Hyponatremia

Question 1

tonicity

Answer 1

the volume behavior of cells in a solution

hypotonic, isotonic, and hypertonic

effective mOsms/total body H2O

Question 2

effective vs. ineffective mOsms

Answer 2

effective means that the mOsms affect transmembrane water flow across an impermeable membrane, while ineffective mOsms do not

Question 3

osmolarity

Answer 3

(effective + ineffective osms)/total body H2O

Question 4

organum vasculosum of the lamina terminalis (OVLT)

Answer 4

circumventricular oran organ in the brain that lacks a blood-brain barrier

may be a site of principal brain osmoreceptors affecting thirst and release of ADH from the posterior pituitary

areas of adjacent hypothalamus near the anterior wall of the third cerebral ventrical may also serve a similar purpose

Question 5

What is the true determinant of osmoreceptor activity?

Answer 5

the degree of stretch of the osmoreceptor cell membrane in the face of changing tonicity that engages stretch-activated or stretch-inactivated channels

Question 6

Where is ADH synthesized and release from?

Answer 6

synthesized in the supraoptic and paraventricular nuclei of the hypothalamus

released from the posterior pituitary

Question 7

What are some of the signals that trigger ADH release?

Answer 7

osmoreceptors in the OVLT responding to plasma tonicity

other neural paths to the brain from non-osmotic stimuli related to volume depletion, nausea, pain, sedation, and slected drugs

Question 8

What components make up urine volume?

Answer 8

electrolyte clearance (C^e) + electrolyte-free H2O clearance (C^efH2O)

Question 9

electrolyte-free H2O clearance equation

Answer 9

CefH2O = solute excretion (mOsm)/urrine mOsm/L

Question 10

What are the three types of hyponatremia?

Answer 10

isotonic hyponatremia - artifactual

hypotonic hyponatremia - decreased C^efH2O

hypertonic hyponatremia - translocational

Question 11

causes of isotonic hyponatremia

Answer 11

hypergammaglobulinemia

hypertriglyceridemia

hyperchylomicronemia

Question 12

cause of hypertonic hyponatremia

Answer 12

the addition of effective mOsms in the ECF such as glucose, mannitol, and sucrose

Question 13

symptoms of hypotonic hyponatremia

Answer 13

most early symptoms occur as the SNa drops below 125 mEqv/L

nausea

fatigue

headache

lethargy

somnolence

coma

seizures

Question 14

brain response to hypotonic hyponatremia

Answer 14

rapid adaptation through loss of sodium, potassium, and chloride

slow adaptation through loss of organic osmolytes

Question 15

What is the only way to get hypotonic?

Answer 15

take in H2O and fail to excrete it

water intake needs to exceed the C^efH2O

never caused by the kidney excreting more Na than H2O

Question 16

What are the three outcomes of hypotonic hyponatremia?

Answer 16

volume depletion

near euvolemia

edema

Question 17

hypotonic hyponatremia with volume depletion

Answer 17

high H2O and low TB Na

caused by vomiting/diarrhea, sweating, exercise, diuretics, Addisons, etc.

reduction of EABV -> increased baroreceptor/sympathetic effect on renal afferent arterioles reducing GFR and activation of RAAS

antiotensin II increases Na/H, Na/K/Cl2, and Na/Cl transporters

persistent aldosterone effect on the collecting duct associated with ADH release ensures recapture of both Na and H2O, resulting in oliguria

patient takes hypotonic fluids increasing TB H2O relative to TB effective mOsms

Question 18

hypotonic hyponatremia with edema

Answer 18

increased H2O and increased TB Na

caused by cirrhosis, nephrosis, heart failure, and renal failure

perception of a reduced EABV stimulates retention of Na and H2O through RAAS and ADH

patient then takes in hypotonic fluids increasing TB H2O relative to TB effective mOsms

Question 19

near euvolemia

Answer 19

increased H2O and normal TB Na

common causes are:

psychogenic polydipsia

potomania

thiazide induced SIADH

reset osmostat

Question 20

What is the maximum rate at which two normal kidneys can two normal kidneys excrete absent ADH?

Answer 20

28L of H2O/day

or

1.2 L/hr

Question 21

psychogenic polydipsia

Answer 21

near euvolemia hypertonic hyponatremia

seen in the setting of psychosis, particularly schizophrenia with OCD

patient drinks a lot of water, 3-4L/hr, retain 2-3L/hr, seizure, excrete H2O, and then repeat the cycle

Question 22

potomania

Answer 22

near euvolemia hypertonic hyponatremia

alcohol inhibits ADH and thus increases urine volume

if a patient takes in a steady diet or drinks water beyond thirst while on a crash diet, they will develop potomania

Question 23

thiazide-induced hyponatremia

Answer 23

near euvolemia hypertonic hyponatremia

usually occurs within a couple weeks of starting the drug

more common in those with lower GFRs

no obvious volume deplesion

increased water reabsorption in the intermedullary collecting duct

Question 24

syndrom of inappropriate ADH secretion (SIADH)

Answer 24

leads to near euvolemia hypertonic hyponatremia

no evidence of heart disease, liver disease, edema, or orthostasis

Uosm is inappropriate for the Posm

if the Posm is lower than normal range (275-290 mOsm/L), the Uosm should be below 100 mOsm/L

urine Na should reflect dietary Na intake

BUN and uric acid levels are low

cannot secrete a H2O load

Question 25

reset osmostat

Answer 25

leads to near euvolemia hypertonic hyponatremia

the set point for the osmoreceptors is lower

looks like SIADH except the patient with a reset osmostat can excrete a H2O load, ex. pregnancy

Question 26

treatment of hypotonic hyponatremia

Answer 26

safest approach is water restriction to allow for evaporation

if volume depleted, administer normal saline (0.9%) intravenously until euvolemic

if there is edema, treat primary disease to try and increase C^eH2O

oral V2-receptor antagonists can be used to raise tonicity in SIADH and in heart failure, ubt are contraindicated in liver disease

3% Na administration in acute intoxication with symptoms of cerebral edema

SNa should not rise faster than 8 mEq/L/day to avoid osmotic dmyelination

should avoid normal saline in SIADH unless coupled with loop diuretics because the kidney will dumpy the Na asnd keep the H2O

Question 27

formula for calculating serum Na

Answer 27

(infusate Na - serum Na)/(total body water + 1)

or

[(infusate Na + infusate K) - serum Na]/(total body water + 1)

Question 28

guidelines for using the spot urine electrolytes

Answer 28

if (UNa + UK)/SNa is greater or equal to 1.0 don’t give any H2O

if between 0.5-1.0, restrict to 500 mL/day

if less than or equal to 0.5 restrict to 1000 mL/day