Disorders of Plasma Sodium and Potassium
Jaudat Khan
Eric Iida
Pang-Yen Fan
I. DISORDERS OF PLASMA SODIUM
A. General principles.
1. Plasma sodium (PNa) is the major determinant of plasma osmolality (Posm), which can be estimated as follows:
Posm = 2×(Na mEq / L) + (glucose mg / dL) /18 + (BUN mg / dL) / 2.8
2. PNa disorders generally indicate abnormal water metabolism, rather than abnormal sodium metabolism.
3. Posm is tightly regulated by antidiuretic hormone (ADH).
4. Increased ADH leads to water absorption via urinary concentration (high Uosm).
5. Decreased ADH leads to water excretion through urinary dilution (low Uosm).
B. Hyponatremia.
2. Pathophysiology.
a. Hypovolemia.
i. Kidney retains Na and water in response to hypoperfusion from volume depletion.
ii. Urinary indices reflect both sodium avidity with low urine Na and low fractional excretion of Na (FENa) and water avidity (high Uosm).
iii. Sodium deficit exceeds water deficit.
b. Hypervolemia.
i. Kidney retains Na and water in response to hypoperfusion despite volume expansion (ineffective circulating volume).
ii. Urinary indices reflect both sodium avidity (low urine Na and low FENa) and water avidity (high Uosm).
iii. Water excess exceeds sodium excess.
c. Euvolemia.
i. With syndrome of inappropriate antidiuretic hormone secretion (SIADH), kidney retains water inappropriately, but handles Na normally.
ii. Urinary indices typically reflect water avidity (high Uosm), but urine Na and FENa are not low.
TABLE 58-1 Causes of Hyponatremia | ||||||||||||||||||||||||||||||
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iii. With reset osmostat, water metabolism occurs normally, but maintains an abnormally low PNa.
iv. With psychogenic polydipsia and inadequate solute intake, water intake exceeds renal water excretory capacity and Uosm will be low.
v. With renal failure, water excretion is limited by low urine output.
3. Diagnosis.
a. Clinical presentation.
i. Symptoms principally neurologic, but rarely focal.
ii. Range from no symptoms to fatigue, lethargy, gait disturbances, confusion, nausea, vomiting, or, in severe cases, seizures and coma.
iii. Severity of symptoms relates to both level of hyponatremia and the rapidity of its development.
TABLE 58-2 Causes of Syndrome of Inappropriate Antidiuretic Hormone Secretion | ||
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b. History and physical examination.
i. Assessment of volume and neurologic status.
ii. Estimation of acuity of hyponatremia.
iii. Review of medications.
iv. Evaluation of solute and water intake and losses.
c. Laboratory studies.
i. Measured Posm: Normal value with a low calculated Posm indicates pseudohyponatremia (hyperlipidemia, hyperproteinemia).
ii. Uosm: Maximally dilute urine with Uosm (50 to 100 mOsm/kg) suggests primary polydipsia.
iii. Urine sodium.
(a) Typically <20 mEq/L with volume depletion or ineffective circulating volume.
(b) >20 mEq/L with SIADH.
iv. For euvolemic hyponatremia, assess renal, adrenal, and thyroid function, and obtain chest radiograph (CXR) and head computed tomography (CT).
v. Uric acid: Levels <4 mg/dL suggest SIADH.
4. Treatment.
a. Rate of correction over the first 24 to 48 hours may be more important than rate over a single or first few hours.
i. Avoid rapid correction due to risk of osmotic demyelination.
b. Asymptomatic patients: increase PNa < 0.5 mEq/hour and <8 mEq/24 hours.
i. If mildly symptomatic, increase Na up to 1.0 mEq/L/h for 3 to 4 hours then slow the rate of correction to raise PNa ≤ −10 mEq/L over the initial 24 hours.
ii. If severely symptomatic (seizures, coma), increase PNa 1 mEq/L/h × 4 to 6 hours then slow rate of correction to total of 8 to 10 mEq/L over 24 hours.
c. Monitor PNa every 2 hours for rapid correction.
d. Do not correct to normal PNa; target PNa should be 120 to 130 mEq/L.
e. Rapid correction requires hypertonic saline (512 Na mEq/L) infusion.
f. Calculate Na deficit (amount of Na to raise PNa to target).
Weight × 0.6(target PNa − current PNa) = Na deficit
Volume(mL)of hypertonic saline needed to correct Na deficit = (Na deficit/512)×1,000
Infusion rate(L / hour)= volume of hypertonic saline needed / (target PNa − current PNa) / desired correction rate
g. Hypovolemia.
i. Isotonic saline to correct volume deficit.
ii. In the setting of hypovolemia, each liter of saline will increase PNa ˜1 mEq/L.
iii. Correction of hypovolemia will suppress ADH and subsequently lead to rapid water excretion.
h. Hypervolemia.
i. Water restriction.
ii. Loop diuretics (avoid thiazides, which may exacerbate hyponatremia).
iii. Vasopressin antagonists.
(a) Only indicated in euvolemic/hypervolemic hyponatremia.
(b) NOT indicated for hyponatremic emergencies.
(c) Expert consultation recommended for initiation of therapy.
i. Euvolemic.
i. Water restriction.
ii. Hypertonic saline.
iii. Vasopressin antagonists.
iv. Correction/treatment of precipitating disorder.
C. Hypernatremia.
