Page 93 - Clinical Application of Mechanical Ventilation

Classification of Mechanical Ventilators 59

Ambient Pressure

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Subambient
Chest Expands Pressure

Figure 3-8 A schematic illustrating negative pressure ventilation.

changes in the patient’s compliance or resistance. That is, the pressure level that
is delivered to the patient will not vary in spite of changes in patient compliance
or resistance.

Volume Controller

To be classified as a volume controller, volume must be measured and used as a
feedback signal to control the output (volume) delivered. A volume controller al-
lows pressure to vary with changes in resistance and compliance while volume de-
livery remains constant.
Volume controllers can measure volume by the displacement of the piston or
bellows that serves as the ventilator’s drive mechanism. If the displacement of the
bellows or piston is controlled, volume therefore is also controlled.
Flow Controller

Flow controllers allow pressure to vary with changes in the patient’s compliance
and resistance while directly measuring and controlling flow. Flow may be mea-
sured by vortex sensors, heated wire grids, venturi pneumotachometers, strain
gauge flow sensors, and other devices. What is important is that the ventilator
directly measures flow and uses the flow signal as a feedback signal to control
its output.
Many ventilators are incorrectly classified as volume ventilators. Even though a
tidal volume is set or displayed, many ventilators measure flow and then derive vol-
ume from the flow measurement [Volume (L) 5 Flow (L/sec) 3 Inspiratory Time
(sec)]. However, if a ventilator is operated in pressure support or pressure-controlled
mode, the ventilator then becomes a pressure controller, since pressure is the vari-
able that is measured and controlled.

Time Controller

Time controllers are ventilators that measure and control inspiratory and expira-
tory time. These ventilators allow pressure and volume to vary with changes in
pulmonary compliance and resistance. Since neither pressure nor volume is directly
measured or used as a control signal, time (inspiratory, expiratory, or both) remains
the only variable that may be controlled.

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