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Objectives
Distinguish between the pneumatic and electronic powering systems of the ventilator.
Describe the variables, breath sequence, and targeting scheme used to control mechanical ventilator operation.
Compare pressure control, volume control, flow control, and time control.
Distinguish between trigger, limit, and cycle.
Define spontaneous and mandatory breath types.
Compare continuous mandatory ventilation, intermittent mandatory ventilation, and continuous spontaneous ventilation.
Compare set point, dual servo, adaptive, optimal, and intelligent targeting schemes.
Use the equation of motion to describe patient-ventilator interaction.
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Current generation mechanical ventilators are sophisticated life support devices. The ventilator must be reliable, flexible, and relatively easy to use by the skilled clinician. This chapter describes the ventilator system, and then covers ventilator classification and breath types during mechanical ventilation.
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The Ventilator Powering Systems
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Because ventilators deliver gases to the patient, they must have a pneumatic component. First-generation ventilators were typically pneumatically powered, using gas pressure to power the ventilator as well as ventilate the patient. Current generation ventilators are electronically controlled with the aid of a microprocessor. A generic block diagram of a ventilator is shown in Figure 5-1.
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The pneumatic system is responsible for delivery of a gas mixture to the patient. Room air and 100% oxygen are delivered to the ventilator at 50 lb/in2. The ventilator reduces this pressure and mixes these gases to provide a prescribed Fio2 and flow into the ventilator circuit. The ventilator circuit not only delivers gas to the patient, but also filters, warms, and humidifies the inspired gas. During exhalation, gas flows through the expiratory limb of the circuit, a filter, the exhalation valve, and then into the atmosphere. The exhalation valve closes during inspiration to allow inflation of the lungs and is responsible for controlling positive end-expiratory pressure (PEEP). In the past, the exhalation valve was closed completely during the inspiratory phase. Newer-generation ventilators use an active exhalation valve during pressure-controlled ventilation, meaning that it opens if the pressure exceeds the pressure set during the inspiratory phase.
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The pneumatic system can be either single circuit or double circuit. With single-circuit ventilators, the gas that powers the ventilator is the same gas that is delivered to the patient. With double-circuit ventilators, the gas delivered to the patient is separate from the gas that powers the pneumonic system.
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Ventilators can be positive-pressure or negative-pressure generators. Positive-pressure ventilators apply a positive pressure to the airway. Negative-pressure ventilators apply a negative pressure to the chest wall. Critical care ventilators are positive-pressure generators. Negative-pressure ventilators are used infrequently, but may be used in some patients receiving prolonged mechanical ventilation.
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