Dehydration

Physical exam is unreliable - A 2015 Cochrane review in elderly patients tested dry axilla, dry mucous membranes and tongue, capillary refill, skin turgor, sunken eyes, orthostatic blood pressure drop, dizziness, thirst, urine color, weakness, blue lips, altered mentation, tiredness and appetite. Only fatigue and missed drinks between meals predicted the diagnosis. Skin tenting is not what you build a preop assessment on.

What actually measures it - There is no gold standard test. Serum osmolality at or above 295 mOsm/kg is a reasonable definition, and the Cochrane review used above 294 mOsm/kg. Weight loss of 3% or more over 7 days counts where you have the weights. A BUN-to-creatinine ratio above 10:1 supports it but is mimicked by high urea production, low creatinine from low muscle mass, or urea reabsorption after an upper GI bleed. Low urine sodium, fractional excretion of sodium under 1%, and urine osmolality above 450 mOsm/kg point the same way, but are also abnormal in heart failure, cirrhosis and nephrotic syndrome.

IVC ultrasound, with its limits - Greater than 50% variation in inferior vena cava diameter with respiration marks a collapsible IVC and may correlate with right atrial pressure and intravascular volume. It is influenced by cirrhosis, chronic heart disease, and whether the patient is breathing spontaneously or being ventilated, and it predicts fluid responsiveness poorly. Read it as one input, not the answer.

Hypotension is a late sign - Blood pressure does not fall until dehydration is significant, and tachycardia may be absent in a patient on a beta-blocker. Normal preop vitals do not clear the patient; by the time they are abnormal the deficit is large.

Induction unmasks the deficit - Thirst, ADH and the renin-angiotensin-aldosterone axis are all conserving mechanisms propping the pressure up. Induction takes away the sympathetic contribution and the pressure settles to what the circulating volume actually supports. Replete before induction where the case allows, titrate the dose, and have a vasopressor ready rather than reaching for one afterward.

Water loss or blood loss - The first fork in an adult, because blood loss is replaced with blood and water loss with fluid. Get it backwards and you either transfuse a dry patient or resuscitate a bleeding one with crystalloid.

How to replace it - Isotonic crystalloid in most cases, boluses scaled to severity, larger boluses for the more severely dehydrated. In elderly patients and in heart or kidney failure, small boluses with reassessment between each. Follow blood pressure, heart rate, serum lactate, hematocrit and urine output to judge both the deficit and the response.

Crystalloid choice - No fluid has proved superior. Large-volume normal saline causes hyperchloremic metabolic acidosis; buffered crystalloids can cause hyponatremia; lactated Ringer's contains potassium, so keep it out of renal failure and hyperkalemia. Albumin has a place in specific situations but does not improve outcomes.

Dehydration plus severe hyponatremia - Rapid volume repletion drives a rapid rise in sodium and can precipitate central pontine myelinolysis. Weigh continued dehydration against that risk and follow volume status and serum sodium repeatedly, not once.

Both directions hurt - Under-resuscitation gives organ hypoperfusion: altered mental status, renal failure, shock liver, lactic acidosis, hypotension, death. Over-resuscitation gives peripheral and pulmonary edema. Either way expect electrolyte fallout - uremia, hyponatremia, hypernatremia, hypokalemia, hyperkalemia, metabolic acidosis and metabolic alkalosis.

Keep the fast short - Dehydration is simply water loss outrunning replacement, and obligate losses continue through an NPO period. In a patient already behind, every extra hour without clear fluids or maintenance IV adds to the deficit. Schedule early and start fluid when the IV goes in.

Children - Graded mild 3% to 5%, moderate 6% to 10%, severe above 10% of body weight, with blood pressure staying normal until the severe category and mental status, pulses, capillary refill, respirations, eyes, fontanelle and urine output degrading progressively. Infants carry 70% to 80% total body water against 60% in older children, so an infant must lose proportionally more weight to reach the same grade. Most pediatric dehydration is hyponatremic, metabolic acidosis is common, and the child who has been vomiting and not tolerating oral fluids may be hypoglycemic - check a glucose.

Who arrives dehydrated - Older adults, reported at 17% to 28% in the United States and 20% to 30% more prone than younger patients through immobility, an impaired thirst mechanism, diabetes, renal disease and falls. It is also over-diagnosed, which means fluid can be poured into a patient whose real problem is something else.

Dehydration is loss of body water in excess of replacement, through skin, lungs, kidneys and GI tract. The body is 55% to 65% water: two-thirds intracellular, one-third extracellular, and only one-fifth of the extracellular share is intravascular. A total-body water deficit is spread across all three compartments, so only a small fraction comes directly out of the vascular space - which is why a large deficit can hide behind a normal blood pressure.

Three patterns. Isotonic, water and sodium lost together: vomiting, diarrhea, sweating, burns, intrinsic kidney disease, hyperglycemia, hypoaldosteronism. Hypertonic, water loss exceeding sodium: fever, increased respiration, diabetes insipidus, with serum sodium and osmolality always elevated. Hypotonic, sodium loss exceeding water: mostly diuretics, with low sodium and osmolality. Osmoreceptors drive thirst and ADH; falling pressure drives renin, angiotensin II and aldosterone. All of that conserves - none of it replaces.