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 HyperventilationRQ
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 SIMV45
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

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