Section 6: Fluid Overload

The guidelines task force has proposed that fluid intolerance may be the more appropriate term for this condition, as this term more accurately describes how the amount of fluid needed to overload a patient is dependent on their tolerance for a given amount of fluids. “Fluid intolerance” also encompasses both iatrogenic overload and overload due to underlying comorbidities. However, given that “fluid overload” is still widely used and recognized within the veterinary medical profession, it will be the primary term used in these guidelines to refer to this condition.

All patients receiving fluid therapy are susceptible to fluid intolerance/fluid overload; however, excessive administration is the most common cause of fluid overload. Hypervolemia might be more common than perceived, as early signs of increased body weight and positive fluid balance are difficult to assess. Typically, hypervolemia is recognized when advanced signs such as edema and effusions occur.¹ It is important to understand the harms of fluid overload and the need to prevent this complication, given the limited effectiveness of treatments.

Top Three Takeaways

1. First, do no harm. Fluid overload is a potentially life-threatening complication for which no universally effective therapy exists.
2. Excessive iatrogenic fluid administration is the most common cause of fluid overload. Prevention and vigilant monitoring are crucial to mitigate the potential risks associated with fluid therapy.
3. Patients with impaired renal function are at an increased risk of fluid overload because of their kidneys’ inability to increase urine output and effectively excrete excess fluid.

Recognizing and Managing Fluid Overload

As discussed earlier, fluid movement is influenced by hydrostatic pressure, oncotic pressure, and vascular permeability. Patients who receive excess fluids during resuscitation, maintenance, or anesthesia first develop hypervolemia, which causes an increase in hydrostatic pressure. A decrease in oncotic pressure can occur with dilution of plasma proteins associated with excessive fluid administration or protein losing disease. Hypervolemia, as well as inflammation and injury, can damage the glycocalyx and increase vascular permeability. Together, these changes in fluid dynamics promote movement of fluid out of vessels into the interstitial space. As the interstitial space expands, it becomes more compliant, accommodating larger quantities of interstitial fluid, further exacerbating the problem of fluid overload.

Changes in fluid dynamics favoring accumulation of interstitial fluid are often seen in combination with the body’s physiological response to stress. Hypovolemia, illness, and injury lead to water and sodium retention and increased thirst. This response exacerbates the risk of fluid overload as resuscitation fluids are administered to critically ill patients when their bodies are primed to retain fluid.²

By the time clinical edema or effusions are noted, internal edema will also be present that can negatively impact organ function (Figure 11).³ Once edema has occurred, it is very difficult to reverse even with ultrafiltration. Thus, prevention and early recognition are keys to minimizing the morbidity and mortality associated with fluid overload.

Although all patients receiving IV fluids are at risk of fluid overload, those with impaired renal function, heart disease, or liver disease or those receiving large fluid volumes are at highest risk (Box 5). Pets with oligoanuric renal failure are particularly susceptible to fluid overload because they are unable to excrete excess fluid. Yet these patients often receive aggressive fluid therapy necessitating close monitoring for weight gain, hypertension (in presence of AKI/CKD), or early signs of edema.^4,5,6

• To reduce the risk of fluid overload:
• Use a low-volume strategy for fluid resuscitation.
• Tailor ongoing fluid therapy to meet each patient’s needs.
• Include all fluids delivered as medications, infusions, flushes, and enteral feedings to improve the accuracy of fluid balance calculations. Cats and small dogs with smaller blood volumes can quickly become fluid overloaded if clinicians are not mindful of the volumes administered.
• Limit the total fluid volume delivered in anesthetized patients during lengthy procedures. As a general rule, patients receiving anesthetic fluids should receive no more than the daily recommended total of 20 mL/kg/24 hr.⁷
• Consider enteral water supplementation.

To recognize hypervolemia sooner, weigh patients frequently (every 6–12 hr) to identify a 10% increase in body weight compared with the initial baseline.^8,9 Regularly document all fluid inputs and outputs to help identify positive fluid balance. Additional signs of fluid overload are summarized in Table 15.

Once edema has started, it can become life-threatening and be very difficult to resolve. See Box 6 for common misconceptions that can lead to fluid overload. There is no definitive therapy. General therapeutic approaches include sodium and water restriction, discontinuing IV fluids, administering diuretics, and increasing patient mobility (Figure 12). The interventions and success of those interventions are limited. Thus, the best approach is to avoid fluid overload.^10,11

Figure 11

Impact of Fluid Overload on Organ Function

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1. Continued IV fluid therapy in a feline patient with renal disease. A cat is dehydrated, uremic, and anuric on presentation. Although IV fluid administration improves hydration status, uremia persists. IV fluids are continued with the misguided goal of improving GFR. However, no effective increase in GFR will occur no matter how much fluid this patient receives.
2. IV fluid therapy in a patient anesthetized for a lengthy procedure. A dog receives 10 mL/kg of fluids throughout a 6 hr procedure and develops respiratory distress during anesthetic recovery.
3. SC fluid therapy in a cat with occult or fulminant heart disease. A cat presents for evaluation of vomiting associated with malaise of congestive heart failure and is given SC fluids despite no evidence of dehydration.

AKI, acute kidney injury; Ao, aorta; CKD, chronic kidney disease; La, Left atrium; SpO₂, oxygen saturation

1. Cole LP, Jepson R, Dawson C, Humm K. Hypertension, retinopathy, and acute kidney injury in dogs: A prospective study [published correction appears in J Vet Intern Med. 2020 Nov;34(6):3168]. J Vet Intern Med. 2020;34(5):1940-1947.
2. Park S, Lee CJ, Lee M, et al. Differential effects of arterial stiffness and fluid overload on blood pressure according to renal function in
patients at risk for cardiovascular disease. Hypertens Res. 2019;42:341–353.

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Misconception: IV fluids increase GFR in normally hydrated patients.
Correction: IV fluid administration will not increase GFR in a patient that is hydrated and euvolemic.

Misconception: Hypertension can be used to diagnose hypervolemia.
Correction: Hypertension is rarely associated with hypervolemia or edema. Some patients with renal disease may develop hypertension concomitant with hypervolemia.

Misconception: If a patient is hypotensive, keep giving fluid boluses until their blood pressure improves.
Correction: Some hypotensive patients will not respond to IV fluid therapy. If a patient is not responding to fluid therapy, discontinue fluid boluses and correct any electrolyte or glucose disturbances and consider vasopressor treatment.

Misconception: Hemodialysis is always a backup option for treatment of fluid overload if diuretics do not work.
Correction: Hemodialysis has a very limited therapeutic role in treating edema because the movement of edema fluid into the vascular space is limited.

Figure 12

Fluid Overload Therapy Algorithm
ACE, angiotensin-converting enzyme; ARBs, angiotensin receptor blockers; GFR, glomerular filtration rate

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