Basics of parenteral nutrition

Metabolism in critical conditions and therapy objectives

The prescription of intravenous nutritional support is strictly regulated. The main indications are severe conditions where natural feeding is impossible or critically impaired: shock, massive blood loss, and extensive surgical operations.

In such stress situations, metabolism undergoes major changes. The body's energy demand increases sharply. If a deficiency of energy and plastic materials occurs, this directly reduces the reparative capacity of tissues, which is especially dangerous in the early postoperative period.

With pronounced energy deficiency, the body begins to consume not only its own tissue proteins, but also the amino acids or hydrolysates administered from the outside. Instead of going toward building new cells and healing wounds, the protein substrate is "burned" to cover energy needs. This is precisely why transfusion therapy must solve three global tasks simultaneously:

1. Replenish circulating blood volume deficiency.
2. Restore the disrupted water-electrolyte balance.
3. Provide cells with a balanced complex of energy and plastic substrates.

Calculation of baseline requirements

To ensure that nutrition is truly balanced and covers all the needs of an exhausted body, dosages must be calculated precisely.

Minimum daily requirement of an adult patient is calculated based on body weight:

• Energy: 25 kcal per kilogram of weight.
• Protein: 1–1.5 grams per kilogram.
• Fats: 1–2 grams per kilogram.

The total volume of infused fluid usually ranges within 2500–3000 ml per day. For a more precise individual calculation, the formula is applied: 1500 ml per each 1 m² of body surface area. Additionally, 500 ml is added for each degree of body temperature elevation above normal, as fever increases fluid loss.

Important rule: the total caloric content of the administered solutions should roughly correspond to their volume. In the ideal proportion, 1 milliliter of administered fluid should provide the body with 1 calorie of energy.

Components of infusion support

A rational nutritional support regimen necessarily includes crystalloids, Dextrosum (dextrose) solutions, vitamins, and agents for correcting acid-base status (e.g., sodium bicarbonate or trometamol). All macronutrients are divided into energy sources and plastic material.

Energy sources:

• Carbohydrates (dextrose). Require caution when choosing concentration. The administration of isotonic solutions is associated with infusing huge fluid volumes, which threatens hyperhydration. At the same time, highly concentrated solutions create a risk of plasma hyperosmolarity.
• Fat emulsions. Excellent for covering energy deficits, but have a strict feature: their isolated use rapidly leads to ketosis. Fats must be combined with carbohydrates.
• Ethanol (alcohol). Has a high energy value (about 7 calories per 1 gram). Used at a concentration of 5–7% within other infusion media. The daily dose is strictly limited — no more than 50–100 ml.

Plastic material: Protein hydrolysates or amino acid solutions are used for tissue building. It is worth noting that the protein content in hydrolysates is about 5%, whereas in blood plasma or serum it reaches 7.5–9%.

Important nuance: whole blood, native plasma, protein, and albumin are not used as nutrition. They have a very long half-life (from 14 to 30 days) and circulate in the bloodstream as volume replacement agents. They are administered only in urgent need to restore circulating blood volume, not as a building material.

Indicative daily regimen

The daily dose of drugs is divided into two equal parts, each of which is administered intravenously by drip. The infusion process for each portion should be slow and take at least 4–5 hours.

First half of the day:

• 500 ml of 20% dextrose solution.
• 50 ml of 70% ethanol solution.
• 500 ml of protein hydrolysates or amino acids.
• 500 ml of Ringer-Locke solution.
• B vitamins (B₁, B₂) and ascorbic acid.
• If necessary (by indications), potassium chloride and agents for correcting acid-base status are added.

Second half of the day:

• 500 ml of 20% dextrose solution.
• 500 ml of fat emulsion (based on soybean oil and triglycerides).
• 500 ml of protein hydrolysates or amino acids.
• 50–100 ml of 20% albumin solution (or plasma/protein).
• Additional vitamins within the daily allowance.
• To improve protein utilization, anabolic hormones (e.g., nandrolone) may be prescribed.

Briefly: answers to topical questions

What else to study on this topic

• Pathogenesis of protein metabolism disorders in critical and stress states
• Mechanisms of ketosis development during unbalanced parenteral nutrition
• Dangers of plasma hyperosmolarity: clinical picture and prevention
• Role of anabolic hormones (nandrolone) in stimulating reparative processes
• Rules for correcting acid-base balance using trometamol and sodium bicarbonate

Other topics of "CHAPTER 7. BLOOD SUBSTITUTES"

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