Parenteral nutrition (PN) refers to the provision of nutrients through a route other than the gastrointestinal tract, specifically via intravenous administration. It is a vital therapeutic measure for patients with intestinal failure, significantly improving survival rates and demonstrating clear efficacy in critically ill individuals. Indications for parenteral nutrition include patients who require nutritional therapy but cannot meet their nutritional needs via the gastrointestinal tract or for whom intestinal administration is not feasible.
Components of Parenteral Nutrition Solutions
Parenteral nutrition consists of basic nutrients such as carbohydrates, lipid emulsions, amino acids, water, vitamins, electrolytes, and trace elements. These components provide the energy and nutrients needed daily to sustain normal metabolic processes.
Carbohydrate Preparations
Glucose is the primary energy source in parenteral nutrition, characterized by its abundant availability, low cost, lack of compatibility issues, alignment with physiological requirements, and nitrogen-sparing effects. The general glucose dosage in parenteral nutrition is 3–3.5 g/(kg·d), accounting for approximately 50–60% of total caloric intake. In patients under severe stress, the glucose dosage is reduced to 2–3 g/(kg·d) to avoid metabolic side effects associated with excessive intake.
Amino Acid Preparations
Amino acids serve as the nitrogen source in parenteral nutrition and are substrates necessary for protein synthesis in the body. Since proteins are composed of specific amino acids, the amino acid solutions used in PN should be formulated with a balanced composition to enhance amino acid utilization and promote protein synthesis. An ideal amino acid preparation for PN includes essential amino acids in a well-balanced solution. The recommended amino acid intake in PN is 1.2–1.5 g/(kg·d), with increased requirements during severe catabolic states.
Lipid Emulsions
Lipid emulsions are an ideal energy source in parenteral nutrition, providing energy, carbon atoms for biosynthesis, and essential fatty acids (EFAs). Lipid emulsions are characterized by high energy density, isotonicity, absence of urinary excretion, abundance of EFAs, non-irritating effects on vein walls, and the ability to be infused via peripheral veins. Lipid emulsions generally account for 30–40% of total caloric intake in PN, with a dosage of triglycerides at 0.7–1.3 g/(kg·d). The infusion rate is 1.2–1.7 mg/(kg·min). In patients with hyperlipidemia (serum triglycerides >4.6 mmol/L), the dosage of lipid emulsions should be reduced or discontinued. Commonly used lipid emulsions include long-chain lipid emulsions, medium/long-chain lipid emulsions, lipid emulsions containing olive oil, and lipid emulsions containing fish oil, each with distinct characteristics.
Electrolyte Preparations
Electrolytes play critical roles in maintaining water, electrolyte, and acid-base balance in the body, ensuring the stability of the internal environment, preserving enzyme activity, and supporting the excitability of nerves and muscles.
Vitamin and Trace Element Preparations
Vitamins and trace elements are essential nutrients for sustaining normal metabolic and physiological functions. Water-soluble and fat-soluble vitamins, along with trace elements, need to be added to PN formulations to prevent deficiencies.
Preparation of Parenteral Nutrition Solutions
To optimize the metabolism and utilization of nutrients in the body and reduce complications such as contamination, the preparation of parenteral nutrition solutions often follows the principle of "total nutrient admixture" (TNA). This method involves mixing various nutritional components into a single infusion solution. The preparation process for PN solutions must adhere to strict requirements regarding the preparation environment, aseptic techniques, procedures, and mixing order.
In many hospitals, dedicated intravenous admixture centers have been established to ensure the safety of PN solutions. To maintain the safety and efficacy of admixtures, the addition of other medications to PN solutions is prohibited.
Advancements in technology and materials have led to the development of standardized, industrially produced PN products. These products often feature multiple separated chambers containing amino acid solutions, glucose solutions, and lipid emulsions, separated by membranes to prevent interaction. Before use, the membranes can be broken by manual pressure to allow immediate mixing of components. Standardized multi-chamber PN solutions reduce the need for specialized preparation equipment, simplify the preparation steps, and can be stored for extended periods at room temperature, making them highly promising for clinical applications.
Routes of Parenteral Nutrition Administration
Parenteral nutrition is delivered via central or peripheral vein routes:
Central Venous Route
This route is suitable for patients requiring long-term PN or those needing infusion of hyperosmolar solutions. Commonly used central venous access routes include:
- Internal jugular vein.
- Subclavian vein.
- Peripherally inserted central venous catheter (PICC).
Peripheral Venous Route
This route involves using superficial veins, most often the distal veins of the upper limbs. It is notable for its convenience, high safety, and low risk of complications, making it suitable for patients requiring short-term PN (less than two weeks).
Methods of Parenteral Nutrition Infusion
Parenteral nutrition can be administered using two methods: continuous infusion and cyclic infusion.
Continuous Infusion involves the uninterrupted and uniform delivery of nutritional solutions over a 24-hour period. This method provides a steady supply of nitrogen sources, energy, and other nutrients to the body, thereby exerting minimal impact on metabolism and the internal environment. However, it has the disadvantage of requiring lengthy infusion times, potentially interfering with the patient's daily activities or the administration of other fluids and medications. Continuous infusion is generally recommended for patients who are initiating parenteral nutrition therapy.
Cyclic Infusion shortens the infusion time compared to continuous infusion, making it more convenient for patients engaging in daily activities. However, it has a greater impact on metabolism and poses risks such as hyperglycemia and transient excessive fluid load. Cyclic infusion is typically suitable for patients with stable conditions who require long-term parenteral nutrition and whose nutritional needs remain consistent.
Complications of Parenteral Nutrition and Their Prevention
Complications of parenteral nutrition include catheter-related complications, metabolic complications, organ dysfunction, and metabolic bone disease.
Catheter-Related Complications
Catheter-related complications can be classified into non-infectious and infectious complications:
Non-Infectious Complications occur primarily during the placement of central venous catheters and include pneumothorax, air embolism, and vascular or nerve injury. In rare cases, complications such as catheter dislodgment, breakage, or blockage may result from prolonged use, improper catheter care, or mishandling during removal.
Infectious Complications primarily refer to catheter-related infections (CRI), which are associated with factors such as catheter insertion site, indwelling duration, catheter type, and the patient's underlying condition. These infections may involve localized skin or tissue at the catheter site or systemic bloodstream infections. Catheter-related bloodstream infections or sepsis are severe forms of venous catheter infections, often presenting with symptoms like chills, high fever, tachypnea, and hypotension. Severe cases may lead to altered consciousness and require prompt medical intervention. Peripheral venous complications mainly include thrombophlebitis.
Metabolic Complications
Since nutrients provided through parenteral nutrition are delivered directly into the bloodstream, both excess and insufficient supply of substrates can result in metabolic disturbances and organ dysfunction, leading to complications such as hyperglycemia, hypoglycemia, amino acid metabolism disorders, hyperlipidemia, electrolyte imbalances, acid-base disturbances, essential fatty acid deficiency, refeeding syndrome, and deficiencies in vitamins and trace elements.
Organ Dysfunction
Long-term parenteral nutrition can lead to liver damage, with major pathological changes including hepatic steatosis and cholestasis. These conditions are often associated with factors such as the absence of intestinal stimulation due to prolonged fasting, suppressed secretion of gut hormones, overfeeding, or inappropriate nutrient composition. Furthermore, prolonged fasting can cause atrophy of intestinal mucosal epithelial villi, increased intestinal permeability, and impaired intestinal immune function, resulting in abnormal gut microbiota, bacterial translocation, and enterogenic infections.
Metabolic Bone Disease
Patients receiving long-term parenteral nutrition may develop metabolic bone disease, characterized by clinical manifestations such as calcium loss from bones, osteoporosis, elevated alkaline phosphatase levels in the blood, hypercalcemia, increased urinary calcium excretion, limb joint pain, and even fractures.