needs to conserve sodium, it can reduce renal excretion to less than 1 mEq/day. Daily homeostasis is easily

maintained with 1–2 mEq/kg/day.

Potassium. The normal daily intake of potassium is approximately 40–120 mEq/day, with about 10%–15%

excreted in urine. An amount of 0.5–1 mEq/kg/day is appropriate to maintain homeostasis.

What is a good maintenance IV? (Table 1-3) Using the previous estimates for a 70 -kg male, the weight

formula for IV f luid would equal 110 cc/hour. Minimal sodium maintenance would require 70–140 mEq/day,

and minimal potassium requirements would be 35–70 mEq/day. In 0.5% normal saline (NS), there is 77

mEq/L sodium, and if one adds 20 mEq/L of potassium, then using 0.5% NS with 20 mEq/L KCL at 110

cc/hour would equal about 2.6 L of f luid, 200 mEq of sodium, and 52 mEq of potassium … pretty close!

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TABLE 1-3 Electrolyte Concentration in Various Intravenous Fluids

Fluid

Na+

mEq/L

K+

mEq/L

Mg++

mEq/L

Ca++

mEq/L

Cl-

mEq/L

Lactate

mEq/L

Osmolarity

mOsm/L

Normal saline (0.9%NaCl)

154 0 0 0 154 0 308

1/2 normal saline(0.5% NaCl)

77 0 0 0 77 0 154

Hypertonic saline (3%saline)

513 0 0 0 513 0 1027

Lactated Ringer's 130 4 0 2.7 98 28 525

Plasmalyte* 140 5 3 0 98 0 294

*Plasmalyte also contains 27 mEq/L acetate and 23 mEq/L gluconate.

C Water and electrolyte deficits and excesses

Water

Hypovolemia:

Signs of acute volume loss include tachycardia, hypotension, and decreased urine output.

Signs of gradual volume loss include loss of skin turgor, thirst, alterations in body

temperature, and changes in mental status.

Replacing water deficits. Acute deficits should be replaced acutely; chronic deficits should be

replaced more slowly, with half of the deficit replaced over the first 8 hours and the rest in 24–

48 hours. In the case of hypernatremia with hypovolemia, do not allow the sodium

concentration to drop more than 0.5–1 mEq/hour.

Hypervolemia: Well tolerated in healthy patients––they wil l just urinate the excess.

Signs of acute hypervolemia: Acute shortness of breath, tachycardia.

Complications of acute CHF can arise in patients with poor cardiac function given too

much fluid acutely. Therefore, i t is important to monitor these patients closely.

Signs of chronic hypervolemia: Peripheral edema, pulmonary edema.

Diuresis may be needed in some patients to reduce volume.

Sodium: close relationship to volume status

Hyponatremia

Definition and categories. Hyponatremia is defined as a serum sodium level of 130 mEq/L or

less. The first step in diagnosis and treatment is to assess the osmolar and volemic state.

Hyperosmolar: Dilutional hyponatremia from hyperglycemia, mannitol infusion, or

presence of other osmotically active particles.

Normo-osmolar: Pseudohyponatremia. Hyperglycemia, hyperl ipidemia, and

hyperproteinemia interfere with the lab measurement of sodium.

Hypo-osmolar: True hyponatremia.

Hypovolemic: Most common. Normally, hypovolemia leads to ADH secretion and

the inabil i ty to excrete free water. Intake of free water via thirst mechanisms or

infusion of hypotonic solution leads to hyponatremia. Total body sodium usually

is low.

Hypervolemic: Total body sodium usually is high. The pathology is often related

to low cardiac output (the kidneys see less blood flow, and free water is not

excreted) or hypoalbuminemic (e.g., cirrhosis) or other edematous states where

salt (renin-angiotensin system) and free water (ADH) cannot be excreted by the

kidneys.

Euvolemic: Could be either of the states above, or more frequently in the

perioperative patient, syndrome of inappropriate antidiuretic hormone (SIADH)

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secretion. ADH secretion can be stimulated by the stress response to

trauma and surgery. Free water is retained.

Symptoms. Acute hyponatremia is associated with acute cerebral edema, seizures, and coma.

Chronic hyponatremia is well tolerated to Na concentrations of 110 mEq/L. Symptoms generally

include confusion/decreased mental status, irr i tabil i ty, and decreased deep tendon reflexes.

Diagnosis and categorization. Clinical exam and lab determination of osmolar state are often

enough for diagnosis, but if in doubt, especially with hypo -osmolar hyponatremia, check urine

osmolarity and sodium concentration.

Hypovolemic, hypo-osmolar hyponatremia: Urine osmolarity high; Na low.

Hypervolemic, hypo-osmolar hyponatremia: Similar picture.