Disorders of Sodium Concentration

Question 1

Blood sodium concentration is approximated by:

Answer 1

ratio of total body sodium to total body water

Question 2

How is ADH regulated?

Answer 2

• Osmotically: small (1-2%) increase in effective osmolality leads to thirst and increased ADH release
• Non-osmotically: large (10%) decrease in blood volume or pressure leads to increased ADH release (can override effect of osmolality); also drugs, pain, and stress

Question 3

Do we measure total body sodium or total body water in clinical practice?

Answer 3

No! When evaluating disorders of sodium concentration, we must decide if the total body sodium and/or water are high, normal, or low. This can be assessed through H&P and lab values.

Question 4

Hyponatremia almost always indicates what?

Answer 4

impaired renal water excretion

Question 5

How are volume status and orthostatic changes related?

Answer 5

symptom of hypovolemia = orthostatic changes (increased HR, decreased BP upon standing)

Question 6

Do symptoms of hypovolemia occur when there is a decrease in ICF or ECF?

Answer 6

ECF

Question 7

Does volume depletion alone typically result in symptoms of hypovolemia?

Answer 7

No, water depletion alone will only lead to clinically-evident hypovolemia if it is very severe. This is rare, though.

Question 8

What does a low urine sodium (<10 mmol/L) suggest?

Answer 8

extrarenal loss of Na+ or edematous disorder in which the kidneys are avidly reabsorbing Na+

Question 9

What does a high urine sodium (>20 mmol/L) suggest?

Answer 9

renal loss of Na+ or excess ADH (as in SIADH)

Question 10

What is the treatment for severe, symptomatic hyponatremia with neurological involvement?

Answer 10

hypertonic saline with or without diuretics

Question 11

Will total cessation of water intake lead to hypernatremia?

Answer 11

YES! Insensible water loss through respiration is so significant (500-700 mL/day) that, even with maximum diuresis, total cessation of water intake WILL lead to hypernatremia.

Question 12

What are the causes of hypervolemic hypernatremia?

Answer 12

(rare)

• hypertonic fluid administration
• salt poisoning / seawater ingestion
• mineralocorticoid excess states

Question 13

What are the causes of euvolemic hypernatremia?

Answer 13

• central diabetes insipidus (ADH release impaired/absent)
• nephrogenic diabetes insipidus (kidney does not respond to ADH)
• decreased thirst and water intake

Question 14

What are the causes of hypovolemic hypernatremia?

Answer 14

• Renal Na+ losses: diuretics w/ inadequate water intake, tubular injury
• Extrarenal Na+ losses: sweating, diarrhea, vomiting (w/ inadequate water intake)

Question 15

How is hypervolemic hypernatremia managed?

Answer 15

It can be very difficult and may require both water administration plus either diuretics or dialysis to remove the excess Na+.