Page 132 - Clinical Application of Mechanical Ventilation

98 Chapter 4

60 Mechanical Breaths
Spontaneous Breaths
50
Airway Pressure (cm H 2 O) 30
B
40
A
20
10
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3
1
5
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Figure 4-7 Synchronized intermittent mandatory ventilation (SIMV) pressure tracing with
two mandatory breaths and five anticipated spontaneous breaths (only three active). SIMV mode
does not cause breath stacking since the mandatory breaths are delivered slightly sooner or later
than the preset time interval but within a time window. Mandatory breaths (A) and (B) occur
during a spontaneous inspiratory effort. The anticipated spontaneous breaths #2 and #4 did not
occur as they turned into mechanical breaths during the mandatory cycle.

spontaneous inspiratory effort just prior to the delivery of a time-triggered breath.
For example, if the SIMV mandatory frequency is set at 10/min, then the ventilator
would time-trigger a breath every 6 sec if the patient is not attempting to inspire
spontaneously. However, if the patient is breathing spontaneously between the
mandatory breaths, and if by random chance, the patient begins to inspire just prior
to the point at which the ventilator would be expected to time-trigger, then the
ventilator senses this spontaneous effort and delivers the mandatory breath as an
assisted patient-triggered breath. The mandatory breath, whether time- or patient-
triggered, is controlled by all applicable mechanical tidal volume settings.
Synchronization Window. The time interval just prior to time triggering in which the
ventilator is responsive to the patient’s spontaneous inspiratory effort is commonly
referred to as the synchronization window (Sassoon et al., 1991). Although the exact
time interval of the synchronization window is slightly different from manufacturer
to manufacturer, 0.5 sec is representative. For example, given an SIMV mandatory
frequency of 10/min, the ventilator would be expected to time trigger every 6 sec.
If the synchronization window is 0.5 sec, then at 5.5 sec from the beginning of the
previous mandatory breath, the ventilator automatically becomes sensitive to any
spontaneous inspiratory effort, i.e., the synchronization window becomes active.
If the patient makes a spontaneous inspiratory effort when the synchronization
window is active, the ventilator is patient-triggered to deliver an assisted mandatory
breath. Patient triggering may be based either on pressure or flow. If, however, no
spontaneous inspiratory effort exists while the synchronization window is active,
the ventilator will time trigger when the full time-triggering interval elapses.

SIMV Spontaneous Breath-Triggering Mechanism

In between the mandatory breaths, SIMV permits the patient to breathe sponta-
neously to any tidal volume the patient desires. The gas source for spontaneous

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