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. - Body Composition
Lean muscle has a greater energy requirement than fat. - 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.
- Protein > carbohydrate > fat
- Starvation decreases the BMR
- Digestion increases BMR by ~10% due to the energy required to assimilate nutrients
- Exercise
- Skeletal muscle is the largest and most variable source of energy consumption
- Environment
- Physiological states
- Disease states
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 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
- This ratio will vary depending on the substances metabolised:
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
- Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
- ANZCA Feb/April 2006
- LITFL - Indirect Calorimetry