EE: Respiratory Quotient (2005)
Newer indirect calorimetry equipment uses an assumed RQ value and does not provide individual RQ data making it impossible for the health care professional to use the RQ ratio to detect measurement error.
RQ within the 0.7 to 1.0 is not the only criterion for judging that a test is valid [see other Evidence Analysis Questions, particularly steady state] to assure valid testing conditions.
DEFINITIONS
Coefficient of variation
A measure of relative variation, based on the size of the standard deviation relative to the mean, expressed as a percentage. The coefficient of variation is a ratio of the standard deviation (s) to the mean and multiplied by 100 to express as a percentage.
Fasting – Going without food long enough that energy requirements of body metabolism are supplied by the oxidization of fats which, if glucose is not supplied, results in the products of incomplete fat combustion.
Flow rate – the volume of gas expired per time
Haldane transformation: The formula used to report the difference between inspired and expired ventilation.
Non-protein Respiratory Quotient: The ratio between the nonprotein VCO2 and the nonprotein VO2 (i.e., that portion of the total VCO2 and VO2 that is attributable to carbohydrate and lipid disappearance). When the RQ is close to 0.80, the NPRQ and RQ are essentially identical, and a correct estimate of protein disappearance is not crucial to calculate relative disappearance rates of carbohydrate and lipid.
NPVCO2 = VCO2 - (6.25 X 0.774) N
NPRQ = ----- ---------------
NPVO2 = VO2 - (6.25 X 0.966) N
Post-absorptive – Going without food for a long enough time to allow absorption of water, alcohol and some salts through the stomach and the digestive products in the small intestine via blood or lymph; but not an extended time that promotes oxidization of fats.
Post-prandial- An RQ measure following a meal reflects the dietary composition of meal eaten. If 100% carbohydrates consumed (i.e., a sweetened, flavored beverage, simple carbohydrate snacks such as a candies or fruits), the RQ approaches or exceeds 1.0. In contrast, in individuals who have extensively fasted, an RQ can fall below 0.70 due to lipolysis or fat oxidation.
In critical care settings, where an indirect calorimetry measure is taken during a continuous TPN, PPN, enteral nutrition feeding, the RQ includes an approximately 10% increase beyond the true fasting measurement due to metabolism of continuously fed nutrients.
Respiratory Quotient:
Healthy adult calculation
RQ = VCO2
----
VO2
Critically ill patient on a mechanical ventilator
RQ = PECO2 (1-FIO2)
---------------------
PIO2 – PEO2 – (FIO2 Í PECO2)
PIO2 and FIO2 are inspired O2 partial pressure and concentrations, respectively.
PECO2 and PEO2 are mean expired CO2 and O2 partial pressures, respectively.
(This is equivalent to RQ in healthy adults).
Weir Equation (Modified)
REE = [3.9 (VO2) + 1.1 (VCO2)] 1.44
Weir Equation (Complete)
[3.94 (VO2) + 1.106 (VCO2)]1.44 – 2.17 (Urinary nitrogen)