Fluid and electrolyte balance depends upon the ingestion and excretion of fluids as well as electrolytes. Water is main source of fluids. Water is provided by drinking or ingested in solid food. Sodium, potassium and magnesium are main electrolytes in body.
Body fluid volume and electrolyte concentration are normally maintained within very narrow limits despite wide variations in dietary intake, metabolic activity, and environmental stresses. Homeostasis of body fluids is preserved primarily by the kidneys.
Inorganic salts in the extraracellular or intracellular fluids of the body dissociate into ions.
Cations, which are electropositive ions and anions, which are electronegative ions. Collectively these are the electrolytes.
The cations include sodium, potassium, calcium and magnesium.
The anions include chloride, phosphate, bicarbonate and sulphate.
Together their distribution controls the passage of water through the cell walls and maintains acid—base equilibrium.
Water and Na balance are closely interdependent.
Water balance in the Body
Total body water is about 60% of body weight in men (50-70%) and about 50% in women.
About two-thirds of total body water is intracellular and the other one-third is extracellular (extracellular fluid)
Normally, about 25% of the extracellular fluid is intravascular compartment and the other 75% is interstitial fluid
Two kinds of sources provide the fluid – exogenous and endogenous.
It has wide variations but averages 2—3 litres per 24 hours, of which nearly half is contained in solid food.
Per Kg water requirements of infants and children are greater than adults because of larger surface area per unit of body weight, greater metabolic activity and more excretion due to poor concentrating ability of the immature kidney.
Endogenous water is released during the oxidation of ingested food amounting to about less than 500 ml in 24 hours.
Loss of Fluid
About 400 ml of water is lost in expired air each 24 hours. Loss can be greater when atmosphere is dry, respiratory rate is more and in intubated patients.
pPerspiration attempts to reduce heat of surface of body and invisible perspiration is always occurring. The loss of fluid through skin varies with the atmospheric temperature and humidity, muscular activity and body temperature.
In a temperate climate the average loss is between 600 and 1000 ml in 24 hours.
Feces and Urine
Between 60 and 150 ml of water are lost in faeces daily. In diarrhoea this amount is greatly multiplied.
Urine output depends on blood volume, hormonal and nervous influences,
A person with good renal health has urinary output approximately 1500 ml per 24 hours. The urinary output is dependent on the water intake. But a minimum urinary output of approximately 400 ml per 24 hours is required to excrete the end products of protein metabolism.
Sate of water depletion is usually due to diminished intake. This could be due to lack of availability, conditions that make swallowing difficult or structural obstruction in the oesophagus.
Conditions that increase water loss such as tracheostomy which increases loss from the lungs may also cause water depletion.
Loss of water causes weakness and intense thirst. There is decrease in urinary output and specific gravity of the urine is increased. Concentration of serum leads to rise of serum osmotic pressure which cause causes water to leave the cells leading to intracellular dehydration. This is done to compenstate hypovolemia.
Water depletion is treated by increase in water intake or fluid replenishment if oral intake is not feasible.
Water intoxication occurs due to excessive amounts of water or low amount of sodium taken or given by any route.
Common causes are
- Overload of fluids postoperatively [most common cause]
- Colorectal washouts with plain water, instead of saline
- Excessive uptake of water (and glycine) from irrigation fluid in TURP
- Syndrome of inappropriate antidiuretic hormone secretion [found in lobar pneumonia, empyema, oat-cell carcinoma of bronchus, and head injury.]
Water intoxication leads to drowsiness, weakness, sometimes convulsions and coma. Lab investigations show a falling hematocrit, serum sodium and other electrolyte concentrations.
The water intake should be restricted. A transfer to an intensive care or high dependency unit should be done for more invasive monitoring . The administration of diuretics or hypertonic saline should not be undertaken lightly as rapid changes in serum sodium concentration may result in neuronal demyelination which is a serious condition.
Sodium is the principal cation content of the extracellular fluid. The total body sodium amounts to approximately 5000 mmol, of which 44 per cent is in the extracellular fluid, 9 per cent in the intracellular fluid and the remaining 47 per cent in bone.
More than 50% of sodium is osmotically inactive and requires acid for its solution. The remainder is water soluble and exchangeable.
Daily intake of sodium is varies and an equivalent amount is excreted daily, mainly in the urine and some in the faeces. Profuse sweating results in a considerable loss of sodium — as much as 85 mmol/hour and just water replenishment may lead to serious sodium depletion can occur from excessive sweating.
The output of sodium in the urine and sweat glands, is under the control of the adrenal corticoids. When the adrenal glands have been destroyed by disease or removed, there is an unbridled loss of sodium in the urine.
Following trauma/surgery there is a variable period of reduced excretion of sodium up to 48 hours and due to increased adrenocortical activity and accordingly the replacing fluid should contain measured amount of sodium.
The serum sodium value is normally between 137 and 147 mmol /litre.
Hyponatremia occurs due to depletion of sodium. The causes are
- Obstruction of the small intestine
- Uncontrolled vomiting
- Duodenal, total biliary, pancreatic and high intestinal external fistulae
- Severe diarrhoea [ hyponatraemia with acidosis]
- Adrenocortical insufficiency [along with raised potassium]
- Syndrome of inappropriate ADH secretion
- Postoperative hyponatraemia [prolonged administration of a sodium-free solution]
Patient of hyponatraemia is a case of extracellular dehydration. In established cases the eyes are sunken and the face is drawn.
In infants the anterior fontanelle is depressed.
The tongue is coated and dry and in advanced cases it is brown in colour. Thirst is not particularly in evidence [compare with simple water depletion].
The skin is dry and often wrinkled, making the patient look older than his or her years. The subcutaneous tissue feels lax. Peripheral veins are contracted and contain dark blood. The arterial blood pressure is likely to be below normal.
The urine is scanty, dark in colour.
Investigation reveals specific gravity of urine increased and nil chloride except in case of s of salt-losing nephritis.
sodium levels are decreased in serum as well as urine.
Laboratory investigations would show normal or slightly reduced serum sodium with low urinary output and low urinary sodium.
Hypernatermia means excess of sodium and most commonly occurs when a patient is given an excessive amount of 0.9 per cent saline solution intravenously during the early postoperative period
The patient has no complaints but slight puffiness of the face is the only early sign. Dependent parts may show pitting edema [When present, shows 4-5 litres of excess fluid]
In case of infants there would be increased tension in the anterior fontanelle, increased weight, an increase in the number of urinations and oedema.
Potassium is an electrolyte that is almost entirely intracellular [98%].
Skeletal muscles harbor 75% of potassium.
The normal range is 3.5—5.0 mmol/litre.
Each day a normal adult ingests approximately 1.0 mmol/kg of potassium in food. Fruit, milk and honey are rich in potassium.
An amount corresponding to the intake is excreted in the urine.
It is the state of potassium depletion
Main causes are
- Following surgery [tissue damage]
- Diarrhoea from ulcerative colitis
- Villous tumours of the rectum
- Duodenal fistula
- Prolonged gastroduodenal aspiration with fluid replacement
Most patients of hypokalemia are asymptomatic, but at risk of the sequelae of hypokalaemia such as cardiac arrythmias.
Listlessness and slurred speach, muscular hypotonia, depressed reflexes and abdominal distension [paralytic ileus] are found in severe hypokalemia.
ECG may show a prolonged QT interval, depression of the ST segment and flattening or inversion of the T-wave.
Depending upon the severity, the treatment is by oral or intravenous potassium.
Hyperkalemia means high levels of potassium in the blood.
Common causes are
- Renal failure
- Addison’s disease (adrenal failure)
- Rhabdomyolysis [drug related, alcoholism]. Break down of muscles release potassium in blood.
- Angiotensin-converting enzyme (ACE) inhibitors
- Angiotensin II receptor blockers (ARBs)
- Severe injury or burns
- Excessive use of potassium supplements
- Type 1 diabetes
Patients with hyperkalemia may be without symptoms or may show
- Generalized fatigue
The treatment would depend on the cause.
Chloride and bicarbonates are major anions. They should be estimated simultaneously because variations in the one may be accompanied by opposite changes in the other. The normal level of chloride is 95—105 mmol/litre, and of bicarbonate 25—3 0 mmol /litre. The sum of the two remains roughly constant at 120—135 mmol/litre.
Calcium is an extracellular cation with a plasma concentration of 2.2—2.5 mmol/litre. It exists in three forms: bound to protein, free nonionised and free ionised. Ionized calcium is required for blood coagulation and affects neuromuscular excitability.
The ionised proportion falls with increasing PH. Therefore in respiratory alkalosis due to hyperventilation there may be tetany — with an apparently normal total serum calcium level.
Rise in urinary pH may lead to stone formation .
Serum levels of calcium are affected by its absorption from the bowel, its storage in bone or its elimination by the kidneys. such factors include vitamin D, parathormone and calcitonin, the state of renal and small-bowel function affect calcium metabolism.
Magnesium is an intracellular cation. The normal magnesium concentration is 0.7—0.9 mmol /litre. The average daily intake is approximately 10 mmol.
Causes of decreased levels of magnesium are
- Gastrointenstinal fistulae
- Ulcerative colitis
- Small bowel resections
- Cirrhosis of the liver
- Parathyroid disease
Deficiency of magnesium leads to nervous system irritability, ECG changes, lowered blood pressure and lowered protein synthesis.
The deficiency can be replenished by supplementation.