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