Basal Metabolic Rate

Describe basal metabolic rate and its measurement

Outline the factors that influence metabolic rate

Basal Metabolic Rate is the energy output required to sustain life at rest.

  • 'Resting' is defined as an individual who is:
    • Fasted for 12 hours
    • In a comfortable external environment
    • At mental and physical rest
  • Normal values are:
    • 100W.day-1
    • 70kcal.hr-1

Metabolic rate is the actual energy consumption of an individual, and is greater than BMR due to a number of factors.

Factors Affecting Metabolic Rate

Metabolic Rate is affected by:

  • Age
    BMR decreases as age increases.
    • Neonates have a BMR twice that of an adult
    • Children have an increased BMR relative to that of an adult
    • BMR declines by 2% for each decade of life
  • Body Composition
    Lean muscle has a greater energy requirement than fat.
    • Higher body fat percentage results in a lower BMR
      • Females have a lower BMR for this reason - when adjusted for lean mass there is no difference
  • Diet
    • Digestion increases BMR by ~10% due to the energy required to assimilate nutrients
      This is known as the specific dynamic action of food.
      • Protein > carbohydrate > fat
        Note that the Specific Dynamic Action for each macromolecule is not related to the respiratory quotient for that food type.
    • Starvation decreases the BMR
  • Exercise
    • Skeletal muscle is the largest and most variable source of energy consumption
  • Environment
    • Cooler environments increase BMR
    • Temperate environments decrease BMR up to 10%
  • Physiological states
    • Pregnancy increases BMR up to 20% in 2nd and 3rd trimester
    • Lactation increases BMR
    • Catecholamines increase BMR
    • Corticosteroids increase BMR
  • Disease states
    • Malignancy increases BMR
    • Sepsis increases BMR
    • Hyperthyroidism increases BMR

Measurement of BMR using Indirect Calorimetry

BMR is measured using indirect calorimetry, which calculates heat production via measurement of VO2 and VCO2. A number of methods exists depending on whether the patient is intubated or not, or whether they are requring supplementary oxygen.

In general:

  • Patients should be relaxed and fasted
  • FiO2 needs to be calculated (or taken from the ventilator settings), and ETCO2 and ETO2 must be measured
  • Steady-state should be achieved across a five minute period
    • The average MVO2 and MVCO2 changes by <10%
    • The respiratory quotient () change by <5%
      • This ratio will vary depending on the substances metabolised:
        • Carbohydrates = 1
        • Protein ≈ 0.8
        • Fat ≈ 0.7

Resting Energy Expenditure is given by the abbreviated Weir equation:

in Watts per unit time of measurement.

Errors in Indirect Calorimetry

  • Air leaks and measurement errors
  • Measures consumption (rather than requirements)
  • Point estimate of a dynamic process

Footnotes

The respiratory quotient is the value of at steady-state, whilst the respiratory exchange ratio is affected by metabolic rate.


References

  1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
  2. ANZCA Feb/April 2006
  3. LITFL - Indirect Calorimetry
Last updated 2017-09-11

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