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 requiring supplementary oxygen.

In general:

• Patients should be relaxed and fasted
• FiO₂ needs to be calculated (or taken from the ventilator settings), and E_TCO₂ and E_TO₂ must be measured
• Steady-state should be achieved across a five minute period
  • The average MVO₂ and MVCO₂ changes by <10%
  • The respiratory quotient ($RQ = { VCO_2 \over VO_2 }$) 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:

$REE = 3.94 \times (FiO_2 - E_TO_2 ) + (1.1 \times VCO_2)$ 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 $VCO_2 \over VO_2$ 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