The Massachusetts General Hospital Handbook of Neurology

Authors: Flaherty, Alice W.; Rost, Natalia S.

Title: Massachusetts General Hospital Handbook of Neurology, The, 2nd Edition

Copyright 2007 Lippincott Williams & Wilkins

> Table of Contents > Medicine > Electrolytes

Electrolytes

A. Tests

• Arterial blood gases and serum electrolytes:

  • pH <7.38 = acidosis; pH >7.42 = alkalosis.

    • Respiratory vs. metabolic pH changes: If pH change is purely respiratory, for every change in pCO₂ of 10 torr, there should be 0.8 pH unit change. Greater or less than this implies superimposed metabolic process.

  • Bicarb <24: Implies metabolic acidosis.

    • Anion gap = Na - Cl - bicarb. Normally is <12 4.

      • Correct for low albumin: Normal anion gap = 2(alb) + 4.

      • Correct for alkalosis: A pH >7.5 causes a high anion gap just by uncovering negative sites on albumin.

    • Respiratory compensation? If 1.5(bicarb) + (8 2) > pCO₂, then there is respiratory compensation for the metabolic alkalosis. But if it is < pCO₂, then there is a superimposed respiratory acidosis.

  • Bicarb >24: Implies metabolic alkalosis. Look at pCO₂ (below) to see if there is a concurrent respiratory alkalosis.

  • pCO₂: If pCO₂ <40, there is an added primary respiratory alkalosis. If pCO₂ >50, there is a primary respiratory acidosis.

    • Acute if for every 10 mm Hg pCO₂ above (or below) normal, the pH is below (above) normal by 0.08.

    P.195

    • Chronic if for every 10 mm Hg pCO₂ above (or below) normal, the pH is below (above) normal by 0.03.

• Venous blood gases can be used to estimate arterial blood gases:

  • Venous bicarb + 2 = arterial bicarb.

  • Venous pCO₂ - 6 = arterial pCO₂.

  • Venous pH + 0.04 = arterial pH.

• Urine electrolytes: Get Na, K, Cr, osms. Get serum Cr and serum osms at same time. Diuretics must have been held for at least 6 h.

  • Urine Na <9 is consistent with dehydration (hanging onto salt).

  • Urine Na >20 but low serum Na and normal osms might be SIADH or renal failure.

  • Urine anion gap: Useful in hyperchloremic metabolic acidosis. Measure the urine sodium + urine potassium - urine chloride. The remainder is ammonium ion.

    • Positive urine anion gap: Implies renal wasting of bicarb.

    • Negative urine anion gap: Implies GI wasting of bicarb.

B. Electrolyte abnormalities

• Acidosis:

  • Causes of anion gap acidosis:

    • Increased production of acid:

      • Lactate.

      • Ketosis: DM, alcohol, starvation.

      • Ingestion: Salicylates, methanol, ethylene glycol.

    • Decreased excretion of acid: Renal failure.

  • Causes of non anion gap acidosis: Diarrhea, dilutional acidosis, carbonic anhydrase inhibitors, renal tubular acidosis .

  • Rx: Treat cause. Respiratory acidosis usually requires intubation. For severe acidosis, pH <7.2, consider 44-88 mEq Na bicarbonate IV, in IV solution appropriate to the pts. fluid status. Do not correct to pH >7.2.

    • Complications of bicarbonate rx: Fluid overload, precipitation of acute tetany in pts. with renal failure, and relative CSF acidosis (which can cause coma).

• Metabolic alkalosis:

  • Causes: Vomiting, NG suction, dehydration, low K, loop diuretics, mineralocorticoids, compensation for chronic respiratory acidosis .

  • Rx: Alkalosis usually resolves with volume correction and KCl be careful with the latter in renal failure. Correct alkalosis promptly in pts. with neuromuscular or myocardial irritability.

• High sodium: See Figure 29.

  • H&P: Confusion, thirst, signs of dehydration.

  • Rx: Correct Na slowly to avoid brain edema, CHF. See Dehydration, p. 197.

• Low sodium: See Figure 30.

  • H&P: Confusion, seizures, signs of dehydration.

  • Rx: Correct Na slowly to avoid central pontine myelinolysis. see p. 197 for correction algorithms.

  P.196

  Figure 29. Dx and rx of high serum sodium.

• SIADH:

  • Criteria: Euvolemic hyponatremia from inappropriately concentrated urine, without renal or adrenal dysfunction.

  • DDx of SIADH: Dehydration, overhydration, renal failure, adrenal failure, hypothyroidism, cerebral salt wasting.

  • Causes of SIADH:

    • Intracranial processes: E.g., trauma, infection, tumor. However, you must rule out cerebral salt wasting see below.

    • Lung processes: E.g., trauma, infection, tumor.

    • Other: Drugs, e.g., carbamazepine; malignancies, anemia, stress, porphyria.

    Figure 30. Dx and rx of low serum sodium.

  P.197

  • Rx of SIADH:

    • Acute: Free water restriction; if symptomatic hyponatremia, then consider slow correction with hypertonic saline.

    • Chronic: Fluid restriction; high-salt diet. Consider furosemide, demeclocycline 300 mg bid-qid.

• Cerebral salt wasting (CSW): Hyponatremia from inappropriate excretion of salt in the kidney, via an unknown central mechanism. Unlike SIADH, pts. with CSW are usually hypovolemic. Physical exam, central venous pressure, pulmonary wedge pressure, plasma and urine osms, etc., can help tell if the pt. is hypovolemic. CSW is not uncommon in SAH, and inappropriate fluid restriction for supposed SIADH can worsen SAH vasospasm.

C. Dehydration

• H&P: HR, SBP, urine output. Pt. is orthostatic if SBP drops >15 mm Hg or HR increases >20 bpm from lying to sitting after 2 minutes. Look at skin turgor, mucosal hydration.

• Labs: Electrolytes, BUN, glucose, osms, UA.

  • Specific gravity: <1.015 suggests a renal concentration defect, >1.030 = moderate dehydration, >1.035 = severe.

  • Units: In the following, Na is always measured in mEq/L.

• Rx of dehydration:

  • Discontinue diuretics.

  • Replacement rate = (deficit/time desired to replete) + maintenance rate.

    • Calculate deficit:

      • Total deficit = (old wt - new wt) or = 0.35 (old wt) (1 - old Hct/new Hct). Third-spacing will make these two numbers different; use the Hct estimate.

      • Free water deficit (in hypernatremia) = TBW (Na - 140)/140, where TBW = total body water = 0.5 (wt) for women, 0.6 (wt) for men.

    • Calculate maintenance per kg body weight:

      • For 1st 10 kg: 4 cc/h/kg.

      • For 2nd 10 kg: 2 cc/h/kg.

      • Above that: 1 cc/h/kg.

  • Type of fluid to use:

    • For Na 130 150 (isotonic dehydration): Give hypotonic dextrose solution (30 55 mEq Na/L). Give 50% of replacement in 1st 8 h; rest during next 16 h.

    • For Na <130 (hypotonic dehydration): Give isotonic Na.

      • Beware of central pontine myelinolysis, which may result from overrapid correction of hyponatremia. It presents as mutism, oculobulbar palsies, and quadriparesis. Correct Na slowly, not more than 1 U/h.

      • Beware of seizures when Na <120.

    • For Na >150 (hypertonic dehydration): Give D5 1/2 NS.

      • Beware of cerebral edema: Correct Na slowly; it should not fall >10 mEq/24 h.

• Low potassium:

  • H&P: EKG changes (see Electrocardiogram, p. 207), arrhythmia, muscle twitching, weakness.

  P.198

  • Causes: Vomiting, diarrhea, diuretics, alkalosis (intracellular K shift), hyperaldosteronism, familial periodic paralysis .

  • Complicating conditions: It is important to keep serum K >4.0 in cardiac pts., asthmatics on ₂-agonists, and type 2 DM.

  • Rx: Be cautious if there is renal failure. With normal pts., KCl 40 mEq PO q4h 3 doses is usually enough. IV K correction should usually not exceed 10 mEq/h (note: 40 mEq KCl/L at 100 mL/h is 4 mEq KCl/h).

• High potassium:

  • H&P: EKG changes (see Electrocardiogram, p. 207), arrhythmia, flaccid paralysis.

  • Causes: Acidosis, diuretics, renal failure, hemolysis, rhabdomyolysis, Addison's, familial periodic paralysis .

  • Rx: Stop K supplements. For serum K >5.5 in renal failure, give sodium polystyrene sulfonate (Kayexalate) 30-60 g PO or enema. For emergent rx, consider 1 amp CaCl or Ca gluconate IV, or 2 amps Na bicarbonate with 2 amps D50 given with 10 U regular insulin IV.

• Low calcium:

  • H&P: Confusion, papilledema. Ca <7 can cause tetany (Chvostek's and Trousseau's signs), laryngospasm, and seizures.

  • Correction for low albumin: For every 1 g/dL albumin deficit, lower limit of normal Ca will decrease 0.8 mg/dL. Can also check ionized Ca, which is not influenced by protein, on an ABG. It is normally >1.0.

  • Rx: Ca carbonate 500 mg PO tid, or Ca gluconate 1-2 amps IV in 250 cc NS, given over 2-4 h. Beware giving Ca to pts. on digoxin.

• High calcium: Consider medicine consult.

  • H&P: Abdominal pain, nausea, confusion, muscle weakness.

  • Causes: Cancer, endocrine, granulomatous dzs, renal failure.

  • Rx: Emergent if Ca >15. Aggressive hydration, then diuretics; specific drugs such as bisphosphonates, but caution that these effects last up to a month.

• Low magnesium:

  • H&P: Nausea, tremor, fasciculations, tetany.

  • Rx: MgCl₂ 10 cc PO 3 days; or MgSO₄ 2 g IV in 250 cc D5 W IV. Be careful in renal failure.

• Low phosphate: Treat with PO phosphate 2 tabs tid 3 d, or Na- or K-phosphate 10 mmol over 6-8 h IV. Be careful in renal failure.

• High phosphate: Treat with PhosLo 1-2 tabs PO tid.