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CHAPTER 54: Acute-on-Chronic Respiratory Failure 493

Of particular interest is the development of Neurally Adjusted The purpose of initial ventilatory rest is to facilitate resolution of
Ventilatory Assist technology (NAVA, Maquet, Inc, Wayne, NJ) that uses accessory muscle fatigue and to partially ameliorate hyperinflation-
diaphragm electrical activity (EAdi) from a multiple-array esophageal induced diaphragm shortening. This should reestablish respiratory
electrode to control the timing and level of assist delivered instead of pneu- muscle strength and facilitate timely and expedited liberation. However,
matic signals used in standard modes. EAdi-triggered ventilation has been there is increasing appreciation of the importance of early mobilization
demonstrated to enhance breath-by-breath patient-ventilator synchrony in and reanimation for the critically ill to avoid critical illness–associated
ACRF from COPD without compromising alveolar ventilation. 185 neuromuscular dysfunction, delirium, and prolonged ventilator depen-
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Ensuring Rest and Recovery Following intubation, most patients are exhausted, dence (see Chap. 24). A program of exercise should be initiated after
will sleep for the first day and experience significant diuresis. Little or no 48 to 72 hours in conjunction with daily evaluations of readiness for
sedation is typically necessary although close monitoring for delirium liberation from mechanical ventilation. The goal is to encourage skeletal
and alcohol or substance withdrawal may be required. The respiratory muscle power, tone, and coordination by allowing the patient to assume
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muscles will require 48 to 72 hours for initial recovery, so that resump- nonfatiguing respirations, possibly in combination with inspiratory
tion of breathing efforts before that point is counterproductive and is resistive training. This can be achieved by progressively lowering the
likely to lead to recurrence of respiratory muscle fatigue. However, triggered sensitivity on assist control, lowering the inspiratory pressure
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as discussed below, this does not preclude extubation to NIV if there is on pressure support, or through graded T-piece sprints. After a period
convincing evidence that extrapulmonary organ dysfunction has stabi- of work, the patient is returned to full rest to facilitate sleep at night. As
lized and cognitive function has improved. We continue to encourage strength improves, the amount of exercise can be increased in step, until
rest by maintaining ventilation, adding sedation and antidelirium agents the breathing can be sustained and the patient passes a trial of spontane-
when necessary—particularly if high respiratory drive, PEEPi, and dys- ous breathing.
synchrony cannot be managed by optimizing ventilator settings. Rest During this phase that can be prolonged beyond 10 days, meticulous
can be achieved using any mode of ventilation, including bilevel NIV, as attention should be paid to harm reduction and risk avoidance. Prevention
long as settings are chosen that minimize patient effort. It is important to and early recognition of venous thromboembolism, gastrointestinal stress
emphasize that having the patient connected to a ventilator is no guaran- ulceration, ventilator-associated pneumonia, integument breakdown
tee that the patient is relieved of the work of breathing. (including nasal bridge integrity in NIV patients), corneal desiccation,
Sputum production can be copious and of a tenacious consistency. drug side effects, drug-drug interactions, substance withdrawal, and
Airway humidification is essential, manual or mechanical chest percus- delirium are recommended.
sion may be beneficial particularly if sputum volume is copious or lobar Decreasing Load Efforts to decrease load should continue. Once the patient
collapse develops. Neither N-acetylcysteine nor rhDNAse offer clini- is ventilated, it becomes possible to apportion the load into resistive and
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cally meaningful benefits in improving mucus clearance or resolution of elastic components (see Chap. 48). These determinations may provide
ACRF over inhaled saline in COPD patients but may be of some value insight into the precipitants of respiratory failure and serve to guide
in patients with cystic fibrosis and bronchiectasis. therapy. For example, if the resistive load and PEEPi are minimal, but
Even when the ventilator is set at a very sensitive trigger, the presence the elastic load is excessive, there is little to be gained from more aggres-
of PEEPi causes the patient to have to make a substantial inspiratory sive use of bronchodilators. Rather, the source of the elastic load (lung,
effort to get a breath, even on volume assist-control mode. For example, chest wall, abdomen—see Fig. 54-2) should be determined and corrected.
with a triggered sensitivity of 1 cm H O and PEEPi of 10 cm H O, the It is important to continue treatment with bronchodilators, but
2
2
patient must lower airway pressure by 11 cm H O to trigger a breath. It is whether MDIs and nebulizers are equally effective is controversial. 192,193
2
incumbent on the physician to ensure that the patient is, in fact, rested. On the one hand, in a study of drug deposition in ventilated patients,
When optimal ventilatory rest is achieved, respiratory muscle strength an MDI (plus holding chamber) was more efficient than a nebulizer.
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usually improves demonstrably over the first few days. In another trial of ventilated patients, MDIs were completely ineffective,
Early Mobilization and Improving Neuromuscular Competence Each of the factors despite a cumulative dose in 1 hour of 100 puffs. The magnitude and
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discussed in phase 1 (and in Fig. 54-2) that contribute to depressed duration of MDI and nebulizer effects appear similar. There may be
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neuromuscular competence should be reviewed daily in the ventilated substantial differences related to method of administration or to the
patient. In this phase, the signal importance of nutrition must be rec- specific equipment including humidification used to deliver drug. We
ognized. Malnutrition is a common partner of advanced COPD ; 38% recommend that these drugs be given to clinical effect, whether by MDI
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of 78 patients admitted for AECOPD had a BMI <20 or fat-free mass or nebulizer. If MDIs are used, the usual number of puffs should be
index of ≤16 with a further 40% having features of malnutrition risk. doubled to compensate for the reduced delivery of drug to the patient as
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Malnutrition may contribute to respiratory muscle dysfunction as well as a starting point, and the dose increased, as needed, until bronchodila-
to immune suppression. In a randomized trial of standard feeding versus tion is achieved (assessed by determining respiratory mechanics).
supplementation (1000 kcal above usual), malnourished in-patients with Other contributors to increased load, such as congestive heart failure,
COPD were shown to develop greater respiratory muscle endurance and pulmonary embolization, and respiratory infection, may be easier to
strength in only 16 days when given extra calories. Excessive refeeding discern once the patient is mechanically ventilated, and they should
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should be avoided, however, since unnecessarily high levels of carbon be sought during this phase. Congestive heart failure can usually be
dioxide production (V ˙ CO 2 ) may result. Harris−Benedict predictions excluded by the physical examination and chest radiograph, although
of resting energy expenditure provide a reasonable estimate in stable pulmonary edema may have an atypical appearance in patients with
COPD patients, however, detailed nutritional information, including advanced emphysema. Only occasionally is the additional information
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indirect calorimetry, may be helpful to guide nutritional management from pulmonary artery catheterization useful. Pulmonary embolism
in ACRF (see Chap. 20). Especially with refeeding, hypophosphatemia (PE) is much more difficult to exclude. The incidence of PE as a pre-
commonly develops while the patient is in the ICU, and serum phos- cipitant of ACRF is unknown but may be a concurrent diagnosis in as
phate content should be assessed on a daily basis. Our practice is to many as a quarter of patients admitted with AECOPD. The reported
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encourage enteral intake of protein enriched, moderate carbohydrate frequency of deep venous thrombosis ranges from 9% to 45%. 116,197
and fat diets whenever possible appreciating that severely dyspneic Large pulmonary emboli are much less common although the incidence
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patients infrequently achieve caloric or nitrogen intake goals during of smaller emboli may not be. Nevertheless, PE is commonly found at
the acute phase of illness. Similarly, and in the absence of compelling autopsy. In patients with ACRF, pulmonary hypertension is virtually
evidence that achieving goal caloric nutrition early in medical critical universal and diagnosis of PE is difficult. Perfusion lung scanning nearly
illness improves outcome we provide only 20% to 25% of caloric goals always gives abnormal results, and CT angiography has been incom-
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for the first 5 days for intubated and mechanically ventilated patients. pletely evaluated in patients with underlying structural lung disease

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