Nitrogen requirement is evaluated by measuring daily nitrogen loss corrected for body urea pool variation according to the following equations: males: urea(g/24h)+[D(blood urea(g/1/day)) x body weight (kg) x 0.50] + 3 females: urea(g/24h)+[D(blood urea(g/1/day)) x body weight (kg) x 0.60] + 3
The determination of basal nitrogen loss must be performed during a 24-h fast, owing to the mandatory contribution of protein amino acid intake to the urea production rate (about 30-40 per cent of the supply (Iapichino etaf 1984)). The value of the nitrogen loss represents the need to maintain the actual lean body mass, i.e. the nitrogen balance, which is a reasonable goal for non-depleted injured patients in the acute stress phase.
In malnourished patients, nitrogen support could be increased to replace body protein stores. Energy-nitrogen relationship
Although nitrogen excretion and retention are affected by calorie intake in both stressed well-nourished and non-stressed malnourished patients, nitrogen intake is the main determinant of nitrogen balance (Iapichino etal 1984). In both classes of patients the effect of calorie intake is comparable and accounts for a nitrogen retention of about 1 mg/kg/kcal/day. Also, the effect of nitrogen intake is comparable and results in the retention of about 60 per cent of nitrogen supply.
It is easier to obtain nitrogen balance in malnutrition because malnourished patients show nitrogen losses that are consistently less than those of stressed patients. The simple replacement of fasting nitrogen losses leads to a negative nitrogen balance owing to incomplete utilization of the load. Depending on the patient's metabolic environment (stress or malnutrition), nitrogen clearance ability (renal and hepatic function), and planned nitrogen balance (replacement or maintenance) it may be necessary to add nitrogen to fasting losses. Even in the most favorable conditions (non-stressed depleted patient), it is unlikely that a nitrogen balance of more than 2 to 3 g/day will be obtained.
A rough estimate of 0.11 to 0.3 g/kg/day nitrogen supply is reasonably adequate in clinical practice. Timing
Critically ill patients show daily negative cumulative energy and protein balance (1-2 per cent of body cell mass). The aim of treatment is to provide early support, preventing tissue wasting and nutritional deficit rather than correcting them later. After the start of treatment, cell energetics (sodium pump and calcium kinetics) rapidly recover, resulting in an immediate improvement in cell function (W!lm.oi®.§0d Carpentier..1993).
Obviously, it is mandatory to start treatment when hemodynamic, water, electrolyte, and acid-base balances reach equilibrium.
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