Hello and welcome to the Animal Care Learning Alliance webinar, an introduction to infusion therapy. My name is Felicity Caddick, and I'm one of the vets that work here at Animal Care. The main focus of this webinar is around dehydration, what causes it and how we recognise it.
We'll also take a look at some practical elements of treatment in practise, including setting up, administering, and monitoring an animal on infusion therapy. Dehydration simply means that the animal has fluid depletion across multiple compartments. So let's first look at this fluid.
An adult dog or cat has 60 to 70% of total body water. This can be slightly different in young and old animals. In a fully hydrated animal, there is constant flushing of fresh fluid around the body cells.
Because the walls of the capillary vessels are very permeable, fluid coming into the microcirculation can then transfer nutrients and gases from the blood to the body cells. Conversely, there will also be removal and transportation away of waste products of cell metabolism. It is important to remember that adequate fluid exchange will only occur if there is adequate blood pressure, which is dependent not only on the heart but also on the presence of adequate fluid in the circulation.
The majority of body fluid, 2/3, is inside the body cells. The remaining 1/3 is extracellular. Of this extracellular fluid, approximately 25% is in the bloodstream in the form of plasma.
And the remaining three quarters is institial fluid which the cells are bathed in. Because of the constant exchange of fluid between the blood vessels and interstitial compartments, the constituents of plasma and interstitial fluid are very similar, the only main difference being that plasma contains proteins and blood cells which are normally too large to pass through the capillary walls. On a day to day basis, fluid is lost via various routes.
Some can be controlled by the body, such as urine output, but some have only limited control, such as loss via faeces or evaporation. Balanced against this are inputs of fluid from drinking and eating. A mild disturbance in this balance can be easily corrected, but we shall take a look at the problems caused by more severe disturbances and how to deal with them.
The major organ responsible for controlling this balance of fluid are the kidneys. They can regulate the extracellular fluid by controlling volume, electrolyte composition, and pH whilst also filtering out toxins and waste products. However, the kidney's ability to perform its vital role is affected if the blood delivery to the kidney is reduced.
This can be caused, for example, by the animal not consuming sufficient water or electrolytes, or if the animal has a medical condition, which means it is losing abnormal amounts of water and electrolyte. This leads to dehydration. Therefore, our primary goal of fluid therapy is to restore blood flow to the kidneys and provide fluid and electrolytes.
Dehydration is a fluid deficit across all compartments. Extracellular compartments are most predominantly affected. There are different types of dehydration.
Hypertonic dehydration is when there is pure water loss, so no electrolytes are lost. From the extracellular fluid, then the fluid loss will be spread evenly across the extracellular fluid and the intracellular fluid, and the fluid loss will only be minimally felt. The concentration gradient created between the pure water loss of the ECF and the ICF will draw fluid by osmosis into the extracellular area.
Therefore, the fluid loss is shared between both compartments. In isotonic dehydration, where there is mixed water and electrolyte loss, you can get a more rapid decline in a patient. This is more commonly seen.
A loss of a mixture of water and electrolytes, for example, in cases of vomiting and diarrhoea, leads to little actual change in the composition of the extracellular fluid, even though there is a depletion in volume. For example, it remains isotonic. If no fluid is drawn out of the cells by osmosis.
There is no, so the loss is borne almost entirely by the extracellular compartments, such as the interstitial fluid and plasma. There are several classical signs we can look for on clinical examination to give an indication of hydration status. Weight loss is an often overlooked sign, as there may be no reference weight to compare to.
However, it is important to reweigh all hospitalised animals on fluids as their weight may significantly change with treatment. In the early stages, below 5% weight loss, very few signs are seen. You may get a dull demeanour and possibly an increase in thirst.
Above 5%, we start to expect to see some physical changes. Changes in skin tenting, causing reduced pliability can be easily assessed. However, this is not reliable in certain breeds such as this very loose skinned pug.
Or very young animals which also have excessive skin. Elderly animals can lose their skin elasticity, and obese animals may have very tight skin. Dry tachy mucous membranes is a fairly consistent finding in most dehydrated animals, but requires you to be able to put your fingers into the animal's mouth.
Loss of fluid behind the eye can cause the eyes to sink into their sockets, but it should also be remembered that in some animals such as this skinny cat, we may also have significant body fat loss, which can also be causing a similar sign. This dehydration triangle shows the signs associated with the increasing levels of dehydration. We can see that below 5%, there are very few physical signs to see.
From 5 to 7%, we get the classical signs of dehydration previously described. In severe dehydration above 7%, we often start to see hypovolemia signs as well as the dehydration signs as the fluid shifts out of the circulation and into the intestitial compartment. These include reduced blood pressure and signs of reduced oxygen delivery to tissues.
We can see with what seems a relatively small increase in the percentage dehydration up to 1012, 15%, the consequences become increasingly life-threatening. Dehydration is one of the most common indications for fluid therapy, and our aim is to optimise the kidney function, restoring the normal fluid balance across all compartments and ideally within 24 hours. We can achieve this by replacing the estimated deficit of the animal.
We will approximate that using our physical examination. We also need to supply the patient's maintenance needs, and to and we also need to compensate for any ongoing losses. So when we are calculating the 24 hour fluid requirements, we will use the calculation that the total replacement volume equals the deficit plus maintenance plus ongoing losses, and we'll discuss each of these components in turn over the next few slides.
To discuss these calculations, we're going to look at a case which involved dehydration. This is actually my own dog, Sprocket, who developed a pyothorax and became dehydrated. She had to be hospitalised and her dehydration corrected.
To effectively manage her dehydration, we need to calculate her deficit, her maintenance requirements, and her ongoing losses. So first of all, let's look at her deficit. As we've already said, an accurate weight is vital.
This means no feet, no feet hanging off the edge of the scales. This we will not only be used for our calculations, but will be used as a comparator when monitoring the patient recovery. So remember to record it on the kennel sheet.
It's also often a good idea in practise to use the same set of scales for each measurement. Sprocket weighed 10 kilogrammes on admission. Next, we estimate the percentage dehydration of the patient based on the clinical signs.
Sprocket is showing weight loss. Depression. A mild skin tint, dry mucous membranes, and sunken eyes.
So let's remind ourselves of the percentage dehydration triangle. And we can see that she's 7% dehydrated. Using the deficit calculation of weight, in this case 10 kilogrammes, multiplied by dehydration of 7% and again by 10, we conclude her deficit is 700 mL.
Next, we need to look at her maintenance requirements. We need to compensate for the natural losses which occur in any animal through the day, as these animals are unlikely to be drinking and eating as normal, whilst they're ill and hospitalised on a drip. The average water requirement is often quoted as 50 mL per kilogramme per day or the equivalent of 2 mL per kilogramme per hour.
Due to the surface area to body weight ratio, smaller dogs, such as toy breeds, tend to have slightly higher water requirements at 60 mL per kilogramme per day. And large dogs have slightly lower requirements at 40 mL per kilogramme per day. For cats, we still use 50 mL per kilogramme.
Since although they are smaller, they have slightly less body water compared to dogs. Sprocket is 10 kilogrammes. So at 50 mL per kilogramme, she will need 500 mLs to cover her normal maintenance needs.
This table and our dehydration triangle are both on a poster available to order from the Animal Care practise Assistant centre. Finally, we need to consider any ongoing losses. There can be additional fluid losses each day attributable to the ongoing medical condition of the animal.
Factors that cause this fluid loss should be considered. These include evaporation associated with panting, fever, or open wounds, or increased urination from conditions such as chronic renal failure or diabetes. Some of these additional losses can be accurately measured using a urinary catheter or weighing litter trays before and after urination.
However, the most common scenario faced in practise is extra losses due to vomiting and diarrhoea. Kennel liners can be weighed before and after soiling to judge fluid loss, but typically a calculation of 4 mils per kilogramme per episode is used. Sprocket has vomited 5 times since she was admitted, so we can approximate this extra loss as 4 mL per kilogramme times 10 kilogrammes times 5 episodes at 200 mLs.
So now we have the volumes needed to create a fluid plan. Sprocket's 24 hour fluid requirements will therefore be deficit plus maintenance plus ongoing losses. So 700 mLs plus 500 mL plus 200 mLs totaling 1400 mLs.
There are 2 options for how the calculated volume should be administered. The simplest approach is to divide the total volume by 24 to give an hourly rate. In this case, 58.3 MPH.
This is acceptable, particularly in cases with mild dehydration. Another approach to consider, particularly suited to the more severely dehydrated patients, is to replace half of the volume in the 1st 4 hours and the remaining over the next 20 hours. This approach of using a faster rate initially allows for a more rapid correction of any deficits in blood volume, ensuring restoration of sufficient blood flow to the kidneys.
However, this requires more intense patient monitoring during the initial 4 hours and so may not always be practical in general practise. An alternative method, often used to calculate fluid requirements in dehydration is to use multiples of the standard maintenance fluid rate of 2 mL per kilogramme per hour. This can prove to be adequate in a lot of cases.
However, there are a few assumptions involved with using this system. Firstly, it assumes that rates are used for the full 24 hour period. In this situation, twice maintenance or 4 mL per kilogramme per hour will manage the maintenance and deficit requirements in 5% dehydration.
2.5% maintenance will manage 7.5% dehydration, and 3 times maintenance will manage 10% dehydration.
But these calculations also assume a maintenance rate of 50 mL per kilogramme per day, so they don't take into account differences in small or large breed dogs. Finally, this method also assumes there are no ongoing losses to take into account, and so it can underestimate fluid needs in some animals. As we can see using Sprocket as an example, the difference between the two calculations is 200 mLs.
It is important to record the total volume calculated on the animal's kennel sheet and check this against the actual amount of fluid administered to ensure there is no over or under administration. Likewise, continuous patient evaluation is essential and may lead to recalculation of rates. Now that we have the volume of fluid and the rate to give it at, we will look at the more practical aspects of the fluid plan for Sprocket.
The next thing to consider is how we're going to deliver the fluids at the rate calculated and which fluids are the most appropriate for the patient. The use of infusion pumps in general practise has become more widespread, and they have significantly aided practises in their delivery of fluid therapy. They ensure a very accurate flow rate is achieved and often contain useful safety features.
Some given sets will allow a rate to be set on a flow controller in mils per hour. However, if this is not available, then a conversion calculation to drops per minute will be required in order to use the drip chamber to control the rate of flow. The first part of this calculation converts mils per hour into drops per hour.
Each giving set has a designated rate, also known as the giving set factor. This is usually 6 drops per mL for paediatric sets and 20 drops per mL for standard sets. Drops per hour is therefore the drops per mil multiplied by the mils per hour.
For Sprocket with a standard set, this is 20 drops per mL by 58 mL per hour, giving 1160 drops per hour. To make this more usable, we then convert it to drops per minute by dividing by 60. So for Sprocket, 1160 drops per hour, divided by 60 is 19.3 drops per minute.
You can see that there are also some shortcuts that can be used to speed up these calculations. Having calculated the volume and the rate, we need to decide on the type of fluid needed. If we look at crystalloids first, Crystalloids contain a combination of water and electrolytes, usually in similar proportions to extracellular fluids.
When administered intravenously, approximately 2/3 will eventually leave the bloodstream into other fluid compartments. Because they diffuse into other fluid compartments, they make an ideal choice for management of dehydration. If our primary goal, however, is to restore blood volume, we need to use fairly high rates and volumes because of the fluid movement out of the bloodstream.
That being said, they are very effective at restoring blood volume when used correctly. If we now look at the typical features of colloids, colloids contain water and some electrolytes, but they also contain much larger molecules which are unable to easily leak out of the capillary walls, so they remain in the bloodstream for longer. When administered This makes them very effective at restoring blood volume.
For for example, in haemorrhage, as lower rates and volumes are needed compared to crystalloids. But they will not restore deficits in other fluid compartments, and so are not suitable for dehydration management. For most cases of dehydration management, we only need to consider the crystalloids, and the remainder of our talk will focus on them.
We can further classify crystalloids into replacement or maintenance solutions. Replacement fluids are used to replace body water and electrolyte deficits. Their electrolyte balance is similar to extracellular fluid.
The various types of fluids contain different electrolytes, giving them different properties. Looking at the Aquafarm range, numbers 139, and 11 are all replacement solutions. Maintenance fluids are used to maintain a patient in a normal state of hydration, who already has a stable fluid volume.
They are not suitable for volume replacement and not indicated for dehydration. Aquafarm 18 is a maintenance solution. Which has a very low sodium content compared to plasma.
As she is dehydrated, Sprocket will need a replacement fluid. So how are we going to administer these fluids? Well, we know the type and size of fluid bag that we need, but we need now to consider our choice of catheter, giving set, and connector.
Although there are various types of catheter available, the most common catheter used when catheterized in a peripheral vein is the over the needle catheter, meaning the cannula is on the outside of the insertions that stilelet. It can be seen that the cannula is finely tapered and bonded to the stilet. This ensures smooth entry through the skin and into the vein.
It is very important not to be tempted to loosen the cannula from the stilet prior to placement, since the taper end of the cannula can become ruffled, making it much harder to insert through the skin and into the vein. When selecting catheters, we have a number of different diameters or gauges to choose from. With the gauge classification system, the smaller the gauge, the larger the diameter.
The wider the bore of catheter, the higher the maximum flow rate which can be achieved. Therefore, it is advisable to place the largest size that it is possible to insert. The composition of the cannula and the subsequent properties influences the type of gravity used.
For example, an FEP catheter may feel firmer and therefore easier to insert and less prone to kinking, while a PU catheter that softens on insertion may be more comfortable for longer term placement. The shape of the point on the stilet may also influence the sharpness of the catheter. Various connectors of different configurations are available to connect between the catheter and the given set.
As are various different styles and lengths of giving set. If using an infusion pump, it is advisable to ensure the set is calibrated to the pump for increased accuracy. Having selected our equipment, we now need to catheterize sprocket and set up a given set and fluid bag ready for attachment.
It is useful to prepare everything needed in advance. The giving set should be primed with fluid and all the air removed. The most common site for catheter placement is the cephalic vein in the forelimb.
This vein is easily accessible although there can be issues of occlusion of the catheter flow at the elbow if the catheter is placed too high up the leg. In some animals, it is more appropriate to use the back leg and a sofina vein. This has a wide bore and so may not be as collapsed in cases of hypovolemia.
However, it is very mobile and can prove difficult to secure the catheter in place for longer term use. Other, less commonly used routes can be the marginal air vein. More often seen in rabbits.
But this does pose a risk of skin sloughing in the area. Or the interosseous route. Special catheters are required for this route, but it can be the only route available in very small animals with very fine veins.
Good catheter site preparation reduces the risk of complications in your catheterized patients. A wide clip and a surgical scrub is advised. There are various techniques employed to secure the catheter in place.
The majority of these involve taping the hub of the catheter. In all cases, the tape should not be too tight as to reduce circulation in the lower limb, but secure enough to hold the catheter in place. Additional connectors will need securing themselves, and it is worth applying extra padding to prevent any rigid plastic rubbing on the animal's skin.
The same applies to the giving set connection to the catheter. As accurate as our calculations seem to be, we still need to continually monitor any patient on fluid therapy, carrying out a thorough head to tail check. Checking their nose, their gums, their eyes, their face in general.
Their ears, their neck, their chest, and their distal limbs. Rectal temperatures can be useful and urine output is also a good parameter to measure. So how do we monitor the patient's response to fluid therapy?
We need to compare the results of our initial assessment, so we should record as much information as possible on initial examination. We can't rely on any one particular parameter, so a combination of rehydration parameters are used. There is often an improved improvement in general demeanour.
Weight gain can also be seen, which is another good reason for an accurate starting weight. If your patient on fluid therapy is losing weight every time you measure it, this should cause concern. Since it suggests the animal is losing fluid faster than you are replacing it, and you may need to modify your fluid pan.
There should be a reduction in the skin tenting scene and a moistening of the gums. And urination frequency should improve with more dilute urine being produced. So back to Sprockett's case.
We started with a 10 kilogramme dehydrated dog. We assessed her dehydration. We calculated the volume of fluid she required.
We selected the fluid for her and the equipment that we needed. We gave the appropriate fluids over the appropriate time and we monitored and reassessed her. And now she's rehydrated and ready to go home.
There can be complications associated with fluid therapy, and it is recommended that regular checks are made on all patients. The insertion site of the catheter can become inflamed or infected and may require changing to use an alternative limb. Air bubbles should be prevented from entering the circulation.
And most fluid pumps now have alarms to detect these bubbles. Although the volume of fluid required for each animal can be calculated accurately, this doesn't mean that the physical state of the animal can cope with the volume or rate of administration. Monitoring for signs of volume overload is very important.
These signs can include edoema of the face or limbs, rapid, crackly breathing, and a watery nasal discharge. Thank you for watching our webinar, and we hope it was useful. If you have any questions, please get in touch by calling customer services on the number below or emailing the animal care technical email address.
Just to prove that Sprocket really is OK, this was us completing the Yorkshire Three Peaks last month.
