Intravenous fluid therapy (IVFT) is commonly found in plans for treating emergent and critical patients (King and Boag, 2018), to support the cardiovascular system. IVFT can be used in the treatment of absolute or relative hypovolaemia (Yildiz and Karakoc, 2013), hydration deficits and to maintain normal water, electrolyte and acid–base balance. However, there is no one-size-fits-all approach to be used; all fluid therapy calculations are based on estimates (Malbrain et al, 2020) and what works well for one patient might not for the next. Both the inciting cause and ongoing pathology can influence the effect of IVFT. Goal-directed therapy is the gold standard approach for patients receiving IVFT (Voldby et al, 2018), making the nurse's role of monitoring the patient a very important part of providing the best treatment to the patient.
Water is a nutrient
King and Boag (2018) stated that the average patient's body weight is made up of the weight of dry matter (40%), such as tissues and bones, and also fluid (60%). Fluid can then be further subdivided into intracellular and extracellular portions, with extracellular being further divided into interstitial and intravascular compartments. Some texts (Hughston, 2016; Brandis, 2023) also include a transcellular compartment for fluid, such as vitreous fluid in the eye or cerebrospinal fluid surrounding the brain and spinal cord (Adam et al, 2001).
Water is a nutrient that is vital to the survival of patients, but one that must be replenished daily. The movement of water around the body and between compartments is dynamic and is influenced by various factors, such as changes in osmolality or electrolyte concentrations in any given compartment (Bassert, 2023). Fluid loss from all compartments is called dehydration. Clinical signs (Box 1) reflect this total body water loss. Dehydration is not immediately life-threatening and can be corrected over 24–48 hours. Hypovolaemia is fluid loss from the intravascular compartment. Although this is the smallest fluid compartment, it has the most profound effect on the body. The intravascular compartment is responsible for supplying oxygen and nutrients to the cells and removing waste; these mechanisms are reliant on adequate blood pressure and tissue perfusion. Clinical signs (Box 2) seen reflect the perfusion status of the patient. This condition is life-threatening and requires immediate treatment.
Box 1.Clinical signs of dehydration and percentage of fluid loss at which point they may be seen
- Dry mucous membranes ~5%
- Loss of skin turgor ~6–8%
- Sunken eyes ~6–8%
- Perfusion parameters >10%
- Acute weight loss relative to % dehydration
Box 2.Clinical signs of hypovolaemia
- Altered mentation
- Altered capillary refill time
- Tachycardia (dogs)
- Bradycardia (cats)
- Poor pulse quality
- Low blood pressure
- Hypothermia
Development of dehydration
Dehydration develops when free water loss is greater than free water intake. Causes vary and include periods of hot weather, illness – such as protracted vomiting or diarrhoea – or when it is not possible for the animal to take on water through normal means (Gregory, 2004). Alongside this free water loss, electrolyte disturbances can also occur, or these electrolyte disturbances then cause the free water loss, depending on the inciting cause. Most commonly, disturbances with sodium chloride (NaCl) and potassium (K) can be seen (Bassert, 2023). It is important to measure electrolytes in patients with dehydration or hypovolaemia, as these disturbances can provide important information on the causative disease process. As a patient continues to dehydrate, losses from the intravascular compartment become enough that signs of hypovolaemia develop. Severely dehydrated patients develop hypovolaemia concurrently as dehydration becomes life-threatening (Llewellyn et al, 2020).
Development of hypovolaemia
Hypovolaemia is a loss of water from just the intravascular space (Llewellyn et al, 2020). This could be free water as a result of a progressive dehydration or disease state, or it could be through loss of whole blood as a result of haemorrhage. In its early stages, mild hypovolaemia is detected by baroreceptors and sets off a sympathetic stimulation (King and Boag, 2018). This results in an increase in heart rate and contractility, called the hyperdynamic phase. Pulses will feel tall and bounding and capillary refill time will be fast and snappy. As the hypovolaemia continues, a peripheral vasoconstriction is set off to ensure blood pressure and perfusion to the major organs – the heart and brain – is maintained. This will cause weaker peripheral pulses and cooler extremities. Perfusion to organs, such as the gastrointestinal tract, is reduced. Continuing loss to the intravascular space will lead to hypovolaemic shock (Yildiz and Karakoc, 2013), where the body can no longer compensate. Blood pressure will drop and delivery of nutrients to cells is reduced, leading to irreversible organ injury and cell death (Llewellyn et al, 2020).
Fluid losses from the body
Water loss from the body can be divided into normal and abnormal losses. Normal losses include water in urine, faeces and breathing. Abnormal losses include water in vomit, diarrhoea, polydipsia, wound exudation, fluid from thoracic or abdominal drainage post-surgery or loss into third spaces (King and Boag, 2018). These losses can be classed as sensible and insensible losses. Sensible losses can be measured, such as urine production or wound exudate production. Insensible losses cannot be easily measured, such as normal breathing or panting with pyrexia.
Nursing the patient on fluid therapy
When assessing hypovolaemic patients, perfusion parameters are measured, these include mucous membrane colour and capillary refill time, heart rate and auscultation for the presence of murmurs, blood pressure and assessment of pulse profile (Llewellyn et al, 2020). It is good practice to get used to feeling normal peripheral pulses in dogs. The pulse profile of cats can be much harder to assess and they tend to respond differently to hypovolaemia; they can develop bradycardia rather than tachycardia. For dogs, in the face of hypoperfusion, peripheral pulses – such as the metatarsal or metacarpal – will change before more central pulses, such as the groin. A normal pulse profile can be described as wide and easily palpable. During the hyperdynamic phase of hypovolaemia, the pulse profile is snappy and tall. It can be described as a flick and is hard to occlude. As the patient decompensates, the pulse becomes smaller and weaker and is much easier to occlude. When dealing with an uncomplicated hypovolaemia, these perfusion parameters will change in very predictable ways when treated with resuscitation IVFT. This allows for goals and end points of resuscitation to be set.
Llewellyn et al (2020) suggest lab tests that can be useful when assessing dehydrated or hypovolaemic patients include packed cell volume/total solids and blood lactate measurements. As fluid is lost from the intravascular compartment during dehydration, the packed cell volume/total solids and lactate levels will rise. As fluid therapy is administered and that fluid is replaced, these measurements will decrease, giving end goals for resuscitation. For example, lactate is a measure of perfusion and returning to normal blood levels indicates improved perfusion and so is a desired end point of fluid therapy (King and Boag, 2018). Blood gas and electrolyte measurements will help identify any electrolyte or acid–base derangements and fluid therapy can be adjusted to correct these. In some hypovolaemic patients, such as those who have suffered blood loss, these levels are not immediately affected and may read within normal range. For these patients, assessment of perfusion parameters are preferred to assess end points of fluid therapy.
Monitoring the patient during initial fluid resuscitation requires close attention from the registered veterinary nurse. The patient status can change quickly and the registered veterinary nurse should be available to detect those changes. Monitoring the dehydrated patient is much less intensive and should initially be performed every few hours and once more stable, twice daily (Ballantyne, 2018). Monitoring should include individual goals set for that patient, a physical examination should be performed and any extra tests run. The physical examination should include assessment of perfusion parameters and hydration status (Box 1 and Box 2), along with assessment of body weight at least once a day. As the patient rehydrates, body weight should increase, if this increase is more than 10% in the first 24 hours, fluid overload should be suspected (Hansen, 2021). Extra tests include blood pressure, urine output and packed cell volume/total solids. Urine output should be between 0.5–2 ml/kg/hour. Placement of an indwelling catheter will allow close assessment of volumes voided but weighing inco sheets/bedding can also be performed. When no urine is produced (anuria), kidney injury should be suspected (King and Boag, 2018). Excessive fluid therapy can also lead to volumes voided >2 ml/kg/hour. Packed cell volume/total solids can be high when the patient is dehydrated and will return to normal levels as the patient rehydrates. The most common mistake with fluid therapy is a failure to adjust the plan as the patient condition changes (Malbrain et al, 2020). A dehydrated cat who begins to eat and drink a few hours into fluid deficit replacement, will then require less than the original calculation, as fluid is then also taken in via the enteral route.
Types of fluid
There are many types of fluids available but only a few that are suitable for initial stabilisation of hypovolaemic patients, and so, rather than changing fluid bags, only a few tend to be used for the correction of dehydration as well. The mainstay of initial resuscitation fluids is an isotonic crystalloid solution such as 0.9% saline or balanced crystalloids such as Hartmann's solution (Lewis et al, 2018). After 1 hour, 20% of the infused crystalloid solution remains in the circulation, the rest having moved out of the circulation into the interstitial compartment. In particular, Hartmann's solution lends itself to both fluid resuscitation and correction of fluid deficit and electrolyte imbalances while supporting ongoing losses. Hartmann's solution is similar in electrolyte composition to extracellular fluid, though it has slightly higher sodium and slightly lower potassium. If the patient is not eating, the solution will need to be further supplemented with potassium to avoid hypokalaemia. 0.9% saline may be chosen for economic reasons, as it is a relatively cheap fluid. It can, however, worsen a metabolic acidosis, but is useful for patients that are hyponatraemic or hypochloraemic (Malbrain et al, 2020).
Colloids are available in synthetic and natural forms. Synthetic forms include gelatins, dextrans and hydroxyethyl starches. Natural colloids, including human albumin, are growing in popularity for the treatment of hypoproteinaemia in critical care patients (Malbrain et al, 2020). Colloids will remain in the intravascular system for longer than crystalloids, supporting the circulation for longer and providing colloid osmotic pressure (Lewis et al, 2018). However, colloids have lost popularity in recent years and are less often reached for when supporting the critical patient. There have been concerns over contraindications and allergic reactions in humans and this has fed into the veterinary world (Yildiz and Karakoc, 2013; Lewis et al, 2018).
Fluid rate and time
When deciding the rate of dose of fluids for emergent and critical patients, calculations for providing fluid resuscitation should be separated from those for correcting fluid deficits, maintenance requirements and electrolyte correction and support. It is important to consider which fluid type is indicated for the patient's condition. All fluid therapy calculations are based on estimates taken from measuring perfusion parameters and hydration status. With this in mind, it is important to consider the best monitoring for the patient to ensure goals and fluid resuscitation end points are met without causing fluid overload to the patient.
Correction of hypovolaemia and dehydration
Intravenous resuscitation is when a bolus of fluid is given over a short period of time, the results of which increase the intravascular volume and aid in restoration of blood pressure. Shock doses of fluids (Box 3) are designed around total blood volumes: it would be rare to use this whole dose (King and Boag, 2018). Instead, smaller boluses are used and the patient response is evaluated before further boluses are given. Initial resuscitation is usually delivered over a short time frame of 30 minutes to 1 hour. With the smaller boluses being delivered over 5–10 minutes. At the end of each bolus, the chosen goals, such as reduction in heart rate or increase in blood pressure, are evaluated and the decision can then be made as to whether further boluses are needed. Before to delivering this sort of aggressive fluid therapy, consideration should be made as to whether the patient can handle the boluses. For patients with existing lung or heart disease, or those with suspected lung or brain injury, aggressive fluid therapy should be avoided (King and Boag, 2018). Cordemans et al (2012) suggested that cautious fluid therapy led to better outcomes in patients with lung injury.
Box 3.Shock doses of crystalloids
- Dog 80–90 ml/kg
- Cats 40–60 ml/kg
- (Bassert, 2023)
A successful outcome for the patient would be normovolaemia, through reaching individual resuscitation goals and achieving adequate tissue perfusion; leading to a cessation of clinical signs. This would be achieved without causing fluid overload. Fluid overload is when too much fluid is in the intravascular compartment and starts to leak out. This leaking usually occurs into the lungs, reducing oxygen exchange. Clinical signs including clear fluid from the nose and a cough can occur, and lung auscultation will reveal crackles as the fluid moves around during respiration (Hughston, 2016). Reduced renal function and wound healing can also be seen, so for critical patients at risk of fluid overload, end goals should be aimed at reaching a very mild hypovolaemia, rather than reaching normovolaemia (Malbrain et al, 2020).
An inadequate result from fluid resuscitation should prompt further investigation into fluid loss, such as the patient still haemorrhaging or development of complications such as sepsis or systemic inflammatory response syndrome. They may require different fluids such as colloids or blood products. A colloid dose of 20 ml/kg in dogs is equivalent in effect on the cardiovascular system to shock doses of crystalloid fluids; this is also given as smaller doses over short time periods. It is important to remember that although smaller doses are needed, the effect they have is just as profound and lasts longer, so should be avoided in heart or lung disease and lung or head injury.
Correction of dehydration is a much longer process. Once the patient is haemodynamically stable, the process of correcting a fluid deficit, electrolyte or acid–base imbalance can be started (King and Boag, 2018). Factors to be considered include:
- Estimated percentage of dehydration
- Daily maintenance requirements of the patient
- Any on-going losses with that patient.
The calculation comprises the sum of these three parts; deficit, maintenance and on-going losses. Box 4 shows an example calculation for correcting a 7% hydration deficit for a 35 kg dog over 48 hours.
Box 4.Example calculation for correction of 7% dehydration
- 7% dehydration = 7% body weight
- 35 kg × 0.07 = 2.45 kg
- 2.45 kg ~ 2.45 L
- 2450 ml over 48 hours
- Half in first 24 hours = 1225 ml Maintenance requirements
- 2 ml/kg/hr
- 60 ml/kg/day
- 35 kg × 60 = 2100 ml/day
Total calculation1225 ml + 2100 ml = 3325 ml/day3325 ml/24 = 138 ml/hr
Conclusions
Fluid requirements for emergent and critical patients fall into one of two categories: correction of hypovolaemia or correction of dehydration. These two conditions are quite different and prompt patient assessment and identification of these conditions is essential in selecting the right treatment for the patient. Response to treatment may be influenced by the inciting cause and though care of patients on IVFT may seem routine, it is essential to monitor response to treatment and development of unwanted side effects.
KEY POINTS
- Fluid therapy forms part of many treatment plans in emergent and critical patients.
- It is important to understand the difference between dehydrated and hypovolaemic patients.
- Dehydrated patients have lost fluid from all body fluid compartments.
- Hypovolaemic patients have lost fluid from just the intravascular compartment.
- Hypovolaemia is life-threatening and should be treated quickly with fluid boluses.
- Dehydration should be addressed second along with any electrolytes and acid–base disturbances, over a longer period of time, ie 12–24 hours.
- Gold standard fluid therapy is given to meet end-points while avoiding fluid overload.