2004 OPEN FORUM Abstracts
EXHALATION VALVE RESISTANCE OF MODERN VENTILATORS
Thomas J. Wing, BS
RRT, Lonny J. Ashworth, MEd RRT Boise State University, Boise,
ID.
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.