Page 75 - Clinical Application of Mechanical Ventilation

Effects of Positive Pressure Ventilation 41

Muscle Fatigue

The work of breathing can be affected by mechanical aberrations such as changes
This caloric cost of in airway resistance and lung or chest wall compliance. In clinical conditions where
breathing for COPD patients is
about 10 times that of normal there is a persistent increase in airway resistance (e.g., COPD) or reduction in com-
individuals (normal 5 38 pliance (e.g., atelectasis), the respiratory muscles must work strenuously to over-
to 72 kcal/day) Over time,
these abnormalities may come the abnormal resistance and compliance. For instance, COPD patients use
cause fatigue of the respira-
tory muscles, and ventilatory 430 to 720 kcal/day to carry out the work of breathing. This caloric cost of breath-
failure with concurrent CO 2 ing for COPD patients is about 10 times that of normal individuals (normal 5 38
retention and hypoxemia
(Brown, 1994). to 72 kcal/day) (Brown, 1983) because of the increased work of breathing necessary
to overcome the high airway resistance and V/Q abnormalities.
Other than the mechanical aberrations that can lead to increased work of breath-
ing and eventual muscle fatigue, there are nonmechanical factors as well. Malnu-
trition is an example of a nonmechanical cause of muscle fatigue that may lead to
ventilatory failure (Fiaccadori et al., 1991).
Table 2-9 shows the major mechanical and nonmechanical factors that may lead
to reduced respiratory muscle efficiency and eventual muscle fatigue.

Diaphragmatic Dysfunction

Prolonged positive pressure ventilation can induce diaphragmatic dysfunction. For
Prolonged positive pres-
sure ventilation can induce patients undergoing prolonged mechanical ventilation, atrophy of the diaphragm
diaphragmatic dysfunction. muscles can occur as a result of muscle proteolysis and a decrease in myofiber content.
Furthermore, the loss of diaphragm force is time-dependent (Haitsma, 2011). For this
reason, weaning from mechanical ventilation should be initiated as soon as feasible.

Nutritional Support

Adequate nutrition is a therapeutic necessity in order to provide and preserve inspiratory
muscle strength and prevent ventilatory failure. Patients who have respiratory disorders
are likely to lose weight due to increased work of breathing, decreased nutritional intake,

TABLE 2-9 Factors Leading to Respiratory Muscle Fatigue

Mechanical Factors Nonmechanical Factors

High airway resistance Malnutrition

Low lung compliance Endocrine diseases (high metabolic rate)
Low chest wall compliance Electrolyte disorders
Drugs
Persistent hypoxemia

(Data from Fiaccadori et al., 1991; Grassino et al., 1984; Rochester, 1986.)
© Cengage Learning 2014

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.