Page 391 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 357

breathe and is trying to “inspire” during expiration and, hence, is slowing the
expiratory flow and reducing expiratory pressure as pointed to (arrow a) on the
pressure wave below it. It could also indicate that airway obstruction is slowing
flow. As a result of hyperinflation or trapped gas some patients cannot expand
their thorax or lungs enough to create negative pressure in the ventilator-patient
circuit. Their ineffective inspiratory efforts may only slow the flow of gas being
released from areas of the lungs, which have longer time constants. They may be
able to reduce the driving pressure (P ALV ), but may not be able to create sufficient
negative P to trigger a breath. The drop in flow or pressure may be obstruction
AO
(Figure 11-35) Double to flow, which might be caused by excess secretions, structural damage to airways,
arrow b shows that the
expiratory flow is increasing or dynamic airway compression. Obstruction from a more homogeneous condi-
suddenly with a correspond-
ing rise in pressure. This tion (bronchoconstriction) may present a different pattern (Figure 11-32).
suggests that the patient is The second double-headed arrow b points to a sudden increase in flow and cor-
trying to actively expire or
there is a sudden relief of an responding expiratory pressure. This can occur if the patient attempts to actively
obstruction. expire or the obstruction is momentarily relieved with a change in flow dynamics
and time constants.
A third double-headed arrow c points to an obvious inspiratory effort during expi-
Lack of ventilator ration because flow drops momentarily to zero and an insufficient negative pressure
response may be caused by
dysfunction of the inspira- (above sensitivity threshold) is recorded. The patient continues to expire more of
tory valve or inappropriate the trapped gas (arrow d ) and flow increases and some positive pressure is recorded.
sensitivity setting.
A controlled mandatory breath is then time-triggered before the patient has com-
pleted expiration as indicated (arrow e).
The patient needs to be assessed for respiratory movements whenever such pat-
(Figure 11-35) Double
arrow c shows an inspiratory terns are observed. V and T may have to be reduced to allow more T , and the
T
I
E
effort during expiration. Note sensitivity threshold may have to be increased (20.5 versus 22 cm H O). The
that the flow momentarily 2
drops to baseline and a nega- respiratory effort needs to be properly documented during evaluation for weaning.
tive pressure is recorded. Patients often trigger two or three times as many breaths as recorded by the ven-
tilator. Inspiratory efforts during expiration can be felt by placing a hand over the
patient’s abdomen. Contraction of the abdominal muscles can be felt during forced
expirations following the ineffective inspiratory efforts. Visual signs of chest move-
ment, supra- or subclavicular retractions, and interruption of expiratory flow can
be heard on auscultation during inspiratory efforts. The graphics can be monitored
at the same time to confirm the coordination of efforts with the physical signs of
respiration.

TROUBLESHOOTING VENTILATOR FUNCTION

Lack of Ventilator Response

Another use of graphics is to check proper ventilator function. For example, inspi-
ratory valve dysfunction (i.e., a sticking valve) or an out-of-calibration sensitivity
threshold can be readily observed when the sensitivity tracing on the pressure-time
waveform does not match the sensitivity setting on the ventilator.

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.