Fluid and Electrolyte Disturbances¶
Chapter 56 | Part 2: Cardiovascular and Fluid/Electrolyte Disorders
KEY CLINICAL POINTS¶
- Fluid and electrolyte balance is regulated by AVP, renal function, and intake/output.
- Hyponatremia is the most common electrolyte disorder, with 22% of hospitalized patients affected.
- Hyperkalemia is a medical emergency due to cardiac arrhythmia risk, requiring immediate intervention.
- Diabetes insipidus (DI) is classified as central (AVP deficiency) or nephrogenic (resistance to AVP).
- Potassium homeostasis involves renal excretion, dietary intake, and hormonal regulation (aldosterone).
1. DEFINITION & OVERVIEW¶
Fluid and electrolyte disturbances encompass disorders of water and solute balance. Key mechanisms include AVP regulation, renal excretion, and intake/output. Disorders range from hypovolemia to hypernatremia, hypokalemia, and hyperkalemia. The body maintains homeostasis through hormonal (AVP, aldosterone) and renal mechanisms.
Table 56-1: Causes of SIAD¶
| MALIGNANT DISORDERS OF THE CENTRAL | PULMONARY DISORDERS | NERVOUS SYSTEM | DRUGS | OTHER CAUSES |
|---|---|---|---|---|
| Carcinoma Lung | Bacterial pneumonia | Infection | Drugs that stimulate AVP release | Hereditary mutations in V2 receptor |
| Small cell | Viral pneumonia | Encephalitis | SSRIs | Idiopathic |
| Mesothelioma | Pulmonary abscess | Meningitis | Tricyclic antidepressants | Transient |
| Oropharynx | Tuberculosis | Brain abscess | Nicotine | Endurance exercise |
| Gastrointestinal tract | Rocky Mountain spotted fever | Subdural hematoma | Antipsychotic drugs | General anesthesia |
1.1 Central vs. Nephrogenic Diabetes Insipidus¶
Central DI results from AVP deficiency (hypothalamic/pituitary dysfunction), while nephrogenic DI involves AVP resistance (renal tubule dysfunction). Both cause polyuria and polydipsia with low urine osmolality.
1.2 Role of AVP and Osmoregulation¶
AVP (vasopressin) regulates water reabsorption in the collecting duct. Osmoreceptor activation in the hypothalamus triggers AVP release, increasing renal water reabsorption. Disordered AVP signaling leads to DI or inappropriate antidiuresis (SIAD).
2. EPIDEMIOLOGY¶
Hyponatremia affects 22% of hospitalized patients, while hyperkalemia occurs in 10% of hospitalized patients. Hypovolemia is common in trauma, burns, and GI losses. Hypernatremia is less common but associated with high mortality (40-60%) due to underlying disease severity.
2.1 Hyponatremia Prevalence¶
Hyponatremia is the most common electrolyte disorder, with 22% of hospitalized patients affected. Severe cases (>125 mM) are associated with significant morbidity/mortality.
2.2 Hyperkalemia Risk Factors¶
Hyperkalemia occurs in 10% of hospitalized patients. Risk factors include renal insufficiency, ACE inhibitors, NSAIDs, and conditions like diabetic ketoacidosis.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Fluid/electrolyte disorders arise from imbalances in intake/output, hormonal regulation (AVP, aldosterone), or renal dysfunction. Pathophysiology includes osmotic diuresis, renal tubular dysfunction, and hormonal resistance.
Table 56-2: Causes of Acute Hyponatremia¶
| Iatrogenic | Postoperative | Hypotonic fluids | Colonoscopy preparation | Recent thiazide use |
|---|---|---|---|---|
| Premenopausal women | Glycine irrigation (TURP, uterine surgery) | Cause of › vasopressin | Recent institution of thiazides | Polydipsia |
3.1 AVP and Osmoregulation¶
AVP secretion is regulated by osmoreceptors in the hypothalamus. AVP acts on V2 receptors in collecting ducts to increase water reabsorption. Defective AVP signaling causes DI or SIAD.
3.2 Renal Sodium Handling¶
Sodium balance depends on Na+/K+-ATPase activity, ENaC function, and aldosterone. Disorders like Bartter's syndrome or Gitelman's syndrome disrupt renal sodium reabsorption.
4. CLINICAL FEATURES¶
Clinical manifestations vary by disorder. Hyponatremia presents with neurological symptoms (confusion, seizures), while hyperkalemia causes ECG changes (peaked T waves, widened QRS). Hypokalemia leads to muscle weakness and arrhythmias.
4.1 Hyponatremia Symptoms¶
Neurological symptoms include confusion, seizures, and coma. Severe cases (plasma Na+ <125 mM) risk cerebral edema and herniation.
4.2 Hyperkalemia ECG Changes¶
ECG findings include tall T waves, shortened QT interval, and eventually sine wave pattern. Severe hyperkalemia (>8.0 mM) can cause asystole.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnosis includes SIAD, hypovolemic/hypervolemic hyponatremia, and pseudohyponatremia. Hyperkalemia differentiates between renal excretion failure and extracellular shifts.
5.1 SIAD vs. Hypovolemic Hyponatremia¶
SIAD presents with euvolemia and low urine Na+ (>30 mM). Hypovolemic hyponatremia shows low urine Na+ (<20 mM) with volume depletion.
5.2 Hyperkalemia Causes¶
Differentiate between renal excretion failure (ACE inhibitors, NSAIDs) and extracellular shifts (acidosis, insulin, β -blockers).
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnostic workup includes serum electrolytes, urine electrolytes, osmolality, and AVP/copeptin levels. TTKG and urine-to-plasma ratios help distinguish etiologies.
Table 56-3: Management of Hypernatremia¶
| Water Deficit | Free-Water Deficit | Administer Over 48–72 h | Ongoing Water Losses | Insensible Losses |
|---|---|---|---|---|
| Estimate TBW: 50% of body weight in women | [(Na+ – 140)/140] × TBW | Avoid >10 mM/24 h correction | Calculate free-water clearance | ~10 mL/kg/day |
6.1 Urine Electrolyte Analysis¶
Urine Na+/K+ and osmolality help differentiate renal vs. extrarenal losses. TTKG >8 suggests distal delivery issues, while <3 indicates proximal tubule dysfunction.
6.2 AVP and Copeptin Testing¶
AVP levels are elevated in SIAD but not in central DI. Copeptin (AVP precursor) correlates with AVP and helps diagnose inappropriate antidiuresis.
7. MANAGEMENT & TREATMENT¶
Treatment depends on etiology. Hyponatremia requires fluid restriction, AVP antagonists, or hypertonic saline. Hyperkalemia is managed with calcium, insulin, and dialysis.
Table 5, 56-5: Causes of Hyperkalemia¶
| Pseudohyperkalemia | Intra- to Extracellular Shift | Inadequate Excretion |
|---|---|---|
| Cellular efflux; thrombocytosis, erythrocytosis | Acidosis | Inhibition of RAAS |
| Hereditary defects in red cell transport | Hyperosmolality | Decreased distal delivery |
| b-Adrenergic antagonists | Hyporeninemic hypoaldosteronism |
7.1 Hyponatremia Correction¶
Correct slowly (<10 mM/24 h) to avoid ODS. Use hypertonic saline for severe cases. SIAD responds to vaptans or fluid restriction.
7.2 Hyperkalemia Management¶
Immediate treatment includes calcium, insulin/glucose, and β -agonists. Chronic cases require diuretics, dialysis, or potassium binders (patiromer, sodium zirconium cyclosilicate).
8. PROGNOSIS & COMPLICATIONS¶
Prognosis varies by disorder. Severe hyponatremia or hyperkalemia can be fatal. Chronic disorders like SIAD or PHA may lead to renal failure or metabolic alkalosis.
8.1 Hyponatremia Complications¶
Severe hyponatremia (>125 mM) risks cerebral edema, herniation, and death. Chronic cases may lead to ODS with rapid correction.
8.2 Hyperkalemia Outcomes¶
Severe hyperkalemia (>8.0 mM) can cause cardiac arrest. Chronic hyperkalemia is associated with renal failure and metabolic acidosis.
9. SPECIAL CONSIDERATIONS¶
Special populations include pregnancy (gestational DI), elderly (increased risk of dehydration), and renal failure (impaired electrolyte excretion). Drug interactions (e.g., NSAIDs, ACE inhibitors) affect electrolyte balance.
9.1 Pregnancy and Hyponatremia¶
Gestational DI occurs in late pregnancy due to vasopressinase activity. DDAVP is effective for treatment.
9.2 Elderly Patients¶
Elderly are at higher risk for hypernatremia due to reduced thirst and impaired renal concentrating ability.
10. KEY POINTS & CLINICAL PEARLS¶
- Hyponatremia is the most common electrolyte disorder, with 22% of hospitalized patients affected. 2. Hyperkalemia is a medical emergency requiring immediate intervention. 3. AVP and aldosterone regulate fluid/electrolyte balance. 4. SIAD is diagnosed by low urine Na+ and high urine osmolality. 5. Chronic correction of hyponatremia should be <10 mM/24 h to avoid ODS.