The Science Journal of the American Association for Respiratory Care

1998 OPEN FORUM Abstracts

A COMPARISON OF THE EFFECTS OF DEMAND FLOW, FLOW-BY AND PRESSURE SUPPORT ON IMPOSED WORK OF BREATHING AND VENTILATORY EQUIVALENT.

David C. Shelledy, PhD, RRT and Lynda Thomas Goodfellow, MBA, RRT. The University of Texas Health Science Center at San Antonio, San Antonio, Texas and Georgia State University, Atlanta, Georgia.

BACKGROUND: Flow-by is a newer technique that is designed to reduce the spontaneous work of breathing during mechanical ventilatory support. We compared the imposed work of breathing and ventilatory efficiency of flow-by (FB), demand flow (DF) and pressure support (PS) at 5 and 10 cm H_{2}O in spontaneously breathing volunteers being ventilated via mouthpiece using the Bennett 7200ae ventilator. METHOD: A randomized blocks, repeated measures design was used to compare DF, FB and PS at 5 and 10 cm H_{2}O. Twenty healthy volunteers served as their own controls, breathing in each of the experimental conditions in a random sequence. For DF and PS, sensitivity was set a -1 cm H_{2}O. FB was set with a base flow of 10 L/min and a flow sensitivity of 3 L/min. Data was collected continuously for 5 minutes and the mean values at the end of that time were reported. Ventilatory equivalent (VEQ) for oxygen is a measure of the efficiency of ventilation at various work loads and was calculated by dividing VE (BTPS) by VO_{2} (STPD). Imposed work of breathing (WOBI) was estimated by subtracting VO_{2} during spontaneous breathing without the ventilator from VO_{2} during DF, FB and PS. Results: The mean VEQ and WOBI for the four conditions was:

Demand Flow Flow-by PS 5 cm H_{2}O

Mean WOBI (SD) 8.2 (23.3) -3.3 (32.0) 3.25 (29.9)

(ml/min)

Mean VEQ (SD) 27.0 (3.7) 32.5 (5.4) 35.1 (8.5)*

(L/L VO_{2})

PS 10 cm H_{2}O

Mean WOBI (SD) 4.8 (29.4)

(ml/min)

Mean VEQ (SD)

(L/L VO_{2}) 41.9 (10.7)*,++

* significantly greater than DF (p < .05); ++ significantly greater than FB (p < .05)

There were significant differences in VEQ for DF, FB and PS at 5 and 10 cm H_{2}O when compared using ANOVA. Pairwise followup comparisons found that there were no significant differences in VEQ between FB and DF and FB and PS at 5 cm H_{2}O (p>.05). There were significant differences in VEQ between DF and PS of 5 cm H_{2}O; DF and PS of 10 cm H_{2}O; and FB and PS at 10 cm H_{2}O (p < .05). Flow-by produced the lowest WOBI, however, this difference was not statistically significant (p>.05). Conclusions: There was no significant difference between FB and PS at 5 cm H_{2}O in terms of ventilatory efficiency. PS at 10 cm H_{2}O produced the greatest VEQ and was the most efficient of the four conditions studied. FB produced the lowest WOBI, however, this finding was not statistically significant. We believe measurement of ventilatory equivalent may provide a sensitive method for assessing ventilatory efficiency during mechanical ventilation.

The 44th International Respiratory Congress Abstracts-On-DiskĀ®, November 7 - 10, 1998, Atlanta, Georgia.

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