# Fluid, Electrolyte, and Acid-Base Management in the Surgical Patient

## Overview

Perioperative fluid and electrolyte management is a core competency for surgical residents, and errors in this domain contribute directly to postoperative morbidity -- including pulmonary edema, acute kidney injury, and anastomotic complications. Equally important is the ability to interpret and correct acid-base derangements in critically ill surgical patients, where a systematic approach can be the difference between effective resuscitation and clinical deterioration.

## Body Fluid Compartments

Total body water accounts for approximately 60% of body weight in men and 50% in women. This water is distributed between two major compartments: the intracellular fluid (ICF), which holds two-thirds of total body water (about 40% of body weight), and the extracellular fluid (ECF), which holds the remaining one-third (about 20% of body weight). The ECF is further divided into the interstitial fluid (three-quarters of ECF, or 15% of body weight) and the intravascular plasma volume (one-quarter of ECF, or 5% of body weight). For a typical 70 kg male, this translates to approximately 42 liters of total body water, with 28 liters intracellular, 10.5 liters interstitial, and 3.5 liters of plasma volume. Understanding this distribution is essential because different intravenous fluids distribute differently across these compartments.

## Intravenous Fluid Types

### Crystalloids

| Solution | Na | Cl | K | Ca | Lactate | Osmolality | pH |
|----------|-----|-----|-----|-----|---------|------------|-----|
| Normal Saline (0.9% NaCl) | 154 | 154 | 0 | 0 | 0 | 308 | 5.0 |
| Lactated Ringer's | 130 | 109 | 4 | 3 | 28 | 273 | 6.5 |
| Plasma-Lyte | 140 | 98 | 5 | 0 | 0* | 294 | 7.4 |
| D5W | 0 | 0 | 0 | 0 | 0 | 252 | 4.0 |

*Plasma-Lyte contains acetate (27) and gluconate (23) instead of lactate.

Normal saline contains supraphysiologic concentrations of both sodium and chloride (154 mEq/L each), and large-volume resuscitation with it produces hyperchloremic non-anion gap metabolic acidosis. The SMART and SALT-ED trials demonstrated that balanced crystalloids (Lactated Ringer's, Plasma-Lyte) are associated with reduced major adverse kidney events compared to normal saline, making balanced solutions the preferred choice for most resuscitation. Lactated Ringer's is slightly hypotonic, and the lactate it contains is metabolized to bicarbonate in the liver. While it contains 4 mEq/L of potassium, the clinical significance of this in hyperkalemic patients remains debated. D5W distributes across all body compartments and is used for free water replacement rather than volume resuscitation. Importantly, only about 25% of infused crystalloid remains in the intravascular space after one hour.

### Colloids

Albumin (available as 5% and 25% solutions) remains intravascular longer than crystalloid but is expensive and showed no mortality benefit over crystalloid in the SAFE trial -- with the important caveat that albumin may be harmful in traumatic brain injury. Hydroxyethyl starch (HES) is associated with acute kidney injury and coagulopathy and should be avoided, as the CHEST and 6S trials led to an FDA warning. Dextran is rarely used due to interference with crossmatching and platelet function.

### Hypertonic Saline

Hypertonic saline (3% or 7.5%) is used for symptomatic hyponatremia and cerebral edema. Three percent saline contains 513 mEq/L of sodium. When correcting hyponatremia, sodium should not be raised faster than 8 mEq/L per 24 hours to avoid the devastating complication of osmotic demyelination syndrome.

## Perioperative Fluid Management

Maintenance fluid requirements follow the Holliday-Segar formula: 4 mL/kg/hr for the first 10 kg, 2 mL/kg/hr for the next 10 kg, and 1 mL/kg/hr for each kilogram above 20. For a 70 kg patient, this works out to approximately 110 mL/hr.

The debate between liberal and restrictive fluid strategies has been largely settled by recognizing that both extremes are harmful. Liberal fluid management leads to weight gain, edema, pulmonary complications, and anastomotic leak, while the RELIEF trial showed that an overly restrictive approach increased acute kidney injury without benefit. The current preferred approach is goal-directed fluid therapy (GDFT), which uses dynamic hemodynamic parameters -- stroke volume variation, pulse pressure variation, and cardiac output monitoring -- to guide fluid boluses. This approach is central to Enhanced Recovery After Surgery (ERAS) protocols. The aim is euvolemia, avoiding both hypo- and hypervolemia.

When assessing volume status, dynamic parameters (stroke volume variation, pulse pressure variation, and the passive leg raise test) are far superior to static measures. Central venous pressure, despite its historical prominence, is a poor predictor of volume responsiveness and should not be relied upon for fluid management decisions. Clinical assessment of heart rate, blood pressure, urine output (target 0.5 mL/kg/hr in adults), skin turgor, and mucous membranes remains important.

## Electrolyte Disorders

### Sodium

Hyponatremia (Na below 135 mEq/L) is the most common electrolyte abnormality in hospitalized patients. Classification by volume status guides treatment: hypovolemic hyponatremia (from GI losses, diuretics, or adrenal insufficiency) is treated with normal saline; euvolemic hyponatremia (most commonly from SIADH in postoperative patients) is treated with fluid restriction, salt tablets, or vasopressin receptor antagonists; and hypervolemic hyponatremia (from CHF, cirrhosis, or nephrotic syndrome) is treated with fluid and sodium restriction plus diuretics. Acute symptomatic hyponatremia presenting with seizures or altered mental status requires urgent treatment with 3% hypertonic saline (100 mL bolus over 10 minutes, repeatable up to twice). Correction must not exceed 8 mEq/L in 24 hours to prevent osmotic demyelination syndrome. Postoperative hyponatremia in young women from hypotonic fluid administration is a surgical emergency.

Hypernatremia (Na above 145 mEq/L) typically reflects a free water deficit. The deficit can be calculated as 0.6 multiplied by body weight in kilograms multiplied by the quotient of serum sodium divided by 140, minus 1. Replacement should occur over 48 to 72 hours, correcting no faster than 10 mEq/L per 24 hours, using D5W or half-normal saline.

### Potassium

Hypokalemia (K below 3.5 mEq/L) results from GI losses, diuretics, alkalosis, insulin, or refeeding syndrome. It manifests as muscle weakness, ileus, and cardiac arrhythmias (U waves, flattened T waves, ST depression). A critical clinical point is that magnesium must always be checked and corrected concurrently -- hypomagnesemia makes hypokalemia refractory to replacement. Oral replacement is preferred; intravenous replacement should not exceed 10 to 20 mEq/hr peripherally or 40 mEq/hr centrally with cardiac monitoring. As a rough guide, 10 mEq of KCl raises the serum potassium by approximately 0.1 mEq/L.

Hyperkalemia (K above 5.5 mEq/L) is a life-threatening emergency when ECG changes are present. The progression of ECG findings -- peaked T waves, then widened QRS, then sine wave pattern, then cardiac arrest -- should be known by every surgeon. Treatment follows a logical sequence: calcium gluconate (10 mL of 10%) stabilizes the myocardial membrane immediately but does not lower potassium; insulin (10 units regular) with D50 (50 mL) and sodium bicarbonate shift potassium intracellularly; beta-2 agonists provide an additional intracellular shift; loop diuretics increase renal excretion; and hemodialysis provides definitive removal in refractory cases. Newer GI potassium binders such as patiromer and sodium zirconium cyclosilicate have a safer profile than the older sodium polystyrene sulfonate (Kayexalate), which carries a risk of intestinal necrosis and should be avoided in postoperative patients.

### Calcium

Hypocalcemia in surgical patients is most commonly seen after thyroidectomy or parathyroidectomy, in pancreatitis, after massive transfusion (where citrate chelates calcium), and in hypomagnesemia. Clinical signs include perioral tingling, Chvostek sign (facial nerve tap producing facial muscle contraction), Trousseau sign (carpal spasm with blood pressure cuff inflation), QT prolongation, seizures, and laryngospasm. Treatment is with intravenous calcium gluconate (10 mL of 10% over 10 minutes); calcium chloride is more potent but must be given through a central line to avoid tissue necrosis if extravasated. After thyroidectomy, ionized calcium should be checked every 6 to 8 hours for the first 24 hours.

Hypercalcemia is caused most commonly by hyperparathyroidism in the outpatient setting and malignancy in the inpatient setting. The mnemonic "stones, bones, groans, moans, and psychiatric overtones" captures the clinical manifestations. Treatment begins with aggressive intravenous normal saline resuscitation, followed by loop diuretics (only after rehydration), bisphosphonates (zoledronic acid), calcitonin for rapid but transient effect, denosumab, or dialysis for severe cases.

### Magnesium

Hypomagnesemia is common in surgical patients (particularly with alcoholism, diuretic use, GI losses, or refeeding) and is critically important because it causes refractory hypokalemia and hypocalcemia. Manifestations include tremor, hyperreflexia, and cardiac arrhythmias including torsades de pointes. Treatment is with intravenous magnesium sulfate (2 grams IV over 1 hour).

### Phosphate

Hypophosphatemia is most commonly encountered in refeeding syndrome, during DKA treatment, and in chronic alcoholism. Severe hypophosphatemia (below 1.0 mg/dL) can cause respiratory muscle weakness, rhabdomyolysis, and hemolytic anemia. Treatment is with intravenous sodium or potassium phosphate.

## Acid-Base Physiology

The Henderson-Hasselbalch equation (pH = 6.1 + log([HCO3-] / (0.03 x pCO2))) defines the relationship between pH, bicarbonate, and carbon dioxide. Normal values are pH 7.35 to 7.45, pCO2 35 to 45 mmHg, and HCO3 22 to 26 mEq/L.

A systematic approach to acid-base disorders involves five steps: determine the primary disorder from the pH; identify whether the disturbance is metabolic or respiratory; assess the appropriateness of compensation; if metabolic acidosis is present, calculate the anion gap; and if the anion gap is elevated, calculate the delta-delta ratio to identify mixed disorders.

| Primary Disorder | Compensation |
|-----------------|-------------|
| Metabolic acidosis | Winter's formula: Expected pCO2 = 1.5 x [HCO3] + 8 (+/- 2) |
| Metabolic alkalosis | Expected pCO2 = 0.7 x [HCO3] + 21 (+/- 2) |
| Acute respiratory acidosis | HCO3 increases 1 mEq/L per 10 mmHg rise in pCO2 |
| Chronic respiratory acidosis | HCO3 increases 3.5 mEq/L per 10 mmHg rise in pCO2 |
| Acute respiratory alkalosis | HCO3 decreases 2 mEq/L per 10 mmHg fall in pCO2 |
| Chronic respiratory alkalosis | HCO3 decreases 5 mEq/L per 10 mmHg fall in pCO2 |

The anion gap (AG = Na - (Cl + HCO3), normal 12 +/- 4) must be corrected for albumin: for each 1 g/dL decrease in albumin below 4, add 2.5 to the calculated AG. Elevated anion gap metabolic acidosis follows the MUDPILES mnemonic (Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates), while non-anion gap (hyperchloremic) metabolic acidosis results from diarrhea, renal tubular acidosis, normal saline resuscitation, ureteral diversion, or carbonic anhydrase inhibitors.

The delta-delta ratio ((Measured AG - 12) / (24 - Measured HCO3)) helps identify mixed disorders: a ratio below 1 indicates combined anion gap and non-anion gap metabolic acidosis; 1 to 2 indicates a pure anion gap metabolic acidosis; and above 2 indicates a concurrent metabolic alkalosis.

Several acid-base scenarios are particularly common in surgical practice. Pyloric stenosis and protracted vomiting produce hypochloremic, hypokalemic metabolic alkalosis with paradoxical aciduria (the kidney excretes hydrogen ions to retain sodium in severe volume depletion). Large-volume normal saline resuscitation causes hyperchloremic non-anion gap metabolic acidosis. Small bowel fistula and diarrhea cause non-anion gap metabolic acidosis from bicarbonate loss. Sepsis and shock produce lactic acidosis. The most common acid-base disturbance in the postoperative surgical patient is respiratory alkalosis, driven by pain and anxiety-induced hyperventilation.

<image>Diagram of body fluid compartments showing total body water distribution: intracellular (2/3) and extracellular (1/3) compartments, with extracellular further divided into interstitial (3/4 of ECF) and intravascular (1/4 of ECF). Include arrows showing how different IV fluids distribute: D5W across all compartments, normal saline/LR in ECF only, and colloids primarily in intravascular space.</image>

<image>Clinical algorithm for the systematic approach to acid-base disorders. Start with pH determination, branch to metabolic vs. respiratory, calculate anion gap for metabolic acidosis (with MUDPILES mnemonic), assess compensation adequacy, and calculate delta-delta ratio. Include a reference table of normal values and compensation formulas on the side.</image>

<image>ECG rhythm strip comparison showing progressive changes of hyperkalemia: normal baseline tracing, peaked T waves at K+ 6.0-6.5, widened QRS at K+ 7.0-7.5, sine wave pattern at K+ 8.0+, and asystole. Label each stage with corresponding potassium level and recommended treatment interventions at each stage.</image>

## Clinical Pearls

Balanced crystalloids (Lactated Ringer's, Plasma-Lyte) are preferred over normal saline for most resuscitation -- the SMART trial showed reduced major adverse kidney events with balanced solutions. Central venous pressure is a poor predictor of volume responsiveness; dynamic parameters such as stroke volume variation, pulse pressure variation, and the passive leg raise test are far superior. Always check and correct magnesium when treating refractory hypokalemia. Pyloric stenosis produces the classic "paradoxical aciduria," in which the kidney excretes hydrogen ions to retain sodium in the setting of severe volume depletion and chloride loss. Hyponatremia must be corrected no faster than 8 mEq/L per 24 hours to avoid osmotic demyelination syndrome. Calcium gluconate in hyperkalemia stabilizes the myocardial membrane but does not lower serum potassium. Post-thyroidectomy hypocalcemia is the most common surgical cause of hypocalcemia, and ionized calcium should be monitored every 6 to 8 hours. Citrate in banked blood chelates calcium, and massive transfusion can cause clinically significant ionized hypocalcemia. Large-volume normal saline causes hyperchloremic non-anion gap metabolic acidosis, which can be confused with worsening clinical status. The most common acid-base disturbance in the postoperative surgical patient is respiratory alkalosis from pain and anxiety-driven hyperventilation.

## References

- Semler MW, Self WH, Wanderer JP, et al. Balanced crystalloids versus saline in critically ill adults (SMART trial). *N Engl J Med*. 2018;378:829-839.
- Myles PS, Bellomo R, Corcoran T, et al. Restrictive versus liberal fluid therapy for major abdominal surgery (RELIEF). *N Engl J Med*. 2018;378:2263-2274.
- Finfer S, Bellomo R, Boyce N, et al. A comparison of albumin and saline for fluid resuscitation in the intensive care unit (SAFE study). *N Engl J Med*. 2004;350:2247-2256.
- Miller RD, Eriksson LI, Fleisher LA, et al. *Miller's Anesthesia*. 9th ed. Philadelphia: Elsevier; 2020.