Volume Assessment & Intravenous Fluid Therapy

Assessing Volume Status

Volume depletion vs dehydration. Volume depletion is loss of sodium and water from the extracellular fluid (bleeding, vomiting, diarrhea, diuretics) and is treated with isotonic fluid. Dehydration, strictly, is pure water loss that raises serum sodium and is treated with free water.

Orthostatic vital signs. Measure BP and pulse supine, then after 1–3 minutes standing. A pulse rise ≥30/min, dizziness severe enough to prevent standing, or a systolic fall ≥20 mmHg indicates meaningful hypovolemia; supine tachycardia and hypotension appear only with larger losses. Elderly patients and those on β-blockers may not become tachycardic, and autonomic neuropathy causes orthostatic hypotension without hypovolemia.

Examination. Hypovolemia: flat neck veins, dry mucous membranes and axillae, reduced skin turgor (unreliable in the elderly), capillary refill over 2 seconds in children, and oliguria. Hypervolemia: elevated JVP, hepatojugular reflux, S3, crackles, and edema. No single sign is precise, so combine them.

Laboratory clues. Underfilled kidneys conserve sodium: urine Na <20 mEq/L, FENa <1%, concentrated urine, and BUN:creatinine >20:1 because urea is reabsorbed with salt and water; hemoconcentration and contraction alkalosis may accompany. Diuretics raise urine sodium (FEUrea <35% is more reliable), vomiting forces sodium out with bicarbonate (urine chloride <20 is the better marker), and GI bleeding or steroids raise BUN without hypovolemia.

Bedside ultrasound. A small IVC that collapses >50% with inspiration suggests low right atrial pressure; a plethoric, noncollapsing IVC suggests high filling pressures, and diffuse lung B-lines indicate interstitial edema. Dynamic tests predict fluid responsiveness better than static ones: a passive leg raise that increases cardiac output or stroke volume by ≥10% predicts benefit from a bolus.

Edema is not intravascular volume. In heart failure and cirrhosis, total body sodium and water are increased, yet effective arterial blood volume is low, so RAAS and ADH are activated, urine Na is <20, and prerenal azotemia develops; nephrotic syndrome also retains sodium avidly, largely through primary renal sodium retention. Treatment is sodium restriction and diuretics, not saline.

Where Infused Fluid Goes

Compartments. Total body water is about 60% of body weight (less in women and the elderly): two-thirds intracellular, one-third extracellular, and plasma is one-quarter of the ECF. Isotonic crystalloid stays in the ECF, so only about 250 mL of each liter remains intravascular. D5W becomes free water once the glucose is metabolized and spreads through all compartments, leaving about 80 mL intravascular: useful for free-water deficits (hypernatremia), useless in shock. Hypertonic (3%) saline draws water out of cells and is reserved for cerebral edema and severe symptomatic hyponatremia.

Choosing a Crystalloid

Normal saline. 0.9% saline carries 154 mEq/L of chloride, far above plasma (~100). Large volumes produce a non-anion-gap (hyperchloremic) metabolic acidosis and chloride-mediated renal vasoconstriction linked to AKI.

Balanced crystalloids. Lactated Ringer's and Plasma-Lyte have near-plasma chloride plus a buffer (lactate, acetate, gluconate) that the liver converts to bicarbonate, and they are preferred for most resuscitation, including sepsis. LR's lactate does not cause lactic acidosis. LR contains only 4 mEq/L of potassium and is acceptable in hyperkalemia, CKD, and kidney transplantation; saline's acidosis shifts potassium out of cells and can raise it more.

Avoid hypotonic fluids (D5W, 0.45% saline, even slightly hypotonic LR) in traumatic brain injury or cerebral edema; use 0.9% or hypertonic saline. Albumin increased mortality in traumatic brain injury. LR's calcium binds citrate in stored blood, so transfuse only alongside 0.9% saline.

Maintenance and Replacement

Maintenance. Needed only while a patient cannot eat or drink. The 4-2-1 rule gives 4 mL/kg/h for the first 10 kg, 2 mL/kg/h for the next 10 kg, and 1 mL/kg/h for each kg above 20 (≈ weight + 40 mL/h); adults need roughly 25–30 mL/kg/day, about 2 L at 70 kg.

Colloids and Fluid Overload

Albumin. Albumin is no better than crystalloid for routine resuscitation but has defined roles in cirrhosis: about 6–8 g per liter removed after large-volume (>5 L) paracentesis to prevent post-paracentesis circulatory dysfunction; 1.5 g/kg on day 1 and 1 g/kg on day 3 in spontaneous bacterial peritonitis, lowering renal failure and mortality; and, with vasoconstrictors (terlipressin, norepinephrine, or midodrine plus octreotide), for hepatorenal syndrome. In septic shock it may be added when very large crystalloid volumes are needed.

Hydroxyethyl starch increases AKI, need for dialysis, bleeding, and mortality in sepsis and critical illness (boxed warning); do not use it for volume expansion.

Fluid overload. Fluid has a narrow therapeutic window. A persistently positive balance worsens AKI (renal venous congestion and edema within an encapsulated kidney), ARDS, bowel edema, and wound healing. Give fluid as boluses (for example, 30 mL/kg crystalloid for sepsis-induced hypoperfusion), reassess after each with examination, ultrasound, and dynamic tests, and remove fluid with diuretics once shock resolves. In ARDS, a conservative fluid strategy increases ventilator-free days without increasing the need for dialysis.