Original ContributionEffect of inspiratory time on tidal volume delivery in anesthesia and intensive care unit ventilators operating in pressure control mode
Introduction
Most older anesthesia ventilators provide only a volume control mode of positive pressure ventilation, in which tidal volumes (Vts) and inspiratory gas flow rates are preset and airway pressures vary with changes in lung mechanics. Several newer anesthesia ventilators, however, also offer a pressure control mode where inspiratory airway pressure is set, and gas flow rates are continually adjusted to maintain that airway pressure. Although long-term use of pressure control ventilation in the intensive care unit (ICU) has not improved mortality, such a mode may confer short-term advantages in patients with acute respiratory distress syndrome [1], in pediatric patients or those with partially occluded endotracheal tubes [2], [3], or during 1-lung ventilation [4].
In pressure control mode, the inspiratory phase is characterized by an initial transition period where relatively high gas flow rates rapidly increase airway pressures from expiratory to inspiratory levels. In theory, gas flow during this period depends only on the difference between inspiratory and expiratory pressures and respiratory system resistance [5]. In practice, however, many ventilators deliver less inspiratory flow than this equation demands [5], slowing the delivery of Vt and prolonging the transition from expiratory to inspiratory airway pressure. When inspiratory times begin to encroach on the time required for the ventilator to reach the full preset inspiratory pressure, Vt delivery can depend on inspiratory flow capacity, which may differ between ventilators.
Flow capabilities of most ICU ventilators are sufficiently similar that standard anesthesia and critical care textbooks [6], [7], [8] do not identify ventilator-based inspiratory flow limitations as an important issue in the use of pressure control mode. When airway pressures are high, however, most anesthesia ventilators deliver significantly less flow than do their ICU counterparts [9], [10]. We hypothesized that an anesthesia ventilator operating in an environment with short inspiratory times and high airway pressures would produce smaller Vts than would an ICU ventilator with identical settings. To test our hypothesis, we compared the effect of reducing inspiratory time on Vt delivery during pressure control mode in a Puritan Bennett 7200 ICU ventilator (Puritan Bennett, Pleasanton, Calif), an anesthesia/ICU hybrid (Narkomed 6000, Dräger Medical, Inc, Telford, Pa), and an anesthesia ventilator (Datex-Ohmeda Aestiva 5, Datex-Ohmeda, Inc, Madison, Wis).
Section snippets
Materials and methods
All experiments were performed in the respiratory therapy laboratory at the University of Chicago. We tested the Datex-Ohmeda Aestiva 5 anesthesia ventilator (standard bag-in-bottle design), the Narkomed 6000 anesthesia/ICU hybrid (mechanical piston design), and the Puritan Bennett 7200 ICU ventilator (microprocessor-controlled solenoid valve design). All ventilators were inspected and certified before use by the Department of Respiratory Therapy at the University of Chicago. Each ventilator
Results
At the initial ventilator settings described above (RR = 6 breaths per minute, I/E ratio = 1:2, PIP = 40 cm H2O, positive end-expiratory pressure [PEEP] =5 cm H2O, and test lung compliance = 0.02 L/cm H2O]), Vt delivery for all ventilators was close to the predicted value of 700 mL (Table 1). Throughout the range of RRs tested, neither the I/E ratio nor the end-expiratory pressure changed significantly. The time required for each ventilator to transition from expiratory to inspiratory airway
Discussion
Pressure control ventilatory modes have recently been introduced as readily available options on newer anesthesia ventilators. Differences between pressure and volume control modes, however, raise unique monitoring issues for the anesthesiologist. In a volume control mode, changes in airway resistance or lung compliance are reflected in changes in PIP [12]. With pressure control mode, however, airway pressures are fixed by design, and detecting changes in airway resistance or respiratory system
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