• 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 Naglucose = 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 = (Naurine × Crserum)/(Naserum × Crurine)
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 Naglucose = 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 = (PlasmaOsm × UrineK)/(PlasmaK × UrineOsm)
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)