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Interpretation of Indirect Calorimetry

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1 Interpretation of Indirect Calorimetry
Charles McArthur BA RRT RPFT Mankato, MN

2 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

3 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

4 Antoine Lavoisier 1775

5 Combustion 1: The process of burning
2: a chemical change, especially oxidation, that produces heat ; also : a slower oxidation (as in the body)

6 Human Internal Combustion
O2 + C6H12O6 CO2 +H2O CELL HEAT

7 Each Substrate has Unique Stoichiometry
C6H O2 6CO H20 + Energy Heat + Work RQ = VCO2/VO2 = 1.0

8 Direct Calorimeter Heat = Energy Expenditure( kcal) REE
At Rest Heat = Energy Expenditure( kcal) REE Resting Energy Expenditure = Kcal/day

9 Indirect Calorimetry O2 & CO2 Measured at the Airway
Respiratory Exchange Ratio RER = CO2/O2

10 Measurement of VO2 & VCO2 VO2 = VE x ( FIO2 – FEO2)
VCO2 = VE x (FECO2)

11 Energy Equivalents and RQ’s
SUBSTRATE Kcal/LO2 RQ CHO 5.05 1.0 Protein 4.46 0.8 Fat 4.74 0.7

12 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

13 Energy Equivalents and RQ’s
SUBSTRATE Kcal/LO2 RQ CHO 5.05 1.0 Protein 4.46 0.8 Fat 4.74 0.7

14 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

15 IC Assumptions Subject is in resting state RER = RQ
Disappearance of substrates = oxidation of substrates CHO, Fat, and Protein are the only substrates oxidized

16 Effect of Procedures Biopsy Damask et al CCM

17 RER = RQ Hyperventilation/Hypoventilation Acute metabolic acidosis

18 Hyperventilation/Hypoventilation
Change in CO2 Body Stores

19 Transient Hyperventilation
RQ MINUTES

20 Acute Metabolic Acidosis
HCO3− + H+ ⇌ CO2 + H2O

21 Lipogenesis Disappearance of Substrate without Oxidation
RQ = 2.75 – 8.67

22 RQ = .69 Ketones ETOH Small Effect on REE

23 Adult PREDICTEDS Harris-Benedict 1919

24 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%

25 Effect of BMI on H-B Prediction Using Ideal Body Weight

26 Effect of BMI on H-B Prediction Using Adjusted Body Weight

27 Interpretation Steps Patient Information Quality of Measurement
Demographics Medications Quality of Measurement Length of measurement CV of VO2 & VO2 REE & RQ

28 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

29 Measurement Interval During Mechanical Ventilation
AARC CPG 2004 Revision During Mechanical Ventilation 5 min with <5% CV Sufficient length to account for variability

30 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

31 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

32 Metabolic States Hypometabolic <90% predicted
Normometabolic 90% % predicted Hypermetabolic > 110% predicted

33 Foster et al Metabolism 1988
20% HYPOMETABOLIC 59% HYPERMETABOLIC NORMOMETABOLIC 21% Foster et al Metabolism 1988

34 Metabolic States Lower than expected <90% predicted
Expected Range 90% % predicted Higher than expected > 110% predicted

35 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

36 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

37 METHODS Spontaneous Breathing
Mouthpieces, Noseclips, Masks increase VE Canopy method preferred Supplemental Oxygen must have a consistent FiO2

38 CASE EXAMPLE Patient: Outpatient, 46 yr old man , BMI 46, Fasting
Method: Canopy, Room Air Measurement: 10 min, last 5 min CV 2%

39 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

40 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.

41 Factors that effect outcome of measurements
Eating Increases REE by 10% Increases RQ

42 Measurements During Mechanical Ventilation
Unstable FiO2 Leaks Bias Flow

43 FiO2 Instability

44 FiO2 Variability INTERBREATH INTRABREATH

45 FiO2 Measurement Error

46 FiO2 Measurement Error Most common problem when attempting VO2 measurements on mechanically ventilated subjects Artifactually increases VO2 Artifactually decreases RQ

47 Haldane’s Transformation
VO2 = VE x ( FIO2 – FEO2) FIO2 x (1-FIO2-FECO2) 1-FIO2

48 Error Increases with Increasing FiO2
VO RQ 250ml/min 0.80 -22% +28% -35% +54% -69% +220% 35% FiO2 error 0.5% 60% 80%

49 Causes of Variable FiO2 Fluctuation of Gas Line Pressure Leak
Contaminates in the Proportional Solenoids Ventilator algorithms for gas mixing Patient-Ventilator Dysynchrony

50 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

51 Unstable FiO2 during SIMV
45 Spontaneous Breath Spontaneous Breath with Increase in Rise Time FIO2 40

52 VE = VCO2 x .863 Components of Minute Ventilation PaCO2 x ( 1- VD/VT)
BOHR EQUATION VE = VCO2 x .863 PaCO2 x ( 1- VD/VT)

53 CASE STUDY 70 Kg Male Pneumonia
Vent settings A/C 800 , RR 12/20, FiO , PEEP 3cm

54 Flow CO2 O2

55

56 flow CO2 O2

57 Indirect Calorimetry REE 1540 RQ .78 VO2 320 VCO2 250 PaCO2 40 VD/VT
.40 kcal Started on Kcal/day RQ .85

58 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.

59 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

60 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

61 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.

62 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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