Fluid Therapy for Dogs and Cats

Chapter 5 Fluid Therapy for Dogs and Cats

Shane W. Bateman, Dennis J. Chew

Fluid therapy can be the single most important therapeutic measure used in seriously ill animals. Effective administration of fluids requires an understanding of fluid and electrolyte dynamics in both healthy and sick animals. An overview of a general approach to fluid therapy decision making can be found in Figure 5-1.

Figure 5-1 General decision tree for fluid therapy.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Finco DR: A scheme for fluid therapy in the dog and cat. J Am Anim Hosp Assoc 8:178–180, 1972.)

DECISION-MAKING CHECKLIST

Answer these questions to decide when fluid therapy is needed:

• Does the patient have a form of shock that requires fluid resuscitation?

• Does a serious deficit (or excess) of fluid or water exist?

• Is there an anticipated loss of substantial fluid volume?

• Are there serious electrolyte disturbances (excesses or deficits)?

• Is there a serious disturbance in the acid-base balance?

• Is there a serious disturbance in oncotic pressure or excessive protein loss?

• Is there an immediate need for full nutritional support with calories and protein?

• Does the patient have an anemia that may be affecting oxygen delivery?

• Does the patient have a coagulopathy?

Answer these questions in order to provide appropriate fluid therapy:

• What type(s) of fluids will be given?

• What volume of fluids will be given?

• How fast will fluids be given?

• Through what route will fluids be given?

• What, if any, supplements will be added to commercially available fluids?

• If more than one fluid type is being given, can they be administered together?

• If medications are to be administered with fluids, are they compatible?

If no laboratory testing is immediately available, start correction of dehydration with isotonic-balanced electrolyte solutions.

DISTRIBUTION OF BODY WATER AND ELECTROLYTES

Electrolytes

• Sodium and chloride exist in high concentration in ECF and low concentration in ICF.

• Potassium, magnesium, and phosphorus exist in high concentration in ICF and low concentration in ECF.

MAINTENANCE REQUIREMENTS

Maintenance is defined as the volume of fluid (ml) and the amount of electrolyte (mEq or mg) that must be taken in on a daily basis to keep the volume of TBW and the electrolyte content normal. Obligatory losses of water and electrolytes occur daily as a consequence of normal metabolism. Water taken into the body in all of its forms is equal to the loss of water in the normal animal. See Figure 5-2 for specific maintenance requirements of electrolytes and water in caged dogs and cats that have normal food intake.

• Water can be taken in by drinking, combined mechanically with food, or derived from metabolism of food or tissue breakdown.

• Water is lost from the body through evaporation during breathing, from feces, and from urine.

• Obligatory loss of electrolytes occurs in fecal water and in urine, but the overall loss of electrolytes is proportionally less than the loss of water.

• In most situations, the kidneys conserve sodium and excrete excess amounts of potassium, resulting in urine that typically has a much lower concentration of sodium and a higher concentration of potassium compared to plasma.

• The volume of fluid required daily varies with the size of the animal. Large patients (with low mass-to-body surface area ratios) require less fluid per kilogram on an individual basis than small animals (with high mass-to-body surface area ratios), as shown in Table 5-1 for dogs and Table 5-2 for cats.

• Maintenance volume is comprised of two subcomponents:

• Insensible losses (not readily measured losses from respiratory evaporation and the passage of normal feces) are estimated at 20 ml/kg/day in normal animals but can increase during febrile states, panting, and high environmental temperatures.

• Sensible losses (readily measured as urine production) are estimated to be 20 to 40 ml/kg/day in normal animals that are consuming food.

• Urine output can dramatically decrease during:

• Acute intrinsic renal failure—Oliguria or anuria

• Urinary obstruction—Anuria

• Severe dehydration—Oliguria

• Urine output can dramatically increase during:

• Polyuric renal failure

• Post-obstructive diuresis

• Consider maintenance fluid therapy for hospitalized patients who are not eating and drinking on their own but are not dehydrated. Maintenance fluids should be low in sodium, chloride, and osmolality and high in potassium compared to normal plasma.

Figure 5-2 Maintenance fluid and electrolyte requirements for caged normal dogs and cats.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Finco after Harrison JB: J Am Anim Hosp Assoc 8:179, 1972.)

Table 5-1 DAILY FLUID REQUIREMENTS FOR HOSPITALIZED NON-ANORECTIC DOGS^*

Table 5-2 DAILY FLUID REQUIREMENTS FOR HOSPITALIZED NON-ANORECTIC CATS^*

Lactated Ringer’s and 0.9% sodium chloride solutions are not ideal maintenance solutions because they contain too much sodium and chloride, are too high in osmolality, and do not contain enough potassium.

A 0.45% sodium chloride solution (in water or in 2.5% dextrose) is more suitable as a maintenance solution due to its sodium and chloride content and its osmolality. It does not contain adequate potassium; therefore, potassium must be added.

DEHYDRATION (REPLACEMENT NEEDS)

Dehydration exists when TBW decreases to less than normal.

• Technically, dehydration refers to loss of pure water, but clinical fluid loss is usually accompanied by some loss of electrolytes.

• Acute fluid and electrolyte loss in any disease process is initially lost from intravascular fluid.

• Compensatory shifts of water and electrolytes from intracellular and interstitial compartments subsequently occur.

• The magnitude of these shifts depends on the tonicity and hydrostatic pressure of the remaining extracellular fluid. Consequently, dehydration can occur to different degrees in the various compartments.

Characterization of Dehydration (Type)

Disease processes can display a spectrum of fluid and electrolyte loss combinations, from mostly water loss (hypotonic loss) to water loss with significant quantities of accompanying electrolytes (isotonic or hypertonic). Evaluation of the tonicity and sodium concentration of the extracellular fluid in a dehydrated patient will give clues to the nature of the fluid that was lost and helps determine the type of fluid that will be given as replacement during treatment.

The type of dehydration is defined based on the serum sodium concentration at the time of dehydration.

• Isotonic dehydration is the type that occurs most commonly. It is defined by finding a normal serum sodium concentration (145–157 mEq/L) in the presence of dehydration. It occurs because of loss of water and electrolytes in proportion to that found in normal serum (isotonic loss).

• Hypertonic dehydration is the next most common type. It is defined by finding an elevated serum sodium concentration (158 mEq/L or greater) in the presence of dehydration. It occurs as a consequence of predominantly water loss or water lost in excess of solute found in normal serum (hypotonic loss).

• Hypotonic dehydration is the least common type. It is defined by finding a low serum sodium concentration (143 mEq/L or less) in the presence of dehydration. It theoretically occurs as solute is lost in excess of the concentration found in normal serum (hypertonic loss), but this is probably not the most significant mechanism. More likely, it is the loss of isotonic fluid and concurrent intake and absorption of hypotonic fluids (such as drinking of water) with the net dilutional effect on the remaining extracellular sodium concentration below normal.

Detection of Dehydration

Clinical tools to detect dehydration are limited in both sensitivity and specificity. There is no single test or procedure to accurately assess the magnitude of dehydration. Integration of historical findings, abnormalities on physical examination, and laboratory measurements will be necessary to quantify dehydration. Dehydration is not detectable by clinical means until approximately 5% of body weight in water has been lost. An acute loss of greater than 12% body weight in water is considered life threatening (Table 5-3).

Table 5-3 PERCENTAGES OF DETECTABLE DEHYDRATION

Dehydration	Signs
<5%	Not detectable on physical exam; history is suggestive of losses; acute body weight changes
5%	Subtle loss of skin elasticity
6–8%	Mild delay of skin tent, slight prolongation of CRT, dry mucous membranes
8–10%	Obvious delay of skin tent, slight prolongation of CRT, dry mucous membranes, eyes slightly sunken in orbits
10–12%	Severe prolongation of skin tent, eyes sunken in orbits, dry mucous membranes, signs of shock likely present (prolonged CRT, tachycardia, weak pulses etc.)
>12%	Moribund

^*CRT, capillary refill time.

History

History often leads the clinician to suspect dehydration and to assess its magnitude more accurately. Question the owner about volume of intake (adipsia, hypodipsia, polydipsia, or normal intake of water). Because volume of water intake may, in part, be a function stimulated by food intake, note also the presence or absence of anorexia. Abnormal losses of body fluid may be determined from owner responses to questions about vomiting, diarrhea, polyuria, panting, excessive salivation, or other bodily discharge. The duration of these historical signs and the magnitude of losses affect the magnitude of clinically detectable dehydration.

Physical Examination

Physical examination provides general guidelines for detecting dehydration but is subjective (see Table 5-3). Signs of listlessness and depression may occur from dehydration but may be partially attributable to the underlying disease or to concomitant electrolyte and acid-base abnormalities. As dehydration becomes more severe, decreased skin turgor, sunken eyes, dryness of mucous membranes, tachycardia, diminished capillary refill, and signs of shock may occur. An accurate and recent body weight, when available, can be used for comparison to evaluate change in body weight as an indicator of body water change.

An acute increase or decrease in an animal’s body weight often reflects acute gain or loss of body water. This is the most sensitive clinical tool for assessment of dehydration and rehydration. An acute loss or gain of 1 kg is the equivalent of losing or gaining 1 L of fluid.

Skin Turgor

Skin turgor assessment during physical examination is important for estimating the percentage of body weight loss due to dehydration. Skin turgor is evaluated by determining the time required for skin gently lifted from the body to return to its original position (referred to as the skin pinch or skin tent). Normal skin pliability (skin turgor) depends on hydration of the tissues in the area tested. Skin turgor is largely determined by hydration status of the interstitial tissues, although both vascular and intracellular hydration also contribute. Elastin and adipose within skin and subcutaneous tissues will also influence the apparent skin turgor. Choose skin from the trunk as a test area. Avoid dependent areas and skin from the neck. Normal skin returns immediately to its initial position when lifted a short distance and released. Dehydrated skin shows varying degrees of slow return to the original position. As dehydration progresses, the time required for the return of the skin pinch to its initial position becomes greater. The clinician assigns increasing percentages of dehydration to abnormal skin turgor of increasing severity (see Table 5-3).

Skin Turgor Artifacts

Many artifacts confuse interpretation of skin turgor.

• Skin turgor of obese animals may appear normal despite dehydration, owing to the large amounts of subcutaneous fat.

• The skin of an emaciated animal with normal hydration may fail to return to its normal position owing to a lack of subcutaneous fat and elastic tissue. Consequently, underestimating dehydration in obese animals and overestimating dehydration in emaciated animals can occur.

• Avoid testing cervical skin because redundant skin in the neck area confuses the result.

• Skin turgor changes in longhaired animals are more difficult to detect than those in shorthaired animals.

• Differences in turgor assessment can occur in the same animal in the standing versus recumbent positions.

Dehydration may be as much as 5% of body weight loss in lean dogs and 10% or more in obese dogs before loss of skin turgor is detected.

Laboratory Assessment of Dehydration

Packed Cell Volume and Total Plasma Protein

Simple laboratory testing is helpful in evaluation of intravascular hydration. Packed cell volume (PCV) recorded in percentages (SI unit: L/L) and total plasma protein (TPP) recorded in gm/dl (SI unit: g/L) can be rapidly and inexpensively determined using microhematocrit tubes and a refractometer. These two tests require only a few drops of blood and can be taken by capillary action from a 25-gauge venipuncture. TPP concentration may be more helpful in the detection of dehydration than PCV. Increased TPP and PCV provide documentation for intravascular dehydration. Simultaneous evaluation of PCV and TPP is recommended in order to minimize interpretation errors due to pre-existing anemia or hypoproteinemia. (Table 5-4) Additional value is obtained when PCV and TPP are followed serially as increasing values identify progressive dehydration.

Table 5-4 INTERPRETATION OF CHANGES IN PACKED CELL VOLUME (PCV) AND TOTAL PLASMA PROTEIN (TP)

PCV	Total Protein	Possible Interpretation
↑	↑	Dehydration
↑	N or ↓	Splenic contraction
		Erythrocytosis
		Hypoproteinemia with dehydration
N	↑	Hyperproteinemia
		Anemia with dehydration
		Hypertonic dehydration (RBC shrinkage)
↓	↑	Anemia with dehydration
↓	↑	Anemia with pre-existing hyperproteinemia
↓	N	Non-blood-loss anemia, normal hydration
N	N	Normal hydration
		Dehydration, after secondary compartment shift
		Dehydration with pre-existing anemia + hypoproteinemia
		Acute hemorrhage
↓	↓	Blood loss anemia
↓	↓	Overhydration

Urinalysis

Urinalysis (UA) is important in all cases of suspected dehydration. An elevated specific gravity (SG) represents the healthy kidneys’ response to decreased perfusion. The finding of dilute urine (<1.030 SG) from a dehydrated animal immediately incriminates the kidneys as a major cause of, or contributor to, the dehydration.

Serum Biochemistry

Evaluation of serum electrolytes may help characterize the nature of the fluid that was lost.

Blood Gases

Abnormalities in blood-gas values may appear in dehydrated animals owing to net loss or gain of acid or buffer. The most common acid-base disturbance in dehydrated patients is metabolic acidosis that typically develops because of a decrease in urine production and an impaired ability of the kidneys to excrete the daily acid burden. In addition, moderate to severe forms of dehydration are accompanied by decreased perfusion to major tissue beds, resulting in a switch to anaerobic metabolism and the production of lactic acid.

Animals may be severely dehydrated yet exhibit little or no change in their serum biochemical test results. Likewise, their PCV or TPP can be normal in the face of dehydration.

Anticipation of Dehydration with Specific Diseases

Dehydration should be anticipated in sick animals with certain disease syndromes known to predispose a patient to dehydration regardless of physical exam or laboratory findings. For example, a collapsed diabetic is very likely to be dehydrated, as is an animal with advanced chronic renal failure or a patient with upper intestinal obstruction.

Correction (Replacement) of Dehydration

The volume of fluid to be replaced is calculated as follows based on the assessed percentage of dehydration and the patient’s present body weight:

• % dehydration × weight (kg) = L of fluid to be replaced.

• % dehydration × weight (lbs) × 500 = ml of fluid to be replaced.

• Alternatively, if a known recent body weight is available for comparison, replace 1 L of fluid for every kilogram (500 ml/lb) of acute body weight loss.

• The type of fluid that is chosen to accomplish this rehydration is based primarily on the patient’s ability to tolerate a sodium challenge, the serum electrolyte (sodium, chloride, and potassium) concentrations, and the presence of any acid-base disturbances.

Initial replacement with adequate fluid volume is usually more important than correcting minor electrolyte or acid-base disorders.

• Administer to the patient both the volume and the type of fluid that has been lost or continues to be lost from the body. Base the volume calculation on volumetric measurement of the losses or, more practically, from estimates. Base the type of the fluid on actual measurements from the patient or, more commonly, on the predicted biologic composition of fluid losses associated with a specific disease.

Choose a fluid type or add supplements to stock fluids so that electrolytes deficient in the plasma or body are administered and those that are excessive in the plasma are avoided.

Contemporary (Ongoing) Losses

Although initiation of fluid therapy is aimed at replacing fluid lost from the patient, persistence of uncontrolled signs or untreated diseases will permit the dehydrating process to continue. Consider replacement of contemporary or ongoing losses that begin after fluid therapy has begun. Estimate gastrointestinal fluid loss (e.g., from vomiting or diarrhea). When ongoing loss is from the urinary tract, temporary urethral catheter collection may increase the accuracy of estimating the magnitude of the loss. Less invasive but feasible methods of estimation include collecting urine in a container (free catch from dogs or litter box from cats) and measuring the volume or weight change of the container.

Volume To Be Administered

The initial volume of fluids to be administered over the first 24 hours can be calculated (Figure 5-3). The multiples of maintenance volumes method is less accurate but is acceptable for routine use in uncomplicated cases of simple dehydration (Table 5-5).

Figure 5-3 Two methods of calculating fluid needs.

Table 5-5 ALTERNATIVE METHOD TO CALCULATE MAINTENANCE + DEHYDRATION NEEDS^*^†

FLUID THERAPY FOR SHOCK

See Chapter 156 for a discussion of shock treatment.

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