Fluid Imbalance

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Fluid Imbalance

Fluid Imbalance - Clinical framework for dehydration, volume depletion and volume overload.

Practical orientation: Separate tonicity problems from volume problems. Dehydration is not the same as volume depletion.

Use first: clinical summary and algorithm.

Then: differential diagnosis, workup and management tables.

Escalate: use red flags and biopsy/urgent-care sections.

Clinical summary

Dehydration

Loss of total body water causing hypertonicity.|Hypernatremia and cellular dehydration.

Volume depletion

Loss of ECF sodium-containing fluid.|Reduced effective arterial blood volume and tissue hypoperfusion.

Volume overload

Sodium excess and expanded ECF volume.|Edema, pulmonary congestion, hypertension and venous congestion.

Compartment	Approximate proportion
Total body water	About 60% of ideal body weight in young men; lower in women and older adults.
Intracellular fluid	About two-thirds of total body water.
Extracellular fluid	About one-third of total body water.
Interstitial fluid	About three-quarters of ECF.
Intravascular fluid	About one-quarter of ECF.

Concept	Meaning
Osmolality	Total solute particles per kg water.
Tonicity	Effective osmoles that determine water movement across membranes.
Dehydration	Loss of TBW leading to hypertonicity; not the same as volume depletion.
Hypernatremia	Usually water deficit rather than sodium excess.

Volume status	Salt	Water	Mechanism
Hypovolemic	Low	Very low	Loss of hypotonic fluid.
Euvolemic	Neutral	Low	Electrolyte-free water loss.
Hypervolemic	High	Neutral/high	Sodium gain, often iatrogenic.

Correct chronic hypernatremia gradually to avoid cerebral edema.

Domain	Practical points
Definition	Deficit in ECF volume from loss or sequestration of sodium-containing fluid.
Causes	GI losses, diuretics, osmotic diuresis, salt-wasting nephropathies, burns, hemorrhage, pancreatitis, obstruction/peritonitis, crush injury.
Findings	Weight loss, thirst, orthostasis, tachycardia, oliguria, delayed capillary refill, low UNa unless confounded.
Severe	Hypovolemic shock with cold extremities, cyanosis, altered mentation and organ ischemia.

Domain	Practical points
Causes	Heart failure, cirrhosis, nephrotic syndrome, CKD/ESRD, sodium retention, medications.
Findings	Weight gain, hypertension, JVD, edema, orthopnea, rales, ascites, gut edema and venous congestion.
Kidney effect	Renal interstitial edema and renal venous congestion can worsen kidney function.
Management	Daily weights and balance, sodium restriction, diuretics, treat underlying disease and use dialysis/UF when needed.

Concept	Practical approach
Dry weight	Lowest tolerated postdialysis weight without signs of hypo- or hypervolemia. Reassess regularly.
Overestimated dry weight	Hypertension, pulmonary edema and chronic overload.
Underestimated dry weight	Cramps, hypotension and loss of residual kidney function.
UF rate	Prefer gradual fluid removal; reduce interdialytic weight gain and extend treatment duration when needed.
Residual urine	High-dose loop diuretics may help if meaningful urine output remains.