How can it be used clinically?
The RQ is the ratio of the patient's carbon dioxide production (VCO2) to his or her oxygen consumption (VO2). These values are obtained by indirect calorimetry at the patient's bedside. The RQ is helpful in guiding the planning of nutritional therapy. The physiologic range for RQ values is 0.7 to 1.2 and is influenced by the relative contribution from fat, protein, and carbohydrate. RQ values for fat, protein, and carbohydrate are 0.7, 0.8, and 1.0, respectively. Thus, an RQ of >1.0 might suggest excessive carbohydrate or calorie provision that can result in increased CO2 production and cause difficulty weaning from mechanical ventilation. A RQ of <0.7 might suggest underfeeding and use of ketones as a fuel source. The use of energy fuels by animals and by a flame may be compared by considering the stoichiometry of the chemical reactions. They may also be compared by noting the amount of energy produced with complete oxidation of the fuel in question. The following discussion begins with the stoichiometry of combustion of simple compounds — such as paraffin, methane, and methanol — and then continues onto the combustion of more complex materials — such as glucose and fat. The study of the stoichiometry of various reactions requires the counting of the number of molecules of CO2 produced per molecule of fuel combusted, the number of water molecules produced, and the number of oxygen atoms (or molecules) consumed.
The stoichiometry involved in the combustion of fuels can be used to calculate a very interesting ratio: the number of CO2 molecules discharged from the body per number of oxygen molecules consumed. This ratio (CO2/O2) is called the respiratory quotient (RQ). The biochemical events of CO2 production and O2 utilization are a direct result of the oxidation of various fuels such as fat and glucose. The amount of O2 used is also influenced by the rate of synthesis of new fat in the body. Respiratory gases can be measured and analyzed quite easily. These measurements can be used to calculate the amount of CO2 produced and O2 used by the body over any given period. Calculating the RQ for a subject is quite easy and can provide remarkable insight into the overall behavior of energy fuels in the body. The RQ can provide only limited information on metabolism, but it is a powerful measurement.
The RQ is different for different fuels. For example, the RQ for the complete combustion of glucose is 1.0, and that for the complete combustion of fat is 0.7. These values are determined in three ways:
1.
On paper, by balancing chemical formulas, with no knowledge of metabolic pathways.
2.
By consideration of the biochemical pathways of the body, providing that these pathways have been memorized or are available in some format.
3.
Experimentally, by use of a furnace. The furnace contains an atmosphere of pure oxygen. A mechanism for igniting the fuel or food in question, as well as devices for measuring the amount of CO2 produced and O2 consumed, is available.
Knowledge of the RQ for different energy fuels can be used to determine the predominant fuel used by a particular organ of the body, such as the muscle or brain. To determine the RQ for a particular organ, the blood entering and exiting that organ must be analyzed. Most of the oxygen entering and exiting the organ is bound to hemoglobin. Most of the CO2 entering and exiting the organ is in the form of bicarbonate.
