9/11- Dysnatremias Flashcards
Hypo and hypernatremia are defined by what?
Serum sodium CONCENTRATION
(NOT CONTENT!!!!!!)
What is the equation for serum Na?
Edelman Equation
Serum Na (mEq/L) = (Na + K)/TBW
(Na and K here are “exchangeable”- the ones available across the cell for movement)
What are the CONCENTRATION ranges for hypo and hypernatremia?
Hyponatremia: less than 135 mEq/L
- Relatively more TBW compared to total body Na
Hypernatremia: > 145 mEq/L
- Relatively less TBW compared to total body Na
(thus, normal range = 135-145)
Dysnatremia is caused predominantly by what?
Dysnatremia is predominantly caused by changes in TBW (total body water)
- Think back to Edelma equation and small denominator (TBW)
What is the predictor of volume status in a person?
Salt intake
How may high salt intake present clinically?
Volume overlaod
- Increased JVP
- Edema
- Lung crackles
- HTN
How may low salt intake present clinically?
Volume depletion
- Decreased BP
- Poor skin turgor
- Dry mucus membranes
How may high water intake present clinically?
Hyponatremia
How may low water intake present clinically?
Hypernatremia
KEY: Dysnatremic patients can present with volume overload, volume depletion, or euvolemia!!
yay
Case
- 37 yo man with Na 159 (normal 135-`145)
- He was out in the sun all day long and did not eat/drink much
- His BP is 90/50 (hypotensive)
- He has tenting of his skin
- Urine Na is under 10 mEq/L
Diagnosis?
What is you initial choice of IV fluids?
A. Salt water [normal saline] (0.9%)
B. Dextrose water (D5W)
C. Blood transfusion
Hypovolemic hypernatremia
- Total body Na is decreased
- Total body water is very decreased What is you initial choice of IV fluids:
A. Salt water [normal saline] (0.9%)
B. Dextrose water (D5W)
C. Blood transfusion
You want to fix depleted water status (hypotension and whatnot)
T/F: Normal saline (0.9%) is hypotonic?
False! It is isotonic
What is normal saline?
Isotonic saline (0.9% concentration)
- 0.9g salt in 100cc water
- Na is 154 mEq/L (approximates plasma sodium of 140)
What is dextrose water (D5W)?
Concentration is 5%
- 5g sugar in 100cc water
- Na content is 0!
Given because:
- Avoids hemolysis (entry of water into the cells)
- Dextrose will move into the cells (insulin effect), leaving “free” water
What fluid should be given to a hypovolemic patient?
Normal saline should be used to expand ECF volume
What fluid should be given to patient with relatively low TBW?
Example of this case?
Give oral water or IV D5W to patients with relatively low TBW
- Ex) hypernatremia
What equation is used in calculating water deficit?
Water deficit = TBW x (pt Na - 140)/140
TBW for man ~ 60% of weight (70kg)
Water deficit = 0.6 x 70 x (pt Na - 140)/140
Always add ongoing losses when replacing water, including insensible losses and ongoing urinary losses
Case
- 23 yo med student has polyruia and polydipsia
- While cramming for the renal exam, he sits in the library and forgets to drink water for > 15 hrs
- This is the 1st time he has gone so long without water
- He continues to urinate as he normally does
- He presents to the ER with altered mentation
- Serum sodium is 161 and he weighs 60 kg
Water deficit calculation?
Treatment?
While in the ER, his mother calls and says he left a certain prescription medication at home
- “He sustained head trauma as a child and takes this nasal spray several times a day” to decrease his urine output
Diagnosis?
A. Nephrogenic diabetes insipidus
B. Diabetes mellitus
C. Central diabetes insipidus
D. Osmotic diuresis
Water deficit = 0.6 x 60 x (161-140)/140 = 5.4 L
Tx: Replace water deficit over 48 hours to avoid sudden changes in serum Na concentration
Diagnosis:
A. Nephrogenic diabetes insipidus
B. Diabetes mellitus
C. Central diabetes insipidus
D. Osmotic diuresis Kid was taking vasopressin
- DO NOT stop vasopressin in these pts (can decrease load, but stopping would cause swing FAR to the other side, with much urine excretion and hypernatremia)
What does “gently” imply in treatment?
Over 48 hours
What is diabetes insipidus? Subtypes?
Production of large volumes of very dilute urine (polyuria)
- > 3 L per 24 hrs
- < 300 mOsm/L
There are four major subtypes (2 involve defect in ADH)
- Central DI
- Nephrogenic DI
- Gestational DI
- Primary Polydipsia (Dipsogenic DI)
What differentiates between diabetes mellitus and insipidus?
Glycosuria (hyperglycemia)
What is the mechanism of central DI?
- Genetics?
- Causes?
Central DI = deficiency in ADH production/release
Acquired:
- Brain tumor
- Head trauma
- Granulomatous disease
- Autoimmunity
Inherited:
- AD/AR mutation in AVP gene or X-linked mutation in ? gene
What is the mechanism of nephrogenic DI?
- Genetics
- Causes?
Nephrogenic DI = defective kidney response to ADH
- Genetic: mutations in V2 receptor or AQP2
- Drugs: Lithium
- Electrolytes: hypokalemia, hypercalcemia
Pathologies:
- Obstruction
- Chronic kidney disease
- Sickle cell anemia
What is the mechanism of gestational DI?
Vasopressinase produced by placenta degrades arginine vasopressin (ADH)
