The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

RESPONSE OF THE VENTILATOR SUBSEQUENT TO PATIENT DISCONNECTION DURING DUAL-MODE VENTILATION

Essam Aljamhan RRT,1 David Southwick,2 Lonny J. Ashworth MEd RRT2. King Abdulaziz Medical City,1 Boise State University, Boise, ID.2

Background: In dual-mode ventilation, each breath is a pressure-limited breath in which the pressure level is automatically modulated to achieve a preset tidal volume (VT).The ventilator will deliver a single volume-controlled test breath with a constant flow waveform to determine the compliance and resistance when the patient is initially connected to the ventilator. After the compliance and resistance have been measured, the ventilator adjusts the pressure required to deliver the set VT. Subsequent breaths are delivered as pressure-targeted breaths. The inspiratory pressure is based on the resistance, compliance, set VT and patient effort. If the delivered VT varies from the preset value, the target pressure for subsequent breaths is adjusted incrementally to achieve the set tidal volume.

Study Question: What is the response of various ventilators to a patient/ventilator disconnection while in dual-mode ventilation?

Method: Three ventilators were studied: Viasys Avea (VA), Drager Evita 2 (DE) and Puritan Bennett 840 (840). The values for Peak Inspiratory Pressure (PIP), Inspiratory VT (VT insp) and Expiratory VT (VT exp) displayed on the ventilators were recorded while connected to a mechanical lung model. Ventilator settings: Modes tested were Dual-Mode Assist-Control and Dual-Mode SIMV; VT 500 mL; TI 0.84 - 0.85 seconds; PEEP 0.0 cm H2O; respiratory rate 12 breaths per minute. Each ventilator was disconnected for 10-15 seconds then reconnected; the values for PIP, VT insp, and VT exp were recorded until values stabilized.

Results: With a measured compliance of 20 mL/cm H2O and resistance of 5-7 cm H2O/L/sec, the PIP for each ventilator was as follows: VA 25 cm H2O, DE 28 cm H2O, 840 28 cm H2O. After disconnecting and then reconnecting the ventilator to the mechanical lung model, the 840 ventilated at the previous pressure required to deliver the set VT prior to the disconnection. The VA delivered a low PIP (4-6 cm H2O) for the first breath, but on the second breath the PIP increased to the previous level prior to the disconnection. The PIP varied slightly for 4 – 5 breaths before the VT stabilized at the set value. The DE started with a test breath of 6-9 cm H2O, resulting in a VT of 65 - 69 mL on the first breath. With every breath following the test breath, the PIP increased by no more than 3 cm H2O until the preset VT was reached; this required 11 – 12 breaths.

Conclusion: After sensing a patient disconnection during dual-mode ventilation a significant difference between the three test ventilators was noted. The Drager Evita 2 was the only ventilator that ventilated with an initial test breath, and then incrementally increased the pressure by no more than 3 cm H2O until the set VT was reached. The Viasys Avea and PB 840 started at the pre-disconnection pressure required to deliver the set VT during the first or second breath after reconnection to the lung model. Clinical trials are necessary to assess the significance on patients of a disconnection or of a change in compliance between disconnection and reconnection of the ventilator when utilizing dual-mode ventilation.

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