5: Electrolytes

Published on 06/02/2015 by admin

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CHAPTER 5 Electrolytes

James Duke, MD, MBA

1 What is a normal sodium concentration? What degree of hyponatremia is acceptable to continue with a planned elective procedure?

A normal sodium level is between 135 and 145 mEq/L. Generally a sodium level of 130 mEq/L will not result in cancellation of a planned procedure as long as the patient is not symptomatic and hyponatremia is not an expected result of the procedure. It is important to note that recognizing hyponatremia should prompt an investigation as to the cause. Whether the investigation should take priority over the surgery depends on the urgency of the procedure and an overall assessment of the patient’s condition.

2 How is hyponatremia classified?

Hyponatremia may occur in the presence of hypotonicity, normal tonicity, or hypertonicity; thus it is important to measure serum osmolality to determine the cause of hyponatremia. Assessment of volume status is also important in determining the cause. An excess of total body water is more common than a loss of sodium in excess of water. Table 5-1 summarizes causes and treatment of hyponatremia.

TABLE 5-1 Causes of Hyponatremia

Total Sodium Content	Causes	Treatment (Always Treat Underlying Disorder)
Decreased	Diuretics (including osmotic diuretics); renal tubular acidosis; hypoaldosteronism; salt-wasting nephropathies, vomiting; diarrhea	Restore fluid and sodium deficits with isotonic saline
Normal	SIADH; hypothyroidism; cortisol deficiency	Water restriction
Increased	Congestive heart failure; cirrhosis; nephrotic syndrome	Water restriction, loop diuretics

SIADH, Syndrome of inappropriate antidiuretic hormone.

3 How should acute hyponatremia be treated?

The rate at which hyponatremia develops and the presence of symptoms determine the aggressiveness of treatment. If hyponatremia has developed quickly, the patient may develop nausea, vomiting, visual disturbances, muscle cramps, weakness, hypertension, bradycardia, confusion, apprehension, agitation, obtundation, or seizures; usually the sodium content is found to be less than 125 mEq/L. The aggressiveness of treatment depends on the extent of symptoms.

In the simplest cases fluid restriction may be sufficient. Administration of loop diuretics may also be indicated. Severe neurologic symptoms require careful administration of hypertonic (3%) saline. The dose of 3% saline (513 mEq Na/L) is determined as follows:

Correction should occur slowly, with serial sodium concentrations measured. Below 125 mEq/L, correct at a rate of about 1 mEq/L/hr. Once a concentration of 125 mEq/L has been achieved, the likelihood of continued severe neurologic symptoms has diminished. It should be noted that aggressive correction may result in central pontine myelinolysis.

Seizures require securing a protected airway, oxygenation, ventilation, and perhaps administration of anticonvulsants, although seizures are usually self-limited.

4 Is there a subset of patients who may tend to have residual neurologic sequelae from a hyponatremic episode?

Females of reproductive age, and especially during menstruation, have been noted to be at greatest risk for residual sequelae. There may be an estrogen-related impairment of the adaptive ability of the brain in the setting of hyponatremia.

5 What may cause acute hyponatremia in the operating room?

Administration of hypotonic fluids or absorption of sodium-poor irrigants may result in hyponatremia. Such irrigants are used to facilitate transurethral resection of the prostate or distend the uterus during hysteroscopies. However, isotonic fluids are used increasingly in these settings when bipolar electrical cautery is used for the surgical procedure. Thus hyponatremia as a consequence of these procedures has been observed with decreasing frequency. However, some surgeons may still prefer monopolar cautery because the resection is more rapid. In this instance a hypotonic irrigation fluid is used to prevent dispersal of the electrical current, and in such a scenario hyponatremia is a risk. The most common cause of postoperative hyponatremia is syndrome of inappropriate antidiuretic hormone secretion and subsequent water retention.

6 Discuss hypernatremia and its causes

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