2011 OPEN FORUM Abstracts
PRESSURE AND VOLUME COST OF HIGH FREQUENCY VENTILATION (HFV) OF INHOMOGENOUS LUNGS IN A NEONATAL TEST LUNG MODEL.
Robert Gillette; Neonatology, Wilford Hall Medical Center, San Antonio, TX
Background: HFV may provide effective gas exchange with less lung injury, by using lower pressure ( P) and volume (Vt) excursions and lower peak pressures (PIP). With homogeneous lungs Vt, < P, and PIP are presumably uniformly minimized. However, neonatal lung disease is usually NOT homogeneous in compliance (C) or airway resistance (R), so hypothetically Vt, P, and PIP may be regionally unequal, perhaps markedly. CO2 flux is believed to depend on frequency (F) times Vt2. "Cost" of HFV might be defined as Vt or P required to provide a given CO2 outflow, i.e. Vt or P divided by F x Vt2. Methods: A rigid-container test lung model was used in which C is the result of internal gas compression. HFV (Sensormedics(R) 3100A or Percussionaire(R) Bronchotron(R)) was connected via a 15 mm "Y" to two 3.0 L calibration syringes (Hans Rudolph) used as test lungs (C=0.15-2.1 ml/cmH2O set by varying their volumes), via a 4.0 mm endotracheal tube (ETT) and calibrated Pneuflo(R) (Michigan Instruments) parabolic airway resistors of 5-200 cmH2O/L/s in line with each lung. Vt to each lung as well < P, PIP, and mean pressure (MAP) were measured proximally, at the "carina" or Y, and in each lung with a Florian(R) (Acutronic) hot wire anemometer/monitor or Setra 239 pressure transducer, at various combinations of C, R, F, and proximal P. P and Vt per unit ventilation (F x Vt2) were calculated and compared between settings, holding F, P, or F x Vt2 constant. Results: 1) P attenuation across ETT and R depended on total distal impedance, with more P drop across higher R and with lower distal impedance (higher C, lower R) 2)Vt to each lung depended on the total impedance (R and C) of the path to it, while its P = Vt/C; there was higher P with lower C; lung with higher R had lower Vt thus lower P; Vt could vary markedly 3)MAP in lung fell as proximal P rose and varied with lung impedance and F, impacting PIP. 4)PIP was higher in lungs with lower C and/or lower R. 5)The P and Vt "cost" per unit ventilation depended on each lung's R and C as well as F and proximal P; at equal F / proximal P, a lung's "cost" may also depend on impedance of the other lung 6) Holding total F x Vt2 equal, "cost" of unequal lungs varied with F, often showing improved "cost" of one lung while the other became worse. Conclusion: Even in a simple physical model, distribution and cost of ventilation showed complex (though predictable) dependence on F and P. No optimal F could be identified.
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