32 - Electrolyte Disorders

Question 1

Describe what happens to potassium in the body

Answer 1

• Total body stores = 3000-4000 mmol
• 98% intracellular, 2% extracellular
• Na+/K+/ATPase pump results in 3:2 ratio of Na+ out to K+ in
• Normal serum level = 3.5-5 mmol/L, intracellular = 150 mmol/L
• Intracellular & extracellular levels are in dynamic flux
• Many important roles including protein synthesis, cell metabolism, action potential across cell membrane, & BP

Question 2

Describe potassium homeostasis

Answer 2

• Intake through diet
• Elimination usually 90% renally (secretion from distal tubules) & 10% GI
• Homeostasis affected by – hormones, acid-base status, hyperosmolality

Question 3

Which hormones affect potassium hormones and how?

Answer 3

• Insulin, catecholamines, & aldosterone affect homeostasis
• Insulin stimulates Na+/K+/ATPase pump to transport K+ intracellularly
• Catecholamines (ex: epinephrine) = beta-receptor stimulation, which:
• • Activates Na+/K+/ATPase pump -> drives potassium intracellularly
• • Causes glycogenolysis -> increases glucose -> releases insulin, drives potassium intracellularly
• Aldosterone – acts at distal tubule, increased urinary potassium excretion

Question 4

Describe how acid-base and osmolality affect potassium homeostasis

Answer 4

• Acid-base status affects K+ shifting; in simple terms:
• • Decreased blood pH – body responds by moving H+ into cells & K+ out (0.1 unit decrease in pH = ~0.6-0.8 mmol/L increase in serum K+ = false hyperkalemia b/c not actually changing amount in body, just shifting)
• • Increased blood pH – body responds by moving H+ out of cells & K+ in (0.1 unit increase in pH = ~0.6 mmol/L decrease in serum K+ = false hypokalemia)
• Hyperosmolality – K+ shifts to extracellular fluid

Question 5

Hypokalemia definition

Answer 5

• Definition = serum K+ < 3.5 mmol/L
• • Mild = 3.1-3.5 mmol/L
• • Moderate = 2.5-3 mmol/L
• • Severe < 2.5 mmol/L

Question 6

Causes of hypokalemia

Answer 6

• Due to total body K+ deficit or intracellular shift
• Most common cause = medications
• Other causes = excessive GI loss (diarrhea, vomiting, metabolic alkalosis can develop & decrease serum K+ further); hypomagnesemia (increases renal excretion of potassium; **important – need to correct underlying magnesium deficiency to correct potassium)

Question 7

Why is hypokalemia a problem?

Answer 7

• Moderately low = cramps, weakness, myalgias

- Severely low = EKG changes, arrhythmias, increased digoxin toxicity

Question 8

Hypokalemia tx

Answer 8

• Non-pharms = adequate dietary intake of K+, potassium salt substitutes
• Pharm = oral supplementation when mild & N/V not a concern; IV replacement for severe and/or vomiting; replacement of magnesium first if hypomagnesemia (oral or IV)
• General rule of thumb = 100 mmol of oral replacement increases serum K+ by ~1 mmol/L

Question 9

Hypokalemia monitoring

Answer 9

• Acute inpatient setting correcting severe hypokalemia = can monitor serum levels numerous times/day, EKG w/ high rate infusions
• Inpatient setting w/ mild to moderate acute deficiency = replace & check serum levels daily to q3days
• Remember to check Mg2+ levels if K+ replacement isn’t correcting serum levels
• Ambulatory setting (ex: K+ supplementation w/ diuretic) = check serum level, renal function q1-2 months if levels have been stable

Question 10

Hyperkalemia definition

Answer 10

• Definition = serum K+ > 5 mmol/L
• • Mild = 5.1-5.9 mmol/L
• • Moderate = 6-7 mmol/L
• • Severe > 7 mmol/L

Question 11

Why is hyperkalemia a problem?

Answer 11

• Mildly elevated = usually asymptomatic

- Moderately to severely elevated = EKG changes, arrhythmias, mortality

Question 12

Main causes of hyperkalemia

Answer 12

• Increased K+ intake
• Decreased K+ excretion
• Decreased effect of aldosterone
• Extracellular movement of total body K+ (pseudohyperkalemia)

Question 13

How can increased K+ intake cause hyperkalemia?

Answer 13

• Increased intake usually only a problem in severe renal impairment or dialysis
• Non-compliance w/ diet restriction (ex: products w/ high K+ content like potato, sweet potato, prunes, carrots, plain yogurt, orange juice)
• Unwittingly using KCl salt substitutes (200 mmol K+ pet 5 mL)
• OTC/alternative products containing potassium

Question 14

How can decreased K+ excretion cause hyperkalemia?

Answer 14

• Impaired excretion in renal failure alone or in conjunction w/ medications (ex: ACEis, ARBs, K+ sparing diuretics, NSAIDs; less common = digoxin, TMP-SMX, heparin)
• Caution w/ additive effects of > 1 medications (ex: TMP-SMX added short term to pt already receiving ACEi + NSAID

Question 15

What is important to note about K+ lab tests?

Answer 15

• Blood sample hemolysis = falsely elevated serum K+
• Intracellular K+ can spill from RBCs
• Lab will usually note if sample appear hemolyzed
• Consider false elevation if other labs (blood CO2, renal function) normal
• Redraw sample before making tx decision

Question 16

Hyperkalemia tx

Answer 16

• Mild = may self-correct w/ monitoring, may or may not op to treat
• Hyperkalemia w/ EKG changes = emergency, immediate tx needed
• Available therapies aim to either reduce total body stores of K+ or correct extracellular (serum) levels by temporarily driving K+ intracellularly

Question 17

Severe hyperkalemia tx

Answer 17

• First, stabilize cardiac membrane (reverse EKG changes) w/ IV calcium gluconate
• • Acts w/in 5 minutes, lasts 30-60 mins
• • Doesn’t change serum K+ level
• Next, drive serum potassium intracellularly w/ regular insulin 10 U IV x1 (if not already hyperglycemic, give dextrose 25 g IV x1 concurrently to avoid hypoglycemia) +/or beta-2 agonists (salbutamol 10 mg nebule inhaled x1)
• • Don’t use subcut insulin b/c duration is too long for what its being used for (will cause hypoglycemia)
• For metabolic acidosis (decreased extracellular pH), can also give sodium bicarbonate 50-100 meq IV x1
• • Raises pH, potassium moves into cells
• • Not tx of choice outside of metabolic acidosis px
• • Doesn’t work as well in px w/ ESRD

Question 18

Mild hyperkalemia tx

Answer 18

• Sodium polystyrene sulfonate (commonly known by brand name Kayexalate)
• • Cation exchange resin
• • Exchanges sodium for potassium 1:1 in intestine
• • Takes many hours to see full effect
• • Can give PO or PR, typically 15-45 g per dose
• • CI in bowel dysfunction or constipation
• • Risk of colonic necrosis
• • May bind other medications
• Furosemide (ex: 40 mg IV x1) to increase urinary K+ loss – onset w/in minutes, duration 4-6 h

Question 19

Describe what happens to sodium in the body

Answer 19

• Human body requires ~1.5 g per day
• Predominant extracellular cation
• normal serum concentration 135-147 mmol/L
• Kidneys normally function to excrete excess sodium or conserve it in a deficit situation, keeping concentration tightly controlled

Question 20

What is hyponatremia?

Answer 20

• Serum sodium < 135 mmol/L
• Most common electrolyte abnormality
• Associated w/ significant morbidity & mortality, especially w/ rapid swings in sodium levels

Question 21

Categories of hyponatremia

Answer 21

• Hypovolemic hypotonic hyponatremia (common w/ thiazide diuretics)
• Euvolemic hyponatremia (associated w/ SIADH, syndrome of inappropriate ADH secretion)
• Hypervolemic hyponatremia (associated w/ cirrhosis, HF, nephrotic syndrome

Question 22

Hyponatremia – clinical presentation

Answer 22

• Mild (> 125 mmol/L) = usually asymptomatic; impairment of attention & gait & increased fall risk
• Moderate (115-124 mmol/L) = headache, lethargy, restlessness, disorientation
• Severe (< 115 mmol/L) = seizures, coma, respiratory arrest, brain damage, death

Question 23

Hyponatremia tx

Answer 23

• Treat underlying cause to correct level
• Hypovolemic hyponatremia – treat w/ IV NaCl (0.9% or sometimes 3% if severe)
• Euvolemic or hypervolemic = try fluid restriction first
• Rapid swings in sodium levels (normally caused by correcting deficiency too rapidly) can cause osmotic demyelination syndrome (damage to myelin sheath of brainstem) -> can result in paralysis or death in minutes
• Rule of thumb – never increase serum Na+ level more than 12 mmol/L/24 h

Question 24

Describe the options for K+ replacement

Answer 24

• Oral – consider tablet vs. liquid, tolerance can be an issue (split doses to minimize GI irritation) – potassium chloride = 8 meq per tablet (avoid giving more than 24 meg per dose to minimize GI irritation and wait at least 2 h before giving additional doses; chronic deficiency may often give ~8-16 meq oral replacement daily)
• IV – severe hypokalemia, inpatient setting, high replacement rates need EKG monitoring; ex: 1 L bags of 40 meq KCl in 0.9% NaCl (not in D5W b/c dextrose will increase blood glucose and cause insulin release, which will decrease K+)