2004 OPEN FORUM Abstracts
Ventilator calculated values of Resistance: Are they accurate?
Jeffrey
K. Goldman AS CRT, 1 Russell G. Peterson AS CRT,
2 Lonny J. Ashworth MEd RRT. 3 St. Luke’s
Regional Medical Center; 1 Saint Alphonsus Regional
Medical Center; 2 Boise State University, Boise, ID3
Rationale:
The purpose of this study was to compare the accuracy of the
displayed value calculated by the ventilator for airway resistance to
the airway resistance calculated using the traditional method of
calculating airway resistance in old and new generation mechanical
ventilators.
Methods:
Four ventilators were studied: Viasys Avea (VA), Drager Evita 2
(DE), Puritan Bennett 840 (840) and Puritan Bennett 7200 (7200). Each
ventilator was attached to a mechanical lung model: compliance 0.015
L/cm H2O, #5 parabolic resistors in each lung and a #20
parabolic resistor as the trachea. Ventilator settings:
Volume-targeted, Assist-Control; VT 500-1250 mL (increased
in increments of 250 mL); flowrates 40, 60, 80 and 100 L/minute;
square flow waveform; PEEP 0 cm H2O. Displayed values and
calculated values for airway resistance were recorded after a
breath-hold on five consecutive breaths at each flowrate and VT
setting. Airway resistance was calculated as the (peak pressure –
plateau pressure)/ Flow in LPS.
Results:
At a flowrate of 40 L/minute and at all tidal volumes, the
difference between displayed and calculated airway resistance with
the VA, 840, and 7200 were all within 1 cm H2O/L/second of
the calculated airway resistance; the DE was 4-11.8 cm H20/L/second
greater than the calculated value. As the tidal volume increased, the
difference between the displayed airway resistance and the calculated
airway resistance increased. See figure below.
Conclusions:
When evaluating airway resistance for ventilator patients, it is
important to realize that the values displayed by some ventilators
are not consistent with the traditional method of calculating airway
resistance. The displayed resistance on the DE is actually an
expiratory resistance, rather than the traditional inspiratory
resistance. Future studies may determine whether clinicians should
assess the inspiratory resistance or the expiratory resistance, and
if they have different normal values.
