Page 597 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 48: Ventilator Waveforms: Clinical Interpretation 417

l/s Flow-time l/s Flow-time s
1.2 1.1

0
0 8

−1.2
−1.1
cm H 2 O
40 cm H O Pressure-time s
2
30

0
0 8
FIGURE 48-13. This patient had respiratory failure due to septic shock. Despite ACV at a
high minute ventilation, the patient continued to work hard to breathe, lowering Pao during
inspiration below the set PEEP level (left panel). Notice that Pao only rises at the termination FIGURE 48-15. Signs of patient effort during constant flow, volume-preset ventilation
of inspiration. Following therapeutic paralysis (right panel), Ppk rises to 40 cm H O. The differ- include concavity of the Pao versus time waveform during inspiration; a drop in Pao before the
2
ence in Pao before and after paralysis reflects the very high work of breathing of this patient. breath indicating triggering; and variability in the height of the Ppeak. Notice the first breath shows
marked concavity, the 2nd reveals obvious triggering, and during the third the patient is passive.
of ACV and SIMV: the greater the patient demand, the lower the Pao. impact of sedatives, and other central nervous system/patient features.
One of the advantages of pressure-preset modes is that Pao is main- In the usual setting, the patient desires a longer T than is set so that the
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tained, no matter how high the flow required, and these modes may patient’s inspiratory effort persists, even while the ventilator has cycled to
be more comfortable for the patient with high drive (see Fig. 48-14). It expiration. When neural and mechanical T ’s are dissimilar, patients may
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would even be sensible to augment pressure when the patient effort is be uncomfortable. Pressure support ventilation goes far toward solving
high, and this rationale underlies PAV (see Chap. 50). Of course, meeting this problem by switching off when inspiratory flow falls, since reduced
the patient’s flow demand may run counter to other goals, such as the flow often correlates with the end of neural inspiration. In contrast, many
desire to limit tidal volume in the interest of lung protection. patients ventilated with PCV or volume-preset modes rely on happenstance
Clues to patient effort are often available from the airway pressure or sedatives to accommodate their neural T to the machine T (Fig. 48-16).
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tracing when using a volume-preset mode, such as concavity of the Postinspiratory effort can be recognized as loss of the usually rapid
rise in Pao, variability of Ppeak, and a dip in Pao before inspiration, initial expiratory flow, as illustrated in Figure 48-16, or by double- (or
indicating a triggering effort (Fig. 48-15). Since every breath of a pas-
sive patient should produce identical flow and pressure waveforms, an
important clue to effort is simply a lack of uniformity of breaths. l/s Flow-time s
1.1
Effort Persisting After Machine Inspiration: One of the paradoxes of
mechanical ventilation is that the patient’s desired inspiratory duration
(neural T ) may bear no relationship to the ventilator inspiratory time
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(mechanical T ). Mechanical T is set by the physician, either directly 0 6
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(as with PCV) or indirectly (ACV, SIMV), whereas neural T depends
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on arterial blood gases, the gas exchange efficiency of the lung, the
−1.1
PAV cm H O Pressure-time s
2
30
PSV, PCV
Pao

ACV
IMV 0 6
Patient effort
FIGURE 48-16. Volume assist-control ventilation with a machine T of 0.7 second. Notice
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FIGURE 48-14. This shows Pao as a function of patient effort during various modes that expiratory flow does not follow the first breath until after a substantial delay because
of ventilation. Pressure-preset modes deliver a fixed pressure, whatever the effort, whereas the patient is still contracting the inspiratory muscles. Only after a total neural T of about
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volume preset (constant flow) modes, like ACV and IMV produce ever less pressure with 1.2 seconds does the patient turn off the inspiratory muscles and begin to exhale. The same
increasing effort. IMV forces the patient to work even harder, since not all breaths are sup- phenomenon is seen after the 2nd breath, but this time the patient’s continued inspiratory
ported. PAV responds to increasing patient demand by augmenting flow. effort is sufficient to double-trigger the ventilator.

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