Indirect Calorimetry (171)
Fink Chapter 225
Haugen et al 2007 Nutrition in clinical practice vol22 N4 p377-88
o We are becoming increasingly aware of the importance of nutrition in the ICU
setting
o Definitions
o TEE – total energy expenditure (TEE=BEE+DIT+AEE)
o BEE – basal energy expenditure – energy required to maintain body’s
basic cellular metabolic activity and organ function
 Ideally measured after fasting 5hours, no physical activity, no
stimulants (nicotine, caffeine etc)
o REE – Resting energy expenditure – used in place of BEE in hospitalized
patients because BEE is not usually feasible
o DIT – diet induced thermogenesis – energy used during substrate
metabolism
o AEE – Activity energy expenditure – energy used in physical activity
o Adequate nutritional support is rarely achieved
o Patients are either under- or over- fed
o Parenteral nutrition often results in overfeeding leading to complications
of hyperglycemia, increased inflammation and infection, liver function
abnormalities and CO2 retention
o Enteral nutrition is often associated with feeding intolerance – high gastric
residuals lead to reflux, emesis, aspiration and diarrhea
 Leads to underfeeding which leads to an increase in complications
and increased mortality
o Multiple formulae to predict energy requirements
o Harris-Benedict (1919)
 Men = 66.5 + (13.75 x kg) + (5.003 x cm) - (6.775 x age)
 Women = 65.1 + (9.563 x kg) + (1.850 x cm) - (4.676 x age)
If you get little to no exercise
Daily calories needed= BMR x 1.2
oIf you
O exercise lightly(1-3 days per week)
Daily calories needed= BMR x 1.375
w
If you exercise moderately (3-5 days per week)
Daily calories needed= BMR x 1.55
e
If you exercise heavily (6-7 days per week)
Daily calories needed= BMR x 1.725
n
oIf you
R exercise very heavily (i.e. 2x per day, extra heavy Daily calories needed= BMR x 1.9
workouts)
o
o Robertson
o Reid
o Frankenfield
o Penn State
o Ireton-Jones
o Many studies have been done comparing formulae to calorimetry
 MacDonald - % formulae vs indirect calorimetry

No formula was good for all BMIs in all conditions
o Calorimetry
o Measurement of heat produced to quantify TEE
o Total body heat is measured in a thermally sealed chamber
o Very accurate but cumbersome, expensive and requires technical expertise
o Indirect Calorimetry
o Used in place of direct calorimetry
o Energy expenditure is quantified under specific conditions by measuring
respiratory gases (O2 consumed and CO2 produced)
 These numbers are then inserted into the Weir equation

o
o
o
Energy Expenditure (kcal/day) = [(VO2 X 3.941) + (VCO2 X 1.11) +
(uN2 X 2.17)] X 1440
 The urinary Nitrogen (uN2) factor is usually excluded because it
accounts for <4% of the true energy expenditure
 Can also be done using the Fick equation by comparing the A-V O2
contents and using cardiac output
Period of measurement
 24 hours is the old standard
 Recent evidence suggests that 30 minutes is almost as accurate
 One trial demonstrated that 5 minute measurements were as good as 30
minutes – but the patients had to be stable
One measurement or many
 There is a great deal of day to day variation in energy expenditure (may
be as great as 35%)
 Day to day measurements affected by temperature but not by illness
severity score
 Daily measurements may be necessary
Value of energy balance
o Negative energy balance is associated with
 Longer duration of mechanical ventilation
 Longer ICU stay
 Development of pressure sores
 Renal failure
 Sepsis
 Hemodynamic instability
 *it may be that the sicker patients don’t get fed rather than not being fed
makes them sicker
 Energy balance of -10, 000kcal associated with increased mortality
o
Respiratory Quotient (RQ)
o Ratio between VCO2 and VO2 (VCO2/VO2)
o If glucose is completely oxidized in a system the quotient is 1
o Lipid = 0.69, Protein = 0.81
o If a value for RQ is found to be outside the “normal” range (0.67-1.2) then the
measurement is likely inaccurate
o Poor measurement occurs when
 Poor seal with air leak
 Hemodialysis
 Patient agitation or pain
 Residual anesthetic gasses
 FiO2 >60%
o
Limitations of IC
o VCO2 and VO2 are calculated using volumes of inspired and expired volumes
(Ve) but
 Inspired volumes (Vi) are difficult to accurately measure so they are
estimated using the Haldane transformation which assumes static
volumes of N2 in inspired and exhaled gasses
 Vi = [(FeN2/FiN2) Ve]
o
Types of machines
o Douglas Bag
 GOLD STANDARD
 Person breathes ambient air through a 1-way valve connected to an
airtight container (Douglas bag)
 Expired gas is sampled; relative concentrations of O2 and CO2 are
measured by electronic gas analyzers
 Comparison is made between the composition of inspired and expired
air: VO2 and VCO2 are determined
 Challenges: expensive analyzer equipment, requires technical expertise
o Metabolic Cart (this is what we use)
 Person breathes into mask or through mouthpiece connected to metabolic
cart
 Gas analyzers and airflow measurement
 Mixing chamber: gas samples are continuously obtained and analyzed
from a mixing chamber within the metabolic cart
 Breath-by-breath: analysis from an external sample port on a breathing
valve
 In some instances, the metabolic cart has been designed to collect a gas
sample using either mixing chamber or breath-by-breath procedure
 Accurate and well-accepted in clinical settings
 Challenges: costly and requires technical expertise to maintain
o
Special considerations
o Extreme obesity – (BMI>40)
 It is very difficult to estimate energy expenditure in these patients and IC
may be the onlt useful method
o Post op bariatric surgery
 Energy expenditure may change very rapidly with changing gbody
composistion
o HIV infected patients
 Hypermetabolism may play a role in wasting associated with the disease
 Difficult to estimate using formulae
o IHD
 Hemodialysis removes CO2 from the venous bed and therefore changes
the FeCO2 and it may take 24 hours to reequilibrate - as such wait 24
hours after hemo run
o CRRT
 Similar problem removes CO2 but not O2 can lose up to 35% of CO2
 In addition the temperature of the patient may decrease on CRRT
 No good guidelines for the interpretation of IC in these situations
o Deep sedation/paralysis
 Significantly reduce energy expenditure
 IC should be performed during and post deep sedation