Skip to Main Content





  1. Describe the relationship between ventilation and metabolism.

  2. Discuss the effects of malnutrition on respiratory function.

  3. Discuss the effects of excessive caloric intake on respiratory function.

  4. List markers of nutritional status in mechanically ventilated patients.

  5. Compare open- and closed-circuit indirect calorimetry.

  6. Discuss issues related to indirect calorimetry in mechanically ventilated patients.

  7. Compare enteral and parenteral approaches to nutritional support.


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).

Figure 32-1

The relationship between nutrition and respiration. Either too few or too many calories can result in respiratory failure.

Graphic Jump Location

Oxygen Consumption, Carbon Dioxide Production, and Energy Expenditure


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.


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.


Effects of Starvation


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 ...

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


Create a Free MyAccess Profile

* Required Fields

Note: If you have registered for a MyAccess profile on any of the Access sites, you can use the same MyAccess login credentials across all sites.

Passwords must be between 6 and 40 characters long (no whitespace), cannot contain characters #, &, and must contain:
  • at least one lowercase letter
  • at least one uppercase letter
  • at least one digit

Benefits of a MyAccess Profile:

  • Remote access to the site off-campus on any device
  • Notification of new content via custom alerts
  • Bookmark your favorite content such as chapters, figures, tables, videos, cases and more
  • Save and download images to PowerPoint
  • Self-Assessment quizzes saved for quick review
  • Custom Curriculum access for both instructors and learners

Subscription Options

AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more

$995 USD
Buy Now

Pay Per View: Timed Access to all of AccessAnesthesiology

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.