1995 OPEN FORUM Abstracts
Flow Versus Pressure Triggering In Mechanically Ventilated Adult Patients
Robert L. Goulet, MS. RRT, Dean Hess, PhD, RRT, Robert M. Kacmarek, PhD, RRT. Respiratory Care and Anesthesia, Massachusetts General Hospital and Harvard Medical School, Boston, MA.
Adult mechanical ventilators have traditionally been pressure or time triggered. More recently, flow triggering has become available with adult ventilators and some ventilators allow the choice between pressure or flow triggering. Although bench studies have supported the superiority of flow triggering, there have been few studies that compared pressure and flow triggering in patients. The purpose of this study was to compare pressure and flow triggering during pressure support ventilation (PSV) in adult mechanically ventilated patients. Method: The study population consisted of 10 adult patients ventilated with a Puritan-Bennett 7200 in PSV mode (6 males and 4 females, age 62.9 ± 17.1 y, PSV 12.1 ± 5.7 cm H_2O, PEEP 5.1 ± 1.1 cm H_2O). We compared 4 trigger settings in random order: pressure trigger of -1 cm H_2O and -0.5 cm H_2O, flow trigger of 10/3 L/min and 5/2 L/min. Data were collected for 5 min using a VenTrak Respiratory Monitoring System (Novametrix, Wallingford, CT) with a data acquisition rate of 100 Hz. Pressure calibration was performed at 0 and 10 cm H_2O using a water column. Pressure was measured at the proximal endotracheal tube. At each trigger setting, 10 representative breaths that were free of artifact were chosen for analysis. Raw data points at 100 Hz were exported to a spreadsheet (Microsoft Excel) for detailed analysis of the airway pressure wave form. From the airway pressure signal, trigger pressure ([Delta]P) was defined as the difference between PEEP and the maximum negative deflection prior to onset of the triggered breath. Trigger time ([Delta]T) was defined as the interval between the initial negative deflection and the onset of the triggered breath. Pressure-time product (PTP) was defined as the area produced by the pressure waveform below PEEP from initial negative deflection until the onset of the triggered breath. Statistical analysis consisted of mean ± SD and ANOVA with post-hoc Scheffe test.
trigger [Delta]P (cm H_2O) [Delta]T (ms) PTP (cm H_2O u s)
pressure - 0.5 cm H_2O 1.1 ± 0.4 172.9 ±60.3 0.088 ± 0.039
pressure - 1.0 cm H_2O 1.6 ± 0.5 196.9 ±50.5 0.140 ± 0.041
flow - 10/31.4 ± 0.7 186.9 ±50.8 0.137 ± 0.073
flow - 5/2 1.3 ± 0.7 180.6 ±42.4 0.199 ± 0.054
Pressure trigger of 0.5 cm H_2O was significantly less than the other trigger methods (P < 0.01) for [Delta]P, [Delta]T, and PTP; flow trigger of 10/3 L/min was not significantly different than flow trigger of 5/2 L/min or pressure trigger of 1 cm H_2O. There were also significant differences between patients for [Delta]P, [Delta]T, and PTP for each trigger method (Pɘ.001).
Conclusions: For this group of patients, flow triggering was not superior to pressure triggering at -0.5 cm H_2O. It is of interest to note that prior studies comparing pressure and flow triggering have typically used a pressure trigger of 1.0 cm H_2O. The significant differences that we found between patients suggests that respiratory drive may have an important effect on triggering. (Supported in part by Puritan-Bennett Corporation)