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Special Feature: Oral Presentations

VENTILATORS FOR TOTAL LIQUID VENTILATION

Walti, Hervé; Robert, Raymond

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Total liquid ventilation (TLV) using Perflurochemicals (PFC) is superior to every tested ventilation strategy, but necessitates a dedicated mechanical system, a liquid ventilator (LV). Regardless of the technology used to design the LV, three levels of complexity could be described: Basic, controlled, and advanced controlled functions. Basic functions: Basic functions include: (1) Drive, to insert and withdraw Vt of PFC from lungs; (2) Gas-exchanges, to oxygenate and remove CO2 from PFC; (3) Thermostatic, to maintain PFC at desired temperature; (4) PFC condenser; to minimize evaporative loss of PFC; and, (5) PFC trapping and filtration, to minimize reintroduction of biologic contaminants into the lungs. To assume drive function, active drainage using pumps has been proved to be more efficient and more secure than passive drainage by gravity. The best type of pump to use (roller vs. piston pumps) is still questionable but piston pumps seems to achieve a more precise control of volume together with steady flows nearly exempted of pulsation and a higher drainage volume. Dual as well as single or two independent piston pumps have been used. Devices used for gas exchange functions must have a high CO2 efficiency as well as a low resistance to flow. Moreover, they must minimize heat and PFC loss and have a low priming volume. Membrane oxygenators (ECMO devices) have been used first with a high CO2 removal efficiency but also a high resistance to flow and PFC leaks if using porous membranes. Recently new silicone hollow-fiber oxygenators have been adapted to LV and demonstrate a low resistance to flow, no PFC leaks, and a low priming volume. However, the cost of these devices is high and compatibility with PFC questionable. Bubble gas exchangers permit increase in the residence time of PFC, have a similar CO2 removal efficiency to membrane oxygenators, are modular, use PFC compatible materials, and are easily sterilized between patients. However, they need a higher priming volume. Thermostatic function must be used to maintain PFC at desired temperature, usually that of the patient, but could also be used for controlled therapeutic hyper or hypothermia. Liquid to liquid heat exchangers as well as electrical-resistance heaters have been used and integrated to the oxygenators. PFC condenser function is crucial to minimize evaporative loss of PFC at the outlet part of the gas oxygenator, because of the high cost of PFC and environmental concerns. They have 60 to 80efficiency and may be integrated in a condenser heater oxygenator modular unit. PFC trap and filter must be integrated to the LV circuit in order to assume lung lavage function of TLV and to minimize reintroduction of biologic contaminants into the lungs. It may include an exudate trap and low-pressure loss coarse filtration on the expired limb (20μ), and a fine filtration (2μ) on the inspired limb of the circuit. It is an essential step before clinical use. Controlled Functions:Computer controlled functions of LV are essential to specify and precisely set LV parameters. In addition, they must control lung volume, airway pressure, and flow profiles. They are also essential to optimize LV efficiency and to protect patients against potential side-effects such as volo-barotrauma airway closure (chocked-flow) and cardio-vascular consequences of LV. Controlled functions include: (1) Time-cycled functions via command of valves and pump speed variation; (2) Pressure-limited function via monitoring of airway pressures (proximal or distal to ETT) with the possibility to monitor dynamic or alveolar (stop-flow) pressures; (3) Volume-controlled functions monitored via patient or reservoir weight, end-expiratory (alveolar) pressure or inspiratory/expiratory volumes. They are essential to monitor Vt and end-expiratory lung liquid volume as well as flow-profile (ramp or exponential). Advanced Controlled Functions: Advanced controlled functions were recently implemented in LV to control with high precision the volume of liquid transfer in the lungs in spite of important measurement noises. For example, a control has been implemented in the INOLIVENT LV prototype to correct for small repetitive measurable errors between inspiratory and expiratory Vt. Another example is the feedback controller using end expiratory pressure implemented by Degraeuwe. In conclusion, During the past decade, innovations in LV design boost the acceptance of TLV as a clinical therapeutic avenue. However, many issues, such as PFC material compatibility, PFC filtering, modularity to scale-up LV for adult use, user-friendliness, and advanced controlled function of lung volume need to be adequately addressed before clinical acceptance.

Copyright © 2006 by the American Society for Artificial Internal Organs