2004 OPEN FORUM Abstracts
EXHALATION VALVE RESISTANCE OF MODERN VENTILATORS
Thomas J. Wing, BS
RRT, Lonny J. Ashworth, MEd RRT Boise State University, Boise,
Background: Modern ventilators are used every day in critical care facilities around the world. The exhalation valve can cause an increase in the imposed expiratory resistance (IER) placed upon the patient leading to an increased work-of-breathing and Auto-PEEP. The exact amount of IER has not been well documented in modern ventilators.
Methods: Four ventilators were studied: Viasys Avea (VA), Drager Evita 2 (DE), Puritan Bennett 840 (PB840) and Puritan Bennett 7200 (PB7200). Each ventilator was connected to a mechanical lung model via a 7.5 mm endotracheal tube. PEEP values of 0-25 cm H20 and tidal volumes of 200-1200 mL were used to generate different driving pressures for exhalation. Pressure and flow were measured immediately before the exhalation valve. Pressure after the exhalation valve was ambient. IER was calculated as (P1 – P2)/ V, where P1 = pressure immediately before the exhalation valve, P2 = 0 cm H2O reflecting ambient pressure, V = peak expiratory flowrate, in LPS, immediately before the exhalation valve.
Results: At PEEP ranges from 0-25 cm H2O and VT ranges from 200-1200mL the values for IER were as follows: VA 7.9 - 114.2 cm H20/L/sec, DE 3.7 - 46.8 cm H20/L/sec, PB840 4.0 - 51.0 cmH20/L/sec, PB7200 6.1 - 71.0 cm H20/L/sec.
Conclusion: The range of IER varies greatly among ventilators and at a variety of set PEEP and VT. This study shows that the resistance is surprisingly elevated and should be taken into consideration on a patient-to-patient basis. Further trials are necessary in order to evaluate the impact of the elevated levels of IER, auto-PEEP and work-of-breathing on patients.