Page 90 - Clinical Application of Mechanical Ventilation

56 Chapter 3

CONTROL CIRCUIT

The control circuit is the system that governs or controls the ventilator drive mecha-
nism or output control valve. The control circuit is the system that is responsible for
the characteristic output waveforms, which will be discussed later in this chapter.
Control circuits may be classified as open- or closed-loop control circuits, mechani-
cal, pneumatic, fluidics, and electronic.
An open-loop control circuit is one where the desired output is selected and the
ventilator achieves the desired output without any further input from the clinician
or the ventilator itself.
A closed-loop control circuit is one where the desired output is selected and then
the ventilator measures a specific parameter or variable (flow, pressure, or volume)
continuously, and the input is constantly adjusted to match the desired output. This
type of control circuit may also be referred to as servo-controlled.
servo: A feedback system that typi-
cally consists of a sensing element,
an amplifier, and a servomotor,
used in the automatic control of the Mechanical
mechanical device of a ventilator.
Mechanical control circuits employ simple machines such as levers, pulleys, or cams
to control the drive mechanism. Early mechanical ventilators used these systems to
control their outputs. Being mechanical, some of these control systems were very
durable but lacked flexibility by being an open-loop type control system.

Pneumatic

Pneumatic devices can be used as control circuits. These devices include valves,
nozzles, ducted ejectors, and diaphragms. The IPPB ventilators and the Percussion-
aire IPV and VDR ventilators all use pneumatic control circuits.

Fluidics

Fluidics is the application of gas flow and pressure to control the direction of
gas flows and to perform logic functions. The logic functions of fluidics have
their origin in digital electronics. Fluidic elements, just as do digital electronic
gates, control their outputs according to the inputs received. By combining flu-
idic elements in specific ways, a fluidic ventilator (e.g. Sechrist IV 100B) can be
designed to function in a similar way to other ventilators that are electronically
controlled.
Fluidic elements operate using the Coanda effect. If a jet of gas exits at high
velocity adjacent to a wall (Figure 3-4), the gas flow will attach to the adjacent
wall. An area of reduced pressure forms a separation bubble, which attaches the
flow to the adjacent surface. Fluidic elements use a flow splitter located beside
adjacent walls to control the direction of flow and to perform logic functions
(Figure 3-5).

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