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CHAPTER 49: Management of the Ventilated Patient 429

the ventilator panel, and the work of breathing is reduced if autoPEEP A peak inspiratory flow rate (V ˙ ) of 1 L/s (60 L/min) is a common initial
is present. 25,26 A potentially misleading feature of some ventilators is that setting, but this may require adjustment upward (for patient comfort
the display may show the mode as “CPAP” when it actually is PSV. This or to lengthen expiratory time) or downward. Increased V ˙ may have a
could lead health care providers to draw erroneous conclusions about stimulatory effect on respiratory rate in active patients, paradoxically
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the patient’s ability to sustain spontaneous ventilation. shortening T , although the impact on respiratory rate and work of
■ EFFECT OF INSPIRATORY FLOW PROFILE breathing after the first few seconds has not been studied. Lowering the
E
peak V ˙ will reduce Ppk when there is significant resistance to airflow but
On most ventilators the physician can choose one of several inspira- in this setting will usually worsen autoPEEP, as described above. One
tory waveforms, most commonly square or decelerating. The ratio- often overlooked adverse consequence of setting peak V ˙ at less than the
nale for the use of decelerating flow is to improve the distribution patient wishes is that the patient will actively inspire against the ventilator,
of ventilation and to minimize Ppk. Peak pressures are indeed lower increasing the respiratory work.
because maximal flow (and therefore, flow-related pressure) occurs
early in the breath when lung volume (and elastic recoil pressure) is ■ TRIGGERED SENSITIVITY
minimal, while near the end of the breath lung volume is maximal but In modes that allow the patient to trigger extra breaths, either Pao must
flow is minimal. However, an inescapable consequence of the overall be drawn below a preset threshold (pressure-triggering) or flow must be
lower V ˙ (assuming equal peak inspiratory V ˙ and a passive patient) is a inspired from the circuit (flow-triggering) in order to initiate the breath.
shorter T . Thus, in patients who are obstructed or have high minute Flow triggering has been reported to reduce the work of breathing below
E
ventilations (those in whom a decelerating flow pattern will most that using conventional demand valves, but this is not seen consis-
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reduce Ppk), autoPEEP is likely to be caused or increased (Fig. 49-3). tently. Further, flow-triggering does not solve the problem of triggering
Although the relative contributions of Ppk and PEEP (or autoPEEP) when autoPEEP is present (Fig. 49-4). In several instances we have seen
to barotrauma risk are not clearly defined, it seems likely that, in most physicians suspect machine malfunction and even change the ventilator
patients, barotrauma risk will be worsened with a decelerating profile when they are confronted with an obviously struggling patient seem-
rather than improved. A sine waveform similarly lowers Ppk while ingly unable to get a breath despite a “minimal” trigger threshold. The
shortening expiratory time. solution for this problem is to eliminate the cause for autoPEEP, sedate
When a square waveform is used, the flow-related pressure near end- the patient, or use externally applied PEEP to counterbalance the autoP-
inspiration is nearly the same as at the beginning of the breath and adds EEP (which only occasionally increases autoPEEP, risking barotrauma
to the elastic recoil pressure to give a higher Ppk than during sine-wave and hypotension) 29-31 (see Chap. 54). Alternatively, one can use neurally
or decelerating flow. However, this higher Ppk is largely borne by the adjusted ventilatory assist (NAVA) to better synchronize patient and
robust proximal airways, not the alveoli; by contrast, greater autoPEEP ventilator, both for breath initiation and termination. 32
means greater Palv and risk of alveolar disruption. Since the peak pres-
increased autoPEEP is typically occult, such flow patterns can be insidi- ■ UNCONVENTIONAL VENTILATORY MODES
sure is visibly lowered by decelerating and sine-wave profiles, while
ously threatening. Accordingly, we believe that there is little reason to Airway Pressure-Release Ventilation: Airway pressure-release ventilation
use anything other than the conventional square-wave inspiratory flow (APRV) consists of continuous positive airway pressure (CPAP) that is
profile. When a decelerating flow profile is used, autoPEEP should be intermittently released to allow a brief expiratory interval, producing a
measured diligently and the hidden complexities of this phenomenon form of inverse ratio ventilation. It has been applied to patients with
33
made clear to all caring for the patient. acute lung injury and has proven effective in maintaining oxygenation
34
Pressure alarm
80
Pao

Pao
Palv
Palv
y
0 Pao z
. −2
V a Triggered sensitivity
Pao
Ti Te Ti Te
Palv
Time
0 x
FIGURE 49-3. Effect of flow profile on Pao and I:E ratio in a patient with status asthmati- −2
cus during volume assist-control or mandatory IMV breaths. The left-hand tracings show the Triggered sensitivity
typical high Ppk and slow expiratory flow of a patient with severe obstruction ventilated with FIGURE 49-4. Effect of autoPEEP on triggering. The lower tracings of airway opening (Pao)
a square-wave inspiratory flow profile. Note that expiratory flow just reaches zero before the and alveolar pressure (Palv) represent a patient who is triggering volume-targeted ventilator
next breath is delivered. The Ppk can be dramatically reduced by changing to a decelerating breaths and who does not have autoPEEP. The upper tracing shows a patient similarly ventilated
flow profile (right-hand tracing), since much of the high Pao is flow related. However, in order and with the same triggered sensitivity (−2 cm H O) but who has 4 cm H O of autoPEEP. The
2
2
to deliver the same tidal volume at the now lower mean inspiratory flow, the inspiratory time patient without autoPEEP must lower his or her Palv by x (about 2 cm H O) in order to lower the
2
(T) must increase. At the same respiratory rate, the expiratory time (T ) falls, increasing the Pao by the same amount, triggering a breath. In contrast, the patient with autoPEEP must lower
I
E
I:E ratio. With the now shortened T , there is insufficient time for the respiratory system to his or her Palv by y (about 4 cm H O) before he or she has any impact on Pao and then by a further
2
E
reach FRC, expiratory flow is still detected at the onset of inspiratory flow (a), and autoPEEP is 2 cm H O to trigger the ventilator. In the patient with autoPEEP, the total reduction in Palv (z)
2
present. Therefore, the lower (but not “improved”) Pao comes at the cost of new (or higher) required to trigger the ventilator rises as autoPEEP rises and is occult. In the extreme, autoPEEP
autoPEEP and a higher mean alveolar pressure. Pao is airway opening pressure, V ˙ is flow. may be so elevated that a weak patient is unable to trigger the ventilator despite great effort.

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