Page 358 - Clinical Application of Mechanical Ventilation

324 Chapter 11

V (L/min) 60 I E

2 4 6 8 10 12 14

260
P (cm H 2 O)

40

22 2 4 6 8 12 © Cengage Learning 2014
a b 10 14
Time (sec)
Figure 11-9 Synchronized intermittent mandatory ventilation (SIMV) waveform showing one
controlled (time-triggered) mandatory breath (arrow a), one assist (patient-triggered) mandatory
breath (arrow b), and three spontaneous breaths (second, third, and fifth breaths).

Since the breath is mandatory, it shows that the patient did not attempt to breathe
within the window of time (SIMV period) set up for synchronized (assist) mechanical
breaths, and a timed mandatory breath was delivered as required for the SIMV mode.
Two sets of flow and pressure-time waveforms depicting ideal spontaneous breaths are
recorded next during the time (spontaneous period) allotted for spontaneous breaths
prior to initiation of another SIMV period by the ventilator. Given a rested patient and
synchronous conditions, sine waves are created by spontaneous inspirations during
the spontaneous period. As in prior examples, these waveform examples are ideal, but
roughly similar to clinical presentations. Spontaneous breathing is accommodated on
ventilators during SIMV and continuous positive airway pressure (CPAP) ventilation.
During spontaneous ventilation, the patient controls flow, volume, inspiratory time,
and expiratory time. The ventilator responds to patient effort by supplying the flow
demanded. The spontaneous breaths show that the patient created a peak inspiratory
flow of about 20 L/min and peak expiratory flows of about 15 L/min. The pressure
waves during spontaneous breathing show a negative pressure to trigger demand flow
and some positive pressure recorded in the circuit during expiration. Spontaneous
breaths are explained in more detail in Figure 11-10.
The next mechanical breath in Figure 11-9 is an assist breath. Letter b (arrow) indi-
cates that the patient’s effort occurred to trigger the breath during the SIMV period.
The manufacturer determines the amount of time allotted for synchronized mechanical
and spontaneous breaths. For the next breath in the graphic, sufficient time has not
elapsed for a mechanical breath to be synchronized with the patient’s effort, so flow
delivery is based on demand, and another spontaneous breath is provided. An SIMV
rate of 6/min has probably been set for the patient in this graphic, with 5 to 6 sec
allotted for the spontaneous breaths and 4 to 5 sec allotted for the SIMV breaths.

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