1995 OPEN FORUM Abstracts
EFFECT OF VARIATIONS IN SOURCE GAS PRESSURE ON OBSERVED FLOW OUTPUT OF TWO TRANSPORT VENTILATORS.
Kelvin Mac Donald, RCP, CRTT., Peter Wang, RCP, RRT., Wes Cowan, RCP, RRT. Kaiser Permanente Medical Center, Los Angeles.
INTRODUCTION: Transport ventilators by their very nature, may encounter variations in source or wall gas pressures used as internal working pressure. Many designs, including the ones tested rely on an inspiration solenoid, operating at working pressure to generate flow. During a mechanical breath, these solenoids allow gas at the set inspiratory time to travel through a flow control and on to the patient. We sought to determine what, if any effect varying source gas pressure has on observed flow output during a mechanical breath.
Methods: Two Impact Medical, UniVent 750 (West Cadwell, NJ), and two Bird Medical, Avian (Palm Springs, CA) transport ventilators were used. These were in turn connected via a Professional Medical Devices 6400 disposable ventilator circuit to an Ohmeda Lung Simulator (C = 10 ml/cm H_2O, Resistance = 5 cm H_2O/L/sec). Flow output was measured using a Timeter RT-200 Calibration analyzer. Source gas pressure was controlled with a Western Enterprises single stage pressure
Inspiratory Time (s) Flow rate (Lpm)
regulator. Each ventilator was tested at 30, 40, 50, 60, and 70 PSIG. Each ventilator was set as shown in the table. Flow rate output and tidal volume were record at each setting and PSIG combination. The ventilator's mean flow output @ each PSIG level was compared to itself @ 50 PSIG as a control. Mean flow output between the two type was compared @ 50 PSIG to validate controls.
Results: The mean difference in observed flow output between the two types @ 50 PSIG was 2.1% (S.D. 3.3%). A correlation coefficient of .995 with a covariant of 0.13 was calculated. The mean % difference (SD) from each ventilator's control mean flow output, at each pressure level for each ventilator are shown in the table.
Source PSIG AvianUni-Vent
30 14 % (3%) 46% (3%)
40 4% (2.5%) 30% (3.5%)
60 0.5% (1.5%)-14% (2%)
70 1.5% (1.5%) -29% (7.5%)
We also observed a proportionally corresponding decrease in delivered tidal volume with each decrease in observed flow output. The negative mean percent differences reflect increases in observed flow output. DISCUSSION: The AVIAN features an internal pressure regulator, which reduces ventilator working pressure below 50 PSIG. This accounts for the lower percent difference observed flow output with a low source pressure. This regulator also prevented increased flow output with high source pressure as seen with the Uno-Vent.
Conclusions: While there was good agreement between both ventilator types @ 50 PSIG, flow output variation was greater with the Uni-Vent at other than 50 PSIG. This may be alleviated by using an external pressure regulator and gauge and realizing flow output will decrease with low source gas pressures.