•  Step 4a: Is there an anion gap?

Anion gap acidosis: (Na – (Cl + bicarb)) > 14 (see chart)

Note: Needs to be corrected for albumin; a fall in serum albumin 1 g/dL from the nl value (4.2 g/dL) decreases the anion gap by 2.5 meq/L. Corrected AG = AG + (2.5 × [4.2 – albumin]).

•  Step 4b: If an anion gap is present, is there an osmolar gap?

Osmolar gap: Measured serum Osm – Calculated Osm >10 mOsm/L, where

Calculated Osm = (2 × [Naμ⁺μ]) + glucose/18 + BUN/2.8 + Ethanol/4.6

•  Step 4c: If no anion gap is present, what is UAG?

Urinary anion gap: Na + K – Cl

Note: The UAG can help differentiate GI & renal causes of non-AG (or hyperchloremic) metabolic acidosis, as base can be lost from the gut or kidney (negative UAG: GI loss [ie, diarrhea, small bowel fistula, ileostomy]; positive UAG: Renal loss, particularly RTA types I & IV)

•  Step 5: What is the delta ratio, also known as the “delta/delta”?

(AG – nl AG)/(nl HCO₃ – HCO₃), or simply (AG – 12)/(24 – HCO₃)

•  If delta/delta > +6, suggests concomitant metabolic alkalosis, or prior compensated respiratory acidosis

•  If delta/delta = 0, suggests uncomplicated AG metabolic acidosis

•  If delta/delta > –6, suggests concomitant hyperchloremic non-AG metabolic acidosis

Treatment and Disposition

•  Both will largely depend on severity & underlying etiology of the disorder

•  Limited role for bicarbonate in the absence of hemodynamic collapse

ABNORMAL ELECTROLYTES

Hyponatremia

Definition

•  Na <135, excess of water relative to sodium, generally from elevated ADH; generally not symptomatic at Na >125

History

•  Most sxs are nonspecific & can include fatigue, weakness, muscle cramps, thirst, or postural dizziness. Sxs can range to more severe manifestations including confusion, agitation, delirium, lethargy, somnolence, coma, or szs.

•  Other helpful historical features include h/o CHF, cirrhosis, renal dz, cancer, adrenal or pituitary dysfxn, recent GI surgery, thiazide or loop diuretics use, alcoholism

Physical Exam

•  Look for signs to assess pt fluid status:

•  Hypervolemia: Elevated JVP, peripheral edema, crackles, ascites, anasarca

•  Hypovolemia: Tachycardia, hypotension, dry mucous membranes, oliguria, poor skin turgor, IVC collapsibility

•  Look for signs of profound hyponatremia: Lethargic, disoriented/abnl sensorium, depressed reflexes, hypothermic, pseudobulbar palsy, Cheyne–Stokes respiration

Diagnostics

•  Labs: Chem 7, FSG, urine electrolytes (Na, Cr, Osm), serum Osm, albumin

•  VBG w/ stat sodium & Osm may provide more rapid turnaround

•  Corrected Na_glucose = Serum Na + [0.016 × (serum glucose – 100)] up to 400 mg/dL

•  for glucose >400 mg/dL, 4 mEq/L should be added to every additional 100 mg/dL

Step-wise Approach to Hyponatremia

•  Step 1: What is the serum osmolality?

•  Step 2: What is the pt’s volume status? Hypervolemic, euvolemic, or hypovolemic?

•  Step 3: What are the urine Na, urine Osm, & FeNA values?

•  Fractional Excretion of Sodium = FeNa = (Na_urine × Cr_serum)/(Na_serum × Cr_urine)

Treatment

•  Asymptomatic or mild sxs of hyponatremia: Correct serum Na at ≤0.5 mEq/L/h

•  Severe manifestations of hyponatremia: RAPID correction serum Na at 2 mEq/L/h × 2–3 h OR until sxs resolve

•  Euvolemic hyponatremia

•  Asymptomatic: Free water restrict (500–1000 mL/d)

•  Symptomatic: See above

•  SIADH

•  Free water restrict + treat underlying cause

•  Caution if using hypertonic or nl saline esp if IVF Osm < urine Osm, serum sodium may worsen (higher Osm will draw out fluid)

•  May also consider lithium or demeclocycline (NEJM 2007;356:2064)

•  Hypovolemic hyponatremia

•  Volume replete w/ nl saline, as above (once dehydration resolved, stimulation of ADH will decline & Na will correct)

•  Hypervolemic hyponatremia

•  Free water restrict (0.5–1.5 L/d)

•  Increase arterial volume: W/ vasodilators (Nitro), loop diuretics; consider albumin in cirrhosis

•  Severe hyponatremia: Consider diuresis + Na replacement

Disposition

•  Home: Mild asymptomatic hyponatremia

•  Admit: Symptomatic, comorbidities, elderly. May require ICU admission if severe.

Pearl

•  Rapid correction >10–12 mEq/L/d may result in central pontine myelinolysis (dysarthria, szs, quadriparesis due to focal myelin destruction in pons & extrapontine areas)

Hypernatremia

Definition

•  Na >145, usually from free water loss or sodium gain (ie, infusion of hypertonic fluid)

•  Appropriate response to hypernatremia is increased free water intake stimulated by thirst & renal excretion of a minimal volume of maximally concentrated urine as regulated by ADH

History

•  Mild sxs include increased thirst or polyuria

•  Severe sxs: AMS (irritability, lethargy, confusion, delirium, coma)

•  RFs: Elderly, infants, debilitated. Endocrine pathology; cardiac, renal, liver dzs; psychiatric disorder (see etiology of Central and Nephrogenic Diabetes Insipidus); MEDS (see below chart), living situation (access to free water).

Physical Exam

•  Look for signs to assess pt fluid status:

•  Hypervolemia: Elevated JVP, peripheral edema, crackles, ascites, anasarca

•  Hypovolemia: Tachycardia, hypotension, dry mucous membranes, oliguria, poor skin turgor, IVC collapsibility

•  Severe hypernatremia: Lethargy, muscle spasticity, tremor, hyperreflexia, respiratory paralysis, ataxia

Diagnostics

•  Labs: Chem 7, FSG, urine electrolytes (Na, Cr, Osm), serum Osm, albumin

•  VBG w/ stat sodium & Osm may provide more rapid turnaround

•  Corrected Na_glucose = Serum Na + [0.016 × (serum glucose – 100)] up to 400 mg/dL

•  for glucose >400 mg/dL, 4 mEq/L should be added to every additional 100 mg/dL

Step-wise Approach to Hypernatremia

•  Step 1: What is the serum osmolality?

•  nl serum osmolality = 275–290 mosmol/kg

•  Step 2: What is the pt’s volume status? Hypervolemic, euvolemic, or hypovolemic?

•  Step 3: What are the urine Na & urine Osm values?

Treatment

•  Hypervolemic hypernatremia

•  Treat underlying disorder

•  Replace free water deficit (as above)

•  Euvolemic hypernatremia

•  Replace free water deficit (as above)

•  Treat underlying etiology

•  Central DI: Vasopressin 10 U SQ

•  Hypovolemic hypernatremia

•  Restore volume 1st then replace free water deficit (as above); add 40 mEq KCl IV to fluid replacement once pt is urinating

Disposition

•  Home: Mild hypernatremia which can be corrected in <24 h

•  Admit: Most will be admitted

Hypokalemia

Definition

•  Kμ⁺μ <3.5 mEq/L (ie, decreased intake, shift into cells, loss); 98% of potassium is intracellular.

History

•  Usually not symptomatic until Kμ⁺μ <3 mEq/L

•  Nausea, vomiting, weakness, fatigue, myalgia, muscle cramps. Meds (see Differential table).

•  Pts at highest risk for electrocardiac cx of hypokalemia include those w/ acute ischemia, prolonged QT syndrome, & those taking digoxin

Physical Exam

•  Paresthesias, depressed reflexes, proximal muscle weakness, ileus

•  Severe hypokalemia: Hypoventilation, paralysis, rhabdomyolysis, myoglobinuria

•  ARF, polymorphic VT, asystole

Diagnostics

•  Labs: Chem 7, UA, urine electrolytes, urine Osm; consider blood gas, CPK, serum Osm

•  Urine Kμ⁺μ <15 mmol/d suggests extrarenal, while urine Kμ⁺μ >15 mmol/d suggests renal etiology

•  Transtubular Kμ⁺μ concentration gradient (TTKG) is helpful, but rarely used in the ED: TTKG = (Plasma_Osm × Urine_K)/(Plasma_K × Urine_Osm)

Note: Hypokalemia w/ TTKG >4 suggests renal Kμ⁺μ loss due to distal Kμ⁺μ secretion

•  ECG: T-wave flattening/inversion, ST depression, U-waves, prolonged QT/QU interval; may also see PR prolongation, decreased voltage, QRS widening, atrial/ventricular dysrhythmias

Treatment

•  ED

•  Potassium replacement: Potassium chloride, Potassium bicarbonate, Potassium phosphate

•  Treat underlying cause

•  Replace Mg as needed (*Note: Concurrent Mg & Kμ⁺μ deficiency could lead to refractory Kμ⁺μ repletion)

•  Goal Kμ⁺μ = 4 mEq/L in pts at highest risk

•  Home

•  Counsel pts to increase dietary intake of Kμ⁺μ (dried fruits, nuts, avocados, wheat germ lima beans, vegetables [spinach, broccoli, cauliflower, beets, carrots], fruits [banana, kiwi, etc])

•  Discuss w/ PCP: Decrease diuretic dose; start/substitute for Kμ⁺μ-sparing med (βB, ACE, ARB, Kμ⁺μ-sparing diuretic)

•  Potassium replacement: KCl 20 mEq PO QD for prevention; KCl 40–100 mEq PO QD for tx

Disposition

•  Home: Mild hypokalemia w/ close f/u to recheck labs

•  Admit: Moderate/severe hypokalemia, acid–base abnormalities, arrhythmia

Pearl

•  Avoid dextrose solutions (stimulate insulin & inward shift of Kμ⁺μ)

Guideline: Cohn JN, Kowey PR, Whelton PK, Prisant LM. New guidelines for potassium replacement in clinical practice. Arch Intern Med. 2000;160: 2429–2436.

Hyperkalemia

Definition

•  Kμ⁺μ >5 mEq/L (ie, Kμ⁺μ release from cells, decreased renal losses, iatrogenic)

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