Page 353 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 319

60
V (L/min) 50

1 2 3 4 5 6

230
P (cm H 2 O) 40

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22
1 2 3 4 5 6
Time (sec)
Figure 11-6 Flow- and pressure-time waveforms to show assist mechanical ventilation. Note
the negative triggering pressure (22 cm H 2 O) before the beginning of inspiration.

When patients expand their thorax in synchrony with gas delivery from the ventilator,
(Figure 11-6) Inspiratory this relieves some of the inspiratory pressure that would build up in the circuit and lungs
flow begins as soon as the
negative deflection reaches compared to a passive patient (sedated and paralyzed). Slight reductions (2 to 3 cm H O)
2
the preset sensitivity of in the P pattern that are made apparent by the variability in PIP at end-inspiration are
22 cm H 2 O. AO
unremarkable. Patients commonly perform 33% to 50% of normal workloads of venti-
lation during optimal assist CMV VCV breaths (Marini et al., 1985).
Figure 11-7 demonstrates the CMV mode of ventilation again. The settings are the
(Figure 11-7) The T I is un- same as in the previous examples, except that volume waveforms have been added,
changed during VCV because
V T and average flow are held PEEP is being administered at 10 cm H O, and the patient is receiving two assist and
2
constant (T I 5 V T /Average two mandatory breaths. For a CFW, volume rises linearly over time, as demonstrated.
Flow). I:E ratios differ because
of changing E time. During expiration, volume demonstrates an exponential decay pattern in correspon-
dence with the expiratory flow pattern. The I:E ratios are 1:3 for the second and third
mechanical breaths, as the patient expires and relaxes after the second assist breath and
allows the ventilator to take control (mandatory breaths). The expiratory time for the
first breath is reduced because the patient expires faster and triggers a second breath (as-
sist) prior to a timed mandatory breath taking place as indicated on the second pressure
waveform. Thus, the I:E ratio (1:3) is reduced, because the patient triggers the second
breath before the 3 sec expiratory time (T ) has elapsed. The T is unchanged during
I
E
VCV because V and average flow are held constant (T 5 V /Average Flow).
T
I
T
In the CMV mode, flow, V , and mandatory frequency are set, but the patient is al-
T
lowed to trigger as many breaths as desired. When patients are able to trigger breaths
in the CMV mode, the term assist/control mode is commonly used, but it is no longer
recommended because it does not define a unique mode. When setting CMV, assist

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