Original Contribution
Effect of inspiratory time on tidal volume delivery in anesthesia and intensive care unit ventilators operating in pressure control mode

https://doi.org/10.1016/j.jclinane.2004.02.005Get rights and content

Abstract

Study objective

To compare the effect of inspiratory time and lung compliance on tidal volume (Vt) delivery in anesthesia and intensive care unit (ICU) ventilators operating in pressure control mode.

Setting

Respiratory research laboratory of a tertiary care medical center.

Design

Two anesthesia ventilators with pressure control capability (Narkomed 6000, Dräger Medical, Inc, Telford, Pa, and the Datex-Ohmeda Aestiva 5, Datex-Ohmeda, Inc, Madison, Wis) and one critical care ventilator (Puritan Bennett 7200, Puritan-Bennett, Pleasanton, Calif) were studied under varying inspiratory time and lung compliance conditions using a mechanical lung model.

Intervention

Each ventilator was set to pressure control mode at a fixed inspiratory/expiratory (I/E) ratio. The respiratory rate (RR) was varied between 6 and 28 breaths per minute. Lung compliance and inspiratory time settings were set to simulate clinical conditions known to affect anesthesia ventilator performance.

Measurements

Inspiratory flow, Vts, and peak airway pressures were measured using the on-board monitor for each ventilator, and confirmed with the Bicore CP-100 pulmonary mechanics monitor (Bicore Monitoring Systems, Inc, Irvine, Calif). To assess differences in inspiratory flow between ventilators, airway pressures were continuously monitored during inspiration.

Main results

Increasing RRs caused delivered Vts to decrease for all ventilators. However, decreases in Vts were significantly larger for anesthesia than for ICU ventilators. At a lung compliance of 0.02 L/cm H2O and set Vt of 700 mL, Vt delivery for the Puritan Bennett 7200 ventilator remained at 88% of baseline, but decreased to 76% for the Aestiva 5 when RRs were increased from 6 to 28 breaths per minute (P < .0025). Airway pressure tracings demonstrated a slower increase in inspiratory airway pressure for the Aestiva 5 than for the other ventilators.

Conclusion

Differences in inspiratory flow delivery between ICU and anesthesia ventilators can cause differences in Vt delivery when the pressure control mode is used at high RRs. These differences can significantly impact the perioperative care of critically ill patients requiring ventilatory support.

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

References (20)

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