The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

Does AAC Maintain Targeted Pressures When Endotracheal Tube Size, Pressure Support Level, And Inspiratory Effort Are Varied?



Randy Rose, Jody Lester M.Ed., R.R.T. Boise State University, Boise, ID

Rationale: According to the manufacturer of the Viasys Avea (VA) when Artificial Airway Compensation (AAC) is turned on, the ventilator automatically calculates the pressure drop across the endotracheal tube (ETT) and adjusts the airway pressure to deliver the set inspiratory pressure to the distal (carinal) end of the ETT. The purpose of this study was to measure proximal and carinal pressures and determine if AAC compensates for the resistance of the (ETT) when ETT size, pressure support level, and inspiratory effort are varied.

Methods:
The Viasys Avea was studied. Pressures and flow rates were measured at the proximal and carinal ends of a 6.5 mm and 7.5 mm ETT's connected to a Hans Rudolph Series 1101 Breathing Simulator. The ETT was manipulated to simulate a natural curve and was not subjected to twisting or kinking. Breathing simulator settings were resistance of 10 cmH2O/L/sec, compliance of 40 ml/cm H2O, RR of 20, amplitude (patient effort/peak negative pleural pressure) of 11 and 22 cm H2O. The ventilator was placed in CPAP at 5 cmH2O and PSV levels of 10 and 15 cmH2O, rise time 0.5 and flow trigger of 1 lpm. Proximal and carinal pressures were measured and graphed.

Results:
The following graph is included to demonstrate the patterns we observed. We found that proximal peak pressures increased (1.6-6.3 cmH2O) with AAC on, with the greatest change occurring when amplitude (patient effort) was 22 cmH2O. With the 6.5mm ETT the difference between measured proximal and carinal peak pressures was slightly larger (1-3.5 cmH2O) than with the 7.5mm ETT (0.38 - 2.1 cmH2O). Peak pressures were at or above the PS + CPAP level with AAC on or off. Measured base pressures were 2-6.5 cmH20 less than set CPAP (5 cmH2O) at the carinal ends of the ETT regardless of the amplitude with AAC on or off at the beginning of inspiration. When patient effort increased significant differences (up to 6.5 cmH2O) between proximal and carinal pressures and set CPAP levels were measured.

Conclusions:
When AAC was on we expected proximal pressures to increase, however, the increase in pressure was only minimally transferred to the carina. The pressure transfer was greater with the 7.5mm endotracheal tube. AAC was not effective in maintaining the set CPAP pressure at the carina, especially during increased patient effort. In short, very little difference was noted in pressure compensation for the endotracheal tube with AAC on or off.


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