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Dehydration and Shock

Terms in this set (63)

- 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).
- Designed to maintain hydration in a patient that is not eating or drinking
- These fluids are essentially half free water.
- The osmolality on the bag is actually iso-osmolar (to slightly hyper-osmolar) but they are, in fact, hypotonic.
- The reason that they are hypotonic is that the dextrose is quickly metabolised in circulation, leaving the free water behind.
- The risk of injecting just free water into a peripheral vein is a marked change in osmolality in that small vein (e.g. the cephalic vein) causing water to rush into red blood cells and lyse them - dextrose is added in order for this to be injected safely into a peripheral vein
- Benefit of using these fluids at maintenance rate is the provision of free water to an animal that is not eating or drinking
- These fluids should never be used at rates higher than maintenance rate.
- The addition of free water can cause clinically significant hyponatraemia and cerebral oedema from free water overload.
- We often use a replacement fluid for maintenance of hydration because they are most commonly available and it is convenient if you also have to rehydrate the animal
o If a patient is on a replacement fluid then their Na may start to creep up slowly over several days - they may need a source of free water, such as these replacement fluids.
o Practically, our patients are more often than not drinking water within a day or two anyway - they fix up their own water imbalance.
o In our hospital, we tend to monitor electrolytes in patients that are NPO every day to ensure we are keeping things in balance, so that we know when the Na starts to creep up and they may need a source of H2O. This also guides our K+ supplementation.
- 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.
o Cats
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.
o Includes
Hartmann's (LRS): fluid of choice as it doesn't have Cl excess and contains bicarb precursors
NaCl 0.9%
Plasmalyte-148- not great because it contains acetate which can cause vasodilation
o The dose depends on the blood volume of the animal (80 - 90ml/kg for dogs; 50 - 60ml/kg for cats)
The standard dose is 25% of blood volume within 10 minutes for mild shock; 25-50% for moderate shock and 50 - 100% for severe shock
This equates to 20-22.5ml/kg in the dog given as fast as possible (over 10 mins) for mild shock
For larger dogs, fluid pumps often do not go fast enough; a pressure infusion bag should be used.
The goal is to reassess after the first bolus and if the animal is still in shock, to give another 25% BV dose, as a bolus
o In cats, it is safer to give fluid boluses in the form of 60ml syringe doses.
If you use a pump or open fluid bag, there is a risk of accidental fluid overload which may kill the cat.
A dose of 60ml is about 10% BV.
You can repeat the 60 ml boluses (by hand) up to a full BV (50-60ml/kg).
Cats rarely need a full BV and are prone to fluid overload; therefore shock fluid therapy is done more cautiously
o If you are suspicious of severe electrolyte abnormalities in the patient due to the presenting problem, it is good practice to run electrolytes (if they can be done quickly) before bolusing large amounts of crystalloid which may help to guide the fluid choice
It is also useful to run a PCV/TP before bolusing fluids, as a baseline for future problems (bleeding and hypoproteinaemia)
o Increase in COP helps to maintain blood volume
o They do not add much crystalloid to the interstitium - they are not useful for rehydrating patients or providing maintenance needs in regards to electrolytes and water
o Albumin is the best example of a natural colloid molecule.
When an animal has low COP due to low albumin levels, artificial colloids can be useful.
Albumin is not typically used to increase COP in a patient as it is expensive, possibly allergenic and a large volume is needed to make a difference to the patient's own albumin levels.
o Dose: 10 - 20mL/kg for dogs; 5 - 10mL/kg for cats because they stay entirely in the vascular space
o Artificial colloids include starch solutions and gelatin solutions
Voluven (hydroxyethylstarch)
• Compromised of low MW starch molecules that are about twice the size of albumin suspended in 0.9% NaCl
• Volulyte - a newer solution on the market that is suspended in a Plasmalyte-type crystalloid
• Because the starch molecules can't easily cross the endothelium most of the injected dose stays within the vasculature (cf. isotonic crystalloids where 75% redistributes to the interstitium)
• The dose is 25% of the full blood volume dose of isotonic crystalloids
• Maximum dose given in any one day shouldn't exceed 40-50ml/kg
• Indications: the need for blood volume expansion paired with a low COP in the patient (you often can't measure COP - be highly suspicious if total protein is low or normal-low)
• CRI in animals with ongoing protein losses to maintain circulating blood volume
• An animal with low TP but no signs of hypovolaemia may not need colloids at all
• Risk of adverse effects generally occur proportionately to the dose given, and include platelet dysfunction & worsening of renal function - take care in patients with bleeding problems or renal dysfunction
• Starch solutions also increase urine specific gravity, as the large molecules pass through the kidneys and interfere with refractometry, and large volumes can slightly alter TP towards 35 to 45 g/L.
• As these solutions take time to break down, fluid overload can have more persistent consequences than fluid overload with a crystalloid, therefore care should be taken.
Gelofusine (gelatin).
• Contains gelatin molecules that are smaller than albumin (about half the size) and are readily excreted through the kidneys and metabolised.
• These solutions can be useful for short term blood volume expansion, such as during anaesthesia, at doses similar to starch solutions (5-20ml/kg).
o 7.2% NaCl is a hypertonic solution that, when administered IV, initially draws water by osmosis into the intravascular space from the interstitium and the ICF very quickly (causes hyperosmolality)
o Once the osmolality is the same in both spaces (ICF and ECF) the salt and water will distribute between the intravascular and interstitial space
o Overall, this therapy 'borrows' water from the ICF and expands the ECF.
o The blood volume expansion is small and short lived.
o You cannot exceed the recommended dose of 2-5ml/kg over 5 minutes as this would lead to significant hypernatraemia and therefore neuron shrinkage and neurologic signs.
o But despite its limitations, it can be useful for large animals to resolve shock while allowing time for large volume of crystalloid to be infused.
o It's also useful for traumatic brain injury as it improves perfusion as well as reducing cerebral oedema.
o There are also some other theoretical benefits
Vasodilation in the periphery to enhance perfusion (though, this is why we don't inject it too fast as vasodilation can be detrimental in a hypotensive patient)
Increased cardiac output
Other immuno-modulatory effects.
o Should not be used in dehydrated animals without knowledge of electrolyte levels, as there is a risk that the animal may already have a free water deficit (hypernatraemia).
o Follow up hypertonic saline with some form of isotonic crystalloid therapy to enhance blood volume expansion which will also help balance out the relative free water deficit you have created by giving a high salt solution.