Page 606 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

426 PART 4: Pulmonary Disorders

■ PRESSURE-TARGETED VERSUS VOLUME-TARGETED emphasized that a “safe” maximal alveolar pressure is not known. Further,

MODES OF VENTILATION when patients are active, P does not represent the transpulmonary pres-
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The terminology describing modes of ventilation can be very confusing sure, meaning that gross overdistention of lung is possible on pressure-
targeted modes despite modest ventilator pressures. In addition, the same
and may vary from one company’s ventilator to another. In this chapter, reduction in maximal alveolar pressure can be achieved using volume-
we refer to volume-targeted modes as those in which the physician sets targeted modes, simply by limiting tidal volume, as has been shown in
a desired tidal volume that the ventilator delivers, using whatever pres- ALI/ARDS patients. Nevertheless, pressure-targeted modes make such
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sure is required, and pressure-targeted modes, in which the physician a lung protection strategy easier to carry out by dispensing with the need
sets a desired pressure that the ventilator maintains, delivering a volume to repeatedly determine Pplat and periodically adjust the V .
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that depends on the settings, respiratory mechanics, and patient effort. Pressure-targeted modes also allow the patient greater control over
Some modern modes (dual control modes, see below) attempt to blend inspiratory flow rate and therefore potentially increased comfort. On
pressure and volume targets. Few studies have compared modes directly the other hand, during lung protective ventilation, pressure modes
except with respect to comfort, a measure generally favoring pressure- (including pressure-regulated volume control, see below) did not reduce
targeted modes. At the same time, comparative trials are plagued by work of breathing compared to volume assist-control and did not allow
the details of settings and these are often dissimilar between modes precise control of tidal volume. A disadvantage of pressure-targeted
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(biasing the study) or are not sufficiently specified in the methods. modes is that changes in respiratory system mechanics (eg, increased
For this reason, modes are often chosen based on preference, personal airflow resistance or lung stiffness) or patient effort may decrease
experience, or institutional practice, rather than on evidence relating to the minute ventilation, necessitating alarms for adequate ventilation.
meaningful outcomes. Also, the mechanics cannot be determined readily and partitioned as
The differences between volume-targeted and pressure-targeted modes described in Chap. 48 without switching modes, inserting an esophageal
are fewer than often appreciated, since volume and pressure are related balloon, or using more complex algorithms. 20
through the mechanical properties of the respiratory system, most
notably the respiratory system compliance (Crs). For example, a passive Pressure Assist-Control Ventilation (PACV) In the passive patient, ventilation
patient with a static Crs of 50 mL/cm H O ventilated on VACV at a tidal is determined by f, the inspiratory pressure increment (P − PEEP),
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volume of 500 cc with no PEEP (or autoPEEP) will have a plateau airway inspiratory to expiratory (I:E) ratio, and Crs. In patients without severe
pressure (Pplat; see Chap. 48) of about 10 cm H O, whereas the same obstruction given a sufficiently long T , there is equilibration between
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patient ventilated on PACV at 10 cm H O can be expected to have a tidal the ventilator-determined P and Palv so that inspiratory flow ceases
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volume (V ) close to 500 cc. The difference to the patient between these (Fig. 49-1A). In this situation, tidal volume is highly predictable, based
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modes may be quite trivial, often amounting to small differences in inspi- on P (= Palv) and the mechanical properties of the respiratory system
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ratory flow profile. Thus, while physicians’ comfort level with volume- (Crs). In the presence of severe obstruction or if T is too short to allow
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targeted and pressure-targeted modes may be very different, the modes equilibration between ventilator and alveoli, V will fall below that pre-
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can be similar because they are tied to each other through the patient’s Crs. dicted based on P and Crs (see Fig. 49-1A). One of the advantages of
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■ CONVENTIONAL MODES OF VENTILATION For example, alveolar overdistention can be prevented by ensuring that
PACV is that it may facilitate ventilation with a lung protective strategy.
In the following descriptions, each mode is first illustrated for a passive Palv never exceeds some threshold value (this is often taken to be 30 cm
H O, but a truly safe level is unknown) by simply setting P (alternatively,
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patient, such as following muscle paralysis, and then for the more com- PEEP + PSV) to the desired upper limit. Inspiratory activity can raise
mon situation in which the patient plays an active role in ventilation. On the transpulmonary pressure well above a safe level, despite a modest P ,
some ventilators tidal volume (V ) can be selected by the physician or threatening lung protection. During PACV, T and f are set by the physi- I
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respiratory therapist, whereas on others a minute ventilation and respi- cian and may not approximate the patient’s desired T and f.
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ratory rate (f) are chosen, secondarily determining the V . Similarly, When the patient is active, the tidal volume reflects patient effort
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on some machines an inspiratory flow rate (V ˙ ) is selected, whereas on and the patient may trigger additional breaths. When the patient makes
others V ˙ depends on the ratio of inspiratory time to total respiratory inspiratory efforts synchronized with machine inspiration, the tidal
cycle time (T /T ) and f; on inspiratory-expiratory (I:E) ratio and f; or volume is generally greater than that predicted from the Crs and P
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on rise-time and other parameters. and may exceed targets for lung-protection. However, dyssynchrony or I
Pressure-Targeted Modes: In pressure-targeted modes, a fixed inspira- expiratory effort during machine inspiration may reduce V below that
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tory pressure (P ) is applied to the patient, whatever the resulting V . otherwise expected. Special care must be taken to adjust T to the indi-
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Depending on the particular ventilator, the physician may have to spec- vidual patient (Fig. 49-2); otherwise, heavy sedation is typically needed.
ify the actual level of P or, alternatively, the increment in pressure over When unphysiologic settings are intentionally chosen, as when the physi-
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the expiratory pressure (PEEP). Ventilators designed primarily for non- cian desires an unusually long T (T longer than T results in inverse ratio
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invasive ventilation often require setting P and PEEP independently, ventilation, IRV), deep sedation or therapeutic paralysis is often given.
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while most ICU ventilators require setting the PEEP and an inspiratory Pressure-Support Ventilation (PSV) The patient must trigger the ventilator in
pressure increment. For example, the following settings are identi- order to activate this mode, so PSV is not applied to passive patients.
cal: 1. P = 20 cm H O; PEEP = 5 cm H O (noninvasive ventilator); or Ventilation is determined by P − PEEP, patient-determined f, patient
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2. Pressure increment (eg, “pressure-support” or “pressure-control”) = effort, and the patient’s mechanics. Once a breath is triggered, the venti-
15 cm H O; PEEP = 5 cm H O. In this chapter, we will specify inspiratory lator attempts to maintain P using whatever flow is necessary to achieve
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(P ) and expiratory (PEEP) pressures to avoid confusion. this. Eventually, flow begins to fall due to cessation of the patient’s inspi-
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In pressure-targeted modes, the V is predictable (again, passive ratory effort combined with increasing elastic recoil of the respiratory
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patient) when the Crs is known: system as lung volume rises. The ventilator maintains P until inspiratory
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flow falls an arbitrary amount (eg, to 20% of initial flow) or below an
V = (P − PEEP) × Crs
T I absolute flow rate (set by default or user-configured).
assuming time for equilibration between P and alveolar pressure (Palv) It is useful to first consider what happens if the patient were to trig-
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and the absence of autoPEEP (both of these assumptions are often not ger the ventilator and then remain passive (an artificial situation). Tidal
true in patients in the ICU; see below). volume would be determined by P and the (largely static) mechanical
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Compared with volume-targeted modes, a potential advantage of properties of the respiratory system, as during PACV (see Fig. 49-1B).
pressure-targeted ventilation is greater physician control over the maxi- More typically, the patient makes an effort throughout inspiration,
mal alveolar pressure (P ) in passive patients, although it should be in which case V is determined, in part, by the degree of effort (see
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