1 Sodium Disorders Flashcards
What makes up the electrolyte panel?
Sodium (Na) Potassium (K) Chloride (Cl) Bicarbonate (CO2) Calcium Magnesium* Phosphate*
*Not usually part of BMP, will usually have to order them separately
Total body water = _____% of total body weight
60%
Of the total body water, how much is in the ICF and how much in the ECF?
2/3 ICF
1/3 ECF (interstitial fluid and plasma)
Percentage total body water in Newborns vs Adult Males vs Adult Females vs Elderly
Newborns = 80%
Adult Males = 60%
Adult Females = 50%
Elderly = 45%
Total body water of an obese person is …
Much lower than total body water in a lean person
What does “TIE 60,40,20” mean?
Total body water = 60%
Intracellular water = 40% (2/3 of 60)
Extracellular water = 20% (1/3 of 60)
In the ECF, how much is interstitial fluid and how much is plasma?
3/4 Interstitial fluid
1/4 plasma
Most important ECF cation?
Sodium
Most important ICF cation?
Potassium
Most important ECF anion?
Chloride
And some HCO3- (but mostly Cl-)
Most important ICF anion?
PO4 and organic anions
Oh and some protein
Total solute concentration in a fluid compartment
Osmolality
What are the main solutes that determine the calculated osmolality of the ECF?
Sodium
Glucose
Urea
Normal range of osmolality
280-295 mOsm/kg
How do we calculate Osmolality?
(2 x Na) + (Glucose/18) + (BUN/2.8)
SODIUM is the biggest factor
Symptoms occur if osmolality is > _____ or < ______.
> 320 mOsm/kg or <265 mOsm/kg
Other “osmotically active” substances that aren’t included in the calculated osmolality (because they aren’t supposed to be there)
Mannitol (given as a med for cerebral edema) and various proteins
Ethanol, methanol, and ethylene glycol
High amounts of osmotically active substances in the blood can lead to an elevated…
Osmolal Gap (Measured Osmolality - Calculated Osmolality)
What is the normal Osmolal Gap?
<10
If it’s greater than 10, it’s usually because of ethanol, methanol, or ethylene glycol
The ability of the combined effect of all the solutes to generate an osmotic driving force that causes water movement from one compartment to another
Tonicity
To increase ECF tonicity, a solute must be …
Confined to the ECF compartment (unable to cross from ECF to ICF
Examples: SODIUM, glucose, mannitol
______ easily crosses cell membranes and therefore distributes evenly throughout total body water (contributing to OSMOLALITY , but NOT TONICITY)
Urea
Why do we care about tonicity?
Because it affects the size of cells
Decreased Na+ —> decreased tonicity of the ECF —>
Shift of water from ECF to ICF —> cells (including brain cells) swell with extra water
What is the major determinant of the size of ECFV (extracellular fluid volume)
Total amount of Na+
Increased Na+ —>
Increased ECFV or HYPERVOLEMIA
Think edema
Decreased Na+ —>
Decreased ECFV or HYPOVOLEMIA
think dry patient
90-95% of the total body sodium is located in…
The ECF
Serum [Na+] (lab value) primarily refers to …
The amount of water relative to Na+ in the ECF (NOT the total body Na+ amount)
Abnormal serum Na+ is a sign of …
A disorder of water regulation
High Na+ = too little water relative to sodium
Low Na+ = too much water relative to sodium
The ECFV is determined by …
Overall volume status of the patient
An abnormality with the size of the ECFV is a marker of…
Abnormal sodium control (too much/too little total body sodium)
High ECFV = “too much sodium”
Low ECFV = “too little sodium”
How do we describe a patient’s volume status?
Hypovolemic
Euvolemic
Hypervolemic
Common causes of hypovolemia
Anything that causes dehydration
GI losses (bleeding, NG suction, diarrhea, vomiting)
Renal losses
• Salt and water loss: diuretics
• Water loss: diabetes insipidus
Skin losses: sweat, burns
Sequestration without loss
• Intestinal obstruction
• Pancreatitis
• Rhabdomyolysis
Clinical features of hypovolemia
Increased thirst, decreased sweating
Decreased skin turgor & dry mucus membranes
Oliguria with increased urine concentration
CNS depression
Weakness and muscle cramps
Decreased BP, postural hypotension/dizziness
Increased pulse, postural pulse increase
Common causes of hypervolemia
Acute or chronic renal failure Nephrotic syndrome Primary aldosteronism Cushing’s syndrome Liver disease*** Heart failure*** Pregnancy
Clinical features of hypervolemia
Edema SOB Orthopnea, paroxysmal nocturnal dyspnea Jugular venous distention Hepatojugular reflux Crackles on pulmonary exam
Water retention is influenced by:
Thirst
ADH aka vasopressin
Salt retention is influenced by:
Renin-Angiotensin system***
Also:
ANP and catecholamines
Renal function
Renal blood flow
ADH is produced in the _________, then transported to the ________, from which it is released into the blood stream
Hypothalamus —> Posterior Pituitary
Aldosterone’s main actions are:
Increase renal sodium reabsorption (Na+ retention)
Increase renal potassium secretion (K+ excretion)
What is the most common electrolyte abnormality in hospitalized patients?
Hyponatremia
A hypotonic disorder due to serum sodium <135
What is a normal serum [Na+]?
135-145 (varies by lab)
Mild Hyponatremia is a [Na+] of…
125-135
Moderate hyponatremia is a serum [Na+] of ..
120-125
Severe hyponatremia is a serum [Na+] of …
<120
Life threatening if [Na+] < 120 with SEIZURES
The “danger zone” for [Na+] is …
Below 125 (patients not generally symptomatic until about 125)
Hyponatremia can be acute or chronic, so always compare a low [Na+] to…
The patient’s BASELINE sodium
The faster the Na decreases —> the more severe the symptoms
Hyponatremia is most common in what populations?
The very young and the very old
Can also be seen in association with pulmonary disease or CNS disorders
Symptoms of hyponatremia depend upon …
Level of cerebral edema
Headache, dizziness N/V Lethargy, weakness CONFUSION Hypoventilation, respiratory arrest SEIZURES COMA DEATH!!!
Condition characterized by falsely low serum sodium
Pseudohyponatremia
Serum Na<135 but NORMAL osmolality (Isoosmolar)
This is a LABORATORY ARTIFACT - relative % of water reduced and FLAME photometry reports artificially low sodium
Patients with what conditions can have pseudohyponatremia?
Hyperlipidemia and hyperproteinemia
Can also occur with obstructive jaundice and multiple myeloma
What should you do if you suspect pseudohyponatremia?
Speak to your lab about more specialized tests to confirm true sodium level
What is redistributive or hyperosmotic hyponatremia?
A hyperosmotic state (“relative hyponatremia”) caused by osmotically active solutes in ECF that draw H2O from cell, diluting serum sodium concentration
Increased glucose in ECF causes shift of water from ICF —> ECF, thus lowering serum Na
What is the most common cause of redistributive or hyperosmolar hyponatremia?
Hyperglycemia!
Because it elevates osmolality and makes Na+ look super low
How do you calculate corrected [Na+] in patients with redistributive or hyperosmolar hyponatremia?
Add 1.5mEq/L to sodium value for every 100mg/dL serum glucose > 100mg/dL
Ex: if labs show Na = 125 and Glucose = 400
400-100= 300 so 3x1.5 = 4.5
Corrected Na = 125 + 4.5 = 129.5
Hypervolemic hyponatremia is commonly caused by…
Hepatic cirrhosis
CHF
Renal Failure
Treatment for hypervolemic hyponatremia?
Diuretics, dialysis, fluid restriction
What are some renal causes of hypovolemic hyponatremia?
Diuretics (esp thiazides)
Osmotic diuresis
Addison’s disease*** Primary adrenal insufficiency —> low cortisol (which controls ADH)
What are some non-renal causes of hypovolemic hyponatremia?
External GI: vomiting, diarrhea, NG suction, fistula
Internal GI: pancreatitis, peritonitis
Burns
Treatments for hypovolemic hyponatremia?
Replace fluid losses with isotonic fluid (ie NS) and treat underlying cause
What are some causes of euvolemic hyponatremia?
SIADH
Psychogenic polydipsia (drinking too much water)
Ecstasy intoxication (can cause psychogenic polydipsia)
Hypothyroidism
Adrenal insufficiency
Treatment for Euvolemic hyponatremia?
Fluid restriction, treat underlying cause
Hallmark findings in SIADH
Concentrated urine (>100mOsm/kg) with low serum osmolality and euvolemia (so long as kidneys working well)
Causes of SIADH
CNS disease Pulmonary disease (esp SMALL CELL LUNG CA****) Meds Major surgery Stress Psych Pain
Upwards of 35% of hospitalized patients can develop …
SIADH
How is SIADH diagnosed?
Check urine and serum osmolality - urine should be highly osmolar, but serum should be low osmolar
Determine underlying cause:
• CT/MRI of head to check for CNS disorder
• CXR to check for lung tumor/infection
• Review all patient meds
How do you treat SIADH?
FLUID RESTRICTION (usually sufficient on its own)
Treatment of underlying pathology
For refractory cases: • Hypertonic saline • Democlycline • Urea • Lithium • Vaptan
What labs do you look at first in cases of hyponatremia?
First look at: UA(Na), UA (Osm), Serum Osm, CMP (serum Na)
Second: TSH, serum cortisol
Treatment of hyponatremia depends upon…
The underlying cause
When do you hospitalize a patient with hyponatremia?
If Na<125 OR symptomatic
Chronic hyponatremia must be managed with extreme care
Why is slow, cautious correction of serum sodium important?
Rapid increase in serum sodium can lead to cerebral pontine myelinolysis (CPM) - aka osmotic demyelination syndrome - b/c of quick water shifts in and out of brain
Traditional treatment of chronic hyponatremia is …
Demeclocycline (to induce nephrogenic DI)
“Vaptans” are a new class of treatment agents which are vasopressin receptor antagonists
What should the rate of correction be in patients with severe symptomatic hyponatremia?
6-12 mEq/L in the first 24 hours and ≤18 mEq/L in 48 hours
If chronic hyponatremia, should try to keep it ≤8mEq/L in the first 24 hours
Check serum sodium q2h as you are replacing, to make sure you are not over correcting
What is Central Pontine Myelinolysis
A poorly understood entity characterized by focal demyelination in the pons and extra-pontine areas
IT IS IRREVERSIBLE
Dysarthria, dysphagia, seizures, altered mental status, quadriparesis, hypotension, etc begin 1-3 days after overcorrection
Clinical features of hypernatremia are due to …
Brain shrinkage secondary to increased ECF osmolality
Causes of hypernatremia
GI losses: common in elderly and infants with diarrhea
Skin loss: sweating, fever, etc
Renal loss
Drug related: diuretics (thiazides usually cause hypo but other diuretics can cause hyper), lithium (b/c it can induce nephrogenic DI)
Osmotic diuresis from Hyperglycemia or increased mannitol
In general, which is more common: Hypo- or Hypernatremia?
Hyponatremia
Clinical features of hypernatremia?
Often asymptomatic
Thirst, signs of volume depletion
AMS, weakness
Neuromuscular irritability
Focal neurological deficits
Seizures or coma
Symptoms of hypernatremia are related to…
Rate of onset
If it develops slowly, symptoms may be less dramatic
What is our body’s normal response to hypernatremia
Create thirst and increase fluid intake
Maximally concentrate the urine to prevent further water loss
The vast majority of cases of hypernatremia are due to WATER LOSS, not sodium loss
Nonosmotic urinary water loss in the setting of elevated serum sodium (urine is dilute when it should be concentrated
Diabetes Insipidus
The collecting ducts are impermeable to water so water is not reabsorbed
Central DI is due to…
Impaired secretion of ADH
AKA neurogenic DI
Central DI is typically treated with …
Desmopressin (often an inhaled DDAVP nasal spray or IV DDAVP)
Nephrogenic DI is due to …
Lack of kidney response to ADH, causing continued water loss even though patient is low on water. Adequate ADH is present but useless
Desmopressin will NOT help in nephrogenic DI
What causes Nephrogenic DI
Can be genetic or acquired
If acquired, typically from: • Chronic renal insufficiency • Tubulointerstitial renal disease • Amyloidosis • Lithium toxicity
Treatment for Nephrogenic DI
Thiazides diuretic
Amiloride (potassium sparing diuretic)
Chlorpropamide (antidiabetic oral agent)
NSAIDs have been tried (including indomethacin)
How do we treat hypernatremia in general?
Hospitalize if severe
STOP WATER LOSS
REPLACE WATER DEFICIT
• Orally, NG tube, or IV (hypotonic fluid)
• DO NOT replace too rapidly, esp if patient has been hypernatremic for days —> CPM
How do you calculate a patient’s water deficit?
Water deficit = Normal TBW - current TBW
Normal TBW = 0.6 x body weight in kg
Current TBW = (Normal serum Na x Normal TBW)/(Measured serum Na)
