Measuring BMR in the Pulmonary lab.
All cells in the human body undertake the same basic metabolic processes which are divided into anaerobic and aerobic enzymatic pathways. Glycolysis is the main process in anaerobic metabolism, and for anaerobic metabolism, the processes are the transcarboxylic acid cycle and the electron transport chain. Glycolysis uses simple carbohydrates, such as glucose, as substrate and produces two carbon dioxide molecules per glucose as well as adenosinetriphosphate (ATP) for energy demands in the cellular. level. Nutrients including amino and fatty acids can be brought into glycolysis through the donation of their carbon atoms and glycolysis can be reversed in neural tissue to produce glucose for the brain, the only metabolite it can use. The transcarboxylic acid cycle in aerobic metabolism is the main site of carbon dioxide production and the electron transport chain is the main source of ATP, which is directly used a source of energy for enzymatic and cellular activity in all cells. Aerobic metabolism needs oxygen to function through the removal of hydrogen ions and their associated electrons at the bottom of the electron transport chain.
During rest and very low levels of exercise, the rates of anaerobic and aerobic metabolism are equal. But during higher levels of exercise and especially during anaerobic metabolism, lactate is produced in order to temporarily store the extra carbons not immediately processes through aerobic metabolism. Lactate is produced by the reversal of a segment of glycolysis. The rates of both anaerobic and aerobic metabolism together at rest are directly proportional to the oxygen consumption, or VO2. At basal metabolism, the VO2 determines the overall metabolic rate of the body since the overall metabolic rate is directly coupled with hydrogen removal in the electron transport chain. Thirty or more years ago, calories, or kilocalories (kcal) generated were measured, through water tank immersion for the measurement of heat production, to quantify the BMR. Since the electron transport chain in aerobic metabolism produces metabolic heat in direct proportion to oxygen consumed, it is now the practice of all exercise physiologists, physicians, and pulmonary technologists to measure the VO2 as a direct measurement of total metabolic rate. For the same reason, in pulmonary exercise studies, VO2 is used to directly measure the exercise levels or metabolic rate during all levels of work. The measurement of the respiratory exchange ratio (RER) (VCO2/VO2) gives an indication of the major source or proportion of calories consumed: protein, or carbohydrate, or fat. Respiratory quotient (RQ) is also the measurement of VCO2/VO2, but at the cellular level, while the measurement end tidally is RER. RER and RQ are only equivalent at steady state, which is the requirement during BMR testing. The term indirect calorimetry is a misnomer since the actual VO2 and VCO2 are being measured and the results are more direct than measuring the temperature changes in a water bath after subject immersion.
The measurement of VO2, which is the equivalent of metabolic rate, and VCO2, as well as the measurement of urinary nitrogen (UN), can be used to determine the resting energy expenditure (REE). REE is expressed in kilocalories /day. The calculation of REE makes use of the Weir equation:
REE (kcal/24hr) = 5.68 VO2 + 1.59 VCO2 - 2.17 UN
VO2 = ml/min (STPD)
VCO2 = ml/min (STPD)
UN = urinary nitrogen (g/24 hr)
If UN is not available, the REE can be estimated using the following equation:
REE (kcal/24hr) = 5.46 VO2 + 1.75 VCO2
As all measurements discussed will be made at steady state, I will refer to the ratio of VCO2/VO2 as respiratory quotient or RQ. Although a normal RQ at rest is 0.85, the RQ can vary between 0.70 to 1.10, depending on the proportion of substrates being metabolized. A diet high in carbohydrate can produce a higher RQ because as the carbon rich carbohydrates are brought through metabolism, specifically through the transcarboxylic acid cycle, the high proportion of carbons are released in the form of CO2, which produces a larger RQ ratio. If the diet contains a larger proportion of protein, there is a lower level of CO2 produced as more energy comes through amino acid oxidation.
To determine if the RQ is attributable to carbohydrates or fats, the urinary nitrogen can be subtracted from the VO2 and VCO2 using the following equation to calculate the non protein RQ or RQnp:
RQnp = 1.44 VCO2 - 4.754 UN/1.44 VO2 - 5.923 UN
The following equations are used to determine the proportion of metabolism from fats, carbohydrates, or protein:
carbohydrate = 5.926 VCO2 - 4.189 VO2 - 2.539 UN
fat = 2.432 V02 - 2.432 VCO2 - 1.3943 UN
protein = 6.250 UN
Carbohydrates, fats, and protein are oxidized in grams per 24 hrs (g/24 hr). Finally we can us the following conversion to obtain kilocalories.
4.18 grams of carbohydrate = 1 kcal
9.46 grams of fat = 1 kcal
4.32 grams of protein = 1 kcal
carbohydrate + fat + protein = total kcal
These calculations can only be done when the patient is in steady state.
The forgoing calculations are very useful but a simple observation of the RQ in itself provides information as to whether the patient is eating a significant proportion of carbohydrates or eating a larger proportion of protein. If the RQ ratio is above one, a diet significantly high in carbohydrates is probable and if the RG is below 0.90, there is a diet significantly high in protein.
With the information giving the proportion of carbohydrates, fats, and protein, or even with only the RQ, adjustments in diet can be made to decrease CO2 production, which in turn can have effects on acid base balance and the ease of breathing. The overall measurement of the BMR is useful in specific genetic metabolic disorders or as an indication for possible weight gain or loss. Measuring a patient's body weight and an evaluation of body fat levels through the use of upper arm skin fold measurements can be useful to assess the effects of a patient's nutrition but calorimetry provides a direct measurement of the patient's overall metabolic rate and the specifics of carbohydrate, fat, and protein oxidation.
In preparation for a BMR measurement any drug or other substances which could affect metabolism should be avoided for 24 prior to the test as well as caffeine, nicotine, and alcohol, and methylxanthine-type medications. The patient should be advised to avoid high sugar meals or snacks on the day of the test which would falsely increase overall metabolic rate and RQ. It is much better to schedule all patients and successive tests in the morning when BMR is stable and consistent. The patient should fast that until the testing is complete. As a basal or baseline metabolic measurement is desired, the final measurements should be taken after at least ten minutes of steady state with the patient supine or sitting comfortably. A steady state condition can be ascertained when the VO2, VE, and heart rate do not vary by more than [+ or -]5% over a five minute period.
Types of calorimetry are described as closed-circuit and open circuit. Closed circuit measurements involve a system in which the patient is breathing through a mouthpiece and valve attached to volume displacement spirometer or to a Douglas bag filled with 100% oxygen. Any volume decrease will be the result of oxygen consumed or VO2. The VE can be measured at he same time by means of pneumotachograph. A CO2 scrubbing device must be made part of the breathing circuit so that CO2 will not build within the closed system. The scrubbing device contains a compound such as soda lime which binds the CO2 out of the system.
Open circuit technique involves the patient breathing through a pneumotachograph for volume measurement with an end tidal catheter attached, leading to one of the "off the shelf" metabolic systems which measure VO2, VCO2, and VE. These systems invariably advertise themselves as making use of "breath by breath" analysis but you will notice that they average ten second segments of data for final display and graphing since sighs or single deep breaths cause huge peaks in apparent VO2. Although these peaks seem to accurately represent metabolism at the end tidal level, they in no way represent accuracy in the cellular level, even during steady state. If the device allows, it is best to report data from a ten second average once steady state is established.
Jim Harvey MS, RPFT, RCP works in the Pulmonary Function Laboratory at Stanford Hospital and Clinics in Palo Alto, and teaches Pulmonary Function at Skyline College in San Bruno, California.
by Jim Harvey MS, RPFT, RCP
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|Title Annotation:||PULMONARY FUNCTION TESTING|
|Publication:||FOCUS: Journal for Respiratory Care & Sleep Medicine|
|Date:||Jul 1, 2006|
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