Hypovolaemia versus Dehydration

The first step in guiding fluid therapy is determining whether the patient is dehydrated, hypovolaemic or both. Dehydration is a loss of total body water, manifested as decreased skin turgor, tacky mucous membranes, and, if severe, sunken eyes. Hypovolaemia is a decrease in effective circulating vascular volume; clinical signs include tachycardia (although cats may show bradycardia), prolonged capillary refill and poor peripheral pulses. Hypovolaemia can result from substantial loss of body water (along with severe dehydration) or can occur with no change in body water (for example, with vasodilation from anaesthetics or other drugs).

Sodium Content

The second step in choosing fluids is to consider the sodium content of the fluids, which affects the distribution of administered fluids. Fluids with a sodium concentration approximately equal to that of normal plasma (or about 145 mmol/l) will equilibrate between plasma and extracellular tissues. Fluids with a sodium content much lower than plasma (e.g., 5% dextrose) will tend to 'dilute' plasma sodium; this leads to water either entering cells or being eliminated as excess water by the kidneys. The end result is that very little of a sodium-free fluid remains in the vascular space to expand blood volume. Fluids with a sodium content higher than normal plasma (e.g., hypertonic saline) will tend to draw water out of cells and into the vascular and extracellular spaces. This is beneficial for immediate treatment of a patient with a sudden drop in blood pressure, but is undesirable in a dehydrated patient.

Isotonic Crystalloid Fluids

Isotonic fluids are designated either for fluid replacement, with sodium concentrations close to normal plasma sodium (~140 mmol/l), or for fluid maintenance, with sodium concentrations close to normal total body concentrations (~70 mmol/l) (Figure 1). However, only one-third of the given volume of isotonic replacement fluids remains in the vascular space; the other two-thirds enters the interstitial space. Therefore, large volumes of isotonic replacements fluids can lead to peripheral oedema.

Replacement fluids (Ringer's, lactated Ringer's solution (LRS), Normosol-R) also have potassium concentrations similar to those of plasma, and are buffered with lactate, acetate, or gluconate to maintain a physiological pH. Although normal plasma potassium hovers around 4 mmol/l, most of the total body potassium is inside cells, and is not measured in the plasma. Dogs normally lose 15–20 mmol/l of potassium per day in the urine, and when dehydration is present, the kidney will waste even more potassium in an effort to conserve sodium (via aldosterone release). Therefore, replacement fluids need to be supplemented with potassium for longer-term use. This is especially important in cats.

Normal saline (0.9%) is another isotonic replacement fluid, and contains no potassium or buffers. It is higher in sodium and slightly higher in osmolality than Ringer's and related fluids. Like Ringer's, saline also distributes rapidly to the interstitial space, and only about one-third of given fluid remains in the vasculature after administration. Normal saline is a reasonable choice for shock therapy, and is the fluid of choice for hypercalcaemia, since it is calcium-free, and the high sodium content promotes calcium loss in the urine. Saline is also the fluid of choice for hyperkalaemia, since it is potassium-free. Finally, saline is ideal for sodium-depleted states, such as Addison's disease and diabetic ketoacidosis. Unless diabetic patients are not producing urine, 0.9% saline should be supplemented with at least 20–40 mmol/l of potassium chloride.

Because of the high sodium load, 0.9% saline can exacerbate volume overload or salt-sensitive states, such as heart disease, hypertension, liver failure, hyperadrenocorticism and hypoalbuminaemia. In these cases, intravenous saline can rapidly lead to peripheral or pulmonary oedema.

Maintenance fluids are designed for maintenance fluid administration and diuresis in the patient that is not hypovolaemic. While replacement fluids such as Ringer's are commonly used for this purpose, maintenance fluids are designed to replace daily sodium losses without sodium overload. Dextrose is typically included in maintenance fluids, such as Normosol-M, Plasma-Lyte 56® and half-strength saline with 2.5% dextrose, to approximate the tonicity of plasma and prevent haemolysis on intravenous administration. Maintenance fluids are very helpful for patients that cannot tolerate a sodium load. However, these lower-sodium fluids do not stay in the vascular space and are poor at expanding blood volume. They are often combined with colloids in patients with low albumin.

Figure 1. Sodium and potassium content of crystalloid fluids.

Colloids

Colloids are large molecules that remain in the vascular space and 'hold on' to plasma water. Since almost all of administered colloid fluid stays in the vascular compartment, smaller fluid volumes are necessary to expand vascular volume. Colloids are therefore less likely to cause haemodilution, hypoproteinaemia, oedema and fluid overload. Fluids containing colloids will also draw water out of both the intracellular and interstitial spaces to expand plasma volume. This is beneficial in hypovolaemia or oedema states.

Plasma

Plasma is the most physiological colloid. It contains albumin, which exerts oncotic pressure. Albumin is stable for one year or more in frozen plasma, even if it is not frozen immediately. Fresh or fresh frozen plasma also contains active clotting factors, as long as it is used or frozen within 6–8 hours of collection. The shelf-life of coagulation factors in fresh frozen plasma stored in commercial freezer is 1 year. Fresh or fresh frozen plasma is also a source of antithrombin, a natural anticoagulant produced by the liver. Levels of antithrombin are often depleted in patients with liver failure, disseminated intravascular coagulation (DIC) or protein-losing nephropathy, and this can lead to thrombosis or continued consumption of clotting factors.

Plasma, if available, is useful in any hypoalbuminaemic, volume-depleted patient. Best responses are seen in patients with temporarily impaired production of albumin (e.g., acute liver disease) or limited loss of proteins (surgery, trauma). In patients with ongoing loss of protein (e.g., glomerular protein loss or protein-losing enteropathy), it is difficult to give enough plasma to appreciably raise plasma albumin.

Because of its expense, plasma is best targeted for patients that require clotting factors. Plasma can be used just prior to liver or kidney biopsy to provide some albumin, maintain glomerular filtration rate (GFR) and provide antithrombin to help prevent thrombosis. Plasma is often used to treat pancreatitis, where it may help to prevent complications of hypoalbuminaemia and DIC. Finally, plasma is useful for patients with coagulopathies due to DIC or anticoagulant rat poisons. Cats must be blood typed (both donor and recipient) prior to plasma administration; this is not necessary in dogs. Plasma must be thawed prior to use, and is cost prohibitive to give multiple units.

Hetastarch

Hetastarch (hydroxyethylstarch) is an excellent, cost-effective alternative to plasma for patients that do not need clotting factors. Hetastarch is a synthetic high molecular weight starch polymer that exerts oncotic pressure. It is formulated as 6% solution in saline, and has a relatively long duration of action, as its large molecules are slowly degraded by amylase into smaller osmotically active particles. Hetastarch is useful, along with replacement crystalloids, for volume resuscitation of shock. It is also useful for intraoperative or postoperative fluid replacement where blood loss or lavage of body cavities has led to protein loss. Hetastarch is indicated for patients with hypoalbuminaemia, such as those with protein-losing nephropathy or enteropathy, vasculitis or postoperative abdominal surgery. Hetastarch can also provide oncotic pressure in patients with liver disease, pancreatitis or sepsis, when plasma is unavailable.

Hetastarch is given in boluses of 5–10 ml/kg over 5–10 minutes for hypovolaemia, or given as a continuous rate infusion (CRI) of 10–20 ml/kg/day. Like any colloid, Hetastarch can overexpand blood volume if given too rapidly or in large volumes, and is contraindicated in heart failure. Hetastarch may cause restlessness and salivation in some cats.

Dextrans

Dextrans are an alternative colloid composed of large synthetic sugars that exert oncotic pressure. A single infusion of dextrans can increase blood volume and improve haemodynamics for more than 24 hours in humans. Most emergency veterinarians in the US use Hetastarch in preference to dextrans. However, one experimental study in cats with hypovolaemia showed better volume expansion with Dextran 70 compared to Hetastarch. Dextrans can be combined with hypertonic saline (e.g., Dextran 70 plus 7% NaCl) for rapid and sustained resuscitation of shock. Dextran 70 is dosed similarly to Hetastarch and has similar contraindications. Dextrans may interfere with accurate blood cross-matching. Dextran 40, which has smaller molecules than Dextran 70, can also interfere with clotting and can cause sludging in the renal tubules; therefore, Dextran 40 is usually not recommended in dogs and cats.

Subcutaneous Fluids

Both replacement and maintenance fluids can be given subcutaneously as long as they are isotonic. Ringer's is useful for routine subcutaneous fluid administration, while half-strength saline with 2.5% dextrose is ideal for subcutaneous administration in sodium sensitive patients. Up to 30 mmol/l can be added to subcutaneous fluids without discomfort. B vitamins are reported to sting and are not recommended.

Alcohol swabbing does not need to be performed prior to subcutaneous fluid administration, and can cause stinging. Have clients warm fluids in warm water, or use an in-line fluid warmer, to make administration more comfortable. The following web sites have nice client tutorials on subcutaneous fluid administration: http://www.fabcats.org/owners/kidney/subcutaneous/info.html ; http://www.vetmed.wsu.edu/cliented/cat_fluids.aspx