Page 596 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

416 PART 4: Pulmonary Disorders

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

0 6 0 6

−1.1 −1.1
cm H O Pressure-time cm H O Pressure-time s
2
2
50 50

0 6 0 6

FIGURE 48-11. This is the same patient shown in Figure 48-10, this time showing the measurement of autoPEEP while the applied PEEP is 7 cm H O (left panel) or 0 cm H O (right panel).
2
2
The measured autoPEEP (10 cm H O is identical).
2
roughly 75% of the autoPEEP, there is little effect on Palv, expiratory flow is set by the physician and ventilator during typical volume-preset
flow, or the magnitude of autoPEEP, although there are occasional ventilation, no amount of patient effort can raise the flow (unless the venti-
14
exceptions. 15 lator allows the patient to override the settings). The effect of this is that
Pao may not become positive during the breath or may even become
Effects of Therapy: A reduction in airways resistance or a response to negative (since the Pao reflects the competition between the ventila-
bronchodilators may be signaled by (1) reduced Pres (and lower Ppeak); tor, tending to raise Pao, and the patient, tending to lower it). If Pao is
(2) reduced autoPEEP ; and (3) a more normal expiratory flow-volume not positive during inspiration, the ventilator is not doing work on the
9
curve. Expiratory peak flow often does not increase with bronchodila- patient. That is, the patient’s work of breathing would be no lower if
tors, however, because the lowered alveolar pressure (less autoPEEP) the ventilator were disconnected. In extreme circumstances, the ventila-
reduces the driving pressure for exhalation. tor may even impede respiration. This represents a fundamental problem
Effect of Patient Effort: Flow and pressure waveforms are generally easy
to analyze when patients are fully passive. On the other hand, an active
patient presents numerous challenges and pitfalls. The discussion above
always assumed a passive patient, but this is often not the case, especially 0.6 l/s Flow-time s
in an era of lower tidal volumes and lighter sedation.
Inspiratory Effort Preceding Machine Inspiration: The inspiratory thresh-
old load presented by autoPEEP sometimes leads to a striking delay
between the initiation of the patient’s inspiratory effort and the onset of 0 10
machine inspiration, sometimes consisting of several hundred millisec-
onds (Fig. 48-12). This delay, which signals the presence of autoPEEP, is
often shortened markedly by the addition of externally applied PEEP, a
feature that may aid the setting of PEEP in obstructed patients. −0.6
Effort During Machine Inspiration: It has long been known that patients cm H O Pressure-time s
2
perform inspiratory work throughout an assist control breath. With 30
16
modern ventilator management shifted to greater patient effort, lower
tidal volumes, less sedation, and only rare use of therapeutic paralysis,
most patients exhibit effort on most ventilator breaths, no matter the
mode of ventilation. This effort may not be obvious despite careful
examination of the patient unless measures of intrathoracic pres-
sure (esophageal, central venous, or wedge pressures) are available.
Inspiratory effort may alter Pao (volume-preset breaths), inspiratory 0 10
flow (pressure-preset breaths), or expiratory flow (any mode). Effort
at the end of a volume-preset breath will affect the Ppeak and Pplat, FIGURE 48-12. The pressure waveform in this patient shows a significant fall in Pao
making determination of respiratory system mechanics unreliable. The preceding each breath by several hundred milliseconds and lasting until the ventilator delivers
magnitude of this problem is illustrated in Figure 48-13, where the a breath. This long delay was due to the difficulty experienced by the patient in overcoming a
degree of patient effort is hidden until therapeutic paralysis reveals it. large amount of autoPEEP. A casual inspection of the flow tracing might lead one to conclude
Some patients have extremely high drive, despite being connected to that autoPEEP was not present (since flow near end-expiration is zero), but flow has ceased
a ventilator. They may desire inspiratory flow rates much higher than only because the patient is making inspiratory effort. On other breaths (not shown) the patient
are typically ordered (often in excess of 100 L/min). Since inspiratory failed to trigger at all, even while lowering Pao and stopping expiratory flow completely.

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