2012 OPEN FORUM Abstracts
HIGH FREQUENCY OSCILLATORY VENTILATOR (HFOV) TIDAL VOLUME (VT) VARIES WITH LUNG IMPEDANCE (Z) â BUT WHERE DOES THAT VT GO?
Robert Gillette; Neonatology, SAMMC, San Antonio, TX
Background: Vt to lung from SensorMedics 3100A HFOV changes despite fixed pressure amplitude (?P) and frequency (F) as compliance (C), resistance (R), and/or ETT size (all components of Z) change, but the mechanism is unclear. Part (âVoâ) of the pistonâs oscillatory volume goes out the exhalation valve. Hypothetically as lung Z changes, the piston oscillating volume remains constant and Vo changes to compensate for changes in lung Vt. This study explores that question in a bench model. Methods: A 3100A HFOV ventilated a 3 L calibration syringe used as a rigid-container test lung with C of < 0.1 to 2.1 ml/cmH2O set by varying its volume, via a 4.0 mm ETT. Proximal and lung ?P were measured with a Setra 239 pressure transducer, and lung Vt was calculated from lung ?P. Vo was measured with a TSI model 4043 mass flowmeter in the circuitâs outflow limb. A USB data acquisition device and Labview software were used. The effect of a large lung C change was studied at various F, ?P, mean airway pressure (MAP), and bias flow, with POWER or ?P constant for each C pair. Results: In all cases, changing from high (2.1) to very low (clamped ETT) C only modestly affected Vo. For example, at bias flow 20, MAP 12, 5 Hz, proximal ?P of 20 at C of 2.1, Vo=9.2 ml and Vt=15.9, while with ETT clamped (zero C, no Vt) Vo rose to only 11.5 while ?P changed to 23. In no case was the Vt drop fully diverted to add to Vo; Vt + Vo (total piston output) always fell. Changing either bias flow or MAP with the other constant, keeping proximal ?P unchanged, significantly affected Vo but minimally changed Vt. When POWER instead of ?P was kept constant while making these changes, there was modest effect on Vt but again large effects on Vo. . Conclusion: The hypothesis that increase in lung Z diverts Vt into Vo was disproven. This may be because altered lung Z does not change outflow valve R, so Vo only changes if ?P does; increase in ?P would increase Vo, whether due to increased lung Z or a control adjustment. Proximal ?P did go up at constant POWER when the ETT was clamped, accounting for the change in Vo. Further, a change in bias flow with unchanged MAP (and vice versa) changes outflow R, and thus Vo, with either proximal ?P or POWER constant. These findings suggest that the piston acts as a nearly âideal pressure sourceâ, where ?P changes are modest but Vt+Vo changes can be large with change in Z load on the ventilator due to either outflow R or lung Z changes. Sponsored Research - None