The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

PROXIMAL WAVEFORM CHANGES IN RELATION TO THE PRESSURE-VOLUME CURVE DURING VENTILATION WITH THE VOLUMECTRIC DIFFUSIVE RESPIRATOR IN A MECHANICAL MODEL

Robert Estetter RRT, Parkland Health and Hospital System, Dallas, TX T. Al West MD MPH, University of Texas Southwestern Medical School, Dallas, TX

Background: Lung protective strategy--targeting ventilation to volumes generated at points between the upper and lower inflection points on a static pressure-volume (P-V) curve--may prevent ventilator-associated lung injury. The volumetric diffusive respirator (VDR) may also reduce the complications of mechanical ventilation by aiding in mobilization of secretions. Waveform monitoring during ventilation with the VDR may provide insight into the mechanical properties of the lung. We hypothesize that analysis of the oscillatory component of the VDR waveform can be used to select optimal pressure settings for a lung protective strategy.

Methods:
A static P-V curve was derived for a modified Michigan lung model, and lower (P-flexL) and upper (P-flexU) inflection points calculated. The VDR was connected to the model with oscillatory PEEP held constant at 8 cmH2O. Pressure-time waveforms during an expiratory hold were obtained at different values of demand PEEP (PEEPD) corresponding to different regions of the P-V curve. Likewise, inspiratory hold waveforms were obtained at different values of peak inspiratory pressure (PIP). These waveforms were then examined for qualitative differences.

Results:
The pressure-time waveform demonstrates a noticeable "trough" in mid-cycle when the corresponding baseline pressure (PEEPD for end-expiration, PIP for end-inspiration) falls between P-flexL and P-flexU. In real-time at full sweep speed on the oscilloscope, this trough translates into a "bright zone" that is easily distinguished from the flat curves seen outside the inflection points. Discussion: In this bench model, oscilloscope waveform analysis made it possible to set the VDR for a lung-protective strategy. In practice, this is accomplished by slowly increasing PEEPD until the distinctive bright zone on the oscilloscope is seen at end expiration (indicating PEEPD > PflexL), then adjusting PIP until a corresponding bright zone is seen at end inspiration (PIP < PflexU). Ventilating "between the inflection points" may prevent iatrogenic injury either from alveolar collapse or from overdistension. (See Original for Figure)
Figure 1. Proximal waveforms at end-expiration. Note the pronounced "trough" that appears in the midpoint of the waveform (arrow) as demand PEEP exceeds the lower inflection point. Analogous changes are seen in waveforms at end-inspiration, with a trough appearing as PIP falls below the upper inflection point.

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