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Dehydration and rehydrationDr. saeid Family physician 2014
Physiology Composition of body fluids:Water is the most plentiful constituent of human body.Total body water (TBW) as a percentage of body weight varies with age.In term infants TBW is 75%Premature have higher TBWDuring the first year TBW decreases to 60%, and remains at this level until puberty.
Fluid compartments:Intracellular fluid30-40%Interstitial fluid15%Plasma 5%Extracellular20-25%With puberty, the increased muscle mass of boys causes them to have a higher ICFvolume than girls.There is no significant difference in the ECFvolume between girls and boys postpubertaly
Osmolality:The plasma osmolality is normally 285-295 mOsm/kg.Osmolality = 2* Na + glucose/18If the osmolality in one compartment changes, then water movement leads to a rapid equalization of osmolality.Hypothalamus senses changes in the osmolality through two receptors; one for ADH, the other for thirst.
The balance between hydrostatic and oncotic forces regulates the intravascular volume.Water is drawn to IVC because of its higher albumin concentration.Juxtraglomerular apparatus in the kidneys senses changes in the intravascular volume.
Why is dehydration common in infants?Infants have a different body composition to adults 70-80% body water content versus 60% in adults.High fluid intake: 150 ml/kg/day compared to 30-40 ml/kg/day in adults.Daily water turnover of 10-15% of body weight copared to 3-5% in adults.Relative reduction in renal ability to concentrate urine.
5. Greater surface area/mass ratio resulting in high insensible fluid losses through skin and respiratory tract.6. Higher basal metabolic rate plus greater febrile response to infection.7. Infants have little or no control over fluid intake.
A normal state of hydration is manifested by bright eyes, moist tongue and good skin turgor.
The fluid spaces:First space = the intravascular spaceSecond space = the interstitial spaceThird space = fluid in pleural cavity, peritoneal cavity, GIT etc.Fluid losses in the third space are concealed and may be unconsidered.* Mild-moderate dehydration = interstitial volume* Sever dehydration = intravascular volume
Internationally: diarrheal illnesses with subsequent dehydration account for nearly 4 million deaths per year in infants and children. Mortality/Morbidity: Mortality and morbidity are generally dependent upon the severity of dehydration and the promptness of oral or intravenous rehydration. If treatment is rapidly and appropriately obtained, morbidity and mortality are low. Age: Children younger than 5 years are at the highest risk. Dehydration:
Causes of dehydration
Assessment of the degree of dehydration:History: Intake of fluids, including the volume, type (hypertonic or hypotonic), and frequencyUrine output, including the frequency of voiding, presence of concentrated or dilute urine, hematuriaStool output, frequency of stools, stool consistency, presence of blood or mucus in stoolsEmesis, including frequency and volume and whether bilious or nonbilious, hematemesis
Contact with ill people, especially others with gastroenteritisUnderlying illnesses, diabetes mellitus, hyperthyroidism, renal diseaseFeverAppetite patternsWeight lossRecent antibiotic usePossible ingestions
Mild Moderate Severe Weight loss %Infant Adolescent 5%3% 10%6% 15%9%
General observation Thirsty, alert, restless Thirsty, restless or lethargic but irritable and drowsyDrowsy, cold, sweaty, cyanotic extremities, may be comatose
Signs and symptomsMild Moderate Severe Tachycardia Palpable pulsesBlood pressureRespirationFontanelleTearsMucous membraneSkin turgorUrine output AbsentPresentNormalNormalNormalPresentMoistNormalNormal PresentPresent (weak)Postural hypotensionDeep, may be rapidSlightly depressedPresent or absentDrySlight reductionOliguria PresentDecreasedHypotensionDeep and rapidSunkenAbsentVery dryReducedAnuria and sever oliguria
Types of dehydration:According to the serum level of sodium, dehydration is divided into:Isonatremic: serum [Na+] = 130-150 mEq/LHyponatremic: serum [Na+] is < 130 mEq/LHypernatremic: serum [Na+] is > 150 mEq/L
Pathophysiology:Isonatremic dehydration:It is the commonest, account for 70-80% of cases of dehydration.There is a proportionate loss of water and sodium.The serum osmolality is the same.
Account for 10-20%Disproportionately large net losses of water compared with losses of electrolytes.Causes:Inadequate intake of water, either due to emesis, lack of access to water, or anorexia.Administration of badly home-prepared ORSFeeding a boiled skim milkIncreased evaporative water loss from fever, hot climate, and hyperventilation.Hypernatremic dehydration:
Children with hypernatremia often appear less ill than children with a similar degree of isotonic dehydration. This is because there is movement of water from intracellular to extracellular space protecting the intravascular volume.It is the most dangerous one due to complications of hypernatremia and its therapy.
Children are often lethargic but irritable when touched. Hypernatremia may cause fever, hypertonea, and hyper-reflexia.Hypernatremia can cause serious neurologic damage including hemorrhage and thrombosis. This appear to be secondary to movement of water from brain cells into the extracellular fluid, causing brain cell shrinkage and tearing blood vessels within the brain.
The pinched abdominal skin has a doughy feel.Hypernatremia is associated with hyperglycemia and mild hypocalcemia the mechanism is unknown.Seizures and coma are possible sequelae of the hemorrhage, even though they are more common during treatment.Thrombosis occur secondary to dehydration and possibly hypercoagulability associated hypernatremia.
Hyponatremic dehydration:Account for 10-15% of all patients with diarrheaIt is usually due to combination of sodium and water loss and water retention to compensate for the volume depletion.Causes:Diarrhea has, on average, a sodium concentration of 50 mEq/L. the patient drinks water or formula which has low sodium, there will be a reduction in the serum sodium concentration.
2. Volume depletion stimulates synthesis of ADH thus reducing renal water excretion.3. Loss of fluid with a high sodium concentration, as may occur with renal salt wasting, third space losses, or diarrhea with a high sodium content (e.g. cholera).During hyponatremia there is movement of water into the cells causing increase in intracellular water and cell swelling; this is not problematic in most tissues of the body, but it is potentially catastrophic in the brain, increasing the intracranial pressure.
There is a more substantial intravascular volume depletion than a patient with hypertonic or isotonic dehydrationNeurological symptoms include; nausea, vomiting, confusion, malaise, lethargy, headache, seizure, and decreased reflexes.Acute severe hyponatremia can cause brain stem herniation, apnea, cheyne stoke breathing, and hypothermia.
Lab findings:Laboratory data are generally not required if the etiology is apparent and mild-to-moderate dehydration is present.Serum sodium concentration determines the type of dehydration.Blood PH, CO2, and HCO3 level to determine whether there is acidosis or alkalosis: Low pH, high CO2, and low HCO3 indicate metabolic acidosis; while high pH indicate metabolic alkalosis.
Anion gap to differentiate between the various causes of metabolic acidosis which may occur due to:Stool bicarbonate losses in children with diarrheaSecondary renal insufficiencyLactic acidosis from decreased tissue perfusionMetabolic alkalosis may occur due to emesis and nasogastric losses.
Serum potassium concentration:Causes of hyperkalemia:Metabolic acidosis causing shinfting of potassium into the extracellular fluid.Renal failureCongenital adrenal hyperplasia
Causes of hypokalemia:Emesis (e.g. pyloric stenosis)Metabolic alkalosisUrinary potassium losses
Renal function tests: BUN and serum creatinineBlood urea is increased without renal insufficiency secondary to increased passive reabsorption of urea in the PCT due to appropriate renal conservation of sodium and water.
The baseline normal creatinine concentration increases with age e.g. in infants = 0.2 mg/dl, at 5 year 0.4 mg/dl, at 10 year 0.6 mg/dl, at 15 year 1 mg/dl.
Urinalysis:Urinalysis is most helpful in the measurement of urine specific gravity, a specific gravity of less than 1.020 indicate mild or no dehydration or indicates a urinary concentrating defect, as in chronic renal disease or primary or secondary diabetes insipidus.Hyaline or granular casts, a few WBC and RBCs, and 30-100 mg/dl of proteinuria. These findings remit with therapy and are not usually associated with significant renal pathology.
Hemoconcentration:Hemoconcentration causes an increase in the hematocrit, hemoglobin, and serum proteins in dehydration. These values normalize with rehydration. A normal hemoglobin concentration during acute dehydration may mask an underlying anemia. A decreased albumin in a dehydrated patient suggests a chronic disease, such as malnutrition, nephrotic syndrome, or liver disease, or an acute process, such as capillary leak.
Oral rehydration:ORS is used in many countries and has significantly reduced the morbidity and mortality from acute diarrhea.Mild and moderate dehydration from diarrhea of any cause can be treated effectively using a simpl