The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

THE EFFECT OF INSPIRATORY RISE TIME ON TOTAL PATIENT INSPIRATORY WORK OF BREATHING (TPiWOB), EXHALED TIDAL VOLUME (VT), PEAK INSPIRATORY PRESSURE (PIP), AND MEAN AIRWAY PRESSURE (MAP) DURING PRESSURE SUPPORT VENTILATION (PSV) WITH 6 AND 7 mm ID ENDOTRACHEAL TUBES (ETT) IN A SPONTANEOUS BREATHING LUNG MODEL.

David Vines, MHS, RRT, Arthur Carandang, CRT, Myra Garcia, CRT, Rene Guerra, CRT, EMT, Denise Rodriguez CRT, David C. Shelledy, PhD, RRT. The University of Texas Health Science Center at San Antonio, San Antonio, Texas.

Background:
Some data exist to suggest that shorter inspiratory rise times may lower WOB in patients with high inspiratory demands. The effect of inspiratory rise time on TPiWOB and exhaled VT during PSV with 6 and 7 mm ID ETT has not been reported. We compared various rise time settings during PSV to determine which setting resulted in the lowest TPiWOB, PIP, and MAP, and largest VT using a two-compartment mechanical lung model to simulate spontaneous breathing.

METHODS:
WOB was first measured on lung B at tidal volumes of 200, 400, 600, and 800 mL, with peak flows varying from 40, 60, 80, and 100 L/min with a sine wave flow pattern, using the Ventrak 1550 Respiratory Mechanics Monitoring System (Novametrix Medical Systems, Inc., Wallingford, CT). Then, WOB was measured at these same tidal volumes for lung B to drive lung A at a normal compliance (0.05 L/cm H20) and normal resistance (2.7 cm H20/L/sec), decreased compliance (0.02 L/ cm H20), and increased resistance (17.6 cm H20/L/sec) while lung A received assistance through 6.0 and 7.0 mm ID ETT (Mallinckrodt Critical Care, Glens Falls, NY) via a SERVO 300 (Maquet Inc., Bridgewater, NJ) from either PSV of 5,10,15, or 20 cm H20, while the rise time percentage was varied from 0 to 10 for each level of PSV. TPiWOB was then calculated using this formula: [TPiWOB=WOB(B+A) – WOBB]. TPiWOB was converted from joules to joules per liter (J/L) based on the delivered tidal volume to lung A. Data for exhaled VT, PIP, and MAP was collected from the SERVO 300 for all conditions tested. The SERVO 300 was calibrated prior to its use. An ANOVA with a post hoc follow up test (Newman-Keuls) was used to determine significant differences.

RESULTS:
The table below contains mean values and standard deviations for TPiWOB, exhaled VT, PIP, and MAP during various rise time percentages (%) across all conditions for 6 and 7 mmID ETT.

a) significantly less than 10% rise time, p < 0.05;

b) significantly less than 8% rise time, p < 0.05; N – 384 in each group

CONCLUSION: The rise time % setting of 0 resulted in the lowest TPiWOB during PSV on size 6 and 7 mmID ETTs. Under these conditions, 0% rise time had no adverse effects on exhaled VT, PIP, and MAP on size 6 and 7 mmID ETT in our spontaneously breathing lung model.



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