The Journal of critical illness
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Most modern ventilators have several key features in common: microprocessor control of operational and monitoring functions; electromechanical valves to control and adjust gas flow patterns; and extensive monitoring systems. In addition, these machines can provide a number of different modes of ventilation (including pressure support). Though not microprocessor-controlled, the Siemens Servo 900 series ventilators use feedback electronics to adjust inspiratory flow based on expiratory flow to meet preset volumes. In contrast, the Bennett 7200 units use microprocessor-regulated solenoid valves to deliver preset tidal volume. High-frequency ventilators deliver smaller tidal volumes at rates greater than 60 bpm.
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The Bennett MA-1 ventilator is a volume-cycled, constant flow generator that can act as an assistor, controller, or assist-controller. It is one of the most commonly used ventilators in clinical practice. ⋯ Other basic controls allow you to establish the sensitivity of the ventilator to spontaneous breathing attempts, the maximum flow rate, the frequency of respirations, and the oxygen percentage. Special controls permit delivery of a sigh breath and slowing of exhalation.
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Review Historical Article
A clinician's guide to ventilators: how they work and why they can fail. A classification system to make sense of available options.
To select a ventilator (or a ventilatory mode), consider the most basic characteristics: How is tidal volume generated (with a constant or nonconstant flow or pressure generator)? How does the ventilator trigger a changeover from exhalation to inhalation and cycle back to exhalation? How is tidal volume delivered to the patient (either directly from a power source or indirectly from an intermediate chamber)? What special functions are available? The answers to these questions will not only let you make the best selection but will also help you troubleshoot when a ventilator fails to function properly.
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Currently available ventilators offer a number of special options to meet the needs of critically ill patients. Intermittent mandatory ventilation allows a patient to breathe spontaneously without assistance. CPAP and PEEP ensure that the patient breathes at an elevated pressure either constantly or during expiration. ⋯ Airway pressure release ventilation facilitates venous return and decreases airway pressure. Sophisticated monitors provide detailed information on the patient's status, but alarm features are somewhat unreliable. Thorough knowledge of the controls on modern ventilators can help you provide the optimum form of respiratory support.
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Continuous arteriovenous hemofiltration (CAVH) and continuous arteriovenous hemodialysis (CAVHD) are extracorporeal ultrafiltration techniques that permit ongoing removal of plasma water and uremic toxins. Both techniques are performed in the ICU with a minimum amount of equipment and achieve overall fluid balance more readily than intermittent hemodialysis. CAVH is used to manage hypervolemia, electrolyte imbalance, and/or mild uremia. CAVHD is used in hypercatabolic patients with acute renal failure who are hypervolemic and uremic; a dialysate fluid is used for more efficient solute removal. The most serious complications of CAVH and CAVHD relate to bleeding associated with cannulation or anticoagulation. Excess fluid and electrolyte losses may also occur.