2004 OPEN FORUM Abstracts
COMPARISON OF DUAL MODE TO PRESSURE-LIMITED VENTILATION UNDER SIMULATED CONDITIONS OF VARYING PATIENT DEMAND AND IMPEDANCE
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.