Page 360 - Clinical Application of Mechanical Ventilation

326 Chapter 11

EFFECTS OF FLOW, CIRCUIT, AND LUNG
CHARACTERISTICS ON PRESSURE-TIME WAVEFORMS

Flow and Transairway Pressure

An end-inspiratory pause has been set in Figure 11-11 to delineate the P into the
AO
(Figure 11-11) Increase two distinct pressures, P and P (double-headed arrows), generated during gas
in flow rate causes a higher TA ALV
airflow resistance and thus a flow through a ventilator circuit into a patient’s lungs. For waveforms analysis, it is
corresponding increase in PIP important to know how each of these pressures is affected by changes in flow and
and P TA . Peak alveolar pressure
(peak P ALV ) or plateau pressure lung characteristics. Figure 11-11 demonstrates the effects of increased inspiratory
is not affected by changes in flow or airflow resistance on a pressure-time waveform, which can be compared to
flow rate.
the effects on them created by decreased compliance (Figure 11-12). The pressure-
time graphics in Figure 11-11 depict (double-headed arrows) that there has been a
rise in the P from beginning to end in the second pressure waveform because of an
TA
increase in turbulence when the constant flow was doubled from 60 to 120 L/min
in the second flow wave. As gas flows through a circuit system with different twists
and turns, and with varied lumen sizes such as large bore tubing, humidifiers, and
endotracheal tubes, turbulent flow results. In turn, the circuit system causes an

120

V (L/min) 1 Area 1 = Area 2 2

1 2 3 4 5 6 7

P (cm H 2 O) 2120

80
PIP
P
PIP TA
P TA 20 cm H O 20 cm H O
2
2
P P
ALV ALV
1 2 3 4 5 6 7 © Cengage Learning 2014
Time (sec)
Figure 11-11 Flow- and pressure-time waveforms showing the effects of flow rate on P TA ,
P ALV , and expiratory flow. Increase of inspiratory flow or airflow resistance causes an increase of
P TA , but no changes in peak P ALV (plateau pressure) and expiratory flow.

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