The water content of the adult body is, on average, 60% by weight. However, the water content can vary with age and nutritional status. For example, the water content of an older animal may be as little as 50–55%, whilst the body water content of young animals may be as high as 70–80%.
The body water content is also affected by the proportion of fat to lean tissue within the body. Fatty tissue contains much smaller amounts of water than other organs and tissue. This is an important consideration (to avoid overhydration) when devising a fluid therapy plan for the obese patient; the plan should be based on the patient’s ideal weight, not current weight.
Distribution of Body Water
- Intracellular fluid (ICF): Two-thirds of the total body water is located inside the cells of tissues; this is called intracellular fluid.
- Extracellular fluid (ECF): The remaining one-third of the total body water is located outside the cells. This is further divided into:
- Intravascular fluid: The water contained within the blood vessels.
- Interstitial fluid: The water present in the spaces between the cells (also bone, cartilage and dense tissue).
- Transcellular fluids: Specialised fluids (e.g., cerebrospinal fluid, gastrointestinal secretions). However, this is only a very small amount of the extracellular fluid.
We need to be able to match the amount of water being lost by the body to the amount of water being taken into the body to enable water balance. The normal water balance in a healthy dog or cat is generally calculated at 50 ml per kg bodyweight per day. However, this ranges from 40–60 ml/kg/day (24 h).
This calculation is devised from the amount (on average) that a dog or cat will lose due to sensible and insensible losses. A sensible loss refers to fluid loss that an observer can readily detect with their senses (e.g., can see it coming out).
Source of fluid loss
20 ml/kg/24 h
Urinary loss (normal range)
20 ml/kg/24 h
Faecal loss (normal range)
10 ml/kg/24 h
Total fluid loss
50 ml/kg/24 h
Abnormal Fluid Loss
Causes of abnormal fluid loss include:
- Blood loss
- Evaporation (increased panting, etc.)
Goals of Fluid Therapy
The goals are to replace lost fluid and electrolytes and to maintain a state of normal fluid and electrolyte balance in the presence of inadequate intake and ongoing losses.
When formulating a fluid therapy plan, the following five questions should be considered.
- Is fluid therapy indicated? Consider the clinical indications, laboratory data and patient presentation that signify fluid therapy is indicated.
- How much fluid should be given? Consider rehydration, maintenance and ongoing losses.
- What rate should fluid be given? The rate is very much dependent upon patient presentation and amount of fluid that can be administered to rehydrate without fluid overload occurring. Sometimes a deficit will be replaced over 24 h, while in other cases a portion of the deficit will be replaced rapidly and then the remainder of the deficit at a slower rate. The daily rate is calculated and then the hourly rate to ensure correct administration.
- What route should fluid be given? This has been previously discussed. In this section, we are concentrating on intravenous fluid administration.
- What type of fluid should be given? Several types of intravenous fluids are available. Choice of fluid depends on the nature of the deficit of electrolytes and disease factors.
Each patient should be individually assessed for the level of dehydration they are suffering from and an appropriate fluid therapy plan devised. There are several clinical and laboratory methods of assessing dehydration. These include:
A thorough history assists in correct assessment of the type and amount of fluid deficit present. Areas to be considered include:
- Patient eating and drinking status. It is important to establish if the patient has been eating and drinking normally. If the patient is anorexic or has had reduced food intake, this should be noted. The drinking status should also be recorded; this may include lack of fluid intake or excessive fluid intake (polydipsia).
- Gastrointestinal losses. If the patient has had any vomiting or diarrhoea, the frequency, amount and consistency should be noted. When devising a fluid therapy plan, any ‘ongoing’ losses should be taken into account; vomiting and diarrhoea are ongoing losses until the condition is resolved.
- Trauma. Haemorrhage should be duly noted and attended to, along with a history of when the trauma occurred, circumstances, patient status.
- Urinary status. Notation of the patient’s urinary status should be made. This includes reduced or increased urine output, colour and consistency of the urine.
- Abnormal discharges. Any abnormal discharge should be recorded (for example, an open pyometra or weeping large sore). A history should be taken of how long the discharge has been present, amount and consistency of discharge.
Although they are not always accurate, clinical signs are a useful means of assessing dehydration. It should be noted, however, that clinical signs will not become apparent until the animal is at least 5% dehydrated.
To assess hydration, feel the mucous membranes and perform the ‘skin tenting’ test. Mucous membranes should normally be moist. Dry, tacky mucosa is an indication that hydration is not adequate.
To perform ‘skin tenting’, gently lift the skin of the animal, twist and observe how long it takes before skin returns to its normal position.
Clinical signs of dehydration per percentage
Slight decrease in skin turgor, slightly tacky MM
Delay in skin ‘tent’ to normal, tacky and dry MM, increased CRT, eyes may appear slightly sunken
Tenting of skin, dry and tacky MM, increased CRT, tachycardia, sunken eyes, cold extremities
Severe dehydration, clinical signs of shock are apparent, life threatening, hypothermic, weakness
Blood work may indicate increase in PCV, TPP and BUN.
Routes of Administration
The oral route is the most physiological. This route should not be used in the presence of vomiting or if contraindicated due to illness or surgical procedure. This route is also inadequate for animals that have had acute or extensive fluid losses. Fluid absorption is not sufficiently rapid via the oral route for those cases in which the fluid loss has been extensive and the blood flow is inadequate.
Fluids are usually administered in the subcutaneous tissues over the dorsal neck and cranial trunk. In the absence of vasoconstriction and/or hypovolemia, the rate of absorption is approximately 6–8 h. Fluids should be administered at body temperature to decrease the discomfort to the patient and improve absorption. Only isotonic fluids should be administered by this route. Potassium supplementation up to 40 mmol/L may be added to the fluids. The rate and volume of administration will vary from patient to patient. Skin necrosis and infection are complications associated with this route of fluid administration.
When administering subcutaneous fluids, try to massage the area being injected. This will help prevent formation of a lump. If large amounts of fluid are being administered subcutaneously, divide the total amount into smaller injections and change sites (e.g., if 100 ml is being given, give four separate injections of 25 ml).
This is the route of choice when vascular volume restoration is desired. Fluid absorption is rapid. In addition to isotonic solutions, hyper- and hypotonic solutions may be administered via this route. The rate and volume administered will vary from patient to patient and be based upon the patient condition and desired end-point.
Intravenous catheter placement and fluids are discussed in more detail further in this text.
Fluids are administered via the bone marrow. Like intravenous administration, fluid absorption is rapid. This route is indicated when it is difficult to gain venous access using standard techniques (e.g., in neonate patients or patients with collapsed circulation).
The veins in the bone marrow drain into the systemic venous system and enable quick and effective absorption of fluids.
Sites for placement of an intraosseous catheter in the cat or dog include:
- Tibial crest
- Inter-trochanteric fossa of the femur
- Wing of ilium
- Tibial tuberosity
- Greater tubercle of the humerus
The sites for intraosseous placement in birds are:
- The distal ulna
- Proximal tibiotarsal bone
How Much Fluid?
Once we have assessed how dehydrated our patient is, we can calculate how much fluid is required to replace (rehydrate) the loss. We then look at how much is required to maintain normal hydration, and finally if there are any ongoing losses (e.g., vomiting, diarrhoea, draining wounds, etc.) to take into account.
Types of fluids available will be discussed later in this section.
Replacement fluids are typically used for rehydration.
To calculate the amount of missing fluid (the amount that needs replacing), multiply the bodyweight in kg by the percentage dehydration. Remember that mathematically 5% is expressed as 5/100. This will give you the missing volume in litres. So for a 10-kg animal that is 5% dehydrated: 10 x 5/100 = 0.5 litres (i.e., 500 ml).
A quick rule-of-thumb formula to give you the result in millilitres is: percentage dehydration (just the number) x bodyweight (kg) x 10. So for a 10-kg animal that is 5% dehydrated: 5 x 10 x 10 = 500 ml.
The goal of maintenance fluid therapy is to replace fluid lost in normal body functions.
Maintenance fluid requirements = insensible fluid losses + sensible fluid losses.
Insensible losses (GIT, respiratory tract, skin) = 20 ml/kg/day.
Sensible losses (urine) = 1–2 ml/kg/h (i.e., 24–48 ml/kg/day).
Therefore, combined maintenance volume is generally estimated at 40–60 ml/kg/day. Larger dogs typically have lower requirements than small dogs (due to surface area:body mass ratio).
Ongoing Fluid Losses
Ongoing losses refer to fluid losses over and above the maintenance requirements of a normal patient. Examples include vomiting, diarrhoea, bleeding and wound exudate. These can be difficult to predict and so a best estimate is used.
Calculating Fluid Requirements
When calculating fluid requirements, all three losses/deficits are considered.
Requirement = fluid deficit + maintenance requirements + ongoing losses.
If a fluid pump is used to deliver the fluids, then it has the capacity to have an hourly fluid rate set. If a gravity-fed administration system is used, then the fluid rate should be calculated to drops per second. Conventional administration sets generally administer 20 drops/ml. It will be printed on the packaging.
10-kg dog is 8% dehydrated
Has diarrhoea—estimate it will lose 140 ml of fluid via D+ in next 24 h
Maintenance choice of rate = 50 ml/kg/day
Deficit + maintenance + ongoing losses
- 10 (kg) x 8 (% dehydration) ÷ 100 = 800 ml (replacement amount)
- 10 x 50 (ml/kg/day) = 500 ml (maintenance amount)
- 140 ml over 24 h = 140 ml (ongoing losses)
- 800 + 500 + 140 = 1440 ml
- 1440 ml ÷ 24 h = 60 ml per h
Calculate drops per minute:
- Hourly rate (ml/h) ÷ 60 (min/h) x 20 (drops/ml—giving set factor)
- 60 ÷ 60 x 20 = 20 drops per minute
Calculate drops per second:
- Minute rate ÷ 60 (sec/min) = 20 ÷ 60 = 0.33
- Convert decimal value to fraction – 0.33 = 1/3rd
- 1 drop every 3 sec
Type of Fluid Required
The type of fluid given will depend on many factors, including:
- Composition of lost fluid
- Abnormalities requiring correction
- Severity and type of fluid depletion
Considerations of choice of fluid should also include:
- Need for glucose
- Electrolyte balance
- Osmotic pressure
- Oxygen-carrying capacity
Fluids can be divided into three main categories:
- Blood and blood products
Crystalloids are water-based solutions that contain small particles (ions and molecules). Common constituents include electrolytes (e.g., sodium, potassium, calcium, magnesium and chloride to replace or maintain plasma levels), glucose or dextrose (to provide energy) and sometimes a buffer (to correct or maintain the normal acid-base balance of the plasma).
These small particles readily pass through membranes and into all body compartments. The fluid is rapidly redistributed through the body. Generally, 75% of crystalloid fluids given will leave the intravascular space within 30 minutes of administration.
Crystalloid solutions can be:
Colloids contain large molecules that do not readily pass through membranes. Fluid is drawn into blood vessels. Colloid solutions refer to solutions that have an osmolality greater than the extracellular space; they contain large molecules which are designed to stay in the intravenous space longer than crystalloids. They are sometimes referred to as ‘plasma expanders’ due to their ability to draw fluid out of the interstitial and intracellular spaces into plasma and thus maintain vascular volume more effectively.
Much smaller volumes of colloids are just as effective as large amounts of crystalloids. Their main indications for use are:
- Hypovolaemic shock
They are often administered as a bolus (supported with crystalloid fluids) or over several hours.
Examples of synthetic colloids include:
- Voluven 6%
Voluven 6% contains 6% hydroxyethyl starch in 0.9% NaCl. Its main use is as a plasma volume replacement. It does not replace red blood cells or clotting factors. It is a clear, colourless solution. It should be stored below 25°C and should not be frozen.
- Do not use in patients with pre-existing disorders of haemostasis and coagulation
- Do not use in patients with established renal failure
- Side effects
- Minor allergic reactions due to histamine release
- Transient increase in bleeding time may occur
- Hypersensitivity reactions may occur, including (rarely) severe anaphylactic reactions
- Voluven dose rates (as per manufacturer’s recommendations)
- Up to 50 ml/kg/day—maximum due to coagulation problems if larger amounts given
- Continuous infusion can be calculated at 1–2 ml/kg/h, although not recommended to use as a CRI now due to adverse renal effects
Larger bolus over shorter period can be given at 20–40 ml/kg per day (so larger amount given over 2 h instead of smaller amount over 8 h, but must not exceed 50 ml/kg/day)
Monitoring of and Care of Intravenous Catheters
It must be remembered that intravenous catheters can be a port of entry for bacteria into the vein. The catheter and the dressing must be checked regularly.
Intravenous catheter checks should be performed frequently (at least four times a day) and include:
- Removal and replacement immediately if contaminated in any way (dirty, wet, chewed, etc.).
- Check for swelling of the distal paw.
- Check for swelling or heat proximal to the bandage.
- Ensure catheter is not leaking.
At least once a day, the following check should be employed:
- Remove the catheter bandage.
- Visualise the insertion site of the catheter and check for:
- Any redness
- Any heat or swelling
- Any discharge (note colour and amount)
- Catheter dislodgement
- Adhesive tape or padding dislodgement
- Pain on palpation of area
Re-bandage and note catheter check performed and findings.