Page 94 - Clinical Application of Mechanical Ventilation

60 Chapter 3

PHASE VARIABLES

A ventilator-supported breath may be divided into four distinct phases: (1) the change
from expiration to inspiration, (2) inspiration, (3) the change from inspiration to ex-
piration, and (4) expiration. More detail can be learned by studying what occurs to
the four variables (pressure, volume, flow, and time) during these phases. When the
variable is examined during a particular phase, it is termed a phase variable.

Trigger Variable

The trigger variable is the variable that determines the start of inspiration. Pres-
sure, volume, flow, or time may be measured by the ventilator and used as a vari-
able to initiate inspiration. Many ventilators may use time or pressure as trigger
variables.

Control: Time-Triggered. A time-triggered breath is initiated and delivered by the
time-triggered: Initiation of a
mechanical breath based on the ventilator when a preset time interval has elapsed. The frequency control on the
set time interval for one complete ventilator is a time-triggering mechanism.
respiratory cycle (inspiratory time
and expiratory time). For example, if the ventilator frequency is preset at 12 breaths per minute (60 sec),
the time-triggering interval for each complete breath is 5 sec. At this time-trigger in-
terval, the ventilator automatically delivers one mechanical breath every 5 sec without
regard to the patient’s breathing effort or requirement.
60 sec/12 breaths = 5 sec/breath

Pressure-Triggered. A pressure-triggered breath is initiated and delivered by the
pressure-triggered: Initiation of
a mechanical breath based on the ventilator when it senses the patient’s spontaneous (negative pressure) inspiratory
drop in airway pressure that occurs effort. The patient may trigger the ventilator by generating a pressure gradient or a
at the beginning of a spontaneous
inspiratory effort. flow gradient.
Pressure triggering uses the drop in airway pressure that occurs at the beginning
of a spontaneous inspiratory effort to signal the ventilator to begin inspiration
(Figure 3-9). The amount of negative pressure, below the patient’s baseline airway
pressure (or end-expiratory pressure), that a patient must generate to trigger the
ventilator into inspiration, is the sensitivity level. The range of acceptable sensitiv-
ity levels for pressure triggering varies from 21 to 25 cm H O below the patient’s
2
baseline pressure.
For example, if the sensitivity for pressure triggering is set at 23 cm H O, then
2
Comparing to a sensitiv- the patient must generate a pressure of 23 cm H O at the airway opening to trigger
ity setting of 23 cm H 2 O, 25 2
cm H 2 O requires more patient the ventilator into inspiration. If the sensitivity for pressure triggering is changed
effort to trigger the ventilator from 23 to 25 cm H O, the ventilator becomes less sensitive to the patient’s in-
to inspiration. 2
spiratory effort as more effort is needed to trigger the ventilator into inspiration.
Changing the sensitivity from 23 to 25 cm H O is decreasing the sensitivity setting
2
on the ventilator.
In situations where auto-PEEP is present, the triggering pressure would become
greater. In order for the ventilator to sense a successful triggering effort, the patient
must overcome both the auto-PEEP level and the sensitivity setting. See Figure 12-2

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