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This process
determines the specific needs for energy, protein, micronutrients
and water. |
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Determination of Caloric (Energy) Requirements
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Caloric
requirements can be measured directly using indirect calorimetry
techniques. This approach is usually not used outside of a critical
care unit. Instead, a number of formulas to derive needs based on
the degree of injury are used. Since over 95 percent of the energy
generated requires oxygen, there is a direct proportion between
oxygen consumption and the metabolic rate. The increase in VO2
from the normal valve can be used to calculate the increase in the
metabolic rate and the need for calories. One simple and accurate
system that can be used is to estimate caloric needs according to
the following formula:
Energy
requirements = BMR x activity level (usually 1.25) x stress factor. |
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The energy required is dependent on the energy expenditure, which in
turn has three components: the basal metabolic rate, muscle
activity, and stress induced energy needs. |
Basal
Metabolic Rate
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The term basal metabolic rate describes the energy required to
maintain cell integrity in the resting state and at
thermo-neutrality. The latter term means an ambient temperature,
usually close to 80° F (28°C) at which the heat loss and the need
for increased heat production to maintain the body temperature are
minimal.
The BMR for man has been defined by direct calorimetry. Body size
is a principal factor, and the value is therefore expressed in terms
of square meters of body surface or body weight. Other variables
include age and, to a lesser extent, sex. A significant portion of
this energy goes into thermo-genesis, i.e. the maintenance of normal
body temperature through heat production. The average adult BMR
value is 1500 to 1800 kilocalories per day (850 to 950 kilocalories
per sq m per day or 20 to 24 kilocalories per kg per day |
Activity
Level
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The activity level relates to the energy utilized by muscles for
average daily activity. In the ICU patient activity is usually
limited and is usually considered to be no more than an additional
25 percent of the BMR. Excessive muscle activity from an excessive
work of breathing, or as a result of a combative disoriented state,
markedly increases the energy expenditure. |
Stress
Induced Energy Needs
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The third component is the stress factor, which defines the
hypermetabolism induced by surgical illness. The stress factor is a
multiple of the BMR (normalized to 1.0). The value takes into
consideration the type of injury and the average metabolic response
with this degree of injury. Burn injury leads to the greatest
increase in metabolic demands. |
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Effect of Injury on
Metabolic Demands |
| Stress Factor |
Increase Above Basal
(%) |
| Starvation |
-10-0 = 0.9 |
| Elective Operation |
0-10 = 1.1 |
| Peritonitis, major
infection |
25 = 1.25 |
| Long Bone fracture |
25 = 1.25 |
| Multiple blunt trauma |
50 = 1.5 |
| Thermal Injury |
|
| 10% BSA |
25 = 1.25 |
| 20% BSA |
50 = 1.5 |
| 30% BSA |
50 = 1.5 |
| 40% BSA |
75 = 1.75 |
| 50% BSA |
100 = 2.0 |
| Smoke Inhalation |
50 = 1.5 |
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A significant
portion of the increased calorie need is the result of the increase
in resting body temperature seen in injured patients, thought to be
due to an upward resenting of the hypothalamic
temperature-regulating center. This response appears to be due to
circulating hormones, such as the catecholamines. The increase in
circulating catecholamines also increases the metabolic rate by
further stimulating cell metabolism. Additional stress factors
increase metabolism primarily by a further increase in
catecholamines and other stress hormones.
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Additional
stress insults exclude hyperthermia; increased heat loss
necessitating increased heat production, pain, and anxiety, and
sleep deprivation. |
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Stress |
Stress Factors |
| Minor Injury |
1.2 |
| Minor
Surgery |
1.2 |
| Clean Wound |
1.2 |
| Bone Fracture |
1.3 |
| Major Surgery |
1.3 |
| Infected Wound |
1.3 |
| Major Trauma |
1.5 |
| Major Infection |
1.5 |
| Severe Burn |
2.0 |
| Combination |
2.0 |
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Another
formula described is the Harris Benedict Formula. |
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HARRIS-BENEDICT EQUATION FOR CALCULATIONS RESTING ENERGY EXPENDITURE (REE)
Harris-Benedict Equation
Females: REE = 65 + [4.3 x Wt
(lbs)] + [4.3 x Ht (in.)] - [4.7 x age]
Males: REE = 65
+ [6.2 x Wt (lbs)] + [12.7 x Ht (in.)] - [6.8 x age] |
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Indirect Calorimetry |
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The reference
standard for measuring energy expenditure in the clinical setting is
indirect calorimetry. Indirect calorimetry is a technique that
measures oxygen consumption and carbon dioxide production to
calculate resting energy expenditure and respiratory quotient (RQ).
One liter of oxygen consumed, generates 3.8 kcal (16.32 kJ): one
liter of carbon dioxide produced, generates 1.1 kcal (4.60 kJ).
Indirect calorimetry allows the precise measurement of the daily
caloric expenditure. Use of a stress factor to account for injury
is not necessary because the measured energy expenditure accounts
for the effects of disease state, stress and trauma. However,
because the measurement occurs at rest, it is necessary to multiply
by an “activity” factor of 1.0 to 1.3, depending on whether the
patient is intubated, at bed rest, or ambulatory. In addition,
indirect calorimetric data provides information as to the relative
contributions of carbohydrate, fat, and protein that are being
oxidized for energy by monitoring the RQ or respiratory quotient.
Caloric
requirements in the stressed patient, or the patient with
involuntary weight loss, is about 50% above basal levels (20 cal/kg)
or about 30 cal/kg/day.
Inadequate
nutrition is reflected in continued weight loss or continuing
evidence of impaired lean body mass. Excess calories can be
reflected in increasing body weight, mainly fat, above the normal
level. Glucose intolerance is often a clue to excess glucose
intake. Carbon dioxide product also increases as does the
respiratory quotient. |
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