BMP: Potassium and Sodium

what is serum potassium concentration a general indicator for?

total body potassium

how much of potassium is in the extracellular fluid?

1.5-2%

where is the majority of potassium located?

intracellular fluid

what establishes the resting membrane potential of cells?

the ratio of extracellular to intracellular potassium

what is the maintenance of potassium balance essential for?

the normal function of excitable tissues

what are examples of excitable tissues?

nerves, skeletal muscle, and cardiac muscle

how much of the total potassium excreted from the body are the kidneys responsible for?

90%

where is the remaining potassium excreted from (other than the kidneys)?

sweat or stool

what are the causes of hypokalemia?

transcellular shifts, inadequate intake of potassium, extra-renal losses, and renal losses

what are transcellular shifts?

movement from ECF to ICF

what happens in alkalemia?

H+ exits the cells to counter the rise in pH in the blood, K+ moves in the opposite direction

what drives K+ into cells?

insulin

what do beta 2 agonists do?

stimulates potassium uptake into cells (epinephrine)

what are some causes of extra-renal losses of potassium?

GI origin: vomiting, NG suctioning, diarrhea, laxative abuse

sweating

what are some causes of renal losses of potassium?

loop or thiazide diuretics (lasix, HCTZ)

renal tubular acidosis (start spilling K+ in urine)

hyperaldosteronism (aldosterone stimulates the Na/K pump)

what test can we use to measure for hypokalemia?

24-hour urine measurement can help determine etiology of hypokalemia

measure the total urine sodium and potassium concurrently

what does sodium excretion <100meq/day suggest?

inadequate intake of potassium as well as sodium

what does sodium excretion >100meq/day and potassium excretion <25meq/day suggest?

extra-renal losses

what does potassium excretion >25meq/day suggest?

renal wasting of potassium

what are mild deficits of hypokalemia?

malaise, fatigue, neuromuscular disturbances (weak/hyporeflexia)

what are severe deficits of hypokalemia?

GI disorders (constipation, ileus, vomiting), cardiac arrhythmias, paralysis

what are two kinds of treatment for hypokalemia?

oral therapy, IV therapy

what is used for oral therapy for hypokalemia?

potassium supplements as potassium chloride

slow release tablets are better tolerated than liquid potassium

what can result after oral supplementation for hypokalemia?

severe hyperkalemia

why would patients use IV therapy instead of oral therapy for hypokalemia?

they are unable to take oral supplements

what type of IV solution is preferred for hypokalemia? why?

a non-dextrose solution is preferred because dextrose may decrease the plasma potassium transiently (insulin driving K+ into cells)

where can IV therapy for hypokalemia be given if it is life threatening?

a large vein such as the femoral vein

what are the etiologies for hyperkalemia?

redistribution of potassium from ICF to ECF

potassium loads

pseudo-hyperkalemia

decreased renal excretion

what are manifestations of hyperkalemia?

neuromuscular (weakness, paresthesia, areflexia)

cardiac arrhythmias (bradycardia, risk of V-fib, cardiac arrest)

when is urgent treatment needed for hyperkalemia?

when serum potassium is >7meq/liter OR ECG shows changes consistent with hyperkalemia

peaked T waves, wide QRS complex

what are modalities for correcting hyperkalemia?

shifting potassium from ECF to ICF

reducing total potassium

what is the treatment for hyperkalemia?

calcium administration, glucose/insulin infusions, cation exchange resins, hemodialysis

why do we use calcium administration for hyperkalemia treatment?

temporarily antagonizes the cardiac neuromuscular effects of hyperkalemia

infusion effects occurs within minutes and lasts about 1 hour

why do we use glucose/insulin infusions to treat hyperkalemia?

insulin shifts K+ from ECF to in the cells

use glucose to avoid hypoglycemia

why do we use cation exchange resins for treatment of hyperkalemia?

binds K+ in exchange for another cation Na+ in the intestinal tract to remove K+ from the body

why do we use hemodialysis for treatment of hyperkalemia?

last option if nothing else is working

dialysis so you get rid of it manually

who do we see chronic hyperkalemia in?

patients with renal failure

what is the treatment for chronic hyperkalemia?

loop diuretics or kayexalate

how much of the total body water is found in the ICF?

2/3

how much of the total body water is found in the ECF?

1/3

what dictates the changes in the ECF?

net get or loss of sodium

what is the osmolality of the ECF if the fluid loss is isotonic (blood loss)?

unchanged, no change in ICF volume

what is isotonic?

both solutions have equal concentrations of solutes

what happens to plasma osmolality if fluid loss is hypotonic (sweating, NG suction)?

plasma osmolality increases, shift of the ICF to the ECF

what is hypotonic?

lower concentration of solute

what is serum osmolality?

measure of the concentration of dissolved particles in the serum

what is urine osmolality?

measure of the concentration of dissolved particles in urine

what is urine osmolality used to measure?

kidney function and hydration status

what is hyponatremia?

low total body sodium

who do we commonly see hyponatremia in?

hospitalized patients, especially on IV fluids

what other values do we normally see with hyponatremia?

elevated BUN:Cr ratio, hypovolemic state

what are some reasons for hyponatremia?

renal sodium loss, extrarenal sodium loss, or decreased sodium intake

what is typically associated with renal sodium loss?

high urinary sodium excretion and high urinary osmolality

most commonly due to diuretics

less commonly due to renal tubular disease

what is extra renal sodium loss associated with?

low urine sodium excretion (diarrhea, fever, sweat, exercise)

is decreased sodium intake a common cause of hyponatremia?

no, it is a rare cause

what is typically associated with decreased sodium intake?

patients with extremely poor diet (alcoholics, anorexia), hospitalized patients maintained on IV fluids for prolonged time, hypovolemic, low urine sodium excretion, high urine osmolality (concentrated), high BUN

how do we classify hyponatremia?

based on patient’s volume status and/or their osmolality

what do we need to know to classify hyponatremic patients?

patient’s history (co-existing medical problems)

volume status: hypervolemia (edema/ascites) and hypovolemia (orthostasis, dry mucous membranes, tenting of skin)

BUN/creatinine

urine osmolality/electrolytes

plasma osmolality (one of 1st things you check)

why is looking at plasma osmolality important for classifying hyponatremia?

hyponatremia is typically associated with plasma hypoosmolality, however may be normal or hyperosmolality

what does an elevated osmolality cause?

a shift of water from ICF to ECF by diluting the sodium concentration

what is the most common cause of hyponatremia with high plasma osmolality? why?

hyperglycemia because osmotic diuresis causes renal sodium and water losses, which can further raise the plasma osmolality

what is the treatment for hyponatremia with high plasma osmolality?

correcting the hyperglycemia

what do we NOT give if someone has hyponatremia with high plasma osmolality? why?

saline because it will only cause fluid to leave the ICF

what do we see with hyponatremia with normal plasma osmolality?

hyperlipidemia and hyperproteinemia

what word do we use to refer to hyponatremia with normal plasma osmolality?

pseudohyponatremia

normally, how much of plasma is water?

93%

what else is plasma made of, other than water?

solids (lipids and proteins)

what happens to the volume of plasma with hyperlipidemia or hyperproteinemia?

decreases

what happens to plasma sodium in hyperlipidemia or hyperproteinemia?

plasma sodium is falsely low

is treatment required for hyperlipidemia or hyperproteinemia in hyponatremia with normal plasma osmolality?

no, treatment is not required

what do we call hyponatremia with low plasma osmolality?

hypotonic hyponatremia

what are the “types” of hypotonic hyponatremia?

hypotonic hyponatremia with ECF volume excess (hypervolemic)

hypotonic hyponatremia with decreased ECF volume (hypovolemic)

hypotonic hyponatremia with clinically normal ECF volume (euvolemic)

what is shown with urine sodium in hypotonic hyponatremia with ECF volume excess?

urine sodium >20meq/L (renal failure)

urine sodium <20meq/L (CHF, hepatic cirrhosis)

with edema, where is the fluid movement?

movement of fluid from the plasma into the interstitial fluid

what is the relationship between loss of fluid from plasma and ADH and aldosterone?

the loss of fluid from the plasma leads to an increased production of ADH and aldosterone

what is the treatment for hypotonic hyponatremia?

judicious sodium and fluid restriction

water intake must be less than urine output to raise plasma sodium

what is seen with hypotonic hyponatremia with decreased ECF fluid?

total body sodium is depleted disproportionately to water losses OR sodium deficit is replaced with hypotonic fluids

renal losses: urine sodium >20meq/L (diuretic therapy, adrenal insufficiency/ACE-inhibitors)

extra-renal loss of sodium and water: urine sodium <20meq/L (less common but can be seen with vomiting, diarrhea, sweat)

what is treatment of hypotonic hyponatremia with decreased ECF volume focused on?

re-expansion of the ECF volume with isotonic saline and correction of any underlying disorders

what are the two diagnoses we see with hypotonic hyponatremia with clinically normal ECF volume?

primary polydipsia and SIADH

what is seen with primary polydipsia in hypotonic hyponatremia with clinically normal ECF volume?

urine sodium low, urine osmolality low, seen in psych patients, treatment involves water restriction, monitor plasma sodium closely, if plasma corrects too rapidly, then liberalize the water restriction

what does SIADH do?

low serum sodium and osmolality

adequate urine sodium excretion with high urine osmolality

no edema

no evidence of dehydration

what other diagnoses do we see SIADH in?

lung diseases, malignancies, CNS abnormality, cortisol deficiency

what inhibits ADH secretion?

cortisol

what are the manifestations of hyponatremia?

observed typically at sodium concentrations <120meq/L or if the rate of fall in sodium concentration is rapid

initially: h/a, nausea, malaise, lethargy, cramps

may progress to: delirium, psychosis, seizures, coma

what happens if you correct hyponatremia too quickly?

can cause acute/permanent neurological complications

what are the main causes of hypernatremia?

lack of free access to water (infants, institutionalized patients, patients with neurological disorders may lose their ability to respond to thirst signals)

is urine osmolality high or low with hypernatremia?

high

what is diabetes insipidus?

absence of ADH (central DI) (associated with encephalopathy, head trauma, pituitary surgery, tumor)

renal resistance to ADH (nephrogenic DI) (3rd trimester of pregnancy-rare)

what type of urine do people with DI excrete?

large volumes of extremely diluted urine with low sodium in the urine

what is the treatment for DI?

d-DAVP (desmopressin- manufactured ADH)

what are the very rare causes of hypernatremia?

can be administered of hypertonic fluids: occasionally during resuscitation patients will receive IV hypertonic sodium, chicken soup, salt water