Page 647 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

466 PART 4: Pulmonary Disorders

Respiratory acidosis has many physiologic effects, including cellular and 52-7). In the Amato trial, recruitment maneuvers consisted of applica-
metabolic dysfunction, depression of myocardial contractility, coronary tion of continuous positive airway pressure (CPAP) of 35 to 40 cm H O for
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vasodilation, systemic vasodilation, pulmonary vasoconstriction, 30-second periods. Others have advocated longer periods at the same or
enhanced hypoxic pulmonary vasoconstriction, cerebral vasodilation, higher airway pressures. The justification for recruitment maneuvers is
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increased intracranial pressure, and renal vasoconstriction, among to “recruit” or open totally or partially collapsed alveoli, which then would
others 302,303 (see Table 52-10). Yet even very high levels of P CO 2 seem be kept inflated during expiration by a higher level of PEEP. 314
remarkably well tolerated by adequately sedated patients. Perhaps this Evidence is lacking that recruitment maneuvers alone improve clini-
is related to highly efficient and rapidly acting cellular compensatory cally significant outcomes such as mortality or ventilator-free days. Most
mechanisms that tend to defend intracellular pH. Because respiratory studies of recruitment maneuvers have used physiologic end points,
acidosis raises intracranial pressure, permissive hypercapnia should not such as improvement in oxygenation. The ARDSNet studied recruit-
be used in patients with cerebral edema, trauma, or space-occupying ment maneuvers as a substudy of 96 subjects in the higher-PEEP group
lesions. This and other contraindications are listed in Table 52-11. in the ALVEOLI study (see Table 52-8). There were no clinically rel-
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Prone Positioning Multiple studies have shown that about two-thirds of evant improvements in arterial saturation, but complications occurred,
patients with ARDS exhibit improved oxygenation with prone position- such as transient hypotension and slight drops in arterial saturation
ing (“proned”). 304-307 Hypotheses offered to explain the improvement during the recruitment maneuver. Other studies have shown more
in oxygenation include (1) increased FRC, (2) change in regional dia- consistent improvement in oxygenation after recruitment maneuvers if
phragm motion, (3) redistribution of perfusion, and (4) better clearance relatively low levels of PEEP were being used, 316-318 if larger tidal volumes
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of secretions. FRC has been shown to be increased in the prone posi- were used, or if the patients are paralyzed. 320
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tion in intubated, mechanically ventilated patients without lung injury Given the lack of controlled clinical trials that demonstrate efficacy
who are undergoing general anesthesia for surgery. Animal models in clinically relevant end points and the potential adverse effects,
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of ventilation-perfusion distribution have suggested that gravity has far we reserve the use of recruitment maneuvers for cases of refractory
less influence on the distribution of perfusion in the prone position, and hypoxemia or cases of desaturation due to acute derecruitment that
that the distribution of blood flow to regions of the lung is relatively responds well to re-expansion. Furthermore, because they exceed the
unaffected by the change from the supine to the prone position. This, threshold of 30 cm H O used in the ARDSNet clinical trial that showed
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coupled with the observation that turning to the prone position is associ- improved survival, and the lack of studies that demonstrate improved
ated with a migration of the edema fluid to the dependent portions of the outcomes, 321-323 routine use of “sighs” are not recommended.
lung (as demonstrated by CT scan), has suggested to some investigators Salvage Interventions When treating subjects with severe ARDS, some
that ventilation-perfusion relationships might be favorably altered by clini cians may try unproven interventions if the patient is deteriorating
the prone position. In patients managed in the prone position, special with severe hypoxemia (eg, Pa O 2 <45-50 mm Hg) or needing an Fi O 2 of
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attention is necessary to prevent pressure injury to the nose, face, eyes, 0.9 or more to maintain Pa O 2 above 55 mm Hg. These may be referred
and ears, and to ensure maintenance and patency of the endotracheal to as “salvage” interventions. These clinicians justify their use of these
tube and central venous catheters. Pressure on the eye could lead to interventions on two grounds: (1) the dire condition of the patient
retinal ischemia, especially in hypotensive patients. Some patients expe- and (2) a hope of clinical efficacy. The latter is based on results from
rience cardiac arrhythmias or hemodynamic instability on being turned. basic science studies suggesting a reasonable rationale, from their use
These considerations led to a large clinical trial performed by in animal models, and from clinical usage that showed improvements in
Gattinoni and colleagues. In this study, subjects were placed in the certain physiologic parameters (eg, Pa O 2 : Fi O 2 ).
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prone position for 6 or more hours daily for 10 days. The results, Despite the failure to improve survival in phase III clinical trials
published in 2001, revealed that although oxygenation was transiently of patients with ALI/ARDS who were not necessarily in such dire
improved, prone positioning offered no survival advantage over routine straits, these clinicians may feel ethically obligated through the “Rule of
supine positioning. Further post hoc analyses indicated that a patient’s Rescue” to provide an intervention that may help as long as the risk is
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response to prone positioning may have prognostic value. Patients acceptable. Because of expense, lack of proven efficacy, and potential for
fell by 1 mm Hg or more when placed in the prone position harm, we do not advocate routine use of any of these “salvage interven-
whose Pa CO 2
didn’t fall or rose tions.” Rather, we support a management strategy guided by evidence and
had a lower mortality rate than those whose Pa CO 2
(mortality of 35.1% vs mortality of 52.2%). 312 including active observation of critically ill patients with ARDS. We urge
Based on the rationale that prone positioning may have a role in those who advocate for their use to conduct clinical trials in the targeted
severe ARDS, when combined with lower-tidal-volume ventilation (see population of patients with severe ARDS to assess their safety and efficacy.
Table 52-9), and using prolonged proning sessions early in the course Nonetheless, when there are severe problems with oxygenation in an
of ARDS, a recent trial was conducted which demonstrated a significant otherwise salvageable patient, some clinicians will want to utilize certain
survival benefit. In this multicenter trial, 466 patients with severe adjunctive therapies. It is important to realize that these therapies should
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ARDS (P/F <150 mm Hg) were randomized to traditional, supine ven- not distract caregivers from the fundamentals of good critical care, includ-
tilation or prone-positioning sessions for a minimum of 16 hours on a ing nutrition, aspiration precautions, hygiene and prevention of nosoco-
daily basis until oxygenation improved (P/F ≥150 mm Hg). The use of mial infections, appropriate sedation practices, and careful vigilance for
early prone positioning resulted in significantly reduced 28-day (16.0% complications of critical care.
vs 32.8% in the supine arm of the study, p <0.001) and 90-day mortality Tracheal Gas Insufflation Tracheal gas insufflation (TGI) involves introducing
(23.6% vs 41.0%, p <0.001), without an increased risk of complications. fresh gas near the carina through a modified endotracheal tube. This
Based on these recent findings, we support the recommendation that added flow washes CO -rich gas out of the trachea (and, through turbu-
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prone positioning be prioritized as a salvage therapy for severe ARDS lence, out of smaller airways as well), reducing anatomic dead space.
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325
and recommend the consideration of its use early in severe ARDS in -reducing effect of TGI is lessened by ALI/ARDS, but this is
experienced centers. The Pa CO 2 values used during permis-
partially counterbalanced by the higher Pa CO 2
Recruitment Maneuvers Recruitment maneuvers evolved from traditional sive hypercapnia. In patients with ARDS, TGI with 100% humidified
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“sighs,” which are extra-large breaths of the order of two or three oxygen, delivered throughout the respiratory cycle at a flow of 4 L/min,
normal-sized tidal volume breaths. Sighs normally occur 4 to 10 times successfully lowered P CO 2 from 108 to 84 mm Hg. Potential risks of
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per hour and increase the surfactant’s surface-tension-lowering proper- TGI include tracheal erosion, oxygen toxicity related to the unknown
ties, thus stabilizing small alveoli and resisting atelectasis. Fi O 2 , hemodynamic compromise or barotrauma due to the occult pres-
Recruitment maneuvers were part of the “open-lung” strategy in the clin- ence of auto-PEEP, and a larger tidal volume than the ventilator is set to
ical trial of low-tidal-volume ventilation by Amato et al (see Tables 52-6 deliver (ie, potentially increasing the risk of VILI).
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