Page 109 - Clinical Application of Mechanical Ventilation

Classification of Mechanical Ventilators 75

volume waveform with respect to time) assumes the characteristic rectangular
shape.
The ramp waveform can be ascending or descending. If flow rises as the breath
is delivered, it is termed ascending. If flow falls during the ventilator-supported
breath, it is called a descending ramp.
The sinusoidal waveform resembles the positive portion of a sine wave. It is gener-
ated by a rotary-driven piston drive mechanism.

ALARM SYSTEMS

Alarm systems are designed to alert the clinician to undesirable technical or pa-
tient events. Triggering of any alarm requires clinician awareness or action. As the
complexity of mechanical ventilators has increased, so have the number and com-
plexity of the alarm systems. Technical events are those events limited to the per-
formance of the ventilator, while patient events are those relating to the patient’s
condition. Alarms can be visual, audible, or both, depending on the seriousness
of the event.

Input Power Alarms

Input power alarms can be further classified as to loss of electrical or pneumatic
power.
Loss of electrical power usually results in the ventilator activating a backup alarm
that is battery-powered. Most battery backup alarms are powered by rechargeable
nickel cadmium batteries, which are recharged when AC power is available. When
AC power is lost, the backup batteries activate audible and visual alarms.
Loss of either air or oxygen pneumatic sources will result in a technical event
alarm. If either input pressure falls below a specified value from 50 psi, the
alarm will result. Some alarms are electronic (BEAR 1000, BEAR I, II and III,
Puritan-Bennett 840), whereas others are pneumatic reed alarms, such as those
employed in oxygen blenders.

Control Circuit Alarms

Control circuit alarms alert the clinician to settings or parameters that are not
within acceptable ranges or specifications, or they warn the clinician that the ven-
tilator has failed some part of a self-diagnostic test. In the event of an incompatible
setting or parameter, the clinician is allowed the opportunity to change the input
to one that is compatible. Failure of the self-diagnostic test may render the ventila-
tor inoperative, and the clinician is alerted by a message display that test failure
has occurred.

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