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Interpretation of Indirect Calorimetry
Charles McArthur BA RRT RPFT Mankato, MN
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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
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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
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Antoine Lavoisier 1775
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Combustion 1: The process of burning
2: a chemical change, especially oxidation, that produces heat ; also : a slower oxidation (as in the body)
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Human Internal Combustion
O2 + C6H12O6 CO2 +H2O CELL HEAT
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Each Substrate has Unique Stoichiometry
C6H O2 6CO H20 + Energy Heat + Work RQ = VCO2/VO2 = 1.0
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Direct Calorimeter Heat = Energy Expenditure( kcal) REE
At Rest Heat = Energy Expenditure( kcal) REE Resting Energy Expenditure = Kcal/day
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Indirect Calorimetry O2 & CO2 Measured at the Airway
Respiratory Exchange Ratio RER = CO2/O2
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Measurement of VO2 & VCO2 VO2 = VE x ( FIO2 – FEO2)
VCO2 = VE x (FECO2)
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Energy Equivalents and RQ’s
SUBSTRATE Kcal/LO2 RQ CHO 5.05 1.0 Protein 4.46 0.8 Fat 4.74 0.7
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De Weir Equation [( 3.94 x VO2 + 1.11 x VCO2 ) x 1.44] - 2.17 UUN
REE = Resting Energy Expenditure = KCAL/day [( 3.94 x VO x VCO2 ) x 1.44] UUN ml/min ml/min = Kcal/day
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Energy Equivalents and RQ’s
SUBSTRATE Kcal/LO2 RQ CHO 5.05 1.0 Protein 4.46 0.8 Fat 4.74 0.7
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Error Caused by Lack of UUN Measurement
Reappraisal of the Weir equation for calculation of metabolic rate P. I. Mansell and I. A. Macdonald American Journal of Physiology 1990:R1347-R1354
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IC Assumptions Subject is in resting state RER = RQ
Disappearance of substrates = oxidation of substrates CHO, Fat, and Protein are the only substrates oxidized
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Effect of Procedures Biopsy Damask et al CCM
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RER = RQ Hyperventilation/Hypoventilation Acute metabolic acidosis
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Hyperventilation/Hypoventilation
Change in CO2 Body Stores
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Transient Hyperventilation
RQ MINUTES
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Acute Metabolic Acidosis
HCO3− + H+ ⇌ CO2 + H2O
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Lipogenesis Disappearance of Substrate without Oxidation
RQ = 2.75 – 8.67
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RQ = .69 Ketones ETOH Small Effect on REE
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Adult PREDICTEDS Harris-Benedict 1919
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Harris-Benedict Equation (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%
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Effect of BMI on H-B Prediction Using Ideal Body Weight
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Effect of BMI on H-B Prediction Using Adjusted Body Weight
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Interpretation Steps Patient Information Quality of Measurement
Demographics Medications Quality of Measurement Length of measurement CV of VO2 & VO2 REE & RQ
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Measurement Interval Healthy Adults
American Dietetic Association EBG 2006 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
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Measurement Interval During Mechanical Ventilation
AARC CPG 2004 Revision During Mechanical Ventilation 5 min with <5% CV Sufficient length to account for variability
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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
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Interpretation Steps Confirm Patient Demographics
Confirm Resting, Fasting State (or nutritional intake) Confirm and Assess Measurement Method Compare Measured REE to Predicted REE Assess RQ
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Metabolic States Hypometabolic <90% predicted
Normometabolic 90% % predicted Hypermetabolic > 110% predicted
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Foster et al Metabolism 1988
20% HYPOMETABOLIC 59% HYPERMETABOLIC NORMOMETABOLIC 21% Foster et al Metabolism 1988
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Metabolic States Lower than expected <90% predicted
Expected Range 90% % predicted Higher than expected > 110% predicted
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INTERPRETATION OF RQ .7 .8 .9 1.0 Starvation Mixed Substrates
Overfeeding Fat CHO .7 .8 .9 1.0 Hyperventilation Hypoventilation ETOH or Ketones Metabolic Acidosis
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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
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METHODS Spontaneous Breathing
Mouthpieces, Noseclips, Masks increase VE Canopy method preferred Supplemental Oxygen must have a consistent FiO2
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CASE EXAMPLE Patient: Outpatient, 46 yr old man , BMI 46, Fasting
Method: Canopy, Room Air Measurement: 10 min, last 5 min CV 2%
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CASE EXAMPLE 46 yr old man , BMI 46, Fasting
Predicted REE (adjusted body weight) = 1600 kcal/day Measured REE = 1840 kcal/day 115%predicted RQ = .75 RQ .70 to .79 Fasting State Starvation ETOH or Ketones
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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.
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Factors that effect outcome of measurements
Eating Increases REE by 10% Increases RQ
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Measurements During Mechanical Ventilation
Unstable FiO2 Leaks Bias Flow
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FiO2 Instability
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FiO2 Variability INTERBREATH INTRABREATH
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FiO2 Measurement Error
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FiO2 Measurement Error Most common problem when attempting VO2 measurements on mechanically ventilated subjects Artifactually increases VO2 Artifactually decreases RQ
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Haldane’s Transformation
VO2 = VE x ( FIO2 – FEO2) FIO2 x (1-FIO2-FECO2) 1-FIO2
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Error Increases with Increasing FiO2
VO RQ 250ml/min 0.80 -22% +28% -35% +54% -69% +220% 35% FiO2 error 0.5% 60% 80%
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Causes of Variable FiO2 Fluctuation of Gas Line Pressure Leak
Contaminates in the Proportional Solenoids Ventilator algorithms for gas mixing Patient-Ventilator Dysynchrony
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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
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Unstable FiO2 during SIMV
45 Spontaneous Breath Spontaneous Breath with Increase in Rise Time FIO2 40
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VE = VCO2 x .863 Components of Minute Ventilation PaCO2 x ( 1- VD/VT)
BOHR EQUATION VE = VCO2 x .863 PaCO2 x ( 1- VD/VT)
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CASE STUDY 70 Kg Male Pneumonia
Vent settings A/C 800 , RR 12/20, FiO , PEEP 3cm
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Flow CO2 O2
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flow CO2 O2
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Indirect Calorimetry REE 1540 RQ .78 VO2 320 VCO2 250 PaCO2 40 VD/VT
.40 kcal Started on Kcal/day RQ .85
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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.
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Day 4 Increased VE Attending Physician thought the CXR had increased infiltrates Pulm/CC Physician thought the infiltrates were stable and the patient was receiving too many calories
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Indirect Calorimetry REE 1540 2095 RQ .78 .94 VO2 320 420 VCO2 250 396
Day Day 4 REE 1540 2095 RQ .78 .94 VO2 320 420 VCO2 250 396 PaCO2 40 38 VD/VT .40 VE 11.2 19.2 kcal 2450
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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.
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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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