Direct calorimetry: Measurement of the amount of heat (energy) produced by a subject enclosed within a Chamber.

Indirect calorimetry: Measurement of the amount of heat (energy) produced by a subject by determination of the amount of oxygen consumed and the quantity of carbon dioxide eliminated

• The metabolic cart essentially measures the oxygen consumed and the carbon dioxide produced by the patient and then calculates (using the modified Weir equation) the energy expenditure for the patient

• Weir equation: EE = (3.94 x VO2) + (1.1 x (VCO2)

Indirect calorimetry provides two pieces of information:

1. A measure of energy expenditure as reflected by the resting energy expenditure (REE)

2. A measure of substrate utilization as reflected in the respiratory quotient (RQ)

REE = 75 % - 95 % of total energy expenditure
(diet-induced, environmental, activity)

• Metabolism in brain, liver, heart and kidney is relatively constant (60 - 70 % REE)

• Variability in REE:

• • between people

  • during the day 12 %

  • increases with critical illness

  • day to day - 23 %

Variability in REE can be due to:

• Size

• Gender

• Age

• Work of breathing (2 - 3 %)

• Diet-induced thermogenesis

• Sleep

• Illness

• Starvation

• Fever (13 % per degree C)

• Cold

• Activity

• Drugs

• REE correlates closely with fat free lean body mass.

• Work of breathing normally (2 - 3 %) can be as high as 25% of REE with impending respiratory failure

• Diet-induced thermogenesis normally 8-10% drops to 4-8% with continues enteral feeds

• REE drops with sleep

• Most disease states increase REE

• 20-50% increase seen with elective surgery and trauma, 100% increase in REE seen with severe burns

• Increases in REE are seen in the flow phase of injury and can be effected by therapy

• Increases in REE can be expected to reflex severity of illness but the response plateaus at

• 2x the REE

• Increases in REE due to acute illness usually return to base line at 7 to 10 days

• Not all critically ill patients become hypermetabolic (35-60%)

• 15-20 % of ICU-patients are found to be normometabolic.

• 10 -20% of ICU-patients are found to be hypometabolic.

• Hypometabolic state may be do to the disease process: specific cancers ,cachexia ,spinal cord injuries paraplegics decreased REE by 10% quadriplegics by 30%

• Long-term starvation reduces EE by 30-40%

• Fever increases REE (13 % per degree C)

• Exposure to Cold /hypothermia increases REE by shivering and nonshivering thermogenesis

• General Activity is responsible for most of the variability in REE.

• Being awake and alert increase EE by 10%.

• Routine nursing care increases EE by 20 -30%.

Medications that affect REE:

• caffeine, aspirin increase EE

• catecholamines and pressor increase EE

• sedatives,analgesics, beta blockers

* general anesthesia decreases EE
** Acute hyperventilation increases EE, while Hypoventilation decreases EE

Those that increase RQ

• Hyperventilation

• metabolic acidosis leading to increases in carbon dioxide,

• overfeeding leading to lipogenesis

• exercise

Those that decrease RQ

• Hypoventilation

• Mild starvation with ketosis

• Diabetes with ketoacidosis or high rates of urinary glucose lose

• Gluconeogenesis

• ETOH metabolism

• Hypothermia via continued gluconeogenesis

Resting energy expenditure of critically ill patients varies widely over the course of the day and over the course of an illness. Measurements from 10 - 23% of an “average” REE can be seen within a 24 hour period. Test patient at rest in quiet, controlled environment. “Steady state” implies a 5 minute interval where the average V02 and the VC02 changes by less then 10% and the average RQ changes by less than 5%.

• Question validity of the test

• Steady state is not achieved

• RQ falls outside of the physiologic range of 0.67-1.3

• Measurements should fall with in the range of V02 (1.7 to 3.4 mL/min/kg) and VC02 (1.4 to 3.1 mL/min/kg)

• FiO2 >60%

• Air leaks (chest tubes etc).

• Hemodialysis (Co2 loss via the dialysis coil )

Use of the metabolic cart can prevent over-feeding, and under-feeding by accurately measuring energy requirements. Overfeeding critically ill patients results in hyperglycemia, hepatic steatosis, R.E.S. dysfunction and increased septic complications. Under-feeding patients can lead to the complications of malnutrition. The use of a metabolic cart can reduce the amount of “unnecessary” TPN which is administered (use of a metabolic cart reduced TPN use from 33,000 liters to 26,000 liters in one study - Mullen et al., Proc Nutr Sos 1991. 50:239-44).