilaterally or inhomogeneous with R/C impedance mismatch, but if C was higher/equal, or unequal but R/C-matched, TV mostly went to the low-R side, independent of F and minimally reflecting C inequality. With low-moderate, equal to moderately-unequal R, and matching-sided unequal C, TV was more equally distributed at higher F and at more-equal C, as it was when lungs were mismatched for R and C but with moderately inhomogeneous R. Dependence of TV distribution on F was similar whether ΔP or FxTV2 was held constant. Holding F constant while varying ΔP, TV was more evenly distributed at higher ΔP when inhomogeneity was mostly in C, and at lower ΔP if mainly R inequality. MAP and PA were always lower in lungs than proximal, and relative MAP and ΔP between lungs depended on R and C configurations and F. Results were similar for both ventilators. CONCLUSIONS: Distribution of TV between lungs in this physical model depended markedly on inhomogeneities of R and C. Dependence on F and PA varied with specific combinations of R and C. Electrical circuit analog theory approximated the findings. Speculatively these results may favor clinical use of higher F in early (low R, inhomogeneous C) premature lung disease and lower F with airway disease (highly-inhomogeneous R). Sponsored Research - None

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