Page 386 - Clinical Application of Mechanical Ventilation

352 Chapter 11

accompany such a graphic display. The second, third, and fifth flow waves, a, b, and
(Figure 11-31) Letters c (arrows), show that flow demand is high (35 to 45 L/min) throughout inspiration,
a, b, and c are examples of
respiratory distress where the sustaining near-constant flow wave patterns, which are created under conditions
inspiratory flow demand is
high throughout the inspira- of stress. The flow-time waves do not show a relaxed pattern as in the first flow
tory phase. Compare the wave, shown for comparison, where flow gradually descends to cycle the ventila-
abnormal near-constant flow
patterns (a, b, and c) to the tor into expiration at 25% of the initial flow. Instead, flow demand is sustained
normal descending flow pat- (squared off) at a high level and ends abruptly, dropping quickly to zero with the
terns (first flow wave). Letters
d and e show high triggering patient’s expiratory effort and attempts to quickly trigger another breath. The more
efforts. that flow appears to square off and drop perpendicularly through the flow cycling
level, the higher the flow demand, and the less the patient is relaxing during inspi-
ration and pausing between breaths at that set level of pressure support.
The patient’s high respiratory drive is also evident by evaluating the patient-
triggering efforts in the pressure-time waveforms (d and e arrows). Negative pressure
drops substantially below the sensitivity threshold level set below PEEP before the
ventilator demand valve can respond. Air trapping is apparent in the graphic as well,
(Figure 11-31) Letters f which can be seen on the third and fourth expiratory flow waves (f and g arrows).
and g show that the expira-
tory flow waves do not return Thus, expiration is not complete before the patient triggers breaths four and five. Sev-
to baseline before the next eral of the pressure waveforms are not squared off, so, obviously, the pressure limit set
breath. Incomplete expiration
may lead to air trapping. is not being sustained as it should be. Increasing the pressure support level to 15 to 20
cm H O would probably eliminate this patient’s distress and provide adequate sup-
2
port for a normal level of WOB. If pressure-controlled ventilation (PCV) in the CMV
mode were being demonstrated in this example at the same pressure level setting,
similar pressure patterns of dyssynchrony would be manifest, but fluctuations in flow
to accommodate demand would be less likely. The T would be maintained, however,
I
consistent with the settings for PCV in the CMV mode.

USING EXPIRATORY FLOW AND PRESSURE
WAVEFORMS AS DIAGNOSTIC TOOLS

Increased Airway Resistance

The expiratory flow waveform can be used to determine whether a patient has exces-
sive airway resistance (asthma), obstructive, or restrictive disease (ARDS). The solid
line expiratory flow wave in Figure 11-32 demonstrates excessive expiratory airway
resistance. The dashed line represents a relatively normal waveform. Circuit and
endotracheal tube (ETT) resistance (i.e., size 6 mm I.D. ETT) can complicate the
assessment by creating a similar pattern, but RCPs can easily measure circuit resis-
tance and eliminate it as the cause for the abnormal expiratory flow curve. Typically,
circuit resistance is about 6 to 8 cm H O/L/sec at 60 L/min with an 8 mm I.D. ETT
2
and humidifier in line (Dennison et al., 1989), which can be subtracted from clinical
measurements of airway resistance. The low-peak expiratory flow, the average flow
level, and abnormally long expiratory flow time (.5 sec) compared to the normal
curve are obvious signs of severely elevated airway resistance in this example. High

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