2004 OPEN FORUM Abstracts
COMPARISON OF DUAL MODE TO PRESSURE-LIMITED VENTILATION UNDER SIMULATED CONDITIONS OF VARYING PATIENT DEMAND AND IMPEDANCE
Ghazi
Alotaibi, MS, RRT (University of Medicine and Dentistry of New
Jersey, Newark, NJ); Robert Kacmarek, PhD, RRT, FAARC
(Massachusetts General Hospital and Harvard Medical School, Boston,
MA); Craig Scanlan, EdD, RRT, FAARC (UMDNJ, Newark, NJ); Al
Heuer, PhD, RRT (UMDNJ, Newark, NJ).
BACKGROUND: Dual mode ventilation (DMV) is designed to exploit
the benefits of
volume-controlled ventilation (VCV) and pressure-controlled
ventilation (PCV) while minimizing their shortcomings. DMV is a
variant of PCV in which the inspiratory pressure is adjusted
breath-to-breath to accommodate changes in respiratory mechanics,
with the goal a more stable minute volume. Unfortunately, studies of
ventilator perform-ance during DMV are few in number and of limited
scope. To address this shortcoming, we compared DMV to PCV on four
adult critical care ventilators using a programmable lung simulator.
We hypothesized no significant differences between DMV and PCV for
each ventilator on any of 11 measures of respiratory mechanics during
conditions of varying patient demand and impedances.
METHOD: The
Active Servo Lung 5000 (Ingmar Medical, Pittsburgh, PA) simulator was
programmed to produce five breathing conditions: (a) normal
resistance and compliance (baseline), (b) low compliance, (c) high
resistance, (d) high patient demand, and (e) simulated leak. The DMV
feature of the Puritan-Bennett 840 (PB840), Hamilton Galileo
(Galileo), Drager Evita XL (Evita), and Siemens Servo 300 (Servo)
ventilators was implemented using the following settings: VT
= 600mL, Ti = 1 sec, PEEP = 4 cmH2O, flow trigger = 2
L/min, and most rapid rise time. During PCV, inspiratory pressure was
adjusted to deliver 600 mL. Ventilation parameters were acquired,
stored, and analyzed using the ASL5000 software. Dependent variables
included inspiratory trigger airway pressure (Pawt), trigger delay
time (Dtrig), post-trigger delay time (Dptrig), total trigger delay
time (Dtot), patient inspiratory work (Winsp), patient trigger work
(Wtrig), inspiratory trigger muscle pressure (Pmust), inspiratory
time 90% (IT90), VT, PIFR, and PIP. RESULTS: Due to the
high precision of the ASL 5000 measurements, we adopted a decision
rule requiring variations in excess of 2 SD from the group mean to be
considered of ‘practical’ significance. Under baseline
conditions, PCV in Galileo and Servo imposed slightly higher Wtrig
and Pmust than DMV. When impedance was high (low compliance or high
resistance), all ventilators delivered higher VT, PIFR,
and PIP when delivering DMV than when set to PCV. When the compliance
was low, the Evita and Servo exhibited higher Wtrig and Dtot during
PCV than DMV. Under conditions of high patient demand, there were no
differences in the performance of PCV and DMV on any ventilator
studied. During simulated leak conditions, the PB840 and Evita
achieved higher VT, PIFR and PIP with PCV than with DMV,
while the Galileo and Servo exhibited significantly lower VT,
PIFR and PIP when in the PCV mode.
CONCLUSIONS: DMV results in higher
VT and flows than PCV under conditions of increased
impedance (high resistance or low compliance). When patient demand is
high, both modes are comparable. In the presence of a leak, PCV and
DMV differed in VT delivery depending on the ventilator.
PCV may impose more work and delay time than DMV.