Total Parenteral Nutrition (TPN) is a specialized method of providing complete nutritional support directly into the bloodstream when a person’s digestive system is unable to absorb nutrients properly. The solution contains all the necessary components for a full diet, including carbohydrates, proteins, fats, vitamins, and minerals. Because this method bypasses the stomach and intestines entirely, the nutrients are delivered in a highly concentrated liquid form.
Understanding TPN Composition
TPN solutions are manufactured to be nutrient-dense to meet a patient’s full caloric and protein requirements in a manageable fluid volume. The primary components are dextrose (carbohydrates), amino acids (protein), and lipid emulsions (fat), along with a customized mix of electrolytes, trace elements, and vitamins. A defining characteristic of this combined solution is its very high concentration, measured as osmolarity.
Standard TPN formulations are significantly hyperosmolar, often ranging from 1,500 to 2,200 mOsm/L, which is many times greater than that of normal blood. This high concentration is primarily due to the high amounts of dextrose and amino acids. Infusing such a concentrated solution into a small, peripheral vein would cause severe irritation and damage to the vein lining, known as phlebitis or thrombophlebitis. This potential for vein damage determines the administration route.
Central vs. Peripheral Administration
TPN is primarily given through a central line due to the solution’s hyperosmolarity. A central line, or central venous catheter, places the catheter tip into a large central vein, typically the superior vena cava, where blood flow is rapid and voluminous. This blood flow, which can be 2 to 5 liters per minute, provides immediate dilution of the concentrated TPN solution. This rapid dilution prevents the solution from irritating or damaging the vein wall, allowing full nutritional support to be safely delivered for extended periods.
Peripheral Parenteral Nutrition (PPN) is an alternative used only in specific, limited circumstances. PPN is administered through a standard peripheral IV, but the solution’s osmolarity must be kept low, generally limited to 900 mOsm/L or less, to avoid causing phlebitis. Because of this concentration limit, PPN cannot provide a patient’s total nutritional needs and is mainly used as a temporary or supplemental measure, often for no more than 10 to 14 days, until central access is obtained.
Types of Central Access Devices
A central line is a general term for a device that ends in a large central vein. The choice of device depends on the expected duration of the TPN therapy.
Peripherally Inserted Central Catheter (PICC)
For medium-term use, typically weeks to a few months, a PICC is common. A PICC is inserted into a peripheral vein in the arm, such as the basilic or cephalic vein, and the catheter is threaded until the tip rests in the superior vena cava.
Tunneled Catheters
For patients requiring TPN for many months or years, a more permanent device is generally chosen. Tunneled catheters, such as Hickman or Broviac lines, are surgically placed and have a portion tunneled under the skin before entering the vein. This subcutaneous tunnel helps to stabilize the device and provides a barrier against infection, making them suitable for long-term home use.
Implanted Ports
Another long-term option is an implanted port, which is placed entirely under the skin in the chest or arm and accessed with a special non-coring needle. This port is often preferred for intermittent TPN infusions as it leaves no external parts when not in use.
Preventing Complications with Central Lines
Because a central line provides a direct pathway into the bloodstream, the primary risk is a Central Line-Associated Bloodstream Infection (CLABSI). Receiving TPN is recognized as an independent risk factor for CLABSI, making strict preventative measures necessary. Infection prevention focuses on meticulous maintenance of the physical line site and adherence to sterile technique during all line access procedures.
Preventative measures include:
- Performing strict hand hygiene before and after handling any part of the line.
- Using a sterile technique when changing the dressing, which is typically done every seven days or immediately if it becomes dirty, wet, or loosened.
- Monitoring for signs of infection, such as fever, chills, redness, pain, or swelling at the insertion site.
- Daily assessment and prompt removal of the central line when it is no longer necessary.