interpretation of indirect calorimetry charles mcarthur ba rrt rpft mankato, mn
TRANSCRIPT
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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 Lavoisier1775
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Combustion
1: The process of burning2: a chemical change, especially oxidation, that produces heat ;
also : a slower oxidation (as in the body)
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O2 + C6H12O6
CELL
HEAT
CO2 +H2O
Human Internal Combustion
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C6H1206 + 6O2 6CO2 + 6H20 + Energy
Each Substrate has Unique Stoichiometry
RQ = VCO2/VO2 = 1.0
Heat + Work
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Direct Calorimeter
Heat = Energy Expenditure( kcal)
REE
Resting Energy Expenditure =
Kcal/day
At Rest
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Indirect Calorimetry
O2 CO2
Respiratory Exchange Ratio RER = CO2/O2
O2 & CO2 Measured at the Airway
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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
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
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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 rateP. I. Mansell and I. A. MacdonaldAmerican Journal of Physiology1990: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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Biopsy
Effect of Procedures
Damask et al CCM 1987
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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
0 5 10 15 MINUTES
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Acute Metabolic Acidosis
HCO3− + H+ ⇌ CO2 + H2O
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LipogenesisDisappearance of Substrate without Oxidation
RQ = 2.75 – 8.67
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Ketones ETOH
RQ = .69
Small Effect on REE
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Adult PREDICTEDSHarris-Benedict 1919
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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– Demographics– Medications
• Quality of Measurement– Length of measurement– CV of VO2 & VO2
– REE & RQ
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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
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Measurement Interval
• During Mechanical Ventilation– 5 min with <5% CV
– Sufficient length to account for variability
AARC CPG 2004 Revision
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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
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
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Metabolic States
• Hypometabolic <90% predicted
• Normometabolic 90% - 110 % predicted
• Hypermetabolic > 110% predicted
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REE MEASURED BY INDIRECT CALORIMETRY
IN 80 OBESE SUBJECTS
NORMOMETABOLIC
59%
HYPOMETABOLIC
20%
HYPERMETABOLIC
21%
Foster et al Metabolism 1988
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Metabolic States
• Lower than expected <90% predicted
• Expected Range 90% - 110 % predicted
• Higher than expected > 110% predicted
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INTERPRETATION OF RQ
.9.7 .8 1.0
Starvation OverfeedingMixed Substrates
Hypoventilation Hyperventilation
Metabolic AcidosisETOH or Ketones
Fat CHO
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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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METHODSSpontaneous 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
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
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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
250ml/min 0.80
-22% +28%
-35% +54%
-69% +220%
VO2 RQ
FiO2 error 0.5%35%
80%
60%
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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
FIO2
40
45Spontaneous Breath with Increase in Rise Time
Spontaneous Breath
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VE = VCO2 x .863PaCO2 x ( 1- VD/VT)
BOHR EQUATION
Components of Minute Ventilation
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CASE STUDY70 Kg Male
• Pneumonia
• Vent settings A/C 800 , RR 12/20, FiO2 .40 , PEEP 3cm
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Flow
CO2
O2
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flow
CO2
O2
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Indirect CalorimetryREE 1540
RQ .78
VO2 320
VCO2 250
PaCO2 40
VD/VT .40
kcal 0
Started on 2450 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
• 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
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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
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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