The Science Journal of the American Association for Respiratory Care

1997 OPEN FORUM Abstracts

COMPARISON OF IMPOSED WORK OF BREATHING BETWEEN CONSTANT FLOW AND FLOW ASSIST DURING SPONTANEOUS BREATHS WITH THE NEWPORT E100m VENTILATOR.

V. Whyte, RCP, T. Malinowski, RCP, M. Terry, RCP, P. M. Gold, MD. Loma Linda University Medical Center, Loma Linda, CA.

Objective: Determine the effect of Flow Assist (FA) on imposed inspiratory work of breathing (WOB) and compare this effect with constant flow in spontaneously breathing intubated adults requiring ventilatory support. Design: Crossover study, each subject acting as their own control. Method: We studied 17 adult cardiothoracic surgery patients. The data from 6 patients were excluded due to mechanical problems. The remaining patients were ventilated in SIMV or CPAP mode. Post-operative ventilatory support time prior to study ranged from 30 minutes to 8 hours. Patients were stabilized on an E100m ventilator (Newport Medical Instruments) equipped with a disposable ventilator circuit (Hudson RCI) and hygroscopic condensation humidifier (HCH, Vital Signs, Inc.). Flow and airway pressure sensors were placed inline between the HCH and the endotracheal tube. Ventilator settings were appropriate for age and respiratory condition. Patients were initially supported with constant flow delivery. Baseline data on WOB and patient work was collected. FA was activated and data collection repeated. Trigger level was 1- 2 cm H_{2}O below baseline throughout the study. During constant flow, spontaneous flow was kept at 8 L/min (pre-set) and supplemental Flow was added to minimize reservoir bag deflation during spontaneous breaths (range 4 - 10 L/min). During FA, spontaneous flow was zero and supplemental flow was unchanged. FA time ranged from 0.4 to 0.7 seconds. Data Collection: WOB and patient work were measured using the Ven Trak Respiratory Monitor (Novametrix, Inc.). Respiratory mechanics were measured for 5 minutes for each flow method. Ten representative breaths were selected for analysis during each flow period. Analysis Plus software (Novametrix, Inc.) was used for computation and data analysis. Results: Results reflect the cumulative data for 11 patients. A comparison was made between standard flow delivery and FA via two-way analysis of variance with replication (ANOVA). FA decreased WOB and Patient Work significantly (mean values ± SD) p < 0.01.

Work of Breathing (gm-cm/liter) Patient Work (gm-cm/liter)

FA off FA on FA off FA on

0.1888 ± 0.12 0.0321 ± 0.27 0.4273 ± 0.43 0.1548 ± 0.17

Discussion: FA uses the ventilator's set inspiratory flowrate (Mandatory flow control) to deliver gas during the inspiratory phase of a spontaneous breath. FA is delivered through a open exhalation valve while maintaining CPAP. Consequently, much higher flow is available to the patient. Our analysis demonstrates both a statistically significant decrease in WOB (83%) and Patient Work (64%) with the use of FA. Considerable breath-to-breath variations in work indices exist for each subject, probably due to differences in inspiratory effort, comfort, sedation, accounting for the large standard deviation. The breath-to-breath variation in work indices was similar whether FA was on or off, both for individual subjects, and for between-subject comparisons. Conclusion: The imposed WOB is less with FA than constant flow delivery during spontaneous breathing on the Newport E100m ventilator.

OF-97-163

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