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Interpretation of Indirect Calorimetry
Charles McArthur BA RRT RPFT
Mankato, MN
Objectives
• Describe the theory of indirect calorimetry
• Describe the assumptions and pitfalls of indirect calorimetry measurements
• Discuss current guidelines for the interpretation of indirect calorimetry data
Antoine Lavoisier 1743-1791
• Father of Modern Chemistry
• First to define combustion with modern terminology
• First to measure human energy expenditure by analysis of respiratory gases
Antoine Lavoisier1775
Combustion
1: The process of burning2: a chemical change, especially oxidation, that produces heat ;
also : a slower oxidation (as in the body)
O2 + C6H12O6
CELL
HEAT
CO2 +H2O
Human Internal Combustion
C6H1206 + 6O2 6CO2 + 6H20 + Energy
Each Substrate has Unique Stoichiometry
RQ = VCO2/VO2 = 1.0
Heat + Work
Direct Calorimeter
Heat = Energy Expenditure( kcal)
REE
Resting Energy Expenditure =
Kcal/day
At Rest
Indirect Calorimetry
O2 CO2
Respiratory Exchange Ratio RER = CO2/O2
O2 & CO2 Measured at the Airway
Measurement of VO2 & VCO2
VO2 = VE x ( FIO2 – FEO2)
VCO2 = VE x (FECO2)
Energy Equivalents and RQ’s
SUBSTRATE Kcal/LO2 RQ
CHO 5.05 1.0
Protein 4.46 0.8
Fat 4.74 0.7
De Weir Equation
REE = Resting Energy Expenditure = KCAL/day
[( 3.94 x VO2 + 1.11 x VCO2 ) x 1.44] - 2.17 UUN
= Kcal/day
ml/min ml/min
Energy Equivalents and RQ’s
SUBSTRATE Kcal/LO2 RQ
CHO 5.05 1.0
Protein 4.46 0.8
Fat 4.74 0.7
Error Caused by Lack of UUN Measurement
Reappraisal of the Weir equation for calculation of metabolic rateP. I. Mansell and I. A. MacdonaldAmerican Journal of Physiology1990:R1347-R1354
IC Assumptions
• Subject is in resting state
• RER = RQ
• Disappearance of substrates = oxidation of substrates
• CHO, Fat, and Protein are the only substrates oxidized
Biopsy
Effect of Procedures
Damask et al CCM 1987
RER = RQ
• Hyperventilation/Hypoventilation
• Acute metabolic acidosis
Hyperventilation/Hypoventilation
• Change in CO2 Body Stores
Transient Hyperventilation
RQ
0 5 10 15 MINUTES
Acute Metabolic Acidosis
HCO3− + H+ ⇌ CO2 + H2O
LipogenesisDisappearance of Substrate without Oxidation
RQ = 2.75 – 8.67
Ketones ETOH
RQ = .69
Small Effect on REE
Adult PREDICTEDSHarris-Benedict 1919
Estimation of Resting Energy Expenditure (REE) with Prediction
Equations
• Harris-Benedict Equation (1919)– based on gender, weight, height, age
• Errors in estimation:– Standard deviation = 10%
– 95% confidence interval = 20%
Effect of BMI on H-B Prediction Using Ideal Body Weight
Effect of BMI on H-B Prediction Using Adjusted Body Weight
Interpretation Steps
• Patient Information– Demographics– Medications
• Quality of Measurement– Length of measurement– CV of VO2 & VO2
– REE & RQ
Measurement Interval
• Healthy Adults– Discard initial 5
minutes, then 5 min with <10% CV
• Critically Ill, Ventilated Patients– Discard initial 5
minutes, then 5 min with <5% CV
– 25 mins with <10% CV
American Dietetic Association EBG 2006
Measurement Interval
• During Mechanical Ventilation– 5 min with <5% CV
– Sufficient length to account for variability
AARC CPG 2004 Revision
Assessment of RQ for Test Quality
• ADA EBG 2006
• RQ < .70 or > 1.0 suggest inaccurate measurement
• AARC CPG 2004
• RQ should be in normal physiologic range .67 – 1.3
• RQ should be consistent with nutritional intake
Interpretation Steps
1. Confirm Patient Demographics
2. Confirm Resting, Fasting State (or nutritional intake)
3. Confirm and Assess Measurement Method
4. Compare Measured REE to Predicted REE
5. Assess RQ
Metabolic States
• Hypometabolic <90% predicted
• Normometabolic 90% - 110 % predicted
• Hypermetabolic > 110% predicted
REE MEASURED BY INDIRECT CALORIMETRY
IN 80 OBESE SUBJECTS
NORMOMETABOLIC
59%
HYPOMETABOLIC
20%
HYPERMETABOLIC
21%
Foster et al Metabolism 1988
Metabolic States
• Lower than expected <90% predicted
• Expected Range 90% - 110 % predicted
• Higher than expected > 110% predicted
INTERPRETATION OF RQ
.9.7 .8 1.0
Starvation OverfeedingMixed Substrates
Hypoventilation Hyperventilation
Metabolic AcidosisETOH or Ketones
Fat CHO
INTERPRETATION OF RQ
• RQ consistent with fasting state
• RQ consistent with nutritional intake
• RQ higher than expected for nutritional intake
• RQ lower than expected for nutritional intake
METHODSSpontaneous Breathing
• Mouthpieces, Noseclips, Masks increase VE
• Canopy method preferred• Supplemental Oxygen must have a
consistent FiO2
CASE EXAMPLE
Patient: Outpatient, 46 yr old man , BMI 46, Fasting
Method: Canopy, Room Air
Measurement: 10 min, last 5 min CV 2%
CASE EXAMPLE
Predicted REE (adjusted body weight) = 1600 kcal/day
Measured REE = 1840 kcal/day
RQ = .75
46 yr old man , BMI 46, Fasting
115%predicted
RQ .70 to .79 Fasting State
StarvationETOH or Ketones
Interpretation
• Quality: Good, CV 2%
• Conditions: Canopy study, Fasting State
• Summary: REE is 1840 kcal/day (115% predicted) with an RQ of .75 consistent with a fasting state.
Factors that effect outcome of measurements
• Eating– Increases REE by 10%– Increases RQ
Measurements During Mechanical Ventilation
• Unstable FiO2
• Leaks
• Bias Flow
FiO2 Instability
FiO2 Variability
INTERBREATH
INTRABREATH
FiO2 Measurement Error
FiO2 Measurement Error
• Most common problem when attempting VO2 measurements on mechanically ventilated subjects
• Artifactually increases VO2
• Artifactually decreases RQ
Haldane’s Transformation
VO2 = VE x ( FIO2 – FEO2)
FIO2 x (1-FIO2-FECO2)
1-FIO2
Error Increases with Increasing FiO2
250ml/min 0.80
-22% +28%
-35% +54%
-69% +220%
VO2 RQ
FiO2 error 0.5%35%
80%
60%
Causes of Variable FiO2
• Fluctuation of Gas Line Pressure
• Leak
• Contaminates in the Proportional Solenoids
• Ventilator algorithms for gas mixing
• Patient-Ventilator Dysynchrony
Correcting Fluctuating FiO2
• External Blender– Set Vent to FiO2 1.0
• External Gas Source– H cylinder
• External Inspiratory Reservoir– Low Compliance– 1 – 1.5 Liters
Unstable FiO2 during SIMV
FIO2
40
45Spontaneous Breath with Increase in Rise Time
Spontaneous Breath
VE = VCO2 x .863PaCO2 x ( 1- VD/VT)
BOHR EQUATION
Components of Minute Ventilation
CASE STUDY70 Kg Male
• Pneumonia
• Vent settings A/C 800 , RR 12/20, FiO2 .40 , PEEP 3cm
Flow
CO2
O2
flow
CO2
O2
Indirect CalorimetryREE 1540
RQ .78
VO2 320
VCO2 250
PaCO2 40
VD/VT .40
kcal 0
Started on 2450 Kcal/day RQ .85
Interpretation
• Quality: Good, CV 3%
• Conditions: Ventilator study, Fasting State
• Summary: REE is 1540 kcal/day (108% predicted) with an RQ of .75 consistent with a fasting state.
Day 4
• Attending Physician thought the CXR had increased infiltrates
• Pulm/CC Physician thought the infiltrates were stable and the patient was receiving too many calories
Increased VE
Indirect Calorimetry
REE 1540 2095
RQ .78 .94
VO2 320 420
VCO2 250 396
PaCO2 40 38
VD/VT .40 .40
VE 11.2 19.2
kcal 0 2450
Day 1 Day 4
Interpretation
• Quality: Good, CV 4%• Conditions: Ventilator study, Patient
receiving 2450 kcal/day TPN• Summary: REE is 2095 kcal/day (115%
predicted) with an RQ of .94 which is higher than expected for the nutritional intake. Consider acute hyperventilation or overfeeding.
Summary
• There are limited guidelines for the interpretation of indirect calorimetry
• It is important to have a consistent approach to the measurement
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