Intravenous therapy
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Intravenous therapy or IV therapy is the administration of liquid substances directly into a vein. It can be intermittent or continuous; continuous administration is called an intravenous drip. The word intravenous simply means "within a vein", but is most commonly used to refer to IV therapy.
Compared with other routes of administration, the intravenous route is the fastest way to deliver fluids and medications throughout the body. Some medications, as well as blood transfusions, can only be given intravenously.
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Intravenous access devices
Needle and syringe
The simplest form of intravenous access is a syringe with an attached hollow needle. The needle is inserted through the skin into a vein, and the contents of the syringe are injected through the needle into the bloodstream. This is most easily done with an arm vein, especially one of the metacarpal veins. Usually it is necessary to use a tourniquet first to make the vein bulge; once the needle is in place, it is common to draw back slightly on the syringe to see a blood return, thus verifying that the needle is really in a vein; then the tourniquet is removed before injecting.
This is the most common method of intravenous drug use for illegal substances such as heroin, or in any case where a person must self-administer intravenous medication at home. It is also a convenient way to deliver life-saving medications in an emergency. However, in a controlled health-care setting, direct injection is rarely used since it only allows delivery of a single dose of medication.
Peripheral IV lines
This is the most common intravenous access method in both hospitals and paramedic services. A peripheral IV line consists of a short catheter (a few centimeters long) inserted through the skin into a peripheral vein. A peripheral vein is any vein that is not in the chest or abdomen. Arm and hand veins are typically used although leg and foot veins are occasionally used. Pediatricians sometimes use the scalp veins of infants. Part of the catheter remains outside the skin, with a hub that can be connected to a syringe or an intravenous infusion line, or capped with a bung between treatments. The caliber of cannulas is commonly indicated in gauge, with 14 being a very large cannula (used in resuscitation settings) and 24-26 the smallest.
Blood can be drawn from a peripheral IV if necessary, but only if it is in a relatively large vein and only if the IV is newly inserted.
Originally, a peripheral IV was simply a needle that was taped in place and connected to tubing rather than to a syringe. Today, hospitals use a safer system in which the catheter is a flexible plastic tube that originally contains a needle to allow it to pierce the skin; the needle is then removed and discarded, while the soft catheter stays in the vein.
A peripheral IV cannot be left in the vein indefinitely, because of the risk of insertion-site infection leading to cellulitis and bacteremia. Hospital policies usually dictate that every peripheral IV be replaced (at a different location) every three days to avoid this complication.
Central IV lines
Central IV lines flow through a catheter with its tip within a large vein, usually the superior vena cava, or within the right atrium of the heart. This has several advantages over a peripheral IV:
- It can deliver fluids and medications that would be overly irritating to peripheral veins because of their concentration or chemical composition. These include some chemotherapy drugs and total parenteral nutrition.
- Medications reach the heart immediately, and are quickly distributed to the rest of the body.
- There is room for multiple parallel compartments (lumens) within the catheter, so that multiple medications can be delivered at once even if they would not be chemically compatible within a single tube.
- Carers can measure central venous pressure and other physiological variables through the line.
Central IV lines also carry higher risks of bleeding, bacteremia, and gas embolism (see Risks below).
There are several types of central IVs, depending on the route that the catheter takes from the outside of the body to the vein.
Peripherally inserted central catheter (PICC)
A PICC line is inserted into a peripheral vein, usually in the arm, and then carefully advanced upward until the catheter is in the superior vena cava or the right atrium. This is usually done by feel and estimation; an X-ray then verifies that the tip is in the right place.
A PICC may have two parallel compartments, each with its own external connector (double-lumen), or a single tube and connector (single-lumen). From the outside, a single-lumen PICC resembles a peripheral IV, except that the tubing is slightly wider.
The insertion site must be covered by a larger sterile dressing than would be required for a peripheral IV, due to the higher risk of infection if bacteria travel up the catheter. However, a PICC poses less of a systemic infection risk than other central IVs, because bacteria would have to travel up the entire length of the narrow catheter before spreading through the bloodstream.
The chief advantage of a PICC over other types of central lines is that it is easy to insert, poses a relatively low risk of bleeding, is externally unobtrusive, and can be left in place for months to years for patients who require extended treatment. The chief disadvantage is that it must travel through a relatively small peripheral vein and is therefore limited in diameter, and also somewhat vulnerable to occlusion or damage from movement or squeezing of the arm.
Central venous lines
There are several types of catheters that take a more direct route into central veins. These are collectively called central venous lines.
In the simplest type of central venous access, a catheter is inserted into a subclavian, internal jugular, or (less commonly) a femoral vein and advanced toward the heart until it reaches the superior vena cava or right atrium. Because all of these veins are larger than peripheral veins, central lines can deliver a higher volume of fluid and can have multiple lumens.
Another type of central line, called a Hickman line or Broviac catheter, is inserted into the target vein and then "tunneled" under the skin to emerge a short distance away. This reduces the risk of infection, since bacteria from the skin surface are not able to travel directly into the vein; these catheters are also made of materials that resist infection and clotting.
Implantable ports
A port (often referred to by brand names such as Port-a-Cath or MediPort) is a central venous line that does not have an external connector; instead, it has a small reservoir implanted under the skin. Medication is administered intermittently by placing a small needle through the skin into the reservoir. Ports cause less inconvenience and have a lower risk of infection than PICCs, and are therefore commonly used for patients on long-term intermittent treatment.
Forms of intravenous therapy
Intravenous drip
An intravenous drip is the continuous infusion of fluids, with or without medications, through an IV access device. This may be to correct dehydration or an electrolyte imbalance, to deliver medications, or for blood transfusion.
IV fluids
There are two types of fluids that are used for intravenous drips: crystalloids and colloids. Crystalloids are aqueous solutions of mineral salts or other water-soluble molecules. Colloids contain larger insoluble molecules, such as gelatin; blood itself is a colloid.
The most commonly used crystalloid fluid is normal saline, a solution of sodium chloride at 0.9% concentration, which is close to the concentration in the blood (isotonic). Ringer's solution or Ringer's lactate is another isotonic solution often used for large-volume fluid replacement. A solution of 5% dextrose in water, sometimes called D5W, is often used instead if the patient is at risk for having low blood sugar or high sodium. The choice of fluids may also depend on the chemical properties of the medications being given.
Intravenous fluids must always be sterile.
Composition of Common Crystalloid Solutions
| Solution | Other Name | [Na+] | [Cl-] | [Glucose] |
|---|---|---|---|---|
| D5W | 5% Dextrose | 0 | 0 | 252 |
| 2/3 & 1/3 | 3.3% Dextrose / 0.3% saline | 51 | 51 | 168 |
| Half-normal saline | 0.45% NaCl | 77 | 77 | 0 |
| Normal saline | 0.9% NaCl | 154 | 154 | 0 |
| Ringer's lactate | Ringer's solution | 130 | 109 | 0 |
Ringer's lactate also has 28 mmol/L lactate, 4 mmol/L K+ and 3 mmol/L Ca2+.
Effect of Adding One Litre
| Solution | Change in ECF | Change in ICF |
|---|---|---|
| D5W | 333 mL | 667 mL |
| 2/3 & 1/3 | 555 mL | 444 mL |
| Half-normal saline | 667 mL | 333 mL |
| Normal saline | 1000 mL | 0 mL |
| Ringer's lactate | 900 mL | 100 mL |
Infusion equipment
A standard IV infusion set consists of a pre-filled, sterile container (glass bottle, plastic bottle or plastic bag) of fluids with an attached drip chamber which allows the fluid to flow one drop at a time, making it easy to see the flow rate (and also reducing air bubbles); a long sterile tube with a clamp to regulate or stop the flow; a connector to attach to the access device; and connectors to allow "piggybacking" of another infusion set onto the same line, e.g., adding a dose of antibiotics to a continuous fluid drip.
An infusion pump allows precise control over the flow rate and total amount delivered, but in cases where a change in the flow rate would not have serious consequences, or if pumps are not available, the drip is often left to flow simply by placing the bag above the level of the patient and using the clamp to regulate the rate; this is a gravity drip.
In the absence of a pump, if the patient requires a high flow rate and the IV access device is of a large enough diameter to accommodate it, an inflatable cuff can be placed around the fluid bag to force the fluid into the patient.
Intermittent infusion
Intermittent infusion is used when a patient requires medications only at certain times, and does not require additional fluid. It can use the same techniques as an intravenous drip (pump or gravity drip), but after the complete dose of medication has been given, the tubing is disconnected from the IV access device. Some medications are also given by IV push, meaning that a syringe is connected to the IV access device and the medication is injected directly (slowly, if it might irritate the vein or cause a too-rapid effect).
Risks of intravenous therapy
Intravenous therapy has many risks and should therefore only be performed by trained personnel under medical supervision, using proper equipment.
Infection
Any break in the skin carries a risk of infection. Although IV insertion is a sterile procedure, skin-dwelling organisms such as Staphylococcus aureus or Candida albicans may enter through the insertion site around the catheter, or bacteria may be accidentally introduced inside the catheter from contaminated equipment.
Infection of IV sites is usually local, causing easily visible swelling, redness, and fever. If bacteria do not remain in one area but spread through the bloodstream, the infection is called septicemia and can be rapid and life-threatening. An infected central IV poses a higher risk of septicemia, as it can deliver bacteria directly into the central circulation.
Phlebitis
Phlebitis is irritation of a vein that is not caused by infection, but from the mere presence of a foreign body (the IV catheter) or the fluids or medication being given. Symptoms are swelling, pain, and redness around the vein. It does not necessarily mean the IV device must be removed; warmth, elevation of the affected limb, or a change in the rate of flow may resolve the symptoms.
Due to frequent injections and recurring phlebitis, the peripheral veins of intravenous drug addicts, and of cancer patients undergoing chemotherapy, become hardened and difficult to access over time.
Infiltration
This occurs when the tip of the IV catheter withdraws from the vein or pokes through the vein into surrounding tissue, or when the vein's wall becomes permeable and leaks fluid. It occurs frequently with peripheral IVs, and requires replacement of the IV at a different location. The symptoms of pain and swelling are temporary and not dangerous, unless a highly irritating medication was being given.
Fluid overload
This occurs when fluids are given at a higher rate or in a larger volume than the system can absorb or excrete. Possible consequences include hypertension, heart failure, and pulmonary edema.
Electrolyte imbalance
Administering a too-dilute or too-concentrated solution can disrupt the patient's balance of sodium, potassium, and other electrolytes. Hospital patients usually receive blood tests to monitor these levels.
Embolism
A blood clot or other solid mass, or an air bubble, can be delivered into the circulation through an IV and end up blocking a vessel; this is called embolism. Peripheral IVs have a low risk of embolism, since large solid masses cannot travel through a narrow catheter, and it is nearly impossible to inject air through a peripheral IV at a dangerous rate. The risk is greater with a central IV.
Air bubbles of less than 30 milliliters generally dissolve into the circulation harmlessly. A larger amount of air, if delivered all at once, can cause life-threatening damage to pulmonary circulation, or, if extremely large (3-8 milliliters per kilogram of body weight), can stop the heart.
One reason veins are preferred for intravenous administration is because the flow will pass through the lungs before passing through the body. Air bubbles can leave the blood through the lungs. A patient with a heart defect causing a right-to-left shunt is vulnerable to embolism from smaller amounts of air.
Fatality by air embolism is vanishingly rare, in part because it is also difficult to diagnose. The few cases that are known to have stopped the heart occurred not by clinical intravenous therapy, but by self-adminstration of vaginal powders using air-bulb insufflators, when the vagina had severe bleeding.