The importance of nutrition for good respiratory function has been recognized since the early 1900s. Protein energy malnutrition leads to abnormalities in the control of ventilation and respiratory muscle function in addition to pulmonary structural changes and deterioration of pulmonary defense mechanisms. Respiratory muscles, like other skeletal muscles, are subject to catabolic changes during critical illness, causing a significant loss in muscle mass. Furthermore, the impairment in respiratory muscle function exceeds the loss of muscle mass.
In addition to impairment of respiratory muscle function protein energy malnutrition can affect the pulmonary structure. Malnutrition may also predispose to pulmonary infection as alveolar defence mechanisms are impaired due to reduced numbers of alveolar macrophages, with inhibited function and decreased IgA secretion.
Thus, in critically ill patients with lung disease, underfeeding may perpetuate respiratory muscle weakness and predispose to pulmonary infection. Conversely, overfeeding results in excessive CO2 production, leading to an increase in the workload required to achieve steady state ventilation.
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