++
Indirect calorimetry is the calculation of energy expenditure by the measurement of V̇o2 and/or V̇co2, which are converted to energy expenditure (Kcal/day) by the Weir method:
+
++
Indirect calorimetry also allows calculation of the R. Indirect calorimeters operate by using an open- or a closed-circuit method.
++
The open-circuit method measures the concentrations and volumes of inspired and expired gases to determine V̇o2 and V̇co2. The principal components of an open-circuit calorimeter (metabolic cart) are the analyzers (O2 and CO2), a volume-measuring device, and a mixing chamber. The analyzers must be capable of measuring small changes in gas concentrations, and the volume monitor must be capable of accurately measuring volumes from 0.05 to 1 L. Exhaled gas from the patient is directed into a mixing chamber. At the outlet of the mixing chamber, a vacuum pump aspirates a small sample of gas for measurement of O2 and CO2. After analysis, this sample is returned to the mixing chamber. The entire volume of gas then exits through a volume monitor. Periodically, the analyzer also measures the inspired oxygen concentration. A microprocessor performs the necessary calculations. Meticulous attention to detail is required to obtain valid results using an open-circuit indirect calorimeter (Table 32-3).
++
+++
Closed-Circuit Method
++
The key components of the closed circuit calorimeter are a volumetric spirometer, a mixing chamber, a CO2 analyzer, and a CO2 absorber. The spirometer is filled with a known volume of oxygen and connected to the patient. As the patient rebreathes from the spirometer, O2 is removed and CO2 is added. The CO2 is removed from the system by a CO2 absorber before the gas is returned to the spirometer. The decrease in the volume of the system equals V̇o2. Gas from the patient flows into the mixing chamber, and a sample is aspirated for Fēco2 analysis. From the mixing chamber, gas flows through a CO2 absorber and then to the spirometer. The volume of the spirometer is electronically monitored to measure tidal volume. The difference between end-expiratory volumes is calculated by a microprocessor to determine V̇o2. If the patient is mechanically ventilated, a bag-in-the-box system is used as a part of the inspiratory limb of the calorimeter. The bellows is pressurized by the ventilator to ventilate the patient. Measurement time is limited by Fio2 and the volume of the spirometer. When the volume of the spirometer decreases to a critical level, the measurement is interrupted to refill the spirometer.
++
Leaks from the closed-circuit system will result in erroneously high V̇o2 measurements (uncuffed airway, bronchopleural fistula, sidestream capnograph). Another problem with this technique is that system compressible volume is increased and trigger sensitivity is decreased. The advantage of the closed-circuit method over the open-circuit method is its ability to make measurements at a high Fio2 (up to 1).
++
In patients with a pulmonary artery catheter, V̇o2 can be calculated from arterial oxygen content (Cao2), mixed venous oxygen content (Cv̄o2), and cardiac output (Q̇c):
+
++
Metabolic rate can then be calculated from the V̇o2:
+
++
Metabolic rate can also be calculated from V̇co2, which can be used in conjunction with volumetric capnography:
+
++
Normal V̇o2 is 250 mL/min and normal V̇co2 is 200 mL/min (2.6 mL/kg/min).
+++
General Considerations with Indirect Calorimetry
++
Because indirect calorimetry is labor intensive and expensive, it should be reserved for selected patients (Table 32-4). When measuring resting energy expenditure (REE) using indirect calorimetry, one must consider both the duration of each measurement and the number of measurements required for a reliable 24-hour estimate. Ideally, continuous 24-hour indirect calorimetry produces the best estimate of REE. For most critically ill patients, it is impossible to obtain measurements for longer than 15 to 30 minutes once every several days. It is important, however, to recognize that shorter and less frequent measurements will less reliably estimate REE. When indirect calorimetry is performed, the patient should be resting, undisturbed, motionless, supine, and aware of the surroundings (unless comatose). The patient should either be on continuous nutritional support or fasting for several hours before the measurement. Before indirect calorimetry is performed, there should have been no changes in ventilation for at least 90 minutes, no changes that affect V̇o2 for at least 60 minutes, and stable hemodynamics for at least 2 hours. Because REE is measured with the patient at rest, calories must be added due to patient activity. There may be considerable fluctuation in REE throughout the day and from day to day.
++