Clinical pathology:clinical chemistry:(iii)Electrolyte metabolism

Question 1

What is the principal cation in the ECF?

Why is it important?ponatraemia

Answer 1

Sodium is the principal cation in ECF.

It is important in the movement of fluids across epithelial surfaces.

Question 2

What are the five main causes of hyponatraemia?

Answer 2

1. Pseudohyponatraemia
   a. Occurs with hyperlipidemia or hyperproteinaemia
   b. Plasma sample is diluted by the excess fluid or protein and thus the measured sodium concentration is falsely lowered.
2. Hyperosmolal, hypervolaemic conditions include hyperglycaemia and mannitol administration.
3. Hypoosmolal hypervolaemic conditions
   a. Occurs when there is excess water retention with dilution of the plasma
   b. Causes include nephrotic syndrome, chronic liver disease, chronic renal failure, and congestive heart failure.
4. Hypoosmolal euvolaemic conditions include hypotonic fluid infusion, antidiuretic hormone(ADH), and psychogenic polydipsia.
5. Hypoosmolal hypovolaemic conditions include the following:
   a. Dietary deficiency of sodium
   b. GI loss from vomiting or diarrhea
   c. Third-space syndrome(GI obstruction, peritonitis, uroabdomen, ascites)
   d. Urinary loss from hypoadrenocorticism, nonoliguric acute renal failure, diuretics, and Fanconi syndrome
   e. Cutaneous losses(burns)

Question 3

What are the 3 main causes of hypernatraemia?

Answer 3

1. Pure water deficits occur in dietary deficiency, central or nephrogenic diabetes insipidus, primary hypodipsia, heat stress and fever
2. Hypotonic fluid loss occurs with the following:
   a. GI loss owing to vomiting or diarrhea
   b. Third-space syndrome(peritonitis, ascited)
   c. Urinary loss from osmotic diuretucs(mannitolm diabetes mellitus), chronic renal failure, nonliguric acute renal failure, postobstructive nephropathy
   d. Cutaneous loss(burns)
3. Solute gain occur with salt poisoning , hypertonic fluid administration, hyperadrenocorticism, hyperaldosteronism

Question 4

What is the principal anion in the ECF? What is its function?

Answer 4

Chloride is the principal anion in ECF. Chloride usually accompanies sodium to maintain neutrality.
NOTE:
-Normal fractional excretion is less than 1% but may be elevated in large animals fed a diet higher in chloride.
-The same conditions causing hypernatraemia and hyponatraemia also cause hyperchloraemia and hypochloraemia.

Question 5

What is the principle cation of the ICF? What is its function?

Answer 5

Potassium is the principal cation of the ICF. It determines resting cell membrane potential.

Question 6

Give the 3 main causes of hypokalaemia.

Answer 6

Hypokalamia(typically associated wuth alkalosis)

1. Decreased intake
   a. Anorexia
   b. Dietary deficiency
   c. Administration of potassium-free fluids
2. Translocation between ECF and ICF
   a. Metabolic alkalosis, respiratory alkalosis
   b. Glucose or insulin administration
   c. Catecholamines
3. Increased loss
   a. GI loss
   (1) Vomiting
   (2) Diarrhea

b. Third-space syndrome
(1) GI obstruction(especially displaced abomasum)
(2) Peritonitis
(3) Ascites

c. Urinary loss
(1) Hyperadrenocorticism
(2) Acute renal failure(nonoliguric)
(3) Postobstructive diuresis
(4) Chronic renal failure(cats)
95) Potassium-losing diuretics
(6) Fanconi syndrome
(7) Renal tubular acidosis
(8) Prinary hyperaldosteronism

d.Cutaneous loss(burns)

4. Feline kaliopenic nephropathy-polymyopathy syndrome
   a. Characterised by hypokalaemia, increased fractional excretion of potassium, azotaemia, and metabolic acidosis
   b. Chronic decrease of potassium leads to decrease in aldosterone, which leads to distal renal tubular acidosis

Question 7

What are the 4 main causes of hyperkalaemia?

Answer 7

Hyperkalaemia(typically associated with acidosis)

1. Pseudohyperkalaemia(in vitro translocation of potassium to plasma)
   a. Thrombocytosis
   b. Leukaemia
   c. Haemolysis
   d. Akita dogs
2. Increased intake or oversupplementation of fluids with potassium
3. Translocation between ICF and ECF
   a. Respiratory or metabolic acidosis
   b. Hyperkalaemic periodic paralysis
   c. Ischaemia or reperfusion injury
4. Decreased urinary excretion
   a. Anuric or oliguric renal failure
   b. Urinary tract obstruction
   c. Ruptured urinary bladder
   d. Hypoadrenocorticism
   e. Potassium-sparing diuretics
   f. Nonsteroidal antiinflammatory drugs
   g. Angiotensin converting enzyme inhibitors

Question 8

Where is phosphorus most commonly found and how is it usually regulated?

Answer 8

Phosphorus is found mainly in ICF. It is regulated through interactions with calcium and calcium metabolic hormones.

Question 9

What is the function of calcitrol?(3)

Answer 9

-Calcitrol increases phosphorus resorption from bone, increases GI phosphorus absorption and increases urinary fractional excretion of phosphorus
NOTE: The concentrations of calcium and phosphorus are reciprocally related and are kept relatively constant.

Question 10

What is hypophosphataemia and what are 3 causes of such condition?

Answer 10

Hypophosphataemia is increased cellular uptake of phsophorus(glucose administration)

2. Acid-base balance
   a. Respiratory alkalosis
   b. Metabolic acidosis(enhanced urinary excretion of phsophates); often in diabetic ketoacidosis
3. Abnormalities in renal tubular phosphate reabsorption
   a. Hyperparathyroidism
   b. Fanconic syndrome
   c. Aminoglycoside nephrotoxicosis
4. GI absorption
   a. Decreased phosphorus in diet
   b. Vomiting
   c. Diarrhea
   d. Intestinal malabsorption syndromes
   e. Excessive ingestion of phosphate binders

Question 11

What are the 7 causes of hyperphosphatemia?

Answer 11

1. Redistribution between ICF and ECF
2. Cellular damage
3. Acute acidosis(chronic metabolic acidosis causes hypophosphataemua usually)
4. Decreased renal blood flow and GFR(resulting in secondary hyperparathyrodism)
5. Ruptured urinary bladder
6. Hypertonic sodium phosphate enemas
7. Excessive dietary intake(with secondary hyperparathyroidism)

Question 12

What is the function of magnesium?(2)

Answer 12

1. magnesium is an important co factor for amny enzymatic reactions.
2. Influences cell membrane properties.

Question 13

Give 4 causes of hypomagnesemia.

Answer 13

1. Most often occurs after excessive magnesium loss
   a. GI tract(malabsorption syndromes, diarrhea)
   b. Renal loss(fluid diuresis, diuretic therapy, renal disease)
2. Iatrogenic deficiency occurs during fluid therapy as most fluids contain little or no magnesium
3. Metabolic disorders(diabetes mellitus, primary hyperparathyroidism, primary hypoprathyroidism, hyperthyroidism, hyperaldosteronism, third-space syndrome, hypophosphataemia)
4. Ruminatns
   a. Milk tetany, in which calves are fed a magnesium-deficient milk diet
   b. Grass tetany, which occurs in adults fed on lush, green pasture that is high in potassium, which blocks magnesium absorption from the rumen

Question 14

Give 3 causes of hypermagnesaemia.

Answer 14

1. Renal disease(both acute and chronic)
2. Increased renal tubular reabsorption of magnesium during dehydration, salt depletion and hypoadrenocorticism
3. Overadministration if magnesium-containing antacids

Question 15

What are the different roles of calcium?(4)

Answer 15

1. Major structural role in the skeleton
2. Important in regulation of ions across membranes
3. Cofactor in many metabolic processes
4. Major role in signal transmission, skeletal muscle contraction, and cardiovascular function

Question 16

What measurements provide an accurate assessment of the calcium status?
Why should adjustment formulas for total calcium not be used?
What is the effect of acidosis and alkalosis in the ionised calcium concentration?

Answer 16

Ionised calcium should be measured for an accurate assessment of calcium status.
Adjustment formulas for total calcium should not be used because they do not reliably predict ionised calcium concentration.
Acidosis increases ionised calcium concentration and alkalosis decreases ionised calcium concentration.

Question 17

What is the primary electrolyte metabolism in the case of primary hypoparathyroidism and what are the different causes of primary hyperparathyroidism?

Answer 17

1.Primary hypoparathyroidism is characterised by hypocalcaemia with a low or low-normal concentration of parathyroid hormone(an inappropriate response). Hypomagnesaemia may also be seen. Primary hypoparathyroidism can be spontaneously occurring; result from parathyroiditis or parathyroid adenoma infarction; or occur postoperatively after removal of a parathyroid adenoma or any other neck surgery that can interrupt the blood supply to the parathyroud glands.

Question 18

What are some common causes of hypocalcaemia?(6)

Answer 18

• Hypoalbunaemia
• Chronic renal failure(ionised hypocalcaemia)
• Eclampsia
• Acute renal failure
• Acute pancreatitis
• Urethral obstruction in cats

Question 19

What are some occasional causes of hypocalcaemia?

Answer 19

• Soft tissue trauma
• Rhabdomyolysis
• ethylene glycol poisoning
• Phosphate enemas
• Post bicarbinate administration
• Critical illness
• sepsis

Question 20

What are some uncommon causes of hypocalcaemia?(8)

Answer 20

• EDTA contamination
• Dilution with calcium-free IV fluids
• Intestinal absorption
• Hypovitaminosis D
• Pancreatitis
• Citrated blood transfusions
• Hypomagnesaemia
• Tumor lysis syndrome

Question 21

What is the most common cause of hypercalcemia in dogs?

Answer 21

Neoplasia is the most common cause of hypercalcaemia in dogs.
NOTE: Neoplasia is characterised ny an elevation of both total and ionised calcium, wirh parathyroid hormone suppressed into the lower part of or below the reference range(a parathyroid-independent hypercalcaemia)

Question 22

What is the most common neoplasm causing hypercalcaemia in dogs? Give examples of other neoplasms that may cause hypercalcaemia.

Answer 22

In dogs, the most common neoplasm causing hypercalcaemia in dogs is lymphoma.
Other neoplasms include anal sac apocrine gland, adenocarcinoma, thymoma, carcinomas(lung, pancreas, mammary gland, skin, nasal cavity, thyroid, adrenal medulla), and haematologic malignancies(multiple myeloma, lymphoma, myeloproliferative disease, leukemia).

Question 23

What are the most common neoplasias causing hypercalcaemia in cats? Give examples of other hypercalcaemia-causing neoplasms.

Answer 23

In cats, the most common neoplasias causing hypercalcaemia are lymphoma and squamous cell carcinomas. Other neoplasms include multiple myeloma, leukemia, osteosarcima, fibrosarcoma and bronchogenic carcinoma.

Question 24

What is the common cause of cause of ionised hypercalcaemia in cats?

Answer 24

Idiopathic hypercalcaemia is the most common cause of cause of ionised hypercalcaemia in cats.

Question 25

Give an example of a condition which leads to an elevation of serum total calcium but not of ionised calcium.

Answer 25

Renal disease is a common cause of an elevation of serum total calcium but not of ionised calcium. With renal disease, serum ionised calcium concentration is typically normal to low.

Question 26

Give examples of how vitamin D toxicity can arise.

Answer 26

-Oversupplementation with vitamin D
-Ingestion of plants containing calcitrol glycosides
-Ingestion of cholecalciferol rodenticides
-Ingestion of antipsoriasis cream(Dovonex)
Vitamin D toxicity is a parathyroid-independent hypercalcaemia and an elevation in phosphorus is typically observed.

Question 27

Give an example of a condition that gives rise to an elevation of both serum total and ionised calcium.

Answer 27

Primary hyperparathyroidisma is a condition that gives rise to an elevation of both serum total and ionised calcium with lack of suppression of parathyroid hormone production.
Prathyroid hormone concentration may be still within normal limits, or it may be elevated.

Question 28

Give other causes of hypercalcaemia.(12)

Answer 28

• Hypoadrenocorticism
• Osteolytic processes
• Granulomatous disease
• Grape or raisin toxicity
• Dehydration
• Vitamin A toxicity
• Aluminium toxicity
• Excessive calcium carbonate supplementation
• Intestinal phosphate binders
• Thiazide diuretics
• Acromegaly
• Severe hypothermia