can be divided into
o Sensible (easily measured)
Urinary losses which is usually hyper-osmolar
o Insensible (not easily measured) losses.
Lost via respiratory tract, GIT and skin
Variable but in healthy sedentary animal are generally estimated to be 1/3 of daily maintenance fluid/water requirements
These losses will be higher than average if animals are active, exercising or in a hot dry environment.
- Normal losses, overall, are usually isotonic to hypertonic (iso- to hyperosmolar) (salt and water) - not just water loss, but fluid loss (water and other solutes).
- Animals that drink more water than is required, for waste product excretion and replacement of evaporative and gastrointestinal losses, will excrete free water (without sodium) via the urine.
o Renal water conservation or loss is under the control of antidiuretic hormone (ADH or vasopressin).
ADH production is stimulated by changes in serum osmolality outside the individual animal's normal range.
ADH excretion is maximal when osmolality increases by 1 to 3% and it is completely suppressed when serum osmolality decreases by 1 to 2 %.
ADH secretion also increases during hypovolaemia.
If an animal doesn't drink for 1-2 days, ADH secretion and water conservation by the kidneys ensures that the body still has an appropriate water balance, and hence a normal osmolality and serum sodium concentration, even if they are clinically dehydrated.
- Dehydration - a reduction in interstitial volume (ECF) - osmolality does not change
- The vascular space is also reduced and any significant reduction in vascular space will cause shock
o Shock is treated differently compared with dehydration (much faster for shock)
- Dehydration is not shock because it is movement of isotonic fluid from the interstitium over time (Starling has time to fix it)
- Restrict the term 'dehydration' to an isotonic interstitial loss only, which means creating the plan is a lot more straightforward
- Third spacing of fluid into the thorax, abdomen or GIT, will also cause an isotonic loss and dehydration
- Isotonic losses (vomiting, diarrhoea and urine) are the most common
- An animal can also become dehydrated by not eating or drinking for one day or more.
o Initially, the animal will conserve sodium via aldosterone secretion and maintain water balance in the body via ADH secretion.
o After 1-2 days, however, the kidneys can no longer conserve the amount of water needed by the body for metabolic processes, and the animal will start to become hypernatraemic.
o Most patients that you see that have not been drinking, or drinking less, for a day or two will have a normal serum sodium, hence a normal water balance, therefore they have an overall isotonic fluid loss.
- Always remember, water imbalance should be treated differently to dehydration.
o The two concepts are not interchangeable.
o You will deal with dehydration on a regular basis but rarely have to correct a free water imbalance (i.e. abnormal serum sodium concentration).
- A potential risk of fluid therapy
- An increase in ECF volume, or increased interstitial fluid beyond normal.
- Clinical signs suggestive of over-hydration include increased urine output with isosthenuria, distended jugular veins, watery nasal discharge, increased tear production, tachypnoea, peripheral oedema and pulmonary crackles.
- If over-hydration is developing, stop fluid therapy, reassess fluid calculations and, if life-threatening signs are present such as dyspnoea, administer diuretics (furosemide).
- Consider other causes for increased fluid in the interstitium other than fluid overload (back to Starling's forces!)
- Intravenous route
o Most convenient and accurate for hospitalised patients
o Suitable sites are cephalic, jugular, lateral saphenous, medial saphenous vein, ear veins (pigs and Bassett hounds).
o Complications of intravenous catheterisation include phlebitis, cellulitis, bacteraemia, septicaemia, subcutaneous extravasation of fluids, embolism (air, thrombi or catheter), kinking of the catheter due to patient positioning and trauma to the catheter or giving set by the patient which could result in blood loss.
o Catheters should be placed using a sterile surgical prep, and catheterised veins should be examined at least every 24 hours to check for complications.
o Calculation of intravenous infusion rates using different giving sets when fluid pumps are unavailable:
10 drops per mL: Drops per minute = desired infusion rate (mL/hr) ÷ 6
20 drops per mL: Drops per minute = desired infusion rate (mL/hr) ÷ 3
60 drops per mL: Drops per minute = desired infusion rate (mL/hr)
- Paediatric burettes
o 100-150ml chambers between IV fluid bag and patient should be used with animals < 10kg body weight to avoid inadvertent life threatening fluid overload when a pump isn't used.
- Intraosseous route
o Used in small patients when an intravenous catheter can't be placed in an emergency situation.
o They are only a short-term solution for bolusing fluids until an IVC can be placed.
- Subcutaneous route
o Used in patients who are 10% dehydrated and not in shock.
o Don't use when patient is spetic
o Generally used in dogs, cats, birds and reptiles and administered once to twice daily on an outpatient basis.
Good because they have stretchy mobile skin
Maximum amount is 150 - 200mL
Don't add glucose - use isotonic fluids
Not always absorbed as predicted
o Dogs - skin is not as stretchy = can be painful
o Be careful with administering large volumes of SQ fluid as a single dose in cats, to avoid fluid overload.
o This route is not as accurate as IV administration (cannot titrate to effect) and can be painful.
10th EditionHalliday, Resnick, Walker 8th EditionJohn Buck, William Hayt 4th EditionDavid J. Griffiths