The Science Journal of the American Association for Respiratory Care

1997 OPEN FORUM Abstracts

NON-BIAS FLOW TRIGGERING (NBFT) REDUCES INSPIRATORY WORK(Wi) COMPARED TO PRESSURE TRIGGERING (PT) DURING PRESSURE SUPPORT VENTILATION.

JA Alonso RRT; RH Kallet MS RRT; AR Campbell MD; DJ Morabito MPH; RC Mackersie MD, University of California, San Francisco at San Francisco General Hospital, Department of Surgery and Respiratory Care Services, San Francisco CA.

Background: In pressure support ventilation (PS) the Wi during the trigger phase depends upon the trigger threshold and associated time delay. Flow Triggering (FT) has been found to decreased Wi compared to PT. Unlike other FT systems, NBFT available on the Drager Evita does not require a continuous base flow. We compared NBFT to PT during PS to assess if this new method of FT also reduces Wi. Method: A prospective, randomized crossover study was conducted on 6 adult critical care patients with either Acute Lung Injury or Acute Respiratory Distress Syndrome. Static thoracic compliance was 30.2 ± 5.2 ml/cm H2O. Minute Ventilation (V_{E}) was 13.6 ± 5.6 L/min and Maximal Inspiratory Pressure was 46 ± 21 cm H2O. Comparisons with NBFT on the Evita were made using PT on the Hamilton Veolar ( 2 patients) and Siemen's Servo 900C (4 patients). Trigger level was set at 1L/min for NBFT and -0.5 cm H2O for PT. PS level (8.7 ± 5.7 cm H2O) and PEEP (5.8 ± 2.08 cm H2O) were held constant between trigger systems. Measurements were performed with the BICORE CP-100 (Irvine, CA.) incorporating the Campbell Diagram. Wilcoxon Sign Rank Tests were used for statistical analysis. Results: No significant differences were found between NBFT and PT respectively in either V_{T} (386 ± 52 vs.. 377 ± 18 ml), V_{E} (11.3 ± 3.9 vs. 10.9 ± 10.9 L/min), peak inspiratory flow rate (58.3 ± 21.2 vs. 54.1 ± 27.5 L/min), inspiratory time (0.76 ± 0.15 vs. 0.77 ± 0.13) and respiratory drive (P0.1 = 5.0 ± 2.8 vs. 5.6 ± 2.9 cm H2O). Intrinsic PEEP was not found in any patient.

Variable PS-NBFT PS-PT

dPes (cm H2O)^{?} 14.0 ± 12.9 17.6 ± 11.6^{?}

Pressure-Time-Product (cm 196.9 ± 80.4 256.2 ± 90.3^{?}

H2O/sec/min)

total Work (joules/L) 1.19 ± 1.15 1.64 ± 0.98^{?}

Elastic Work (joules/L) 0.62 ± 0.69 0.89 ± 0.85^{?}

Resistive Work (joules/L) 0.57 ± 0.49 0.80 ± 0.47

Measured Sensitivity (cm 3.22 ± 0.98 6.70 ± 2.20^{?}

H2O)*

Pressure Time Index 0.12 ± 0.06 0.18 ± 0.08^{?}

? p < 0.05

* Measured sensitivity = Minimum airway pressure required to initiate flow.

? dPes= Peak deflection from baseline esophageal pressures.

Conclusion: During PS, NBFT significantly reduced inspiratory work and the index of muscle fatigue when compared to PT. The lower elastic work and measured sensitivity at comparable levels of V_{T}, inspiratory time and peak flow rate suggests that this was due to a reduction in imposed work during the trigger phase of inspiration. NBFT could benefit patients with low compliance, high V_{E} and elevated respiratory drive when minimizing imposed work is critical in preventing fatigue.

OF-97-027

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