A Comprehensive Guide to Understanding IV Fluid Therapy in Dogs and Cats, Part 4: What are the Types of Fluid Disturbances?

Introduction

Fluid disturbances within the body can arise from several pathways. Identifying the precise origin of the disturbance is essential in determining the optimal therapeutic strategy. Doing so guides decisions regarding the administration route, the required volume, as well as the specific type of fluid needed for the patient.

Types of fluid disturbances include:

• Volume (e.g. dehydration, hypovolemia, and hypervolemia)
• Content (e.g. electrolyte derangements)
• Distribution (e.g. effusions and edema)

This article outlines some of the most frequent fluid disturbances encountered by a clinician. Each condition can be explored in significantly greater depth.

Volume Disturbances: Dehydration

Dehydration represents a total-body depletion of fluid, primarily driven by a deficit within the interstitial fluid compartment, though it can involve all fluid compartments. This condition presents with classic clinical signs of dehydration and frequently stems from issues like vomiting, diarrhea, and/or polyuria.

Clinical signs of dehydration include:

• 5% dehydration
  • Mildly tacky mucous membranes
  • Mild skin tent
• 8% dehydration
  • Moderately tacky mucous membranes
  • Moderate skin tent
  • Increase in capillary refill time
  • Mildly sunken eyes
  • Weak and rapid pulse
• 10% dehydration
  • Significantly tacky mucous membranes
  • Significant skin tent (skin continues to stand up)
  • Moderate to significant increase in capillary refill time
  • Sunken eyes
  • Increased heart rate
  • Weak and thready pulse
• 12% dehydration
  • All of the above plus shock, hypothermia, weakness, and life-threatening

Volume Disturbances: Hypovolemia

Hypovolemia results from a deficit specifically within the intravascular fluid compartment, manifesting as perfusion deficits like shock. It can occur independently or alongside dehydration, precipitated by factors such as blood loss, decreased oncotic pressure, vasodilation, and/or severe dehydration.

Clinical signs of hypovolemia (perfusion deficit) include:

• Decreased urine output
• Weakness
• Collapse
• Decreased mentation
• Pale mucous membranes
• Prolonged CRT
• Tachycardia, though cats may exhibit bradycardia, and dogs may also experience bradycardia during terminal stages
• Hypotension
• Cold extremity temperature
• Weak peripheral pulses

Volume Disturbances: Hypervolemia

Hypervolemia indicates fluid excess, resulting in clinical symptoms of fluid overload. Common causes include heart failure and iatrogenic over-administration of fluids.

Clinical signs of hypervolemia (fluid overload) include:

• Excessive urine output
• Unintended weight gain
• Hypothermia
• Tachycardia
• Bradycardia
• Pitting edema
• Chemosis
• Tachypnea
• Increased lung sounds
• Lung crackles
• Cough
• Dyspnea
• Serous nasal discharge
• Jugular venous distention

Content Disturbances: Electrolyte Derangements

Key electrolyte derangements involve sodium ion (Na⁺), chloride ion (Cl^–), and potassium ion (K⁺):

• ⬇️ Na⁺
  • Hypotonic dehydration (hypertonic fluid loss)
    • Diarrhea (if NA⁺ loss > water loss), vomiting (if NA⁺ loss > water loss), renal disease, diabetes mellitus, diabetic ketoacidosis, hypoadrenocorticism, congestive heart failure, certain diuretics, or third-space loss
• ⬆️ Na⁺
  • Hypertonic dehydration (hypotonic water loss)
    • Diarrhea (if water loss > NA⁺ loss), vomiting (if water loss > NA⁺ loss), panting, fever, renal disease, diabetes insipidus, hyperadrenocorticism, decreased water intake, or salt toxicity
• ⬇️ Cl^–
  • Typically follows Na⁺; same causes as ⬇️ Na⁺
  • If normal Na⁺, may be due to ⬆️ HCO3^– or from conditions such as pure gastric vomiting of HCl resulting in HCO3^– retention (hypochloremic metabolic alkalosis) and may raise suspicion for a pyloric obstruction
• ⬆️ Cl^–:
  • Typically follows Na⁺: same causes as ⬆️ Na⁺
  • If normal Na⁺, may be due to ⬇️ HCO3^– or from conditions such as NaHCO₃^– fluid loss with diarrhea (hyperchloremic metabolic acidosis)
• ⬇️ K^–:
  • Prolonged anorexia, diarrhea, vomiting, diabetic ketoacidosis, renal disease, diuresis, metabolic alkalosis, or insulin therapy
• ⬆️ K^–:
  • Hypoadrenocorticism, diabetes mellitus, anuric or oliguric renal failure, urethral obstruction, or metabolic acidosis

Additional electrolyte derangements include magnesium ion (Mg²⁺) and calcium ion (Ca²⁺).

Content Disturbances: Osmolarity, Osmolality, and Tonicity

Fluid imbalances can also result from shifts in osmolarity (or osmolality) and/or tonicity.

Shifts in tonicity cause abnormal movement of water into or out of cells, changing cellular size and fluid distribution, and are classified into three primary categories: isotonic, hypertonic, and hypotonic.

Some solutes contribute to both osmolarity (or osmolality) and tonicity, whereas others affect only osmolarity (or osmolality) and do not impact tonicity.

Osmolarity is the total concentration of all solutes, both permeable and impermeable, within a particular volume of a solution, expressed in mOsm/L.

Osmolality is the total concentration of all solutes, both permeable and impermeable, within a particular mass of a solution, expressed in mOsm/kg.

Tonicity is the effective osmolarity or effective osmolality, determined by the impermeable solute concentration (effective osmoles, which are almost entirely dependent on Na⁺) between two solutions on either side of a semipermeable membrane. This establishes the effective osmotic gradient that dictates the direction and extent of solvent diffusion; “water follows salt”.

Content Disturbances: Acid-Base Derangements

Acid-base derangements may include:

• ⬇️ HCO3^–:
  • Diarrhea, vomiting, diabetic ketoacidosis, or renal failure
• ⬆️ HCO3^–:
  • Vomiting with pyloric obstruction

Content Disturbances: Additional Derangements

Additional content disturbances may also involve derangements in glucose, red blood cells, and/or proteins.

Distribution Disturbances

Distribution disturbances include effusions and edema. Effusions are abnormal fluid shifts into body cavities, whereas edema is abnormal fluid shifts into interstitial spaces. These shifts are driven by changes in hydrostatic pressure, oncotic pressure, and/or vascular permeability.

References

1. DiBartola, S. Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice, Fourth Edition. Elsevier Saunders, 2012
2. Ettinger S, Feldman E, Cote E. Textbook of Veterinary Internal Medicine, 8th Edition, Elsevier, 2017
3. Pardo M, Spencer E, Odunayo A, et al. 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats. J Am Anim Hosp Assoc 2024 Jul 1:60(4):131-163
4. Davis H, Jensen T, Johnson A, et al. 2013 AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats. J Am Anim Hosp Assoc 2013 May-Jun;49(3):149-59
5. Silva C, Pedro M. Intravenous fluid therapy: essential components and key considerations. Porto Biomedical Journal 10(4):e296, July/August 2025
6. Levick J. Revision of the Starling principle: new views of tissue fluid balance. NIH. Accessed April 25, 2026.
7. Michel C, Woodcock T. Advances in the Starling Principle and Microvascular Fluid Exchange; Consequences and Implications for Fluid Therapy. NIH. Accessed April 25, 2026.
8. Michel C, Woodcock T, Curry F. Understanding and extending the Starling principle. Accessed April 28, 2026.
9. Levick J, Michel C. Microvascular fluid exchange and the revised Starling principle. European Society of Cardiology. Accessed April 28, 2026.
10. Dr. Danelia de Kock’s Veterinary School notes