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Objectives
Describe the relationship between ventilation and metabolism.
Discuss the effects of malnutrition on respiratory function.
Discuss the effects of excessive caloric intake on respiratory function.
List markers of nutritional status in mechanically ventilated patients.
Compare open- and closed-circuit indirect calorimetry.
Discuss issues related to indirect calorimetry in mechanically ventilated patients.
Compare enteral and parenteral approaches to nutritional support.
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Nutritional assessment and nutritional support are important considerations during mechanical ventilation (Figure 32-1). Assessment of nutritional status and determination of nutritional requirements for mechanically ventilated patients requires the teamwork of physicians, dietitians, respiratory therapists, and nurses. Too few calories cause respiratory muscle catabolism and muscle weakness. Too many calories—particularly carbohydrate calories—increase metabolic rate and can result in respiratory muscle fatigue or hypercapnia due to increased CO2 production (V̇co2).
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Oxygen Consumption, Carbon Dioxide Production, and Energy Expenditure
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The relationship between metabolism, oxygen consumption (V̇o2), and V̇co2 is dependent on the substrate metabolized. V̇co2 divided by V̇o2 is the respiratory quotient (R). R is 1 for carbohydrate metabolism, 0.7 for fat metabolism, 0.8 for protein metabolism, 8.7 for lipogenesis, and 0.25 for ketogenesis. Whole-body R is normally 0.7 to 1. With balanced metabolism, R is 0.8, carbohydrate metabolism raises it toward 1, and fat metabolism lowers it toward 0.7. With lipogenesis, the overall R may be greater than 1, but it seldom exceeds 1.2. With ketogenesis, the overall R may be less than 0.7 but is seldom less than 0.65.
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The principal function of the cardiopulmonary system is to provide the O2 needed for energy production and to clear the CO2 produced. An increase in metabolic rate increases V̇o2 and V̇co2, increases ventilation requirement, and forms the relationship between breathing (V̇o2 and V̇co2) and nutrition expenditure (energy as Kcal). Excessive caloric intake, particularly with carbohydrates, results in increased V̇co2.
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If mechanically ventilated patients receive inadequate nutritional support, they may suffer the effects of starvation. The initial response to starvation is an increase in glycogen and fat metabolism. Glycogenolysis provides glucose, which is necessary for cerebral metabolism. Glycogen stores are depleted after 4 to 5 days of fasting. Lipolysis of adipose tissue triglycerides produces ketones, which can also be metabolized by brain cells. Gluconeogenesis also occurs, primarily due to the breakdown of muscle and visceral proteins. By the third day of fasting, ketogenesis and gluconeogenesis are at maximal rates. There is also a decrease in metabolic rate with starvation that slows the rate at which nutritional stores are ...