2005 OPEN FORUM Abstracts
THE EFFECT OF PATIENT EFFORT, PRESSURE SUPPORT LEVEL AND ENDOTRACHEAL TUBE SIZE ON AUTOMATIC TUBING COMPENSATION (ATC) OF THE Dräger Evita 2 USING A LUNG SIMULATOR.
Jody Lester, M.Ed., R.R.T, Randy Rose. Boise State University, Boise, ID
Background: When inspiratory automatic tubing compensation (ATC) is selected on the Dräger Evita 2, the ventilator calculates the pressure difference between proximal and carinal pressure and increases the pressure in the system by this amount. ATC may be set to completely (100%) or partially (0-99%) compensate for ETT resistance. We hypothesized that patient effort, pressure support level and endotracheal tube size would have no effect on ATC response.
Methods:Pressure and flow transducers were attached to the proximal and carinal ends of a simulated trachea which was "intubated" with an uncut ETT. The proximal end of the ETT was attached to a Dräger Evita 2 with the following settings: CPAP of 5 cm H2O plus pressure support, trigger of 1 lpm, slope of 0.2, expiratory ATC off. The distal end of the ETT was attached to a Hans Rudolph, inc. Series 1101 Breathing Simulator programmed to produce a respiratory rate of 20 BPM at a compliance of 40 ml/cm H2O, a resistance of 10 cmH2O/L/sec, and an amplitude (patient effort/peak negative pleural pressure) of either 11 or 22 cm H2O. Proximal and carinal pressures were measured and graphed for a 6.5 and 7.5 mm ETT, with PS of 10 and 15, amplitude of 11and 22, with ATC off or on (100% compensation with correct ETT size entered). The ETT was positioned to simulate a natural curve with no kinking.
Results:The graph below was selected to illustrate the patterns we observed. With ATC off, peak and carinal pressures were similar (within 1-2.5 cmH2O) and met or exceeded the level of PS + CPAP. Peak proximal pressures with ATC on increased 5-38 cmH2O over peak proximal pressures with ATC off. With ATC on, peak carinal pressures were 3-18 cmH2O higher than peak carinal pressures with ATC off. Very high peak (up to 77.8 cmH2O) and carinal (up to 39.7 cmH2O) pressures were measured when using the 6.5 mm ETT with PS of 15 cmH2O and an amplitude setting of 22 cmH2O. With ATC on, carinal pressures exceeded the PS + CPAP level by 6-22 cmH2O. With ATC on, measured tidal volumes always increased. For example, with the 7.5 mm ETT, settings of PS 15, amplitude 22, the tidal volume increased 410 ml when ATC was turned on. These pressures were not the result of a "test breath" sequence as they remained at these levels for the entire series of breaths. Increased patient effort resulted in carinal pressures at the beginning of inspiration that were 4 - 6 cmH2O lessthan the set CPAP whether ATC was on or off.
Conclusions: Proximal and carinal peak pressures were closest to the PS + PEEP level when ATC was off. With ATC on, increased patient effort and pressure support levels cause large increases in tidal volume, peak and carinal pressures. It is important for the clinician to carefully set the high-pressure limit to prevent high peak pressures and large tidal volumes. When set at 100% compensation, the ventilator significantly overcompensates. At the beginning of inspiration, ATC did not maintain set CPAP levels at the carina when we simulated increased patient effort. Patient effort, pressure support level and endotracheal tube size did have an effect on ATC response.