Electrolyte abnormalities

Etiology of electrolyte abnormalities - think about:

  • Intake
  • Shifting
  • Output

Typical repletion thresholds 4/3/2:

  • K > 4
  • Phos > 3
  • Mg > 2

Na

Na
(135-145 mEq/L)		 Hyponatremia
↓ (< 135 mEq/L)		 Hypernatremia
↑ (> 145 mEq/L)
Symptoms Cerebral edema → HA, n/v, lethargy, confusion, seizure, coma, respiratory failure AMS, weakness, irritability, thirst, coma 
Workup/labs Hyponatremia
  • False hyponatremia
    • High plasma osmolality: hyperglycemia (corrected Na = measured Na + 2.4 x (serum glucose - 100)/100)
    • Normal plasma osmolality: hyperproteinemia, hyperlipidemia (lab measurement error)
  • Plasma calc osm = 2Na + gluc/18 + BUN/2.8; normal = 275-295 mOsm/kg
  • Etiology of hypo-osmolar hyponatremia
    • Measure urine osmolality
      • Appropriately low (< 100 mOsm/L) - primary polydipsia or reset osmostat (lower ADH threshold)
      • Inappropriately high (> 200 mOsm/L) - examine ECF volume
        • Fluid up: CHF, cirrhosis, nephrotic syndrome, renal insufficiency
        • Normal fluid balance: SIADH
        • Fluid down: Na loss (vomiting, diarrhea, burns)
  
  • Volume up: pt was given hypertonic Na
  • Etiology - free water loss
    • Measure urine osmolality
      • Maximally concentrated and minimal volume: 2/2 insensible/renal/GI water loss
      • Not maximally concentrated: diuretics (Uosm excretion rate > 750 mOsm/d) or diabetes insipidus
    • Diabetes insipidus - test renal response to DDAVP/desmopression
      • UOsm increases - central DI
      • UOsm unchanged - nephrogenic DI
Treatment

Hyponatremia

  • q4-6 hr BMP
  • Water restriction
  • Na correction
    • Asymptomatic: 0.5 mEq Na/L/hr, no more than 8-10 mEq/L/day
    • Symptomatic: 3-4 mEq Na/L/hr for first 3-4 hrs, no more than 10-12 mEq/L in first 24 hrs
    • Overly rapid correction → central pontine myelinosis (dysarthria/dysphagia, paralysis, coma)
      • Can give back free water if overcorrected
    • Effect of 1 L IVF on serum Na = (NaIVF - Naserum) / (TBW + 1)
      • TBW = weight (kg) x 0.6 (M) or 0.5 (F)
      • e.g. 1 L of 3% saline (513 mEq Na/L) given to 70 kg patient with Naserum = 120 will raise Naserum by (513-120)/(0.6*70 + 1) = 9.1 mEq Na/L
      • To correct at a rate of 1 mEq Na/L/hr, give 3% saline at 1/9.1 x 1000 mL = 110 mL/hr
  • Typically use hypertonic saline to correct; only use NS if hypovolemic (e.g. not SIADH)
    • NS will worsen hyponatremia in SIADH - UOsm is fixed at UOsm > 308 mOsm/kg (NS), so the kidneys will excrete more Osm than was given in NS
    • In SIADH can also do salt + Lasix (Lasix generates hypotonic urine (UOsm ~75 mOsm) and eliminates free water to enhance solute replacement); demeclocycline (blunts ADH response)
  
  • q4-6 hr BMP
  • Water replacement
  • Free water deficit = TBW x (Nadesired/Naactual - 1)
    • TBW = weight (kg) x 0.6 (M) or 0.5 (F)
  • Na correction
    • 0.5 mEq Na/L/hr
    • Overly rapid correction → cerebral edema

K

 K
(3.5-5.0 mEq/L)		 Hypokalemia
↓ (< 3.5 mEq/L, mod < 3.0, severe < 2.5)		 Hyperkalemia
↑ (> 5.5 mEq/L, mod > 6.0, severe > 7.0)
Symptoms

Fatigue, myalgia, muscle cramps, weakness, respiratory failure
U waves, arrhythmia on EKG

Symptoms uncommon (muscle weakness/paralysis)
Peaked T waves on EKG (→ prolonged PR → prolonged QRS → "sine wave" QRS → VF/asystole)
Workup/labs

Etiology

  • Check Mg and replete to > 2 (↓Mg may exacerbate ↓K)
    • **hide**↓Mg inhibits Na-K ATPase; increases renal K excretion
  • K+ shift (alkalosis, insulin, β-agonist)
  • GI loss (vomiting, diarrhea, NGT)
  • Renal loss (diuresis, hyperaldosteronism) 

EKG findings

  • U waves, T wave flattening/inversion, ST depression, prolonged PR U waves
  • May worsen to arrhythmias (ectopics, afib, VT, VF, torsades)

Etiology

  • ACE inhibitors, K-sparing diuretics, renal failure/missing dialysis

EKG findings

  • K > 5.5 - may show peaked T waves on EKG Peaked T waves
  • K > 6.5 - prolonged PR, flattened/disappearing P wave (atrial paralysis) Prolonged PR
  • K > 7.0 - prolonged QRS → wide/bizarre "sine wave" QRS, bradycardiaSine waves
  • K > 9.0 - VF/asystole (cardiac arrest)
Treatment
  • PO K repletion preferred (can cause nausea) - 40 mEq dose up to q4h
  • IV if severe/symptomatic (burns going in; 10 mEq/h (takes 4 h to give a standard dose))
    • May be given faster (20-40 mEq/h) in emergencies but must be given by central line due to irritation, and ideally through a femoral line to avoid ↑ local concentration around the heart with IJ or subclavian)
  • 10 mEq should increase serum K by 0.1 mEq/L (40 mEq is typical order)
  • Caution when repleting in renal failure (renally excreted)
  CB DIAL K: (give Ca, insulin/glucose first if severe). TRANSIENT IS TRANSIENT!!
  • CaCl/Ca gluconate (1 amp) - stabilize cell membrane and prevent arrhythmia (onset ~min, lasts 30-60 m)
  • Bicarb (1-2 amps IV) - shifts K transiently (onset 15-30 m, lasts 1h)
  • Dialysis - eliminates K from body
  • Insulin (10 U reg IV) - give with glucose (1-2 amp D50W). Shifts K transiently (onset 15-30 m, lasts 0.5-1h)
  • Albuterol (10-20 mg inh) - β-agonist shifts K transiently (onset 30-90 m, lasts 2h)
  • Lasix (20-40 IV) - eliminates K from body (onset 0.5 h)
  • Kayexalate (30 g PO) - eliminates K from body (slow, not very effective, avoid in ileus, a/w colonic perforation, don't give this...)
  • **hide**Patiromer/Veltessa - new resin binder that might work better than kayexalate
  • Treat underlying disorder

Ca

Ca
(8.5-10 mEq/dL)
(ionized Ca 4.5-5.5 mg/dL)		 Hypocalcemia
↓		 Hypercalcemia
↑
Symptoms Perioral paresthesia, cramps, tetany, Chvostek's (tap facial nerve → contraction), Trousseau's (inflate BP cuff → carpal spasm), prolonged QT Stones (nephrolithiasis), bones (fatigue/weakness), abdominal groans (n/v, constipation), psychic moans (confusion, stupor) 
Workup/labs
  • Rule out pseudohypocalcemia with albumin correction
    • For every 1 gm/L ↓ albumin (< 4.0), serum Ca ↓ 0.8 mg/dL
  • Check Mg (↓Mg may exacerbate ↓Ca)
  • Etiology
    • Renal failure
    • Vit D def.
    • Hypomagnesemia induces resistance to PTH - replace Mg first
    • (pseudo)Hypo-PTH
    • Massive transfusion (EDTA in blood products chelates Ca)
    • Pancreatitis (saponification)
  • EKG findings - QTc prolongation
  
  • Check PTH
  • Etiology
    • Cancer
    • Hyper-PTH
    • Milk-alkali
    • Meds (thiazides)
    • Granulomatous disease (e.g. sarcoid)
  • EKG findings - short QT; J waves (Osborn waves - notching of terminal QRS complex) LITFL hypercalcemia EKG
Treatment
  • PO Ca/Vit D repletion preferred; IV if severe/symptomatic
  • Make sure not hyperphosphatemic as PO4 will precipitate with Ca
  • Typical PO dose - 1-2 g elemental Ca TID (Vit D helps intestinal absorption)
    • Ca carbonate is 40% elemental Ca
    • give PO on empty stomach, otherwise will chelate with Phos in food
  • Typical IV dose - 1-2 g Ca gluconate over 10-20 min
    • Ca gluconate has less tissue toxicity w/ extravasation
    • Ca chloride has 3x more calcium but must be given through central line
  • 1 g of Ca gluconate IV should increase serum Ca by 0.5 mg/dL
  
  • Severe/symptomatic:
    • NS → natriuresis and ↑ renal Ca excretion (onset ~ hrs)
    • Lasix - only give if volume overload (onset 30 m)
    • Bisphosphonates - inhibit osteoclasts, caution for nephrotoxicity and jaw osteonecrosis (peak 2-4 d)
  • Asx primary hyper-PTH
    • Surgical candidate: < 50 yo, Ca > 1 mg/dL above ULN, CrCl < 60 ml/min, DEXA < -2.5
    • Not a surgical candidate: q year Ca, Cr, DEXA

Mg

Mg
(1.5-2.5 mEq/L)		 Hypomagnesemia
↓ (< 1.3 mEq/L)		 Hypermagnesemia
↑ (> 2.5 mEq/L)
Symptoms Lethargy, confusion, tremor, ataxia, seizures, arrhythmia Headache, lethargy, ↓DTR 
Workup/labs

May cause or exacerbate ↓K, ↓ Ca

EKG findings - ectopy, long QTc, possible torsades

  
Treatment
  • PO Mg repletion preferred; IV if severe/symptomatic (e.g. arrhythmia)
  • IV Mg very inefficient (IV Mg stimulates renal Mg excretion); use additional oral Mg to sustain correction
  • 2 g of MgSO4 IV should increase serum Mg by 0.5 mEq/L
    • Typical PO dose 1 tab Mag-Ox qd (240 mg elemental Mg/400 mg tablet))
    • Typical IV dose 1 g MgSO4 (96 mg Mg)
  

PO4

PO4
(2-5 mg/L)

 Hypophosphatemia
↓ (< 2 mg/L)		 Hyperphosphatemia
↑ (> 5 mg/L)
Symptoms Weakness, rhabdo, resp. failure, heart failure, paresthesias, confusion, seizures, coma Sx related to associated hypocalcemia
Workup/labs
  • Etiology
    • Shift into cells (alkalosis, insulin, increased glycolysis)
    • ↓ GI absorption (poor PO, malabsorption, antacids)
    • ↑ urinary excretion (hypo-PTH, vit D deficiency, Fanconi)
  
  • Etiology: renal failure, hyper-PTH, rhabdo, tumor lysis
Treatment
  • PO PO4 repletion preferred; IV if severe (PO4 < 1.0) or symptomatic
  • Only worry if low phos on CVVHD (removes phos v efficiently → weakness, difficult to extubate)
  • Typical dose - 0.5 to 1.0 g elemental phosphorus BID/TID
    • Neutra-Phos = 250 mg elemental phos, with Na/K
  
  • PO4 restriction, oral PO4 binders (e.g. sevelamer/Renvela), IVF (phosphaturesis)
author: admin | last edited: Jan. 8, 2019, 8:10 p.m. | pk: 72

  1. http://www.internalizemedicine.com/2014/03/how-to-replace-electrolytes.html