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376 PART 4: Pulmonary Disorders

the physician managing RF frequently employs the principles of muscle rest before resuming an exercise program free from fatigue.
diagnosis and management of hypoperfusion states, as discussed in the Note that many patients being managed for respiratory muscle rest by
next section. mechanical ventilation actually may be working as hard as or harder
than during spontaneous ventilation by breathing actively against the
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RESPIRATORY MUSCLE EXERCISE ventilator. This is easily detected by clinical examination coupled
with observation of the airway pressure, which should rise at the start
AND FATIGUE IN RESPIRATORY FAILURE of each ventilated breath. In many patients, the airway pressure stays
The respiratory muscles share with other major muscle groups of the at or below zero during inspiration, indicating active inspiration by
body the characteristic that excessive work leads to fatigue. 37,56,57 This the patient; the amount and duration of inspiratory effort often can
concept seems to explain why patients with severe airflow obstruc- be assessed by the fall in central venous or pulmonary artery pressure
tion or airspace flooding ultimately stop breathing, and why patients with each inspiration. Alternatively esophageal manometry can be used
requiring mechanical ventilation for these and other causes of RF are to estimate pleural pressure changes leading to unrewarded respiratory
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unable to breathe independent from the ventilator until the load on efforts. When respiratory rest is indicated, it can be achieved in these
their respiratory muscles is reduced, the respiratory muscles become circumstances by increasing the inspiratory flow rate, by increasing the
stronger, or both. Note, however, that while this is a useful paradigm minute ventilation, or, occasionally if these measures of eliminating the
with which to manage patients with RF, it is exceedingly difficult to patient’s respiratory effort are inadequate, by sedating and paralyzing
identify fatigue under clinical conditions. Nonetheless, as a rough guide, the patient to ensure respiratory rest. 56
each spontaneous breath is less than one-third the maximal respiratory ■ EXERCISING RESTED RESPIRATORY MUSCLES
spontaneous ventilation can be sustained indefinitely when the effort of
effort achievable. 56,57 In normal patients, the maximum negative inspi- As soon as the patient with RF is stabilized on the ventilator, the physi-
ratory pressure (MIP) measured at FRC exceeds 100 cm H 2O, whereas cian should make a decision whether to rest the fatigued respiratory
the work of spontaneous breathing is less than 10 cm H 2O, providing muscles or to institute a program of respiratory muscle exercise in those
considerable respiratory muscle reserve before the conditions of fatigue patients in whom fatigue is neither evident nor expected. The objective
are approached. In contrast, patients with acute RF frequently have val- of the respiratory exercise program is to increase the tone, power, and
ues of MIP <30 cm H 2O, while the load on the respiratory muscles, as coordination of the respiratory muscles. 37,64 The efficacy of each pro-
measured by the pressure generated by the ventilator during each breath, gram in increasing tone and power is evaluated by the daily MIP and VC
exceeds 30 cm H 2O. 34-36 Such values predict that the patient’s respiratory measurements and daily spontaneous breathing trials. Coordination is
muscles will fatigue quickly if spontaneous ventilation were required, evaluated by bedside observation confirming that the patient’s respira-
a hypothesis easily confirmed in such patients who breathe rapidly tory efforts interact with the ventilator in a manner that is comfortable
and insufficiently when taken off the ventilator. Another measure of for the patient. Here the goal is to adjust the ventilator such that the
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maximum respiratory effort in the conscious patient is vital capacity patient receives a breath on demand at a volume and frequency within
(VC). As a rough guideline, when VC is three times the tidal volume the ranges expected for that patient off the ventilator. Several differ-
(Vt) required to maintain eucapnia and normal pH, respiratory muscle ent modes of ventilation seek to achieve these goals of increased tone,
fatigue is unlikely. A corresponding alternate measure of respiratory power, and coordination during the liberation process, and each is
and described in detail in Chaps. 49 and 50.
acidosis necessarily increase this ventilation and so promote respiratory ■ LIBERATING THE PATIENT FROM MECHANICAL VENTILATION
load is the minute ventilation required to maintain normal Pa CO 2
pH. Factors that increase CO 2 production, dead space, or metabolic
muscle fatigue. Such fatigue is often signaled by increased respiratory This aggressive approach attempts to liberate the patients as soon as
rate (RR >35 breaths per minute), by paradoxical respiratory motion possible from mechanical ventilation by using the mode as an exer-
(the abdomen moves in with inspiration as the fatigued diaphragm is cise program. Then ventilator mode becomes a thoughtful part of a
pulled craniad by the negative pleural pressure), and by the patient’s larger program addressing about 50 correctable factors constraining the
unexplained somnolence or decreased responsiveness. 37,57 Accordingly, patient’s freedom to breathe (Table 43-4). This effective approach to lib-
evaluation of the patient’s ability to resume spontaneous ventilation erating patients from the ventilator measures and attempts to increase the
includes measurements of MIP, VC, Vt, RR, and V ˙ e, as well as direct values of MIP and VC while simultaneously measuring and reducing
observation of the respiratory motions during a period of spontaneous the respiratory load. 64,65 Table 43-4 lists correctable factors to reduce
breathing 59,60 (see Chap. 60). the respiratory muscle load. As a general rule, these are the abnormal
■ RESTING FATIGUED RESPIRATORY MUSCLES respiratory mechanics associated with the several types of acute RF.
Table 43-4 lists correctable factors increasing respiratory muscle
Current evidence and common sense suggest that the treatment for strength, first in the context of those many disturbances of the circu-
respiratory muscle fatigue is respiratory muscle rest, a strategy that lation or internal environment most common to patients with type
must be balanced in nearly all patients against a thoughtful respiratory IV RF. Attempting to liberate patients with hypoperfusion states or
exercise program. The timing of the move from respiratory muscle rest hypotension is almost never successful. Correction of these hemody-
to an exercise program is not currently guided by objective criteria namic variables complements the correction of anemia, hypoxemia,
identifying fatigue. Accordingly, many physicians confronted with this and acidosis to provide dramatic increases in the objectively measured
problem develop empirical guidelines as to the likely presence of respi- respiratory muscle strength by MIP and VC. Similarly, attempting to
ratory muscles fatigue integrated with the type of early ventilator man- liberate patients who are septic or have body temperatures >38.5°C is
agement necessary for the patient’s overall condition. For example, the often unsuccessful. While these systemic abnormalities are being cor-
cardiovascular stability and optimal ventilator management of patients rected, it is also helpful to initiate adequate daily protein-calorie nutri-
with type I AHRF are frequently enhanced by respiratory muscle rest tion utilizing protein (0.8 g/kg) and nonprotein calories (about 30 kcal/g
during the first 6 hours after elective intubation for severe hypoxemia. of protein), of which calories 20% to 50% should be supplied as lipid.
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During this time, the acutely depleted glycogen stores of the resting Elemental malnutrition is corrected by adjustments of serum potassium,
respiratory muscle are repleted, and the accumulated lactic acid or calcium, magnesium, and phosphate levels; severe abnormalities of each
other metabolites associated with fatigue are washed out 54,55,61 ; then the of these electrolytes is sufficient to cause respiratory muscle fatigue, 67,68
patient is ready to move to a respiratory exercise program. By contrast, so modest abnormalities in the patient already weakened by critical ill-
the patient with ACRF who has developed respiratory muscle fatigue ness may converge to make the patient weaker than necessary. When
over a longer period of time may require up to 72 hours of respiratory all other factors are corrected but the patient remains weak, it is helpful

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