Indirect Calorimetry

Indirect calorimetry is able to quantify energy expenditure based on the physiologic relationship between oxygen intake and carbon dioxide release and heat and energy production. More simply, IC measures O2 consumption and CO2 production [56]. Calculation of the resting energy expenditure (REE) is the end result of IC and is estimated to be approximately 10% greater than the basal energy expenditure (BEE), which can only be measured in deep sleep [57, 58]. REE is thought to account for 75%-90% of total energy expenditure (TEE). The remainder of TEE is made up of thermogenesis resulting from nutritional intake (diet-induced thermogenesis), environment (shivering/non-shivering thermogenesis) and physical activity [59]. It should, however, be noted that IC is unable to differentiate non-protein calories from total calories. Although protein requirements are not measured by IC, they can be easily determined by several different methods (see below).

In addition to determining the 24-h caloric requirements as reflected by the REE, IC also provides the investigator with a measure of substrate utilization by calculation of the respiratory quotient (RQ) [59]. Defined by the ratio VC02/VO2, the RQ is purported to be a measurement of substrate utilization in vivo. An RQ > 1.0 is generally considered to be consistent with overfeeding and an RQ < 0.80 is considered to be consistent with underfeeding. Notably, there is an associated physiologic range for the RQ (0.67-1.3), and values outside this range can only be generated by error. Therefore, in practice this value is used as a determinant of test validity [59-61].

The necessary duration of IC testing to provide accurate and valid results is not known. At present, most procedures continue for predetermined intervals or until a "steady state" is reached [62]. Prior to testing, patients should be maintained on bed rest for 30min and kept in a thermoneutral environment [63]. Patients on oral diets should fast overnight, while patients on total parenteral nutrition (TPN) or enteral tube feeds (ETF) should be placed on a continuous infusion rate up until the point of testing. Analgesics and anxiolytics should be administered appropriately if clinically required [62].

The two instruments currently available for conducting IC include the "classic" metabolic cart and hand-held instruments [64]. The "classic" metabolic cart measures both oxygen consumption and carbon dioxide production and automatically calculates EE and the RQ (Fig. 1.5). The majority of the predictive calorie equations in use today were derived from the use of this device. Unfortunately, "classic" carts are expensive, difficult to mobilize and calibrate, and require additional staffing (i.e., respiratory therapists) to perform the testing.

Recently, small hand-held IC devices have been developed (Fig. 1.6). These devices calculate REE by measuring only oxygen consumption, and therefore no RQ is determined [64]. So far, these devices have only been validated in healthy individuals. Conversely, these hand-held devices are highly portable, self calibrating, require minimal operator training and cost considerably less than "classic" carts. Nutrition enthusiasts are optimistic that the reduced cost and ease of operation of these new devices will make IC more commonplace in the monitoring of energy metabolism in the hospital and outpatient setting.

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