Page 87 - Clinical Application of Mechanical Ventilation

Classification of Mechanical Ventilators 53

and inspiratory flow). These variables are related by the equation of motion for the
respiratory system:
Volume
Muscle Pressure + Ventilator Presssure = + (Resistance * Flow)
Compliance
Compliance is defined as a change in volume divided by a change in pressure,
which is a measure of the elastic forces of the lungs and thorax. Flow, as defined ear-
lier, is a unit of volume divided by a unit of time. Resistance is the force that must
be overcome to move gas through the conducting airways, which is best described
by Poiseuille’s Law.
A mechanical ventilator is simply a machine or device that can fully or partially
substitute for the ventilatory work accomplished by the patient’s muscles. If the
patient’s ventilatory muscles contribute no work (sedation, paralysis, etc.), the me-
chanical ventilator provides full ventilatory support. If the patient’s muscles are
able to sustain all of the patient’s ventilatory requirements, no support is provided
by the machine and ventilatory support is zero. Between the two extremes, partial
support can be provided by the mechanical ventilator in assisting the ventilatory
muscles.

INPUT POWER

Mechanical ventilators may be first classified as to the power source that is used to
provide the energy required to support the patient’s ventilation. As described earlier,
ventilation requires work and therefore, energy.
Pneumatically powered ventilators use compressed gas as an energy source for
their operation. Medical gases are anhydrous (without water), and oil-free at a pres-
sure of 50 psi. Examples of ventilators that utilize pneumatic power include the Bird
Mark 7, Percussionaire IPV, Monaghan 225/SIMV, and the Percussionaire VDR.
Ventilators may also be electrically powered, utilizing 120 V 60 Hz alternating
current (AC) or 12 V direct current (DC) for a power source. The electrical power
can be used to run electric motors to drive pistons, compressors, or other mechani-
compressors: A device capable of cal devices that generate gas flow. Examples of electrically powered ventilators in-
building up pressure by compress-
ing the volume of air. clude the CareFusion LTV 1150 and Puritan Bennett 540.
Some ventilators are powered by a combination of both pneumatic and electric
power sources. Many third-generation ventilators require both an electrical (for
microprocessor: Minute com- microprocessor-controlled systems) and pneumatic power source. These ventila-
puter that is designed to perform
specific functions. tors include the Viasys AVEA, Puritan-Bennett 840, Hamilton-C2, among others.

DRIVE MECHANISM

The drive mechanism is the system used by the ventilator to transmit or convert the
input power to useful ventilatory work. The type of drive mechanism determines
the characteristic flow and pressure patterns each ventilator produces. The use of

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.