Fluid Therapy for Dogs and Cats

Chapter 5 Fluid Therapy for Dogs and Cats

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.


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.


Dehydration exists when TBW decreases to less than normal.

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.

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).


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.

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.

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.


PCV Total Protein Possible Interpretation
N or ↓ Splenic contraction
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

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:


See Chapter 156 for a discussion of shock treatment.

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Fluid Therapy for Dogs and Cats

Full access? Get Clinical Tree

Get Clinical Tree app for offline access