Page 95 - Clinical Application of Mechanical Ventilation

Classification of Mechanical Ventilators 61

A B
Exp. Insp. Exp. Insp.
Tubing Tubing Tubing Tubing

2
0 cm H 2 O –3 cm H O

Pressure Trigger
2
Before 0 cm H O Pressure –3 cm H 2 O © Cengage Learning 2014
Inspiratory at Beginning
Effort Inspiration
Figure 3-9 Pressure-trigger mechanism. (A) Before an inspiratory effort, the pressure in the
airway and ventilator tubing equals 0 cm H 2 O. A mechanical breath is not initiated because there
is no pressure drop to trigger the ventilator sensitivity settings. (B) At beginning inspiration, the
pressure in the airway and ventilator tubing is 23 cm H 2 O. A mechanical breath is initiated be-
cause the pressure drop is sufficient to trigger the ventilator sensitivity setting (assuming it is set
as 23 cm H 2 O or less).

(Chapter 12) for further discussion on auto-PEEP and the strategy to compensate
for the effects of auto-PEEP.
Flow-Triggered. Some ventilators are able to measure inspiratory and expiratory flows.
When the patient’s inspiratory flow reaches a specific value, a ventilator-supported
breath is delivered. Flow triggering has been shown to be more sensitive and respon-
sive to a patient’s efforts than pressure triggering. A flow-triggered breath uses a
flow-triggered: Flow-triggering
strategy uses a combination of strategy that combines the continuous flow and demand flow mechanisms, and it
continuous flow and demand flow. is used to reduce the inspiratory effort imposed on the patient during mechanical
Before inspiration, the delivered
flow equals the return flow. As the ventilation. It is considered to be more sensitive to the patient’s inspiratory effort and
patient initiates a breath, the return therefore usually requires less inspiratory work than pressure triggering.
flow to the ventilator is decreased
and this flow differential triggers a In flow triggering, a continuous flow passes through the ventilator circuit and
mechanical breath.
returns to the ventilator (i.e., delivered flow = returned flow). As the patient initi-
ates a breath, part of the delivered flow goes to the patient and the return flow to
the ventilator is therefore reduced (i.e., delivered flow . return flow). The ventila-
tor senses this flow differential and instantly supplies enough flow to satisfy the
mechanical or spontaneous tidal volume. CMV, SIMV, and PSV can all be flow-
triggered (Figure 3-10).
How hard the patient must work to initiate or trigger a breath is termed the
ventilator sensitivity. If the ventilator is made more sensitive to the patient’s efforts
(pressure, flow, or volume), it is easier for the patient to trigger a breath. The con-
verse is also true.

Limit Variable

During a ventilator-supported breath, volume pressure and inspiratory flow all rise
above their respective baseline values. Inspiratory time is defined as the time interval

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