Page 378 - Clinical Application of Mechanical Ventilation

344 Chapter 11

V (L/min) 120 A B

2 4 6 8 10 12 14
260
P (cm H 2 O) 40 a b

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2 4 6 8 10 12 14
Time (sec)
Figure 11-26 During pressure-limited ventilation, the flow and volume are dependent on
the airflow and compliance characteristics of the patient/ventilator system. A higher flow and
larger volume are observed in A and B (first and third breaths) than their adjacent breaths (second
and fourth breaths). The waveforms show that A has a lower airflow resistance and B has a higher
compliance than their adjacent breaths.

(first and third) are comparatively greater than the adjacent flow waveforms (second
and fourth) in examples A and B. Thus, greater volume is being delivered by the
first and third waveforms.
For each pressure-controlled breath under the dashed lines labeled A and B
(Figure 11-26) characterize different lung conditions during PCV. The peak flow
and area under the inspiratory curve of the flow waveforms (first and third) are com-
paratively greater than the adjacent flow waveforms (second and fourth) in examples
A and B. Thus, greater volume is being delivered by the first and third waveforms.
In example A (first and second waveforms), given no change in C or effort,
LT
greater airway/circuit resistance will tend to cause some reduction in the initial or
average flow rate, and therefore, a reduction in V as demonstrated by the adjacent
T
flow waveform. Greater flow-resistive pressure (P ) causes the set (target) pressure
TA
to be reached at a lower flow rate. A reduction in delivered V may be the result
T
unless the average flow is sustained, which may be demonstrated by a reduction
in initial peak flow, but an elevation in the end-flow rate. An increase in airway or
During pressure- circuit resistance will result in a reduced V because the peak and average flow will
controlled ventilation, an T
increased airflow resistance be reduced, as demonstrated in this example. Letter a demonstrates that the average
will reduce the delivered flow expiratory flow rate is reduced, causing longer T .
and tidal volume. E
Example B (third and fourth waveforms) demonstrates that a reduction in the
C (i.e., ascites, ARDS) will also result in a reduction in V . However, the initial
T
LT
inspiratory flow rate is not affected since resistance to flow is not a factor. The initial
driving pressure for flow, the PC level set, has not changed, so the initial flow rate is
sustained. However, given lower C , more pressure is required for the same volume
LT
to be delivered, and since pressure is held constant, the result has to be a reduction

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