Ins and outs of fluid therapy (Proceedings)

When is fluid therapy used?

Fluid therapy is used when an animal is unable to maintain normal hydration by eating and drinking, when extra fluid is needed to replace lost extracellular fluid, when blood pressure is low and blood perfusion needs extra support, or when increased fluid volume is needed to promote diuresis.

Examples:

•Unable to maintain normal hydration

• Vomiting patients

• Patients with diarrhea

• Anorexic patients

• Patients unable to or unwilling to drink or eat

•Replacing lost fluids

• Hypovolemic shock

•Support perfusion

• Patients under anesthesia

•Promote diuresis

• Patients with renal failure and uremia

• Patients intoxicated with substances cleared by the kidneys.

What types of fluids can be used?

There are many different types of fluids available, but they fall into two main categories, crystalloids and colloids. Crystalloid fluids contain water and soluble electrolytes. Colloid fluids contain larger molecules that are capable of providing oncotic pressure support to help hold fluid within the intravascular space. Colloids could be used in a patient with hypovolemic shock in which a rapid increase in blood pressure is needed. These types of fluids are given fairly rapidly, and are then fluid therapy is continued with crystalloids. Crystalloids are used far more commonly, and are used for all of the fluid therapy aims listed above. Hetastarch is the most commonly used colloid, but plasma (containing albumin), Dextran, pentastarch, and oxyglobin are other types of colloid fluids.

Crystalloid fluids can be further divided into replacement fluids and maintenance fluids. Replacement fluids are used to restore lost body water and electrolytes. Maintenance fluids are given when the patient is normally hydrated. Replacement fluids are isotonic with normal plasma, and maintenance fluids are hypotonic, containing mostly water. Replacement fluids are often used as maintenance fluids, and this is usually not a problem. The usual difference between the two types of fluids is that replacement fluids contain higher amounts of sodium. Sodium is the predominant electrolyte in extracellular fluid. The normal kidney, and even the disease kidney, is usually more than capable of handling extra sodium loads, but the wisdom of using unnecessary sodium is questionable.

•Most Common Replacement Fluids

• Normal Saline

• 0.9 percent NaCl

• Not a balanced electrolyte fluid

• Can be used in many cases

• Especially useful in hyponatremia

• Could be used when other electrolytes are not wanted

• Hypercalcemia

• Hyperphosphatemia

• Can be used safely in hypovolemic shock

• Acidifying

•Lactated Ringers

• More balanced profile of electrolytes than saline (more closely resembles the balance of sodium, chloride, calcium, phosphorus, and magnesium in plasma.

• Non-acidifying

• Used when sodium is normal

• Normosol-R

• Ringers

•Common Maintenance Fluids (contain less sodium)

• Normosol-M

•Glucose-containing fluids

• D5W Another commonly encountered type of fluid in veterinary medicine is D5W (5 percent dextrose in water). This fluid is isotonic and can be used to maintain local intravascular volume, but it used mainly in patients with hypoglycemia.

• Half-strength saline (0.45 percent) is also used to deliver dextrose in smaller concentrations. This fluid is typically made isotonic by the addition of 2.5 percent dextrose and is used for hypoglycemic patients or patients in which low sodium is required.

Fluid Additives

There are many types of additives used for intravenous fluids the common ones are listed below

•Potassium Chloride

• Used commonly for maintenance fluids in dogs and cats not eating and drinking enough.

• Used for hypokalemia

• Overdose can be lethal. Maximum rate of potassium is 0.5 mEq/kg/hour

• Potassium supplementation is done according to the Modified Sliding Scale of Scott depicted below

•Dextrose

• Used commonly for hypoglycemia

• Mistakenly thought to provide nutrition

•Potassium phosphate

• Useful when hypophosphatemia is a concern (eg. diabetic ketoacidosis)

•Sodium bicarbonate

• Not used as commonly as in the past

• Can cause further sodium overload

• No longer recommended in diabetic ketoacidosis

• Possibly useful in some types of acidosis

•B Vitamins

• Benefit is controversial

• Not thought to be harmful

Types of Catheters

IV catheters are made from a variety of plastic and silicone materials. The most commonly used catheters are short, made of silicone, and are used for short term administration of fluids via peripheral veins. These catheters cannot be left in a vein for more than three days because they can cause phlebitis or, worse, thromboembolism or infection. Catheters designed for more extended use are more expensive, and are longer and usually intended for placement in a central vein (venous catheterization sites are discussed below). These catheters are made of softer materials (eg. softer polyurethanes or silicones) and do not cause inflammatory reactions. I have left these catheters in place for many months in some patients.

Sites of Catheterization

•Peripheral Veins

• Cephalic vein in dogs or in cats

• Lateral saphenous vein in dogs

• Medial saphenous vein in cats.

• Used when rapid access is needed

• Used for temporary fluid therapy

• Easier to place and less expensive

•Central Veins

• Jugular Vein (leading to cranial vena cava

• Saphenous veins (longer catheters can reach the caudal vena cava (or nearly)

• Used for long-term catheters

• Blood samples can be drawn from central lines

• Examples of when to use central venous catheters

• Chronic chemotherapy

• Total parenteral nutrition

• Need for multiple blood samples

• Used when central venous pressure is to be monitored

Rates of fluid delivery

•Maintenance

• Dose: depends on size of animal. Smaller animals need higher maintenance fluids.

• In general: Cats need 45 – 60 ml/kg/day, dogs need 35 – 65 ml/kg/day

•Above Maintenance

• Shock dose: 90 ml/kg for dogs, and 60 ml/kg in cats (for the first hour) Typically a shock dose is calculated and half is given to assess effect

• Anesthesia dose: 10 – 15 ml/kg/hour

•Below Maintenance

• Could be used in a patient with congestive heart failure or other volume overload situation

When to be cautious with fluid therapy

•Heart failure

• Heart failure patients need good perfusion but are in volume overload

• Over-hydration is life-threatening in these patients

• Dehydration is also life-threatening, and patients are usually receiving diuretics

• Oral hydration is often the best solution

•Head trauma

• Patients usually present with trauma and are considered to be in shock

• Shock fluids can be extremely dangerous in these patients because intracranial pressure is already elevated and fluids can cause further elevation

• These patients need to be assessed carefully for signs of increased intracranial pressure, and treated appropriately (eg. mannitol and oxygen therapy)

Subcutaneous fluids

Subcutaneous fluid therapy is used commonly in veterinary medicine. It is most useful for chronic delivery of fluids to a patient at home, and owners can usually be taught to give subcutaneous fluids quite easily. Only isotonic, non-glucose-containing fluids can be used subcutaneously. A common mistake is to give unnecessarily small volumes of fluid subcutaneously. Entire daily fluid requirements can be given in one subcutaneous bolus in dogs and cats. Several cases will be discussed, and fluid therapy problems and questions will be explored in an interactive manner.