Page 351 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 317

patient’s lungs, allowing clinicians to measure lung pressure or, more importantly,
alveolar pressure (P ALV ). Since there is no flow, the corresponding pressure created
by resistance to flow dissipates immediately. Pressure drops to the peak alveolar
peak alveolar pressure (P ALV ):
), plateau pressure (P
Pressure obtained by performing pressure (P ALV PLAT ) level, which can be measured because
an end-inspiratory pause, also of the open communication in the ventilator circuit between the alveoli and the
called plateau pressure.
manometer at the ventilator. All lung pressures, not just peak P ALV , are monitored
during this pause in flow. However, P ALV is used in the graphics and text for em-
plateau pressure (P PLAT ): Pres- phasis throughout this chapter. The extremely thin type 1 pneumocytes (0.5 to 2
sure obtained by performing an angstroms) lining alveoli are more sensitive to pressure and trauma than the progres-
end-inspiratory pause, also called
peak alveolar pressure. sively thicker airways. P ALV is a major concern and reason for performing the pause-
pressure measurement during ventilator management. Once peak P ALV is known,
circuit and airway resistance can be determined [Resistance 5 (PIP 2 peak P ALV )/
Flow], provided that a constant flow pattern is present to cause a consistent flow-
resistive pressure throughout inspiration, as shown in Figure 11-4.
Technically, calculation of circuit and airway resistance should be the pressure gra-
(Figure 11-4) dient between PIP and peak P . Pause pressure (P ) is intended to eliminate the
P AO 5 P TA 1 P ALV ALV pause
contribution of the resistance to flow through the airways, and it is synonymous with
peak P ALV in the equation for circuit and airway resistance measurements practiced
clinically. Measuring P enables calculation of the lung-thorax compliance (C )
lung-thorax compliance (C LT ): pause LT
The relationship of volume and (e.g., C 5 volume/pressure, and observing the graphic it can be determined that
LT
pressure (V/P) that is imposed by C 5 1 L/20 cm H O 5 0.50 L/cm H O). The pressure-time waveform in Figure
the property of the lungs and tho- LT 2 2
rax. Also called static compliance. 11-4 shows, as noted earlier, that the P represents two distinct pressures involved
AO
during inspiration with flow: the pressure caused by resistance to flow through the
circuit and airways, and the elastic recoil pressure created by the airways, alveoli
(Figure 11-4) At end- (P ALV ), and chest wall.
inspiration, P TA 5 PIP 2 Under ideal conditions, there is a linear rise in P (dashed line) during the inspi-
Peak P ALV during constant flow ALV
ventilation. ratory cycle since there is a constant rise in volume per unit time with constant flow
delivery. Flow-resistive pressure (arrows above dashed line) created by the ventila-
tor circuit and airways is also constant, assuming that flow through the respiratory
system is constant. Flow-resistive pressure will rise parallel to the rise in P ALV during
transairway pressure (P TA ): constant flow. The P is the dynamic pressure recorded by the pressure manometer
Flow-resistive pressure, the dif- AO
ference between airway opening that clinicians observe at the ventilator as gas is being forced into lungs. Another
pressure (P AO ) and alveolar pres-
sure (P ALV ), or P TA 5 P AO 2 P ALV . term for flow-resistive pressure during inspiration is transairway pressure (P ),
TA
which is the difference between P and P ALV (P 5 P 2 P ALV ). Thus, the P
AO
AO
TA
AO
is equal to the summation of the two distinct pressures during inspiration: P and
TA
P ALV . At end-inspiration, P equals the difference between the PIP and the peak
TA
Airflow Resistance 5 P (P 5 PIP 2 Peak P ) during constant-flow ventilation. Also, P equals
P TA /Flow ALV TA ALV TA
flow times resistance (P 5 Flow 3 Resistance).
TA
Controlled Mandatory Ventilation
controlled mandatory ventila-
tion (CMV): Time-triggered man- Figure 11-5 is a waveform example of controlled mandatory ventilation (CMV).
datory breaths provided by the
ventilator. Also called controlled CMV is a mode that defines the specific control, phase, and conditional variables
mechanical ventilation or continu- of mandatory breaths (Chatburn, 2001, 2007). The CMV waveforms demonstrate
ous mandatory ventilation.
that each breath is a time-triggered mandatory breath, and volume-controlled

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.