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

Avery Tung, Melinda L Drum, Sherwin Morgan
Journal of Clinical Anesthesia 2005, 17 (1): 8-15

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, Drager 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 H(2)O 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.

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