Hypokalemia

See Also

Pathophysiology

See Potassium Homeostasis
Approximate Total body Potassium = 55 meq/kg
Serum Potassium decreased 0.3 mEq/L: 100 mEq K+ total body deficit
Serum Potassium decreased 1 mEq/L: 350 mEq K+ total body deficit
Serum Potassium less than 2 mEq/L: 1000 mEq K+ total body deficit
Images

Causes

Hypokalemia due to Transcellular Potassium Shift
1. See Transcellular Potassium Shift
2. Medications (Beta-agonists, Insulin excess)
3. Metabolic Alkalosis
4. Hypokalemic Periodic Paralysis
5. Thyrotoxicosis
Hypokalemia due to Renal Potassium Loss
1. See Renal Potassium Loss (Hyperkaluria)
2. Hypertension: Obtain Plasma Renin and Aldosterone
  1. Renin high
    1. Renovascular disease, renin secreting tumor or Malignant Hypertension
  2. Renin normal
    1. Liddle's Syndrome
  3. Renin low
    1. Aldosterone High: Primary Hyperaldosteronism, Bilateral adrenal hyperplasia
    2. Aldosterone Low: Congenital Adrenal Hyperplasia, Cushing's Syndrome, Ectopic ACTH
3. Normotensive: Obtain serum bicarbonate and Urine Chloride
  1. Serum Bicarbonate Low
    1. Renal Tubular Acidosis
  2. Serum Bicarbonate High
    1. Urine Chloride Low: Vomiting
    2. Urine Chloride High: Barter's Syndrome, Normotensive primary Hyperaldosteronism, Diuretic use
Hypokalemia due to Extrarenal Potassium Loss
1. See Extrarenal Potassium Loss
2. Normal Acid-Base Status or Metabolic Alkalosis
  1. Gastrointestinal or skin losses, Hypomagnesemia, Penicillin or Aminoglycosides, Acute Leukemia
3. Metabolic Acidosis
  1. Diabetic Ketoacidosis, Diarrhea, Laxatives
Other causes
1. Inadequate Potassium intake (typically in hospitalized patients)
  1. Total Parenteral Nutrition
  2. Anorexia or Starvation
  3. Dementia
2. Pseudohypokalemia
  1. Delayed lab analysis of sample
  2. Severe Leukocytosis (>75,000/mm3)

Findings
Symptoms and Signs (when Serum Potassium < 2.5 mEq/L)

General
1. Malaise
2. Fatigue
Neurologic
1. Weakness
2. Decreased Deep Tendon Reflexes
3. Paresthesia
4. Cramps
5. Restless Legs Syndrome
6. Rhabdomyolysis
7. Paralysis
Gastrointestinal
1. Constipation
2. Ileus
3. Exacerbated Hepatic Encephalopathy
Cardiovascular
1. Orthostatic Hypotension
2. Hypertension
3. Arrhythmias (rare in otherwise healthy patients)
  1. Acute or recent Myocardial Infarction (5 fold increased risk of Ventricular Fibrillation if K+ <3.9 mEq/L)
  2. Digoxin use and Hypokalemia predisposes to Arrhythmia
Renal
1. Metabolic Alkalosis
2. Polyuria, Polydipsia
3. Decreased GFR
4. Glucose Intolerance

Labs

Serum Potassium <3.5 mEq/L
Serum Magnesium
1. Confirm no Hypomagnesemia (Hypokalemia cause)
Complete Blood Count
1. Confirm no severe Leukocytosis (pseudohypokalemia)
24 hour Urine Potassium and Urine Sodium
1. Most accurate evaluation of Urine Potassium excretion
2. However, Urine Potassium-to-Creatinine ratio is typically obtained instead
3. Sample should have total Urine Sodium > 100 meq
4. Urine Potassium <20-30 meq/day
  1. See Extrarenal Potassium Loss
5. Urine Potassium >20-30 meq/day
  1. See Renal Potassium Loss
Urine Potassium-to-Creatinine ratio
1. Ratio <=1.5 mEq/mmol: Hypokalemia due to Extrarenal Potassium Loss
  1. Manage underlying losses
2. Ratio >1.5 mEq/mmol: Hypokalemia due to Renal Potassium Loss
  1. Elevated Blood Pressure or hypervolemia (mineralcorticoid excess)
  2. Metabolic Acidosis
    1. Type I and II Renal Tubular Acidosis
  3. Metabolic Alkalosis
    1. Diuretics
    2. Renal Tubular transport disorders (e.g. Bartter Syndrome)

Diagnostics
Electrocardiogram

Early changes
1. T Waves decreased amplitude to flattened
Later changes
1. Prominent U Waves
2. ST depression (esp. mid-precordial leads, V2, V3)
  1. ST may appear to sag downwards from a normal J Point
  2. May give rise to a biphasic appearing T Wave
3. T Wave Inversion or biphasic T Wave
4. PR prolongation (first degree AV Block)
5. QTc Prolongation
Arrhythmias associated with Hypokalemia
1. Sinus Bradycardia
2. Ventricular Tachycardia or Ventricular Fibrillation
3. Torsade de pointes
References
1. Mattu (2021) Crit Dec Emerg Med 35(3):14

Management
General

Potassium Replacement
1. Goal Serum Potassium >3.5 mEq/L (>4.0 in CAD, CHF)
2. Total body Potassium deficit
  1. Serum Potassium: <3.5 mEq/L = 100 meq total Potassium deficit
  2. Serum Potassium: 3.2 mEq/L = 200 meq total Potassium deficit
  3. Serum Potassium: 2.9 mEq/L= 300 meq total Potassium deficit
  4. Serum Potassium: 2.6 mEq/L = 400 meq total Potassium deficit
3. Avoid rebound Hyperkalemia (over-shooting replacement)
  1. Hyperkalemia with replacement is unlikely if normal Renal Function, adequate fluid intake
4. Dietary Potassium (Potassium phosphate) is less efficient replacement than Potassium chloride
  1. Most Hypokalemia is associated with concurrent chloride depletion
  2. However Potassium chloride compliance is poor (Dyspepsia, Dietary Potassium tastes better)
5. Half of Potassium Replacement is typically excreted by the Kidneys
  1. Aside from Renal Failure and dehydration, at least half of Potassium Replacement (esp. oral) is excreted
6. Approximate oral replacement
  1. Serum Potassium < 3.0 mEq/L (total body deficit 200-300 meq)
    1. Start with KCl 20 meq orally every 2 hours for 4 doses and consider recheck level
    2. Typically continue Potassium Replacement at 20 meq twice daily for 4-5 days
  2. Serum Potassium: 3.0 to 3.5 mEq/L(total body deficit 100-200 meq)
    1. Give KCl 20 mEq orally every 2 hours for 2 doses OR KCl 40 mEq once
    2. Typically continue Potassium Replacement at 20 meq twice daily for 2-3 days
Magnesium Replacement (empirically or based on lab demonstrated Hypomagnesemia)
1. Especially consider empiric Magnesium Replacement in refractory Hypokalemia
2. Take oral Magnesium supplement 400-500 mg tabs 1-2 daily along with Potassium supplement
Emergent replacement indicated for serious findings or risks
1. EKG changes (esp. QTc Prolongation, see above)
2. Severe Hypokalemia (Serum Potassium <2.5 mEq/L)
3. Rapid onset Hypokalemia
4. Serious comorbidity (heart disease, Cirrhosis)
Consider pseudohypokalemia
1. Consider re-drawing lab for confirmation if delayed analysis
2. Confirm no severe Leukocytosis (WBC >75,000/mm3)
Consider Transcellular Potassium Shift
1. See Transcellular Potassium Shift
2. Hypokalemia from transcellular shift is typically transient

Management
Hospital Replacement Criteria

Hypokalemia with Serum Potassium <3.0 mEq/L AND QTc Prolongation >500 ms
Severe Hypokalemia with Serum Potassium <2.5 mEq/L

References

Orman and Slovis in Herbert (2018) EM:Rap 18(8): 4-5
Viera (2015) Am Fam Physician 92(6): 487-95 [PubMed]