3 Fluid and Electrolyte Management

How fluid and electrolyte balance are regulated

Fluid, electrolyte, and acid–base balance are closely linked. The kidneys regulate water, sodium, potassium, and acid excretion; the lungs adjust carbon dioxide; and antidiuretic hormone and aldosterone help maintain circulation and concentration. Disruption can affect blood pressure, brain function, muscle activity, and cardiac rhythm.

Severe symptoms, abnormal ECG findings, or rapidly changing laboratory results require urgent clinical assessment. This guide supports learning and does not provide individual treatment instructions.

Assessing fluid balance

Assess fluid status using the history, examination, trends, and laboratory results together. Ask about intake, thirst, vomiting, diarrhea, fever, sweating, urine output, medications—especially diuretics—and heart, liver, or kidney disease. Assess vital signs, including orthostatic changes when appropriate, as well as mucous membranes, edema, lung sounds, jugular venous pressure, and perfusion. Daily weights and intake/output trends are often more informative than a single observation.

Volume depletion may follow gastrointestinal losses, bleeding, poor intake, burns, or diuretic use. Findings can include thirst, dry mucous membranes, tachycardia, orthostatic hypotension, reduced urine output, and, when severe, poor perfusion or confusion.

Volume excess may result from impaired renal excretion or heart or liver failure. Findings can include weight gain, dependent edema, raised jugular venous pressure, pulmonary crackles, or breathlessness.

Serum sodium reflects water balance relative to sodium, not total-body sodium alone. A person may be volume depleted with low, normal, or high serum sodium.

Useful tests include a metabolic panel with sodium, potassium, chloride, bicarbonate, urea, creatinine, and glucose. Depending on the presentation, assess serum and urine osmolality, urine sodium, magnesium, calcium, phosphate, and an ECG. Interpret results in context: hemolysis can falsely elevate potassium, and abnormal glucose can affect measured sodium.

Principles of fluid management

Treat the cause and choose fluid according to the patient’s circulation, sodium, and water deficit or excess. Oral replacement may suit a stable patient who is able to drink. Clinicians may use isotonic intravenous crystalloid for significant extracellular volume depletion; the type and amount require reassessment, especially when heart or kidney function is impaired.

Correction of water deficit or excess must be planned and monitored to avoid harmful shifts in serum sodium. With fluid overload, address the underlying cause and consider sodium or fluid restriction, diuretics when appropriate, or dialysis if the problem is not controlled medically. Reassess symptoms, vital signs, weight, urine output, and laboratory trends after treatment.

Sodium and water abnormalities

Hyponatremia is a serum sodium concentration below $135\ \mathrm{mmol/L}$ and often reflects excess water relative to sodium. Causes include diuretics, gastrointestinal losses followed by replacement with free water, SIADH, and heart failure or cirrhosis. Possible symptoms include nausea, headache, confusion, seizures, or coma; symptoms depend on both the sodium level and how quickly it falls.

Assess volume status and look for the cause. Management depends on the cause: a volume-depleted patient may need isotonic fluid, while a patient with water excess may need fluid restriction and treatment of the underlying disease. Severe neurologic symptoms may require carefully monitored hypertonic saline. Avoid overly rapid correction, which can cause osmotic demyelination; clinicians must individualize and monitor correction targets.

Hypernatremia is a serum sodium concentration above $145\ \mathrm{mmol/L}$ and usually indicates a water deficit relative to sodium. Causes include inadequate access to water, impaired thirst, excessive water loss, or diabetes insipidus. Thirst is common; neurologic findings may include confusion, muscle irritability, seizures, or coma. Treatment generally involves controlled water replacement and correction of the cause. The rate depends on duration, symptoms, and circulation, so frequent reassessment is important.

Potassium abnormalities

Potassium is essential for nerve and muscle function, especially cardiac conduction. Abnormal potassium can be life-threatening, and ECG findings may be abnormal even when symptoms are limited.

Hypokalemia can result from diarrhea, vomiting, diuretics, poor intake, or movement of potassium into cells, for example with insulin or alkalosis. Findings include weakness, cramps, constipation, and sometimes arrhythmias. ECG changes can include flattened T waves and U waves. Check magnesium because low magnesium can make hypokalemia difficult to correct. Replace potassium according to severity, cause, kidney function, and ability to take oral therapy, and monitor for overcorrection.

Hyperkalemia can result from impaired kidney excretion, potassium-retaining medicines, adrenal disorders, or cell breakdown. Consider whether sample hemolysis could have falsely elevated the result, but do not delay urgent assessment when hyperkalemia is plausible. Obtain an ECG and review kidney function and medicines. Peaked T waves, widening QRS, loss of P waves, or dangerous rhythm changes are emergencies.

Clinical treatment may stabilize the heart, shift potassium into cells, and remove potassium from the body. Treatments that shift potassium into cells do not remove total-body potassium. Dialysis may be needed when severe hyperkalemia persists or kidney function is poor.

Calcium, magnesium, phosphate, and bicarbonate

Low calcium may cause tingling, cramps, tetany, seizures, or a prolonged QT interval. High calcium may cause weakness, constipation, confusion, and a shortened QT interval. Interpret total calcium with albumin or measure ionized calcium when indicated. Magnesium and kidney function can help identify causes. Alkalosis lowers ionized calcium by increasing protein binding.

Low magnesium can cause tremor, muscle irritability, seizures, arrhythmias, and hypokalemia or hypocalcemia that is resistant to correction. High magnesium, usually associated with impaired renal clearance or excess magnesium-containing products, can cause lethargy, weakness, reduced reflexes, hypotension, and respiratory depression. Replace or reduce magnesium with attention to kidney function and repeat levels.

High phosphate is common in advanced kidney disease and may contribute to low calcium and tissue deposition. Low phosphate may cause weakness and, when severe, impaired respiratory or cardiac muscle function. Assess renal function, nutrition, and related calcium and vitamin D results.

Chloride often changes with sodium and fluid disorders. Low bicarbonate can indicate metabolic acidosis; high bicarbonate can indicate metabolic alkalosis or compensation for chronic respiratory acidosis. Confirm the primary disorder using clinical context and blood gases when needed.

Acid–base disorders

Acidemia means blood pH below $7.35$; alkalemia means pH above $7.45$. Acidosis and alkalosis describe processes that push pH in those directions. The primary disturbance is metabolic, involving bicarbonate $(\mathrm{HCO_3^-})$, or respiratory, involving carbon dioxide $(\mathrm{PaCO_2})$.

Typical patterns and causes include:

• Metabolic acidosis: decreased $\mathrm{HCO_3^-}$, usually with decreased pH. Causes include lactic acidosis, ketoacidosis, kidney failure, diarrhea, or toxins.

• Metabolic alkalosis: increased $\mathrm{HCO_3^-}$, usually with increased pH. Causes include vomiting, diuretics, or mineralocorticoid excess.

• Respiratory acidosis: increased $\mathrm{PaCO_2}$, usually with decreased pH. Hypoventilation, such as from severe lung disease or sedatives, is a cause.

• Respiratory alkalosis: decreased $\mathrm{PaCO_2}$, usually with increased pH. Hyperventilation, for example from pain, fever, or some lung disorders, is a cause.

A practical approach is to check pH, identify whether $\mathrm{PaCO_2}$ or $\mathrm{HCO_3^-}$ explains the pH direction, and then ask whether compensation is appropriate. Compensation does not fully normalize pH; an unexpected response suggests a mixed disorder.

For metabolic acidosis, calculate the Anion gap:

A raised gap suggests added unmeasured acids. A normal-gap acidosis commonly reflects bicarbonate loss or impaired renal acid handling.

In metabolic acidosis, Winter’s formula estimates respiratory compensation:

For example, with pH $7.28$, $\mathrm{HCO_3^-}=14\ \mathrm{mmol/L}$, and measured $\mathrm{PaCO_2}=30\ \mathrm{mmHg}$, the primary disorder is metabolic acidosis. The expected $\mathrm{PaCO_2}$ is $1.5\times14+8=29\pm2\ \mathrm{mmHg}$; the measured value of $30\ \mathrm{mmHg}$ is within that expected range.

Management targets the cause and supports circulation, ventilation, and kidney function. Bicarbonate is not a universal treatment for acidosis; whether it is appropriate depends on the cause and clinical state. Severe or refractory acid–base disturbance may be an indication for renal replacement therapy.

Dialysis and its complications

Dialysis removes waste products and excess fluid and can help correct electrolyte and acid–base abnormalities when kidney function is inadequate. It is considered in context—not solely from a single urea or creatinine value—when complications cannot be safely controlled otherwise. Urgent reasons can include refractory hyperkalemia, severe acidosis, pulmonary edema or fluid overload, and uremic complications such as encephalopathy or pericarditis.

Hemodialysis uses a dialyzer and vascular access. Possible problems include low blood pressure, muscle cramps, rapid fluid or electrolyte shifts, and access infection or clotting.

Peritoneal dialysis uses the abdominal lining. Important complications include exit-site infection, peritonitis, hernia, and glucose-related weight gain. Abdominal pain, fever, or cloudy drainage in a person receiving peritoneal dialysis needs prompt assessment.

Applying the assessment principles

Use the whole clinical picture to assess severity. For example, several days of vomiting accompanied by dry mucous membranes, tachycardia, and orthostatic dizziness suggests extracellular volume depletion. Review losses and intake, vital signs and perfusion, urine output, weight, kidney function, and electrolytes.

A person with heart failure may have edema and breathlessness alongside low serum sodium. The low concentration does not necessarily mean total-body sodium is low: it can reflect excess water relative to sodium, even when total-body sodium is increased. Broad management priorities are to treat the underlying heart failure and manage excess water or volume with a monitored plan.

If a person with kidney disease has a high potassium result from a hemolyzed blood sample, hemolysis may have falsely elevated the result; repeat a properly collected sample when appropriate. If hyperkalemia remains likely, assess urgently with an ECG and review kidney function, symptoms, and medicines. When low potassium does not improve as expected, check magnesium because deficiency can promote renal potassium loss and make hypokalemia resistant to correction.

Urgent dialysis consideration can include refractory hyperkalemia, severe acidosis, pulmonary edema or fluid overload, and uremic encephalopathy or pericarditis when these complications cannot be controlled by other treatment. For a person on peritoneal dialysis with abdominal pain and cloudy drainage, peritonitis is a concern and prompt clinical assessment is needed, following the dialysis team’s urgent instructions.