The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

PRESSURE-TIME WAVEFORMS IN RELATION TO THE STATIC PRESSURE-VOLUME CURVE DURING HIGH-FREQUENCY OSCILLATORY VENTILATION OF A LUNG MODEL

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

Background: High frequency oscillatory ventilation (HFOV) is an alternative mode of ventilation for treating patients suffering from adult respiratory distress syndrome (ARDS) and who are failing conventional mechanical ventilation. The use of an open-lung strategy during HFOV--targeting airway pressures between the upper and lower points on a pressure-volume (P-V) curve--mitigates the risk of iatrogenic ventilator associated lung injury. However, a lack of real time monitoring options makes optimizing ventilator settings at the bedside problematic. We hypothesize that location on the P-V curve is reflected in qualitative changes in the shape of pressure-time waveforms generated during HFOV.

Methods:
A static P-V curve was derived for a modified Michigan lung model, and lower (P-flexL) and upper (P-flexU) inflection points were calculated from a fitted logistic curve. The HFOV was connected to the model with rate (4 Hertz), bias flow rate (30 L/m), and fractional inspiratory time (.5) held constant. Mean airway pressure (MAP) was adjusted to values below, between, and above the inflection points of the P-V curve. At each MAP setting, pressure-time waveforms were obtained from the oscilloscope and examined.

Results:
Inflection points calculated from the logistic curve were P-flexL = 17.6 cmH2O and P-flexU=38.3 cmH2O. The pressuretime waveforms obtained at three values of MAP are shown in Figure 1. The waveforms demonstrate a "shoulder" at the beginning of the piston upstroke when MAP < P-flexL or MAP > P-flexU. In contrast, when P-flexL < MAP < P-flexU, the upstroke of the waveform is smooth.

Conclusions:
In this bench model, changes in MAP along the P-V curve appeared to be associated with demonstrable changes in the shape of the oscillatory waveform. Graphical waveform analysis may allow the clinician to select optimal HFOV settings for a lung protective strategy. Our early experience with this method in clinical application has generated similar observations. (See Original for Figure)
Figure 1. Oscillator waveforms measured proximal to the endotracheal tube. Note the pronounced "shoulder" that appears at the beginning of the piston stroke (arrow) as mean airway pressure exceeds the upper inflection point or falls below the lower inflection point.

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