Fluids And Electrolyte
- A good understanding of the physiology of fluids and electrolytes is fundamental to the practice of surgery.
- Most surgical conditions are associated with changes in this balance and it is only appropriate that these are identified and treated effectively.
Read And Learn More: Basic Principles Of Surgery Notes
Table of Contents
Normal Physiology
- The human body consists of about 50–70% liquids and 30–50% solids by weight. The liquid portion varies with age, sex and body habitus.
- The variation is the result of individual differences in the fat content of the body which contains very little water.
- Hence, thin individuals have greater total body water (TBW) content as compared to obese individuals.
- Similarly, the TBW is about 50% in women and 60% in men. Neonates have up to 80% TBW. Of this total body water, intracellular water constitutes 40% of body weight (2/3rd of TBW) and the extracellular portion, 20% of body weight (1/3rd of TBW).
- The interstitial fluid and plasma portions of extracellular fluid constitute 15% and 5% of body weight, respectively.
Composition of Body Fluids:
These fluid compartments are separated by semipermeable membranes allowing their fluid composition to be maintained within distinct limits.
- Shows the composition of the intracellular and extracellular fluid compartments. The composition of the intracellular compartments may vary according to the tissue, example fat contains very little water.
- The tonicity of plasma is determined by the solutes, sodium and its corresponding anions, chlorides and bicarbonate, together with substances such as glucose, urea and proteins. These particles are osmotically active and hence, tonicity is described in terms of osmolality (mOsm/kg H2O).
- Osmolarity is concentration of a solution in terms of osmoles (or mosmoles) of solute per litre of solution (solute + water).
- Osmolality is concentration of a solution in terms of osmoles (or mosmoles) per kilogram of solvent. Osmolality is independent of the temperature of the solution and volume of the solute. Hence, osmolality is the preferred term in clinical practice.
Osmolarity: Osmoles per litre
Osmolality: Osmoles per kilogram
Osmolality of a solution can be measured in two ways:
1. By using the depression of freezing point of the solution:
A solution of 1 Osm/kg freezes at –1.86°C.
Normal plasma freezes at –0.54°C.
2. By estimating the solute concentration:
Osmolality can be estimated from the concentration of major solutes of plasma.
Example: If a patient’s sodium concentration is 140 mmol/L, blood glucose concentration is 180 mg% and blood urea is 30 mg%, his plasma osmolality can be calculated as follows:
From the equation, it is evident that sodium contributes the most to the osmolality of plasma. A change in osmolality is usually due to changes in sodium. The normal range of plasma osmolality is 285–300 mOsm/kg.
Plasma Colloidal Osmotic Pressure:
- The plasma proteins normally do not pass out of the capillaries into the interstitium. These raise the plasma osmotic pressure above that of the interstitial fluid by an amount referred to as colloidal osmotic pressure (plasma oncotic pressure).
- The normal plasma colloidal osmotic pressure is 25 mmHg. Albumin is responsible for 75% of this oncotic pressure.
- The body has mechanisms to regulate and maintain the volume of fluids, their concentration and composition within narrow limits to maintain homeostasis.
- Hence, a systematic assessment of fluid status of a patient involves the assessment of body fluid volume, its concentration and its composition in that order.
Water Regulation (Regulation of Volume)
The primary methods of body water regulation are:
- Regulating the volume of liquid ingested: When the extracellular fluid volume reduces, the thirst centre in the hypothalamus is stimulated which encourages the person to ingest more water.
- Regulating the volume of urine excreted: This is regulated by plasma antidiuretic hormone (ADH). A reduction in plasma volume releases ADH from the posterior pituitary which in turn acts on the ADH receptors in the collecting tubules of the kidney. This results in increased reabsorption of water and reduced production of urine. ADH release may also be stimulated by increased plasma osmolality and angiotensin.
Disturbances Of Volume
A decrease in the circulating volume is called hypovolaemia and an increase, hypervolaemia.
Hypovolaemia
This is common in surgical patients. The assessment of acute loss of blood volume is detailed in Chapter 12.
The reduction in blood volume due to loss of water can be in the following ways:
- Gut—vomiting, diarrhoea, fistulae
- Skin and lungs—0.5 ml/kg/h normally, increases by 12% for every 1°C rise in body temperature.
- Sequestration of fluid in third space refers to noncontributory fluid space that is unavailable to the circulatory system.
Assessment of Dehydration:
This is a clinical assessment based upon:
1. History: Severity and duration of loss of fluid.
2. Examination: Thirst, dryness of mucosa, loss of skin turgor, orthostatic hypotension, tachycardia, reduced jugular venous pressures and decreased urine output in the presence of normal renal function. Dehydration can be classified as given in.
Laboratory Assessment:
Haemoconcentration leads to falsely elevated haemoglobin, packed cell volume estimations and increased blood urea concentration. The kidneys reabsorb more water than usual leading to increased urine osmolality (>650 mOsm/kg).
Hypervolaemia
Hypervolaemia Causes:
- Excessive infusion of intravenous fluids.
- Retention of water in abnormal conditions, such as cardiac, renal and hepatic failure.
- Absorption of irrigation fluid as during transurethral resection of prostate using distilled water.
Hypervolaemia Diagnosis:
- History and physical examination can lead to the cause.
- Physical examination: Distended neck veins, pedal oedema, body weight gain.
- Circulatory overload:
- Hypertension, tachycardia, pulmonary oedema
- Confusion, restlessness, convulsions and coma.
The development of these signs depends on the rate and volume of fluid overload, renal function and cardiovascular reserve.
Management:
- Treat the cause
- Restriction of water and salt
- Diuretics (or dialysis, if necessary) to remove excess water
Regulation Of Sodium Concentration
- Water constitutes the major component of all body fluids but the composition varies with the fluid compartment.
- The most abundant cation of extracellular fluid is sodium and is the prime determinant of ECF volume. Ninety per cent of the ECF osmolality is due to sodium. The human body has no known mechanism to regulate sodium intake.
The body regulates sodium output by:
- Regulating glomerular filtration rate
- Regulating plasma aldosterone levels
Addition or loss of water produces a change in the concentration of the solute. The quantity of solute relative to the volume of water is thereby increased (ECF is concentrated) or decreased (ECF is diluted) with loss or addition of water, respectively.
- Changes in concentration are generally changes in water balance rather than changes in sodium regulation.
- Since the changes in volume and concentration are interdependent and the changes in water content are not easily measured, an estimate of the fluid volume and concentration is usually made by using the measured sodium levels and serum osmolality.
Disturbances In Concentration
Hyponatraemia
- Hyponatraemia is defined as a sodium level less than 135 mmol/L. It may occur as a result of water retention, sodium loss, or both.
- True hyponatraemia is always associated with low plasma osmolality. It may be associated with expanded, contracted or a normal extracellular volume.
Disturbances In Concentration Causes:
- Assessment of hyponatraemia should begin with an estimation of the extracellular fluid volume (clinically and if necessary, using central venous catheters).
- Thus, true hyponatraemia can be of three types: Hypervolaemic hyponatraemia, hypovolaemic hyponatraemia and normovolaemic hyponatraemia.
Hypervolaemic Hyponatraemia:
- Hypervolaemic hyponatraemia may be associated with clinical features of hypervolaemia, such as oedema.
- Acute hypervolaemia (e.g. TURP syndrome—acute absorption of hypotonic fluids into the intravascular compartment) may result in cerebral oedema and pulmonary oedema.
- As plasma osmolality decreases, water moves from the extracellular space into the cells
leading to oedema. The expansion of brain cells is responsible for the symptomatology of water intoxication: Nausea, vomiting, lethargy, confusion, restlessness, etc.
- If severe ([Na+] <100 mmol/L), it can result in seizures and coma. Chronic hypervolaemia as in congestive cardiac failure, cirrhosis and nephrotic syndrome may manifest with pedal oedema and elevated jugular venous pressures until decompensation occurs. The urinary sodium concentration is less than 15 mmol/L.
Disturbances In Concentration Treatment:
- Acute hyponatraemia (duration <72 h) can be safely corrected more quickly than chronic hyponatraemia.
- The following factors must be evaluated: Patient’s volume status, duration and magnitude of the hyponatraemia and the degree and severity of clinical symptoms.
- Fluid restriction, diuretics and correction of the underlying condition may be adequate in most cases.
- A combination of intravenous normal saline and diuresis with a loop diuretic (e.g. frusemide) also elevates serum sodium concentration.
- Acute symptomatic hyponatraemia is a medical emergency. It should be treated with hypertonic saline (1.6% or 3%).
- Concomitant use of loop diuretics increases free water excretion and also decreases the risks of fluid overload.
- The sodium concentration must be corrected to relieve symptoms and to a concentration of 125 mmol/L.
- Patients who are acutely symptomatic, the treatment goal is to increase the serum sodium by approximately 1–2 mEq/L/h until the neurologic symptoms subside.
- The correction should be slow and over a period of 12–24 hours with frequent checks of sodium concentration (every 2–4 h) to avoid overcorrection.
- Avoid an absolute increase in serum sodium of more than 15–20 mEq/L in a 24-hour period.
- If sodium correction is undertaken too rapidly, the resulting osmolality changes in the extracellular fluid can cause central pontine myelinolysis. This condition is serious and can be irreversible.
- The following equation can aid in the estimation of a sodium deficit to help determine the rate of saline infusion:
- Calculated sodium deficit = (140 – current serum Na+) × (body weight in kg) × 0.6
- A litre of normal saline (0.9%) contains 154 mEq sodium chloride (NaCl) and 3% saline 500 mEq NaCl.
- In chronic severe hyponatraemia (i.e. serum sodium <115 mEq/L), the rate of correction should be slow and should not exceed 0.5–1.0 mEq/L/h, with a total increase not to exceed 10 mEq/L/day.
Hypovolaemic Hyponatraemia:
- Hypovolaemia corrected inappropriately with hypotonic fluids such as 5% dextrose may result in hyponatraemia.
- Hypovolaemia may be due to renal causes such as diuresis or a salt-losing kidney. The urinary concentration of sodium is more than 20 mmol/L in these patients.
- Extrarenal loss of volume as in diarrhoea, vomiting or 3rd space loss may result in urinary concentration less than 20 mmol/L. All these are termed depletional syndromes and require saline infusion.
Disturbances In Concentration Treatment:
Based upon the volume status, administer isotonic saline to patients with hypotonic hyponatremia who are hypovolaemic to re-expand the contracted intravascular volume.
Normovolaemic Hyponatraemia:
Occasionally, hyponatraemia may exist with normovolaemia. In such situations, the plasma osmolality must be estimated. If it is low, renal failure or the syndrome of inappropriate ADH secretion (SIADH) may be considered.
Normovolaemic Hyponatraemia Treatment:
- For patients who have hypotonic hyponatraemia and are normovolaemic (euvolaemic), asymptomatic, and mildly hyponatremic, water restriction (1 L/day) is generally the treatment of choice.
- For instance, a fluid restriction to 1 L/day is enough to raise the serum sodium in most patients. This approach is recommended for patients with asymptomatic SIADH.
- Pharmacological agents can be used in some cases of more refractory SIADH, allowing more liberal fluid intake.
- Demeclocycline is the drug of choice to increase the diluting capacity of the kidneys by achieving vasopressin antagonism and a functional diabetes insipidus.
Pseudohyponatraemia:
- Occasionally, hyponatraemia is only an apparent one due to the accumulation of other solutes such as glucose, urea, plasma proteins or lipids.
- The plasma osmolality is either high or normal in these patients. Such hyponatraemia is called pseudohyponatraemia.
- Serum osmolality is governed by contributions from all molecules in the body that cannot easily move between the intracellular and extracellular spaces.
- Sodium is the most abundant electrolyte but glucose, urea, plasma proteins and lipids are also important. Normally, their concentrations are small and contribute to the plasma osmolality only to a small extent.
- However, when the concentrations of these molecules increase to very high levels, the relative concentration of sodium in unit volume of serum may reduce.
- The actual amount of sodium is normal in these patients hence the term pseudohyponatraemia.
- High blood sugar level or uraemia leads to higher plasma osmolality but high plasma protein or lipid levels is associated with normal plasma osmolality.
Pseudohyponatraemia Treatment:
The treatment of pseudohyponatraemia mainly involves treatment of the cause and supportive therapy.
Hypernatraemia
Hypernatraemia is defined as a plasma sodium concentration of more than 150 mmol/L and may result from pure water loss, hypotonic fluid loss or salt gain.
Causes of Hypernatraemia:
The hypertonicity of plasma leads to cellular dehydration. Clinical evidence of dehydration may not be apparent until 10–15% of body weight has been lost. Rehydration should be slow to prevent cerebral oedema.
The diagnosis can be established by measuring plasma and urine osmolalities and urine output.
- Uosm > Posm and ↓ urine output → Extrarenal causes (e.g. diarrhoea, fistulae)
- Uosm > Posm and ↑ urine output → Osmotic diuresis
- Uosm < Posm and ↑ urine output →↑ ADH or renal response to ADH.
Hypernatraemia Treatment:
- Administration of water orally/nasogastric tube
- Administration of IV fluid—5% dextrose or 0.45% saline
- Change in serum sodium should not be more than 1–2 mmol/L/h. Rapid rehydration can cause cerebral oedema.
Disturbances In Composition Of Body Fluid
Potassium Balance
The normal potassium level is 3.5–5.5 mmol/L. Hypokalaemia and hyperkalaemia are two clinically important disturbances.
Hypokalaemia:
This is defined as a plasma concentration of potassium less than 3.5 mmol/L.
Disturbances In Composition Of Body Fluid Symptoms:
- Anorexia, nausea
- Muscle weakness, paralytic ileus
- Altered cardiac conduction: Delayed repolarisation, reduced height of ‘T’ wave, presence of ‘U’ wave, wide QRS complexes and arrhythmias.
Disturbances In Composition Of Body Fluid Management:
- Diagnosis and treatment of the cause
- Repletion of body stores
- Potassium supplements, in the form of milk, fruit juice, tender coconut water.
Causes of Hypokalaemia:
-
- Reduced intake
- Tissue redistribution: Insulin therapy, alkalaemia, β2 adrenergic agonists, familial periodic paralysis
- Increased loss: Gastrointestinal losses—diarrhoea, vomiting, fistulae
- Renal causes: Diuretics, renal artery stenosis, diuretic phase of renal failure
- Syrup potassium chloride orally—15 ml contains 20 mmol of potassium.
- If the patient cannot take orally or the hypokalaemia is severe, intravenous potassium chloride is usually given at a rate of 0.2 mmol/kg/h.
- If there are lifethreatening arrhythmias, it may be given at a rate not exceeding 0.5 mmol/kg/h under electrocardiographic monitoring and serial measurements.
Hyperkalaemia:
This is defined as a plasma concentration of potassium more than 5.5 mmol/l.
Hyperkalaemia Clinical Features:
- Vague muscle weakness, flaccid paralysis
Electrocardiographic Changes:
- Tall, peaked ‘T’ waves with shortened QT interval (6–7 mmol/l)
- Wide QRS complex, widening and then loss of ‘P’ wave (8–10 mmol/l)
- Wide QRS complex, merge into ‘T’ waves (sine wave pattern)
- Ventricular fibrillation (K+ >10 mmol/l)
Treatment of Hyperkalaemia
- Calcium gluconate (10%): 10–30 ml.
- Sodium bicarbonate: 1–2 mmol/kg over 10–15 minutes.
- 100 ml of 50% dextrose with 10–12 units of insulin over 15–20 minutes.
- Hyperventilation
- Salbutamol nebulisation
- Calcium exchange resins
- Peritoneal or haemodialysis
Clinical Notes:
- A 21-year-old lady was found to be collapsed as she was feeding her 15-day-old baby in the nephrology ward. On arrival, the cardiac arrest response team found her to have ventricular tachycardia without pulse.
- Cardiopulmonary resuscitation was given and she was shifted to the intensive care unit after return of spontaneous circulation.
- Investigations showed that her potassium level was 1.6 mmol/L. She had been admitted to the nephrology unit for postpartum acute renal failure.
- She had been dialysed three times following which she had gone into the diuretic phase of recovery from acute renal failure.
- She was putting out about 5 litres of urine per day in the last two days. Her hypokalaemia was corrected over 2–3 days. She recovered completely and could be discharged from the ward in 5 days time.
Magnesium
- It is the second most abundant intracellular cation. The normal serum magnesium concentration is 0.7–1 mmol/L.
- Most of it is present in the muscle and bone. Only about 1% is intravascular. Consequently, the serum concentration does not reflect body stores.
- Role of magnesium in the body: Magnesium is required for the functioning of most enzyme systems including Na, K-ATPase, for synthesis of proteins, DNA, RNA, and parathormone. It also prevents influx of calcium into the cell.
- Magnesium is a muscle relaxant and produces vasodilatation, cardiac depression, bronchodilatation and tocolysis.
Hypomagnesaemia:
Serum concentration <0.7 mmol/L.
Hypomagnesaemia Causes:
- Inadequate intake as in prolonged starvation or malabsorption, inappropriate fluid therapy.
- Excessive losses through nasogastric drainage, diarrhoea or diuresis.
- Redistribution as with insulin infusion or massive transfusion.
Hypomagnesaemia Clinical Features:
- Predominantly neurological or neuromuscular abnormalities—muscular weakness, cramps
- Anorexia, lethargy and weight loss
- Hyperirritability, hyperexcitability, muscle spasms, stridor, tetany and convulsions
- Hypertension, pulmonary oedema
- Prolonged PR and QT intervals, ST depression and flattening of T waves
- Supraventricular and ventricular tachyarrhythmias
- Features of hypokalaemia and hypocalcaemia can also be seen.
Hypomagnesaemia Treatment:
- Magnesium sulphate is available as 50% (500 mg/ml) solution. Each ml contains 2 mmol of magnesium.
- In hypomagnaesemia, 8 mmol can be diluted in 50 ml of 5% dextrose or 0.9% saline and given over 30 minutes.
- If the patient has life-threatening arrhythmias, such as pulseless ventricular tachycardia due to hypomagnesaemia (torsade de pointes), and is unresponsive to defibrillation and epinephrine, it can be given as a bolus of 2 g.
Hypermagnesaemia:
Most common cause: Iatrogenic.
Hypermagnesaemia Clinical Uses of Magnesium:
- Antiarrhythmic agent for ventricular arrhythmias (torsade de pointes)
- As an antihypertensive, particularly for preeclampsia and eclampsia
- As an anticonvulsant
- As a bronchodilator
Clinical features of hypermagnesaemia depend on plasma concentration
- 4–5 mmol/L—muscle weakness and loss of tendon reflexes
- 6–7.5 mmol/L—respiratory arrest
- 10 mmol/L—cardiac arrest.
Hence, when patients are administered magnesium in large doses or as prolonged infusions, their ankle jerks must be monitored. If found sluggish, further doses or the infusion must be stopped.
Calcium
- Calcium is the most abundant mineral in the body. Ninety-nine per cent is deposited in the skeleton. In addition, calcium ions are important for the control of muscular and neural activities, in blood clotting, as cofactors for enzymatic reactions and as second messengers.
- Calcium homeostasis reflects a balance between reserves in the bone, rate of absorption across the digestive tract, and rate of loss from the kidneys.
- The hormones parathyroid hormone (PTH), vitamin D and calcitonin maintain calcium homeostasis in the ECF.
- Parathyroid hormone and vitamin D raise Ca2+ concentrations and calcitonin lowers it.
- Calcium absorption from the digestive tract and reabsorption along the distal convoluted tubule are stimulated by PTH from the parathyroid glands and calcitriol from the kidneys. The average daily requirement of calcium in an adult is 0.8–1.2 g/day.
- Half the serum calcium is bound to albumin and as albumin levels become low, this bound fraction is lower leading to a low total serum calcium concentration.
- Hence, the serum calcium level should be related to the albumin levels and corrected as follows:
- Corrected calcium (mg/dl) = measured total Ca (mg/ dl) + 0.8 (4.0 – serum albumin [g/dl])
- Free ionic calcium is important for the electrical activity of the nerves and muscles and is more reliable (Normal: 1.0–1.4 mmol/L).
Hypercalcaemia:
Hypercalcaemia exists when the Ca2+ concentration of the ECF is above 11 mg%.
Causes of Hypercalcaemia:
-
- Hyperparathyroidism
- Malignant cancers of the breast, lung, kidney or bone marrow
Hypercalcaemia Features:
Severe hypercalcaemia (12–13 mg%) causes symptoms, such as fatigue, confusion, cardiac arrhythmias, and calcification of the kidneys and soft tissues throughout the body (moans, stones and groans).
Hypocalcaemia:
Hypocalcaemia exists when calcium level is <9 mg%. mmol/L).
Causes of Hypocalcaemia:
-
- Hypoparathyroidism
- Vitamin D deficiency
- Chronic renal failure
Hypocalcaemia Features:
Muscle spasms, stridor, generalised convulsions, myocardial depression, cardiac arrhythmia and osteoporosis.
Perioperative Fluid Therapy
- A patient undergoing surgery needs intravenous fluids to replace volume deficit acquired during starvation, normal maintenance for the duration of surgery and volume lost during surgery.
- Depending on the extent of dissection, fluid accumulates in these tissues in the form of oedema (third space losses).
- In addition, blood loss also needs to be replaced. Perioperative fluid therapy in a patient whose body homeostasis is normal.
The replacement is as follows:
1. Fluid requirement during starvation:
- Patients awaiting anaesthesia and surgery are kept fasting for at least two hours for clear fluids. This could be longer in patients requiring bowel surgeries.
- People need fluids to cover insensible losses (through skin and respiratory tract) and urine output. This is 1–1.5 ml/kg/h.
- The volume deficit that occurs due to fasting before surgery is replaced. The current fasting guidelines allow patients to drink clear fluids up to 2 hours prior to surgery. Hence, the patients should not be dehydrated.
- If there are any signs of hypovolaemia due to vomiting, diarrhea, bowel preparation or any other cause, the patient may need to be administered additional fluids.
2. Maintenance requirement:
- The average daily requirement of water for an average-sized adult is 2000 ml. In general, a volume of 25–30 ml/kg/day meets the daily maintenance needs.
- This is calculated as 1–1.5 ml/kg/h. The maintenance fluids are given to cover insensible losses and urine output as mentioned before.
3. Third space losses:
- The third space refers to accumulation of fluid in spaces that are not in continuity with plasma.
- During surgery, there could be loss of fluid into interstitial space leading to oedema. It is difficult to quantify how much fluid seeps into the third space during surgery but it is proportional to the extent of dissection during surgery.
- It was believed that the third space losses could be as much as 4, 6, or 8 ml/kg/h for surgeries with minimal, moderate or large amount of dissection.
- This is currently considered as an overestimation. Excessive fluid administration can lead to loss of endothelial glycocalyx and oedema.
- Inadequate fluid therapy can give rise to hypoperfusion, renal failure and organ dysfunction.
- The current opinion is that maintenance fluids should be given as described earlier and fluids boluses of 250 ml must be given as required by monitoring the haemodynamic effects.
- When in doubt, passive leg raising can increase venous return and help in decision-making.
- Usually, urine output is measured during major surgery and an output of 0.5–1 ml/kg/h is desired.
- However, intraoperative urine output has no correlation to the incidence of postoperative renal failure and hence should not be used as a guide to intraoperative fluid therapy.
- In complicated and extensive surgeries or in patients with compromised cardiac status or renal failure, more sophisticated monitoring techniques such as central venous pressure, stroke volume variation or inferior vena caval diameter measurements may be required.
4. Blood loss is replaced by compatible blood transfusion :(homologous or autologous), if the haematocrit falls below 25%.
- Blood loss is replaced with an equal amount of colloids or 2–3 times the volume with crystalloids, if the haematocrit is >25% in an otherwise healthy individual.
- Crystalloids are electrolyte solutions that distribute themselves throughout the body water and hence, a larger volume needs to be given.
Perioperative Fluid Therapy in Patients with Disturbed Fluid Balance:
Derangements of fluid therapy can be classified as:
- Disturbances of volume
- Disturbances of concentration
- Disturbances of composition.
In the evaluation of a patient with a suspected problem in fluid and electrolyte or acid–base balance, careful sequential analysis of the volume, concentration and composition must be done in that order.
- This must be followed by appropriate therapy (as described earlier in this chapter). Also, refer to Chapter 17 for management of shock.
Types Of Intravenous Fluids
These can be broadly divided into three groups: Crystalloids, colloids and special-purpose solutions.
Crystalloids:
These are essentially solutions of electrolytes in water, e.g. Ringer lactate. Some also contain dextrose, e.g. dextrose saline, 5% dextrose and paediatric maintenance solutions. They vary in the content of different electrolytes.
Crystalloids Ringer lactate:
- This is also called a balanced salt solution as this is a solution of electrolytes in water, with a composition very similar to extracellular fluid.
- This is the solution of choice to replace third space losses. Ringer lactate does not provide any calories.
- It has a pH of 6.5. It contains Na+ 131 mmol/L, K+ 5 mmol/L, Ca++ 2 mmol/L, chloride 111 mmol/L and lactate 29 mmol/L (lactate gets converted to bicarbonate in the body).
- It has an osmolarity of around 270–278 mOsm/L and hence is marginally hypo-osmolar.
Crystalloids Normal saline:
- Normal saline is a misnomer; it is ideally called isotonic saline or 0.9% saline.
- It contains 154 mmol each of sodium and chloride per litre of solution. It has a pH of 5.
- Its osmolarity is 308 mOsm/L. This does not provide any energy.
- Isotonic saline may be used as replacement solution. However, large amounts of isotonic saline infusion can cause hyperchloraemic metabolic acidosis.
Crystalloids 5% dextrose:
- Each 100 ml contains 5 g of dextrose. It has a pH of 4. One litre of 5% dextrose will provide 200 kcal of energy.
- It may be used to replace insensible free water loss. Once the dextrose is metabolised, only water remains. Hence, it is called a hypotonic solution.
- It may be used to dilute inotropes, vasopressors, inodilators, sodium nitroprusside, aminophylline, etc.
- It may be used as part of glucose-insulin-potassium solution.
Crystalloids Newer solutions:
- Ringer’s acetate is similar to Ringer lactate but the lactate has been replaced by acetate. The lactate in Ringer lactate can cause confusion during serum lactate measurements in sepsis. This is avoided by using acetate.
- Plasmalyte is a crystalloid where the amount of chloride has been further reduced and calcium has been replaced with magnesium. It also has gluconate and acetate instead of lactate.
- The newer solutions are increasingly being used especially in critically ill patients and when large amounts need to be given.
Colloids:
- Colloids are suspensions of large molecules in solution, usually in saline.
- Unlike the ions in crystalloids, such as saline or Ringer lactate, these molecules cannot cross the cell membrane. They remain in the intravascular compartment for a longer period and hence become effective plasma expanders.
- These are used to restore blood volume in emergency situations, e.g. polytrauma with severe haemorrhage, massive GI bleed and shock.
1. Albumin:
- It is available as 5 and 20%.
- It is prepared from pooled human plasma. Since it is pasteurised at 60°C for 10 hours, the chances of transmission of diseases is very low. It is also tested using nuclear acid test (NAT) to detect hepatitis B and C viruses as well as HIV1.
- 5% albumin is used as a plasma expander. 20% albumin can be used to replace lost albumin in severe hypoalbuminaemia in addition to plasma expansion.
2. Indications:
- Can be used as a plasma expander to treat shock.
- Used in severe burns—acute severe hypoalbuminaemia.
- Used in nephrotic syndrome.
Contraindications:
- Heart failure
- Severe anaemia
Gelatins:
- Good plasma expander.
- They are of two types: Urea-linked gelatin (Haemaccel) and succinylated gelatin (Gelofusine).
- Plasma expansion lasts for 4–6 hours.
- Severe reactions including anaphylaxis can occur with urea-linked gelatin but less with succinylated gelatin.
Dextran 40:
- Reduces viscosity and red cell sludging.
- Used to maintain patency of microvascular free flaps by reducing thrombosis.
- May affect renal function and coagulation.
Hydroxyethyl Starch (HES):
- Derived from starch.
- Hydroxyethyl radicals are substituted onto the carbon atoms of glucose molecules.
- The effects depend on the molecular weight, molar substitution and the C2–C6 ratio.
- HES solutions are classified as high (>450 kDa), medium (130–200 kDa) and low (<130 kDa) molecular weight starches.
- The term substitution is used to represent the number of hydroxyethyl residues per 10 glucose subunits. Accordingly, they may be highly substituted (molar substitution 0.62–0.75), medium (0.5–0.6) or low (<0.4).
- Thus HES 130/0.4 means the molecular weight is 130 kDa and the molar substitution is 0.4.
- HES with a molar substitution of 0.7 is called hetastarch, 0.6 is hexastarch, 0.5 is pentastarch, and 0.4 is tetrastarch.
Adverse Effects:
- HES may interfere with coagulation.
- Incidence of severe reactions (1:16,000).
- Use in shock is associated with increased risk of renal injury and mortality and hence, must be avoided.
- Although initial studies showed that adverse effects are less with lower molecular weight and lower substitution HES 130/0.4 (tetrastarch), doubts have been raised about the safety of tetrastarch.
- Consequently, the European Medicine Agency has recommended its withdrawal from the market.
Special Purpose Solutions:
Sodium bicarbonate:
It is available as 7.5% (0.9 mEq/ml) and 8.4% (1 mEq/ml) of sodium bicarbonate.
Special Purpose Solutions Uses:
- As an alkalinising agent
- To treat metabolic acidosis
- To treat hyperkalaemia
- Forced alkaline diuresis
Special Purpose Solutions Disadvantages:
- Increased sodium load
- Alkalosis with a shift of oxygen dissociation curve to the left (increased affinity of haemoglobin to O2, reducing its unloading)
- Increased intracranial pressure and intraventricular haemorrhages in neonates
- Circulatory overload leading to cardiac failure
- Carbon dioxide load leading to respiratory failure
Mannitol (10 and 20%):
- It is an osmotic diuretic. Mannitol expands intravascular volume initially by drawing fluid from the interstitium. This is followed by diuresis.
- Its main use is to reduce intracranial pressure by producing diuresis.
- It is also used to reduce intraocular pressure. It should be used with caution in patients with cardiac failure, renal failure, etc.
Hypertonic saline (1.6, 3, and 5%):
These solutions are available to treat hyponatraemia.
Fluids And Electrolytes Multiple Choice Questions
Question 1. The following electrolyte contributes most to the osmolality of plasma:
- Sodium
- Potassium
- Magnesium
- Calcium
Answer: 1. Sodium
Question 2. The normal plasma osmolality is ________ mOsm/kg.
- 190
- 290
- 160
- 260
Answer: 2. 290
Question 3. Plasma osmolality is determined in the laboratory using:
- Freezing point
- Boiling point
- Saturation point
- Isoelectric point
Answer: 1. Freezing point
Question 4. The second major intracellular cation is:
- Sodium
- Potassium
- Calcium
- Magnesium
Answer: 4. Magnesium
Question 5. In chronic hyponatraemia, the sodium concentration should be increased at a rate not exceeding _______ mmol/Ll/h.
- 1
- 5
- 10
- 15
Answer: 1. 1
Question 6. The following is one of the drugs used to treat hyperkalaemia:
- Digoxin
- Magnesium sulphate
- Atropine
- Insulin
Answer: 4. Insulin
Question 7. Tall ‘T’ wave in the electrocardiogram is a feature of:
- Hypokalaemia
- Hyperkalaemia
- Hypocalcaemia
- Hypercalcaemia
Answer: 2. Hyperkalaemia
Question 8. Rapid infusion of the following fluid can cause intraventricular haemorrhage in neonates:
- 5% dextrose
- 8.4% sodium bicarbonate
- 0.9% saline
- Ringer lactate
Answer: 2. 8.4% sodium bicarbonate
Question 9. The following colloid is often used to reduce plasma viscosity:
- Dextran 40
- Hetastarch
- Gelatin
- Albumin
Answer: 1. Dextran 40
Question 10. Hypercalcaemia may be seen in the following conditions e x c e p t :
- Hyperparathyroidism
- Malignant cancers of the breast and lung
- Vitamin D toxicity
- Chronic renal failure
Answer: 4. Chronic renal failure
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