Fluids, Electrolytes, and Acid-Base Bedside Guide for Residents

Any calculation that needs total body water (TBW): free water deficit, expected sodium change per liter of infusate, volume of distribution.

Any admission or change in status where volume state is unclear.

Every basic metabolic panel, before acting on any single abnormal value.

Before writing any continuous IV fluid order on a patient who is not hypovolemic or actively losing fluid.

Isotonic volume expansion when chloride loss is the problem: hypochloremic metabolic alkalosis, hyponatremia with hypovolemia, traumatic brain injury per local neurocritical protocol, and as the diluent for blood products.

Suspected infection with hypotension, lactate above 2 mmol/L, or organ dysfunction.

Grade by hemodynamics and end-organ signs, not by a guessed fluid deficit. Orthostatic change (HR rise of 30/min or more, or SBP fall of 20 mm Hg or more or DBP fall of 10 mm Hg or more within 3 minutes of standing) marks moderate depletion; supine hypotension or shock marks severe.

Serum Na below 135 mEq/L. Mild 130-134, moderate 125-129, profound below 125 mEq/L (European 2014 biochemical severity). Acute means documented onset within 48 h; anything else, or unknown, is treated as chronic.

Serum Na above 145 mEq/L; symptoms are uncommon below about 150. Thirst, weakness, irritability, and lethargy come first; confusion, seizures, and coma follow with higher levels or fast rises.

Serum K below 3.5 mEq/L. Mild 3.0-3.4 is usually asymptomatic; moderate 2.5-2.9 brings weakness, cramps, constipation; severe below 2.5 risks paralysis, rhabdomyolysis, and arrhythmia.

Serum K above 5.0 mEq/L. Mild is 5.5-5.9 (UK Kidney Association; 5.0-5.4 is only above normal), moderate 6.0-6.4, severe 6.5 or higher. Any level with ECG changes is an emergency.

Corrected total Ca <8.5 mg/dL or ionized Ca <4.6 mg/dL (1.15 mmol/L). Perioral and fingertip paresthesias, cramps, tetany, carpopedal spasm, laryngospasm, seizures, prolonged QT, hypotension, and heart failure that does not respond to usual therapy. Severe usually means ionized Ca <3.2 mg/dL (0.8 mmol/L) or corrected total Ca <7.5 mg/dL; symptoms can appear at higher values, especially with fast falls.

Often silent. Look for tremor, tetany, positive Chvostek or Trousseau sign, seizures, QT prolongation, ventricular arrhythmia, and hypokalemia or hypocalcemia that will not correct. Signs usually appear only with marked depletion (normal 1.7-2.2 mg/dL).

Any arterial or venous gas, or a chemistry panel with bicarbonate outside 22-28 mEq/L.

Metabolic acidosis (HCO3 below 22 mEq/L) with an anion gap above about 12 mEq/L.

Arterial pH above 7.45 with a primary rise in bicarbonate (above 28 mEq/L). The lungs compensate by hypoventilating: pCO2 rises about 0.7 mmHg per 1 mEq/L rise in bicarbonate, and is limited in practice by hypoxemia. Expected pCO2 = 40 + 0.7 × (HCO3 − 24); this is approximate and less reliable than in acidosis.

Rising PaCO2 with pH <7.35 and bicarbonate only slightly raised (expected about +1 mEq/L per 10 mmHg PaCO2 rise; range 1-2). Somnolence, headache, asterixis, flushed skin, then coma.

Polyuria, thirst, vomiting, abdominal pain, Kussmaul breathing, and altered mentation in a person with diabetes. The 2024 ADA/EASD/JBDS/AACE/DTS consensus needs all three: glucose ≥200 mg/dL or known diabetes, β-hydroxybutyrate (BHB) ≥3.0 mmol/L (or urine ketones ≥2+), and pH <7.3 and/or bicarbonate <18 mEq/L.

Creatinine rise of 0.3 mg/dL or more within 48 h, or 1.5 times baseline within 7 days (KDIGO), or urine output under 0.5 mL/kg/h for 6 h. The first task is separating a perfusion problem from tubular injury.

Volume overload (heart failure, cirrhosis, nephrotic syndrome, CKD), and forced diuresis in hypercalcemia or hyperkalemia once volume is replete.

Any adult going to or returning from surgery who cannot drink normally, and postoperative patients with falling urine output or a new sodium change.

Hypernatremia where the volume of free water needed to reach a target sodium must be estimated.

Predicting where a given fluid or solute will go, or why a sodium value does not describe volume.

Depletion is suspected or confirmed and the route of loss is unknown.

Any low bicarbonate, and every metabolic acidosis, to separate high-gap from normal-gap causes.

Estimating a starting maintenance rate in mL/h for an adult who cannot take fluid by mouth.

First-line crystalloid for most resuscitation, perioperative and ED volume replacement, including sepsis (Surviving Sepsis 2021 suggests balanced crystalloids over saline).

Hypotension or tachycardia with active bleeding, or a Class III-IV hemorrhage.

ATLS classes estimate blood loss from vital signs (HR ranges are from the older ATLS table; the 10th edition gives direction only). Heart rate rises first; BP falls late, so a normal BP does not exclude class II-III loss. Beta blockers and pacemakers blunt the heart rate signal.

Brain swelling drives symptoms, and they track the speed of fall more than the absolute value. A chronic Na of 118 mEq/L can be nearly silent; an acute fall to 128 can seize.