Physician Assistant Exam: The Kidneys and Sodium Levels
The kidneys, of course, regulate electrolyte concentrations. If you work in a hospital setting or are taking the Physician Assistant Exam, no matter what your specialty, you’re interacting with patients who have electrolyte abnormalities, including abnormalities of sodium balance (hyponatremia and hypernatremia) and potassium balance (hypokalemia and hyperkalemia). In fact, these four electrolyte abnormalities are the most common ones seen in a hospital setting, especially among older people.
Hyponatremia: Not enough sodium
Hyponatremia is defined as a serum sodium of < 135 mEq/L and a serum osmolality of < 280 mOsm/kg.
Pseudohyponatremia means that the serum sodium level is < 135 mEq/L but the serum osmolality is 280–300 mOsm/kg; true hyponatremia involves a low serum osmality as well. Examples of pseudohyponatremia include multiple myeloma and very, very high triglyceride levels of > 1,000 mg/dL.
High blood glucose levels, as in diabetic ketoacidosis (DKA) or in a hyperosmolar state hyperglycemic hyperosmolar nonketotic coma (HHNKC), can cause a low serum sodium level with an elevated serum osmality that’s > 300.
On a test question concerning hyponatremia, look at the serum osmolality and blood glucose first. If the serum osmolality is normal, think pseudohyponatremia. If the glucose level is high and the serum osmolality is high, the patient doesn’t have true hyponatremia. Don’t let the serum osmality trip you up on the test.
To further evaluate hyponatremia, the next two steps after obtaining the serum osmolality are to obtain the urine osmolality and to evaluate the person’s volume status. In addition to findings on physical examination, the urine sodium level is important. After you’ve established that the patient has true hyponatremia, you assess the patient’s volume status so you can identify and treat the hyponatremia:
Hypovolemic hyponatremia: Think diuretics like hydrochlorothiazide, furosemide (Lasix), and decreased oral intake. The treatment is salt repletion, usually with intravenous normal saline. Diuretics often cause hyponatremia and hypokalemia together. The urine sodium in a hypovolemic hyponatremia is < 25 with a fractional excretion of sodium (FENa) that’s < 1.
If the urine sodium is obtained while the person is on a diuretic, it may be falsely high. For test-taking purposes, however, assume that in a scenario where someone has hyponatremia after taking a diuretic, the urine sodium level in the test reflects a volume-depleted state.
Euvolemic hyponatremia: Think hypothyroidism, adrenal insufficiency, and the syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH). The urine sodium is > 25, and the FENa is > 3. For SIADH, the first line of treatment is water restriction.
Hypervolemic hyponatremia: Think congestive heart failure, kidney disease, and cirrhosis. The urine sodium is < 25, or the FENa is < 1. Although there’s excess volume on board, the kidney isn’t being perfused adequately, leading to low urine sodium levels. For congestive heart failure, the treatment is fluid restriction and diuretics. The same is true for cirrhosis and kidney disease.
On the test, you choose to give hypertonic saline (3 percent saline) to someone with hyponatremia only if the sodium dropped quickly or the person is confused.
Hypernatremia: Too much sodium
Hypernatremia is defined as a serum sodium > 145 mEq/L. It commonly refers to losses of water in excess of sodium. Here are two commonly encountered scenarios:
A patient has increased free water losses and isn’t getting enough free water replacement. Examples include a patient who has had a cerebrovascular accident (CVA) and isn’t able to ask for water when thirsty or a person with increased insensible losses (from fever or diaphoresis) who isn’t meeting his free water requirements.
A person has ongoing urinary losses, perhaps from pure water loss (that is, diabetes insipidus), an osmotic diuresis (the effect of high glucose causing urinary losses of free water), or judicious use of diuretics like furosemide (Lasix).
In problems concerning hypernatremia, you often need to calculate a free water deficit and decide on the best type of fluid replacement. Here’s the equation for calculating the free water deficit:
First calculate the total body water (TBW), which is the patient’s weight in kilograms multiplied by the percentage of the body that is water. Depending on age and gender, it can vary from 50 to 60 percent. Suppose you’re dealing with an elderly woman who weighs 72 kilograms with 50 percent total body water content. Here’s her total body water:
Total Body Water = 72 kg × 0.5 = 36 kg
To calculate her free water deficit (in liters), plug the total body water, current sodium, and desired sodium into the formula. The woman’s current sodium is 154, and the desired level is 140:
The free water deficit is 3.6 L.
The type of fluid to give depends on the clues in the question. If the person has visible signs of volume depletion, including tachycardia and hypotension in addition to hypernatremia, he or she may require 0.45% saline. If the vital signs are stable, then you can replace free water only, using a fluid such as D5W.