The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

EVALUATION OF THE CHANGES IN TIDAL VOLUME DURING PRESSURE-TARGETED ASSIST/CONTROL AS TIME CONSTANTS VARY WHEN FLOW-CYCLE IS ACTIVATED ON THE LTV 1000

Coby Steele, Candy Perry, Lonny Ashworth MEd RRT, Boise State University, Boise, ID

Background: During pressure-targeted A/C, time is generally the cycle variable that ends inspiration. The Pulmonetics LTV 1000 allows the option of flow cycling during pressure-targeted A/C. When flow cycling is active, the parameter reached first, pressure or flow, will end inspiration. The affect on tidal volume and pressure during flow cycling was evaluated.

Methods: The LTV 1000 was connected to the Hans Rudolph Breathing Simulator (HR 1101). HR 1101 settings: resistance 15, 20 and 25 cm H2O/L/second; compliance 20, 40 and 60 mL/cm H2O. LTV 1000 settings: pressure-targeted A/C; pressure 30 cm H2O; TI 1.0 sec; flow cycle percent: off, 10, 20, 30 and 40%. Eight breaths were used to stabilize settings and give an average throughout changes in resistance (RAW) and compliance (CST).

Results: Peak pressure was unaffected at all flow cycle percent settings. At CST 20 mL/cm H2O: VT decreased from 485 to 406 mL with RAW 15 cmH2O/L/sec; VT decreased from 467 to 355 mL with RAW 20 cmH2O/L/sec; VT decreased from 482 to 408 mL with a RAW 25 cm H2O/L/sec. At CST 40 mL/cm H2O: VT decreased from 805 to 699 mL with RAW 15 cmH2O/L/sec; VT decreased from 705 to 627 mL with RAW 20 cmH2O/L/sec; VT decreased from 626 to 557 mL with a RAW 25 cmH2O/L/sec. At CST 60 mL/cm H2O, there was no change in VT as flow cycle % increased at any level of RAW.

Conclusion: The time constant is calculated as the RAW X CST. During pressure-targeted A/C on the LTV 1000, if the set TI is equal to or greater than the time constant, tidal volume will decrease as the flow cycle percent increases. At TI less than the time constant, tidal volumes remained constant and the ventilator remained time cycled.

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