Editing Medical-Surgical Nursing (section)

== Fluid, Electrolyte, and Acid-Base Balance ==

=== Fluid Volume ===

Fluid volume deficit presents with thirst, dry mucous membranes, poor skin turgor over the sternum or forehead, flat neck veins, tachycardia, orthostatic hypotension, decreased urine output with concentrated dark urine, elevated urine specific gravity, elevated hematocrit and blood urea nitrogen, and acute weight loss. Fluid volume excess presents with bounding pulse, distended neck veins, crackles, dyspnea, peripheral or sacral edema, and acute weight gain.

Nursing application: daily weight taken at the same time, on the same scale, in similar clothing is the single most reliable indicator of fluid status. A gain of one kilogram equals roughly one liter of retained fluid. Intake and output records support the weight but do not replace it.

=== Key Electrolytes ===

Approximate adult reference ranges are sodium 135 to 145 mEq/L, potassium 3.5 to 5.0 mEq/L, total calcium 8.5 to 10.5 mg/dL, magnesium 1.5 to 2.5 mEq/L, and phosphorus 2.5 to 4.5 mg/dL. Values vary slightly by laboratory.

Hyponatremia produces neurologic findings because water shifts into brain cells: headache, confusion, lethargy, muscle cramps, and with severe or rapid drops, seizures. Hypernatremia produces thirst, dry sticky mucous membranes, restlessness, and agitation. Correct either one slowly; rapid correction of sodium in either direction causes serious neurologic injury.

Hypokalemia produces muscle weakness, leg cramps, decreased bowel sounds and ileus, shallow respirations from weak respiratory muscles, and electrocardiogram changes including flattened T waves, ST depression, and the appearance of U waves. It also markedly increases digoxin toxicity risk. Hyperkalemia produces muscle weakness, paresthesia, and peaked T waves progressing to a widened QRS and cardiac arrest.

Nursing application: potassium is never given by intravenous push and is never given undiluted. It is always diluted and infused on a pump, with the rate limited per facility policy, and the site is checked frequently because potassium is irritating to veins. Adequate urine output should be confirmed before giving potassium.

Hypocalcemia produces tetany, tingling around the mouth and in the fingers, hyperactive reflexes, a positive Chvostek sign (facial twitching when the facial nerve is tapped), a positive Trousseau sign (carpal spasm when a blood pressure cuff is inflated above systolic pressure), laryngospasm, and seizures. Hypercalcemia produces the opposite: lethargy, muscle weakness, decreased reflexes, constipation, and kidney stones.

Hypomagnesemia mimics hypocalcemia with tremors, hyperactive reflexes, and increased risk of torsades de pointes. Hypermagnesemia causes depressed reflexes, respiratory depression, and hypotension; the antidote is intravenous calcium gluconate.

=== Acid-Base Interpretation ===

Normal arterial values are pH 7.35 to 7.45, PaCO2 35 to 45 mm Hg, and bicarbonate 22 to 26 mEq/L.

Use the memory aid ROME: Respiratory Opposite, Metabolic Equal. In respiratory disorders the pH and the PaCO2 move in opposite directions; in metabolic disorders the pH and the bicarbonate move in the same direction.

Respiratory acidosis (low pH, high PaCO2) results from hypoventilation: chronic obstructive pulmonary disease exacerbation, oversedation, opioid overdose, neuromuscular weakness, or atelectasis. Respiratory alkalosis (high pH, low PaCO2) results from hyperventilation: anxiety, pain, fever, early sepsis, or pulmonary embolism. Metabolic acidosis (low pH, low bicarbonate) results from diabetic ketoacidosis, kidney failure, lactic acidosis from shock, or prolonged diarrhea. Metabolic alkalosis (high pH, high bicarbonate) results from prolonged vomiting, nasogastric suction, or excessive antacid use.