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The metabolic response to tissue injury and the stress of critical illness is a cascade of events supported by an intricate, complex network of mediators, cytokines, growth factors, and hormones that significantly alter energy requirements.1 The initial response after injury is manifested by a decrease in energy expenditure (the ebb phase), lasting 2 to 3 days and aimed at the preservation of energy; this is followed by the flow phase, which is characterized by an increase in energy expenditure and a catabolic response that varies in duration, depending of the nature of the initial insult.2 Critically ill patients are characterized by alterations in the metabolism of carbohydrates, proteins, and lipids. During critical illness, typical alterations include increased secretion of cortisol, glucagon, and catecholamines; decreased secretion of insulin-like growth factor 1 (IGF-1); and resistance to the combined effects of insulin and growth hormone3 and growth hormone alone.4




Accurate measurement of energy requirements, when available, must be used to guide energy prescription during critical illness. Energy expenditure may be measured by indirect calorimetry, tracer dilution studies or by the reverse Fick method.

Gas Exchange Measurement – Indirect Calorimetry

Indirect calorimetry (IC) is the method by which metabolic rate and substrate utilization are estimated from respiratory gas exchange measurements and urinary nitrogen excretion. Indirect calorimetry measures minute-to-minute whole-body oxygen consumption (VO2) and carbon dioxide production (VCO2). The gas exchange method provides both VO2 and VCO2 by measuring the absolute amounts of O2 and CO2 in inspired and expired gas. Accurate measurements of inspired as well as expired gas volumes are necessary for this testing, and are potential sources of error. The Haldane transformation is applied to prevent the need to measure both inspired and expired volumes, and hence decreases the potential for error, as follows: (1) VO2 = [(1 − FEO2 − FECO2)/(1 − FIO2) × FIO2] − (FEO2 × VE), where FEO2 is expired oxygen concentration, FIO2 is inspired oxygen concentration, FECO2 is expired CO2 concentration, and VE is expired minute ventilation; (2) VCO2 = VE (FECO2) − VI (FICO2). Then, the VO2 and VCO2 values are converted to a caloric equivalent based on equations developed by Weir: REE = [VO2 (3.941) + VCO2 (1.11)] × 1440.5 These values can be used to calculate the substrate oxidation rates by the Consolazio formulas once total urinary nitrogen is measured in a 24-hr urine collection.6 Gas exchange reflects the composition of oxidized substrates—1 kcal energy requires 0.236, 0.214, and 0.199 liters of oxygen for protein, lipid, and carbohydrate oxidation, producing 0.190, 0.151, and 0.199 liters of carbon dioxide as a byproduct, respectively.7 It is important to ...

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