103 | Everything the vet nurse REALLY needs to know about intravenous fluid therapy
Fluid therapy is one of those things we do every single day in practice.
We hang the bag, we set the rate, we check the pump, we monitor the patient. It feels routine - maybe even safe.
But the 2024 AAHA Fluid Therapy Guidelines remind us of something incredibly important: fluids are drugs.
They are not ‘just supportive care.’ They are pharmacologically active solutions that can either help our patients recover, or contribute to complications if we don’t use them in the right way.
So in this episode, I want to chat with you about what these guidelines actually say, what that means in real life, and why our role as veterinary nurses in fluid therapy is far bigger than we sometimes give ourselves credit for.
Fluids need to be prescribed just like any other medication.
Fluids must be prescribed to achieve specific therapeutic goals and to minimise complications.
That means no more ‘just pop them on twice maintenance.’
There’s even a section essentially titled ‘one fluid rate does not fit all.’ And that’s an absolutely vital shift we need to make in how we think of fluid therapy, because assigning a standard rate without individual assessment can contribute to patient morbidity and mortality.
Instead, we need to focus on individual-directed and goal-directed therapy, considering our patient’s individual fluid needs, and working with our vet to create an appropriate plan.
So how do we do that?
First, we need to recognise which fluid compartment has a deficit.
Then we need to consider the appropriate type of fluid, and route of administration for the loss present.
Then we need to calculate the correct dose and administration rate.
And finally, we need to closely monitor the patient, to ensure our patient responds to the fluid plan appropriately, and to identify any complications.
When you think about it like that, as veterinary nurses and technicians we are key in at least three of those four steps. We are not passive participants in fluid therapy, and while we won’t be prescribing the fluids, we can assess our patients, identify losses, calculate and administer fluids under direction, and monitor their effect.
Before we can treat fluid deficits properly, we need to understand where fluid lives in the body.
Total body water is divided into three main compartments. About 67% is intracellular. The remaining 33% is extracellular, and that extracellular space is further divided into interstitial fluid and intravascular fluid.
So when a patient is ‘dehydrated’ we need to ask: which compartment is affected?
When a patient is in shock, which space is compromised?
The guidelines are very clear about the order of assessment. We assess the intravascular space first - then the interstitial and intracellular spaces.
You cannot correct intracellular fluid deficits if there isn’t sufficient intravascular fluid. And more importantly, you can’t deliver sufficient oxygen and other essential substances to cells - and therefore create energy for cellular functions - if there’s insufficient circulating volume to do that.
This is where detailed nursing assessment becomes so important. Mentation, heart rate, capillary refill time, mucous membrane colour, extremity temperature, pulse quality, blood pressure… these are not just boxes to tick. They are clues about which compartment needs fluids the most, and therefore how quickly we need to act (and the rates we need to use).
Hypovolaemia and dehydration are NOT the same thing.
One of the most important distinctions we can make when assessing fluid balance is this: is our patient dehydrated, or are they hypovolaemic?
Hypovolaemia is a decrease in circulating blood volume that results in reduced tissue perfusion. I.e., intravascular fluid losses.
Dehydration, on the other hand, is a loss of total body water that primarily affects the interstitial space.
They often occur together, but they are not interchangeable terms, and they are not treated in the same way.
A hypovolaemic patient requires rapid intravascular volume replacement. The guidelines recommend buffered isotonic crystalloid boluses of 5–10 ml/kg in cats and 15–20 ml/kg in dogs, delivered over 15 to 30 minutes.
After this, we reassess the patient, recheck their perfusion parameters, and adjust their fluid plan as necessary - and the role we play in this as veterinary nurses and technicians cannot be overlooked.
Dehydration is different - it’s corrected slowly. We calculate the fluid deficit based on the patient’s clinical signs, estimating a percentage dehydration from this assessment. And that deficit is replaced over 12 to 24 hours.
And if both hypovolaemia and dehydration are present (which they often are) hypovolaemia must be corrected first, and then we address dehydration.
Maintenance fluids: what are they REALLY?
Another really important point the guidelines raise is the difference between replacement fluids and maintenance fluids.
‘Maintenance’ is not a rate. It is a type of fluid.
Replacement crystalloids are formulated to resemble extracellular fluid and are intended to replace losses. Maintenance crystalloids contain lower sodium and higher potassium concentrations and are designed to meet daily requirements.
Using replacement fluids long-term as if they are maintenance fluids can predispose patients to sodium derangements and hypokalaemia. In fact, that’s why we often end up adding potassium to our replacement solutions in our anorexic patients - without it, their potassium will drop.
We don’t often use maintenance solutions in practice - but we need to know they exist, and we need to know that replacement solutions, used long term, may cause issues. That means being aware of the hypokalaemia risk, knowing we may need to reassess electrolytes regularly, and monitoring patients for signs of hypokalaemia as required. And given that we’re the ones monitoring and caring for these patients in the wards, this firmly falls into our role as veterinary nurses and technicians.
Let’s look at how to administer fluid therapy.
Fluid therapy is divided into three phases: resuscitation, rehydration, and maintenance.
Total fluid requirement equals the resuscitation rate plus the rehydration rate (including ongoing losses) plus the maintenance rate.
There is no single superior formula for calculating fluid requirements, according to the guidelines. What matters is that the plan is customised and then adjusted based on monitoring findings and ongoing losses.
And those ongoing losses are really important.
Fluid therapy is not a one-time calculation. Fluid balance is dynamic and ever-changing, and that means our fluid rates may need to, too. Vomiting continues, diiarrhoea continues, urine output changes, blood loss occurs, drain output levels change… And we need to incorporate all of those changes into that patient’s fluid plan.
Once we know our losses, we need to look at the route of administration.
Hypovolaemia requires intravenous or intraosseous access, but dehydration does not always require IV fluids. Subcutaneous and enteral routes are appropriate for some patients - and are often underused.
The guidelines strongly encourage using the enteral route whenever possible. Now this won’t always be suitable, for example in patients with uncontrolled vomiting, but water can be delivered orally or via feeding tubes, and this should be considered part of the fluid therapy plan.
Enteral fluids can correct mild dehydration or supplement parenteral routes - and they’re especially helpful for patients at high risk of fluid overload, for example.
In the hospital, we’re typically using intravenous +/- intraosseus (if venous access is impossible) administration. By the time patients are dehydrated enough to be significantly unwell and require hospitalisation, generally IV fluid therapy is indicated. However, once that dehydration (+/- hypovolaemia) is corrected, we need to consider whether continued IV fluid therapy is appropriate, or if we can meet the patient’s ongoing needs via the enteral route.
What about subcutaneous fluids?
Subcutaneous fluids, where appropriate, are recommended at rates of 20–30 ml/kg once or twice daily, delivered to multiple sites, with a maximum of 10–20 ml/kg per site. Importantly, 0.9% sodium chloride should be avoided subcutaneously due to its low pH and potential discomfort - so stick to buffered solutions like Lactated Ringer’s (Hartmann’s) solution for this.
Another important point to note about subcut fluids is that there is no evidence supporting their use in euhydrated patients. In fact, they may be detrimental in patients with diseases negatively impacting fluid balance, such as cardiac disease or hypoproteinaemia.
Generally, the most common indication for SC fluids is in patients with chronic diseases predisposing them to dehydration, such as CKD - and usually these fluids are administered on an outpatient basis. Veterinary nurses and technicians play a vital role in this, from demonstrating at-home administration to caregivers, to administering the fluids for them in nurse clinics.
What about fluid therapy and anaesthesia?
This is probably one of the biggest areas where fluids are inadvertently overused - so let’s bust some myths and look at how we should be approaching fluid therapy in our anaesthetised patients.
Historically, we’ve been told to run fluids at 10 ml/kg/hour during anaesthesia.
The current guidelines support lower rates: 5 ml/kg/hour in dogs and 3–5 ml/kg/hour in cats with normal cardiac and renal function, and lower rates (if any) in patients at risk of fluid overload.
Why? Because not every anaesthetised patient needs fluids (they may have no significant losses!) and excessively high rates predispose patients to volume overload (and the complications that come with it).
We need to think about blood pressure first.
Mean arterial blood pressure should be maintained at (or ideally above) 60 mmHg. If hypotension occurs, we shouldn’t automatically treat this with fluids - we should first assess anaesthetic depth and reduce vapouriser settings where possible.
Before considering a fluid bolus, we need to ask where that hypotension is coming from. Is it drug-related? Is it due to fluid losses? If there are losses, then yes, fluids will be required. In fact, the guidelines suggest that a crystalloid bolus of 5 ml/kg over 10 minutes may be administered.
However, if there’s no change in the patient’s fluid balance, it may be more appropriate to consider vasopressors or anticholinergic agents under the vet’s direction. And if significant haemorrhage is present, blood products will be required.
The guidelines also emphasise monitoring total fluid volume administered during a general anaesthetic. If rates exceed 20 ml/kg, reassessment of intravascular volume status is indicated - so keep an eye on how much fluid your patient is receiving, and calculate blood volume percentages prior to surgery in high-risk procedures.
And given how crucial we are in anaesthetic management, the importance of our role as nurses and technicians in managing perioperative fluid therapy cannot be overlooked.
What about fluid overload? What is it, when do we see it, and how do we manage it as vet nurses?
Fluid overload is - as the name suggests - a complication of fluid therapy due to excessive administration. It’s described as a spectrum ranging from hypervolaemia to life-threatening oedema and cavitary effusions.
Clinical signs include acute weight gain, tachypnoea, crackles and moist lung sounds, pulmonary oedema, chemosis, peripheral oedema, pleural effusion, ascites, shivering, and restlessness.
Under anaesthesia, signs may include gallop sounds or new murmurs, paw swelling, nasal discharge, pulmonary crackles, and decreased oxygen saturation.
If fluid overload is suspected, fluid administration should be stopped or reduced to the minimum required for catheter patency. Furosemide may be required under veterinary direction in patients with pulmonary oedema or pleural effusion.
Patients at highest risk of fluid overload include those with anuric or oliguric kidney disease, cardiac disease, and very small patients where inadvertent overload is easy, especially where an infusion pump or syringe driver is unavailable.
To reduce the risk, administer fluids carefully and monitor patients thoroughly, with regular weight checks and respiratory checks in at-risk patients. Where possible, considering enteral adminisration may also be helpful, since this corrects dehydration without increasing circulating volume.
The guidelines repeatedly remind us that both hypovolaemia and hypervolaemia are detrimental - it is a balancing act, and one that we play a crucial role in maintaining as vet nurses.
So what does all of this mean for us as veterinary nurses?
Fluid therapy is not passive, and its quite possibly one of the biggest areas we can make a difference in as veterinary nurses and technicians.
We’re the ones monitoring these patients in the wards. We assess perfusion. We evaluate hydration. We calculate deficits. We monitor urine output. We track weight trends. We watch respiratory effort. We recognise subtle changes.
We are often the first to recognise that a bolus has improved perfusion (or that it hasn’t) , and we’re often the first to detect early signs of overload.
Fluids are drugs, with indications, contraindications and complications just like any other.
And when we understand the physiology behind them - fluid compartments, phases of fluid therapy and how fluids really affect our patients - we move from simply hanging bags and programming pumps to actively practising evidence-based nursing.
Did you enjoy this episode? If so, I’d love to hear what you think. Take a screenshot and tag me on Instagram (@vetinternalmedicinenursing) so I can give you a shout-out and share it with a colleague who’d find it helpful!
Thanks for learning with me this week, and I’ll see you next time!
References and Further Reading
Pardo, M., Spencer, E., Odunayo, A., Ramirez, M. L., Rudloff, E., Shafford, H., Weil, A., & Wolff, E. (2024). 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats. Journal of the American Animal Hospital Association, 60(4), 131–163. https://doi.org/10.5326/JAAHA-MS-7444
