Heart, Hydration & Electrolytes

While the kidneys are the filters, they are also the master chemists of the blood. They strictly regulate the electrical system of the body by balancing electrolytes.

For a cat with Chronic Kidney Disease (CKD), these numbers tell us about Hydration, Heart Health, and Acid-Base Balance. When these numbers drift, your cat feels physically weak, nauseous, and "off."

Here is your guide to the electrolytes and metabolic markers.

1. The Hydration Indicators: Salt and Water

Sodium and Chloride are the primary markers we use to determine if the blood is too "salty" (concentrated) or too watery (dilute).

Sodium (Na⁺)

Normal: 145–158 mEq/L
What it is: The main salt in the blood. It holds onto water.
High (Hypernatremia): Almost always means Dehydration. The cat has lost more water than salt (free water deficit). This confirms the cat needs more fluids (subcutaneous or oral).
Low (Hyponatremia): Rare in cats, but can happen with severe losses (diarrhea) or if you are giving too much fluid that doesn't contain electrolytes (like plain water).

Chloride (Cl⁻)

Normal: 112–129 mEq/L
What it is: It usually travels with Sodium.
The Vomiting Signal: Chloride is unique because it is a major part of stomach acid (Hydrochloric acid). If a cat is vomiting frequently, they lose massive amounts of Chloride.
- The Pattern: If Sodium is normal but Chloride is very low, your cat is likely losing stomach acid from vomiting or severe nausea.

2. The "Spark Plugs": Muscle and Heart Function

Potassium and Magnesium are the batteries that allow muscles to contract and the heart to beat. CKD cats are notorious for losing these in their urine.

Potassium (K⁺)

Normal: 3.5–5.1 mEq/L
The "Goldilocks" Mineral: It must be just right. Too high stops the heart; too low stops the muscles.
Low (Hypokalemia): Very Common in CKD.
- Why: Dilute urine washes potassium out of the body.
- The Symptom: Cervical Ventroflexion. The cat looks like it is gazing at its navel; it physically cannot lift its head because the neck muscles are too weak. Generalized weakness and constipation are also common.
High (Hyperkalemia): Dangerous.
- Why: Usually seen in "End Stage" (Stage 4) when the cat stops making urine (anuria) or if there is a urinary blockage.
- False Alarm: If the blood draw was difficult and red blood cells burst (hemolysis), potassium spills out of the cells, causing a falsely high reading. Always check if the sample was "hemolyzed."

Magnesium (Mg²⁺)

Normal: 1.5–2.5 mg/dL
The "Forgotten" Electrolyte: Many standard panels do not include this, but you should ask for it.
The Connection: Magnesium is required for the body to absorb Potassium.
- The Tip: If your cat has low Potassium and supplements aren't working, check the Magnesium. If Magnesium is low, you cannot fix the Potassium until you fix the Magnesium first.

3. The Bone & Mineral Group

We discussed Phosphorus in Part 1, but Calcium is its partner.

Calcium (Total vs. Ionized)

Normal (Total): 8.2–11.8 mg/dL
What it is: Essential for nerve firing and bone strength.
The Trap: "Total Calcium" on the panel is often misleading because calcium rides on protein (Albumin). If Albumin is low, Total Calcium looks low, even if the cat is fine.
The Solution: If Total Calcium is abnormal, ask for an Ionized Calcium (iCa) test. This measures the active calcium that actually matters to the heart and nerves.
The Danger (Calcification): If Calcium × Phosphorus > 70, the minerals can precipitate out of the blood and turn into stone in the soft tissues (metastatic calcification), permanently damaging the kidneys.

4. Acid-Base Balance: The pH of the Blood

Kidneys are responsible for removing acid from the blood. When they fail, acid builds up.

Bicarbonate (TCO₂ or HCO₃⁻)

Normal: 16–24 mmol/L
What it is: The body's natural buffer (like baking soda) that neutralizes acid.
Low Levels: Mean Metabolic Acidosis. The kidneys are failing to dump acid, so the body is using up all its Bicarbonate trying to neutralize it.
- The Symptom: Acidosis makes cats feel terrible. It causes nausea, dissolves bone, and wastes muscle mass.
- Treatment: If TCO₂ is consistently < 16, vets may prescribe oral alkali therapy (like sodium bicarbonate or potassium citrate).

Anion Gap

Normal: 10–25 mEq/L
What it is: A calculated number: (Sodium + Potassium) - (Chloride + Bicarbonate).
The Clue: If the Anion Gap is High, it confirms that unmeasured acids (uremic toxins) are building up in the blood. It supports the diagnosis of Metabolic Acidosis.

5. Fat Metabolism

Cholesterol

Normal: 75–220 mg/dL
In CKD: High cholesterol is surprisingly common. It is not a heart risk like in humans.
Nephrotic Syndrome: If cholesterol is extremely high and Albumin is extremely low, it suggests significant protein loss through the kidneys (Glomerulonephritis).

Triglycerides

Normal: 20–110 mg/dL
The Fasting Check: High levels usually mean the cat ate before the blood draw. However, persistent elevation can indicate Lipid Metabolism disorders or Pancreatitis.

Summary for the Caretaker

Weak cat? Check Potassium and Magnesium.
Vomiting cat? Check Chloride and Bicarbonate (Acidosis).
Dehydrated cat? Check Sodium and Total Protein.

Correcting these electrolytes often provides the biggest "quality of life" boost for a CKD cat. A cat with balanced electrolytes feels stronger, eats better, and is more active.

References

DiBartola, S. P. (2012). Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice (4th ed.). Elsevier Health Sciences.
Sparkes, A. H., et al. (2016). ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease. Journal of Feline Medicine and Surgery, 18(3), 219–239.
Riordan, L. L., & Schaer, M. (2011). Potassium disorders. Topics in Companion Animal Medicine.
Schenck, P. A., & Chew, D. J. (2008). Calcium: total or ionized? Veterinary Clinics: Small Animal Practice, 38(3), 497–502.
Polzin, D. J. (2011). Chronic Kidney Disease in Small Animals. Veterinary Clinics: Small Animal Practice.