Body fluids and electrolyte balance
The total body water constitutes 50 – 85% of total body weight depending on age and lean body mass (muscle mass). In regard to this, 55% – 60% of body weight for a 70 Kg young man is water. Females have lower body water (45 –60%) because of the high fat content of their body. The total body water in neonates is 80%-85%, which is higher than in adults.
Total body water is further divided into two:
- Intracellular fluid, comprising 2/3 of total body water
- Extra cellular fluid, comprising 1/3 of total body water. The extra cellular fluid is sub divided into Intravascular (plasma) comprising 2/3 of extra cellular fluid and Interstitial which comprises 1/3 of extra cellular fluid.
Physiologically, these three compartments of body water are interdependent.
Plasma contains proteins (chiefly albumin) and ions (mainly sodium, chloride, and bicarbonate). Water and electrolytes move freely between plasma (intravascular compartment) and the interstitial fluid, but plasma proteins enter the interstitial fluid only when the capillary endothelium is damaged, for example as a result of septic shock or burns. The protein in plasma is responsible for the intra vascular colloid osmotic pressure, a major determinant of the movement of fluid across the capillary endothelium. Only a small proportion of the body’s potassium is present in plasma, but the concentration of potassium ions is crucial to cardiac and neuromuscular function.
Interstitial fluid has an ionic composition similar to that of plasma. If there is a water deficit in the intra vascular compartment, water and electrolytes pass from the interstitial compartment to restore the circulating blood volume. Electrolyte solutions, such as physiological (normal) saline and Ringer’s lactate solution (Hartmann’s solution), can pass into the interstitial space when they are administered intravenously. For this reason, they are effective in raising the intravascular circulating volume for only a short time if there is a deficit of fluid throughout the extracellular compartment. Blood, plasma, and colloids used as plasma substitutes, for example dextran, hydroxyethyl starch, and gelatin solutions (which are known as “plasma expanders”), remain in the intravascular compartment longer and are therefore more effective in maintaining the circulation.
Intracellular fluid has a different ionic composition to extracellular fluid. The main cations are potassium and magnesium, with phosphates and proteins as the major anions.
After intravenous infusion, the water contained in physiological saline tends to remain in the extracellular compartment, but the water contained in glucose solutions is distributed throughout all body fluid compartments, the glucose being metabolized. Never give pure water intravenously, as it causes dangerous haemolysis.
Daily average water and electrolyte exchanges
In the normal individual, the amount of water and electrolytes excreted each day balances what is taken in in foods and fluids. The kidney regulates, to a large degree, the volume and composition of body fluid. To a lesser degree the skin and lungs affect water losses, but do not regulate them.
Urine: 1500 ml
Insensible loss: 1000 ml (up to 1700 in hot climate)
Stool: 200 ml
A minimum urinary output of approximately 400 ml in 24 hours is required to excrete the end products of metabolism
Endogenous: 200 ml (from oxidation of ingested food.)
Net requirement: 2500-3200
|Fluid||Na+||Cl||Ka+||Carbohydrate (gram per liter)||Used for replacement of|
|Physiologic saline ( Normal saline)||154||154||0||0||Blood/ extra cellular fluid loss|
|Hartmann’s solution ( Ringer’s lactate) (Contains lactate and calcium)||131||112||5||0||Blood, intracellular fluid loss|
|5% glucose in water (D/W)||0||0||0||50||Maintenance and for medication|
Disturbances of body-fluid status
Changes in the volume or composition of the body fluids (which may occur before, during, or after surgery) can cause a severe physiological disturbance and should therefore be corrected promptly. The volume changes seen in surgical practice often affect the extracellular fluid. This fluid may be lost not only externally, for example through external haemorrhage, but also internally through sequestration (translocation or redistribution) into injured tissues, as in patients with burns, crush injuries, peritonitis, or an obstructed loop of the bowel. This internal redistribution of the extracellular fluid, at times referred to as fluid loss into the “third space”, is often overlooked, yet it can markedly reduce the circulating fluid volume.
How to assess volume depletion
Take a detailed history from the patient or from his or her relatives and make a careful examination to determine the nature and approximate amount of fluid lost; the diagnosis should be mainly clinical. The clinical state of the patient depends on the amount and rate of fluid loss, the underlying or associated disease, and the efficiency of compensatory mechanisms. Reliable tests for determining the amount of fluid lost are not available; in particular, the concentration of sodium ions in the serum can be misleading.
Nevertheless, the patient’s blood can yield useful information: the blood urea concentration may be elevated if there is an uncorrected deficit of extracellular fluid, and the severity of dehydration (loss of water and electrolytes) may be indicated by the haemoglobin concentration or erythrocyte volume fraction.
The dehydrated patient is usually thirsty with a dry mouth, sunken eyes, and reduced skin elasticity; the blood pressure may be low, associated with a small pulse pressure and tachycardia. If the fluid loss is acute and severe, the patient may develop hypovolaemic shock. Urinary output may be low and the relative density (specific gravity) of the urine high.
Treatment of fluid imbalance
If the patient is suffering fluid loss but with minimal signs, administer fluids orally, unless contraindicated; a solution of oral rehydration salts (ORS) in water is suitable for this. In patients with burns, oral rehydration salts are a useful supplement to fluids given intravenously. The ideal solution to infuse is one whose composition most closely resembles that of the fluid lost. Replace the fluid already lost, administer fluid for daily maintenance, and anticipate and replace any continuing unusual losses. Remember that patients receiving fluid and electrolyte therapy, except those with diarrhoea, are not likely to pass faeces, so daily requirements must be adjusted accordingly.
In patients suffering fluid loss and showing obvious signs, it is convenient to begin replacement by infusing a balanced salt solution such as physiological saline (containing sodium chloride at 9 g/litre) or Ringer’s lactate solution. In hot countries, water loss is proportionally greater than electrolyte loss, so infuse balanced salt solutions with caution and consider infusing 5% glucose (SO gllitre) as well. Insert a bladder catheter and measure the hourly urinary output and its relative density (specific gravity).
Adjust the rate of infusion and the total amount of fluid in accordance with the patient’s response, as indicated by the trend in the symptoms and signs, and in particular by the hourly urinary output and the jugular venous pressure. The ideal urinary output is at least 0.5 ml/kg of body weight per hour. Record clinical observations and assess the effect of therapy hourly. Establish a fluid input/output chart, and give clear, written instructions about the infusion programme; it is preferable to update these instructions every 6-8 hours rather than only once a day, as losses and requirements may change rapidly.
Treatment of electrolyte imbalance
Hypernatraemia (an excess of sodium ions in the serum, which can be confirmed by a blood test) may be caused by infusion of excessive quantities of saline or by tube feeding without sufficient water supplementation. Associated clinical features are restlessness, tachycardia, dry, sticky mucous membranes, and often an elevated body temperature. Correct hypernatraemia by salt restriction and an intravenous infusion of 5% glucose in water.
Hyponatraemia may follow the intravenous infusion oflarge volumes of salt-free fluids, such as glucose solutions. It can also follow oral or rectal administration of large amounts of water or other salt-free fluids. It is a recognized complication of water enema in infants and children, especially in those with Hirschsprung’s disease, and any form of enema in children and infants should therefore be avoided. The affected patient is lethargic and hypertensive, with tachycardia and cold extremities; oliguria or even anuria is present. Treat hyponatraemia by restricting the patient’s water intake. Do not give hypertonic saline infusions in an attempt to “normalize” the level of serum sodium.
Imbalances of serum potassium concentration have more serious clinical consequences than those of serum sodium concentration. Potassium is crucial to cardiac and neuromuscular functions, and its level in serum (3.5–4.5 mmol/litre) varies with the acid-base status and renal function of the individual. Hyperkalaemia may occur after severe trauma (including burns and surgical operations) and in patients suffering from acidosis, various catabolic states, and acute renal failure.
Although the patient may complain of nausea, vomiting, abdominal colic, and diarrhoea, the symptoms are a poor guide to hyperkalaemia. The electrocardiogram usually has a peaked T wave, a widened QRS complex, and a depressed S-T segment; dysrhythmias are more likely than usual and may lead to cardiac arrest. Give specific treatment intravenously, in the following sequence:
- 20 ml of a 10% (100 g,llitrc) solution of calcium gluconate, over a period of 20 min ;
- 100 mmol (8.4 g) of sodium l:>icarbona.te in solution (in an acidotic patient this will encourage the entry of potassium ions into cells);
- 100 ml of a 50% (500 g/litre) glucose solution, with insulin at l Intemarional Unit for every 5 g of glucose.
Recovery of cardiac function is usually prompt with this treatment. If the patient’s hyperkalaemia is due to acute renal failure, refer the patient immediately after resuscitation, if possible. If refexral is not possible, begin peritoneal dialysis.
Hypokalacmia often results from prolonged administration of diuretics or excessive losses of fluid through the gastrointestinal tract, for example in cases of prolonged diarrhoea or vomiting. The patient has flaccid limbs, reduced tendon reflexes, and paralytic ileus. TI1e electrocardiogram shows a flat T wave and a depressed S-T segment.
An adequate urine output (0. 5 ml/kg of body weight per hour) must be established before correction of the potassium deficit is started. Potassium is given as potassium chloride mixed in the drip fluid : add 40 mmol of the salt to 1 litre of either saline or 5% glucose. Infuse this fluid very slowly so as to deliver not more than 40 mmol of potassium per hour, and estimate the serum potassium concentration after giving every 40 mmol. The bottle of fluid containing potassium chloride must be clearly labelled. Never give a concentrated solution of a potassium salt by direct intravenous injection.