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Flashcards in Fluids and Electrolytes Deck (15)
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1

What are the two commonest causes of hyponatraemia in neurosurgical patients?

1. SIADH;
2. CSW.

2

What are the features of SIADH?

1. Dilutional hyponatraemia with normal or elevated intravascular volume;
2. Treated with fluid restriction.

3

What are the features of CSW?

1. Inappropriate natriuresis with volume depletion;
2. Treated with volume and sodium replacement.

4

What feature differentiates SIADH from CSW?

Volume status:
1. Clinical assessment of hydration;
2. Normal saline infusion test, if hyponatraemia corrects then dehydration was the cause;
3. CVP <5cm H2O suggests hypovolaemia;
4. Serum potassium decreased or normal in SIADH, and increased or normal in CSW.

5

What are the symptoms of mild hyponatraemia?

1. Headache;
2. Anorexia;
3. Difficulty concentrating;
4. Irritability;
5. Muscle weakness;
6. Dysgeusia (disturbed taste).

6

What are the symptoms of severe hyponatraemia?

1. Confusion;
2. Muscle twitching and cramps;
3. Nausea and vomiting;
4. Seizures;
5. Respiratory arrest;
6. Permanent neurological deficit;
7. Coma;
8. Death.

7

What three diagnostic criteria are required for SIADH?

1. Hyponatraemia;
2. Inappropriately concentrated urine;
3. Normal renal and adrenal function.

8

What is the risk of rapid correction of hyponatraemia?

Central pontine myelinolysis.

9

What is the pathophysiological mechanism of CSW?

Renal loss of sodium as a result of intracranial disease producing hyponatraemia and decreased extracellular fluid volume from an unknown mechanism in which the kidneys fail to conserve sodium.

10

What are the treatment goals of CSW?

1. Volume replacement;
2. Positive salt balance.

11

What is the commonest cause of hypernatraemia in neurosurgical patients?

Diabetes insipidus.

12

What is the mechanism of DI?

Low levels of ADH (or rarely due to renal insensitivity to ADH).

13

What is the urine output in patients with DI? Serum osmolality and sodium?

High urine output of dilute urine (SG <200 mOsmol/L). Serum osmolality is normal or high, serum sodium is high.

14

What percentage of ADH secretory capacity must be lost before DI ensues?

85%.

15

What is the differential diagnosis of DI?

1. Neurogenic (true DI);
2. Nephrogenic;
3. Psychogenic (polydipsia);
4. Osmotic diuresis;
5. Diuretic use.