Fluid and electrolytes in children

1. Fluid and electrolytes in children Also see http:/paedstudent.uwcm.ac.uk

2. Composition of Body Fluids • Water is 60% of body mass (70% in infants, less in obese people, females and elderly). • The water is divided between extracellular (ECF) and intracellular (ICF) compartments. • In an average 70kg person:

3. Composition of ECF

4. Composition of Fluid in Compartments • Water is held in individual compartments by the osmotic forces generated by the particles restricted to that compartment: Na+ (along with Cl- and HCO3-) maintain ECF volume K+ (alongside large macromolecular anions) determines ICF volume • Particles such as urea cross cell membranes rapidly and distribute equally in ICF and ECF.  • Ions which are regulated by transporters and active pumps and therefore have an osmotic effect on the distribution of water between ECF and ICF are:

5. Determinants of water distribution

6. Movement of water across cell membranes • Water moves across cell membranes under the action of osmotic forces.  Movement of water continues until the osmolality on either side of the membrane is equal. • Tonicity is the effective osmolality and equals the total osmolality minus urea and alcohol concentrations (mmol/l).  Urea and alcohol do not have an osmotic effect as they diffuse freely across cell membranes. • The number of osmotically active particles in the ICF is relatively constant and only changes to help maintain the ICF of brain cells in states of chronic swelling or shrinkage. • As a result: The body content of Na+ determines the ECF volume The [Na+] in the ECF determines the ICF volume.

7. Distribution of ECF Hydrostatic pressure difference ULTRAFILTRATE Colloid osmotic pressure difference Lymphatics ECF is distributed between the interstitial and the vascular compartments.  The volumes in each compartment are determined by the forces driving ultrafiltrate across the capillary wall:

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9. Water physiology In order to maintain the tonicity of body fluids, the body must be able to sense changes in body water and then excrete or conserve electrolyte-free water (EFW). • Sensor Addition of EFW leads to dilution of solutes. Dilution of the ECF leads to hyponatraemia. However in the ICF, it leads to swelling of cells. Cells in the CNS are sensitive to volume changes and act as a "tonicity receptor". These cells are linked to cells producing antidiuretic hormone (ADH) and to the "thirst" centre. • Effects Swelling of these cells tells the "thirst" centre to reduce water intake and stops ADH production, thereby causing the kidneys to produce dilute urine. Thirst is stimulated by an increase in tonicity. Contraction of the ECF volume also stimulates thirst. At the same time the shrinkage of cells in the "tonicity" receptor stimulates the production of ADH by the posterior pituitary.

10. Aquaporins Collecting duct H2O H2O + ADH AQP-2

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12. Sodium physiology • The content of sodium determines the ECF volume, as Na+ and its accompanying anions account for 90% of the ECF osmoles.  As a result, the kidney, through its ability to control the excretion of sodium, is responsible for maintaining ECF volume.  • To maintain the body sodium content there must be a balance between intake and excretion of sodium.  This is achieved through:

13. Monitoring of effective arterial volume • When NaCl is retained, there is an increase in ECF volume.  • The most important part of the ECF is the effective arterial volume and sensors in the main arteries and central veins send messages to the kidney via renal nerves and hormones to adjust renal sodium excretion accordingly.

14. Messages

15. Control of sodium excretion • In a normal adult, approximately 27000 mmol of sodium is filtered each day, of which over 99% must be reabsorbed. • In order to maintain ECF volume, filtration and reabsorption of sodium is coordinated such that the correct amount of sodium is excreted, independent of the GFR.  This is known as "glomerular tubular balance".

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17. Hyponatraemia • Hyponatraemia is defined as a plasma sodium < 130 mmol/l. • It is the result of an excess of water in comparison to sodium.  The increase in electrolyte-free water (EFW) must be accompanied by ADH in order to prevent the excretion of EFW. • There is an expansion of ICF volume, unless the hyponatraemia is secondary to hyperglycaemia. • It is important to differentiate between: (i) Acute hyponatraemia (ii) Chronic hyponatraemia

18. Acute hyponatraemia • Duration of less than 48 hours. • Need to identify the source of EFW. • Main concern is brain swelling and resultant herniation. • Treatment should be prompt and aim at reducing ICF volume using hypertonic saline for the symptomatic patient with a plasma sodium < 125 mmol/l.  Aim to raise plasma sodium to 130 mmol/l. • When calculating sodium deficit assume that the volume behaves as if the sodium is dissolved in total body water as the cell membrane is permeable to water and not sodium.

19. Clinical problem How much 5% saline (856 mmol Na+/L) should be given to a 35kg patient to raise the plasma sodium by 10 mmol/l?

20. How much 5% saline (856 mmol Na+/L) should be given to a 35kg patient to raise the plasma sodium by 10 mmol/l? Total body water = 35 x 0.6 = 21L 21L x 10 mmol/l = 210 mmol Amount of 5% saline needed = 210 / 856 = 0.245 L Plus any ongoing renal losses of sodium.

21. Preventing hyponatraemia The commonest setting for the development of acute hyponatraemia is in the post-operative period.  The cause is administration of EFW as: ● 5% Dextrose or hypotonic saline ● Sips of water ● The generation of EFW by desalination of isotonic saline solutions.  If excessive amounts of fluids are given in the face of ADH release, then hypertonic urine is produced leaving EFW.

22. To avoid hyponatraemia • Give fluids which are isotonic to the urine if polyuria present and isotonic to the body fluids if the patient is oliguric. • Give fluids only to balance ongoing losses and maintain haemodynamic stability. • If urine output is good, be mindful of conditions which may lead to ADH release: ECF volume depletion Blood loss Hypoalbuminaemia Low cardiac output Excessive pain, nausea, vomiting or anxiety CNS or lung lesions Neoplasms or granulomas Drugs that enhance the actions of ADH on the kidney by increasing cAMP activity

23. Hyponatraemia in an infant • The most common cause of hyponatraemia in young children is loss of sodium in conditions such as acute gastroenteritis.  Loss of fluid leads to a decrease in ECF volume and production of ADH.  Commonly hypo-osmolar fluids are given orally and this leads to retention of EFW. • Treatment of the hyponatraemia depends on rapid reexpansion of the ECF volume and a more gradual restoration of ICF volume.

24. Chronic hyponatraemia • Commonly seen in hospitalized patients. • Picked up on routine electrolyte measurement. • Must recognise that adaptive responses have taken place in order to maintain normal ICF volume: • Initially pumping out of K+ and Cl- from cells. • Later, loss of organic molecules such as myo-inositol, amino acids. • Therefore if the sodium concentration rises too quickly in the ECF and time is not allowed for these intracellular osmoles to return, then cells will shrink.  In the CNS this may result in osmotic demyelination syndrome (ODS).

25. Causes of chronic hyponatraemia • There must be: • A source of EFW eg ingestion of water • A restriction in the ability to excrete EFW ie presence of ADH • Main problem to answer is why the secretion of ADH? • What is the stimulus for ADH secretion?

26. Causes of chronic hyponatraemia • The main stimulus for ADH secretion is a low "effective" vascular volume or low ECF volume.  This will also stimulate the "thirst" centre, even in the presence of hyponatraemia.  The difficulty for clinicians is being able to accurately assess the ECF volume.  However ADH may also be released in the face of a normal ECF volume, if there is an inadequate "effective" vascular volume: • Hypoalbuminaeima - leads to loss of fluid from the vascular compartment • Cardiac dysfunction - results in low arterial volume and high venous blood volume

27. Treatment of chronic hyponatraemia • Firstly, if possible identify and treat the cause. • If possible, correct the hyponatraemia slowly.  Too rapid correction will lead to shrinkage of brain cells.  However more rapid correction may be needed if symptoms are serious i.e coma or seizures.  In this circumstance: • Give hypertonic saline to raise plasma sodium concentration to a level at which seizures cease - usually a rise of around 5 mmol/l. • Do not let the plasma sodium concentration rise by more than 8 mmol/l in any 24 hour period.

28. Gradual correction • Raise plasma sodium by no more than 8 mmol/l/day to prevent development of osmotic demyelination syndrome (ODS). • Reduce rate of correction further if patient may have deficiency of potassium or organic osmolytes eg malnutrition, catabolic states. • Create a negative balance for EFW - Cells have an excess of EFW and this must therefore be lost.  Reduce input of EFW. • Return the composition of the ECF to normal - This will require the provision of adequate amounts of sodium in order to maintain ECF volume as EFW is lost. • Return the composition of the ICF to normal - This will require replacement of potential deficiencies of potassium and organic osmoles to the brain cells.  Administration of KCl will lead to replacement of potassium for sodium in the ICF and an increase in sodium in the ECF with an increase in ECF volume.  If the ECF volume was normal, this must be accompanied by a net excretion of NaCl which is isotonic with the patient.

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30. Hypernatraemia • Plasma sodium greater than 150 mmol/l.  • There is an increase in the amount of sodium relative to water and hypernatraemia usually leads to decrease in ICF. The brain is most at risk. • Most people, if their thirst centre is intact, will take in EFW to correct the excessive loss of EFW. • Urine osmolality: • Large volume of hypo-osmolar urine - diabetes insipidus • Large volume of slightly hyper-osmolar urine - osmotic or pharmacologic diuresis • Minimum volume of maximally hyper-osmolar urine - nonrenal water loss without water intake • A rarer cause of hypernatraemia is gain of sodium, in excess of water. This will produce an increase in ECF volume.

31. Hypernatraemia - Aetiology • The true normal plasma [Na+] is 152 mmol/kg water. • If measured per litre of plasma, the plasma [Na+] is 140 mmol/L because plasma contains 6-7% of nonaqueous fluids (lipids, proteins) while sodium is only present in the aqueous part. • If blood proteins or lipids are raised, the measured plasma [Na+] may be lower than the actual [Na+] in the aqueous phase, depending on the laboratory method used. • If the lab use a Na+-selective electrode or a conductance method, which measures the ratio of sodium to water in the plasma, the result will not be affected. • However if a method such as flame photometry is used, which measures the [Na+] per volume of plasma, a ''factitious" hyponatraemia will be recorded. • Thirst is stimulated by a rise in the plasma [Na+] of 2 mmol/l. For hypernatraemia to develop, this thirst response must fail.

32. To assess the cause of hypernatraemia ask: • What is the ECF volume? • Has the body weight changed? • Is the thirst response to hypernatraemia normal? • Is the renal response to hypernatraemia normal?

33. What is the ECF volume? Gain of sodium leads to ECF expansion. All other causes of hypernatraemia are due primarily to water loss.

34. Has the body weight changed? Rarely fluid moves from the ECF to the ICF  e.g. following a convulsion or rhabdomyolysis. Hypernatraemia then occurs with no change in body weight.

35. Is the thirst response normal? A 2% increase in plasma tonicity stimulates thirst. Failure to take on EFW may occur in a baby who does not have control over access to fluids. The absence of thirst suggests a CNS lesion.

36. Is the renal response normal? The appropriate response is a low volume of concentrated urine (> 1000 mOsm/kg H2O). A failure to produce such a response suggests an ADH or renal problem.

37. Causes of hypernatraemia • Hypernatraemia due to water loss • Nonrenal water loss - Hypotonic solutions may be lost through the skin, respiratory or GI tracts. • Renal water loss.  Usually polyuria - diabetes insipidus or an osmotic diuresis. • Hypernatraemia due to sodium gain • Use of replacement solutions containing more sodium than in the fluids being lost ie urine. • Salt poisoning • Ingestion of sea water • Dialysis error

38. Symptoms • Mild confusion • Thirst • CNS dysfunction • Polyuria

39. Polyuria • Polyuria is the excretion of too much water for a given physiological state. • When assessing polyuria consider: • Urine volume • Osmole excretion • Urine osmolality

40. Causes of polyuria • Look at urine osmolality: • Hyperosmolar • Isosmolar • Hypo-osmolar

41. Hyperosmolar urine If a large volume of hyperosmolar urine is excreted there must be the same number of osmoles being taken in. Normally adults excrete approx. 900 mOsm/day. If amounts greater than this are being excreted, an osmotic diuretic such as urea or glucose must be present. During an osmotic diuresis, Na+ (50 mmol/l) and K+ (25-50 mmol/l) will also be found in the urine. This can lead to a depletion of these ions and ECF contraction.

42. Isosmolar urine These patients are characterised by a loss of medullary hypertonicity. The main cause is renal damage secondary to infection, hypoxic injury, obstructive uropathy or drug-induced. The use of loop diuretics will produce a similar temporary picture. There is no significant increase in osmolality following administration of ADH.

43. Hypo-osmolar urine Most of these patients with very dilute urine will have central diabetes insipidus.

44. Treatment of a water deficit • Stop any ongoing water loss • Replace the deficit slowly, if possible by the oral route

45. Stop any ongoing water loss • If this is the result of ADH deficiency then administer ADH. • If the cause is an osmotic diuresis then remove source and address any sodium or potassium deficit.

46. Replace the deficit slowly • If hypernatraemia is acute or there are serious CNS symptoms, then initial reduction of plasma sodium may have to be rapid.  However aim to replace total water deficit over 2-3 days.  • Oral replacement is best, unless unable to administer fluids orally.  Can give water.

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48. Potassium physiology • Potassium ions are important in the maintenance of resting membrane potentials across cell membranes.  Imbalances of potassium homeostasis affect many biologic processes which rely on these membrane potentials. • This is important with respect to cardiac muscle cell contractility and changes in plasma [K+] may lead to arrythmias. • The kidneys are responsible for maintaining plasma [K+]. • Potassium is the main intracellular cation.  98% of body potassium is inside cells.  It is held inside cells by a charge gradient which maintains a negative charge within cells.  This is achieved by: • A Na+K+ ATPase creates a high intracellular [K+].  3 Na+ are pumped out and only 2 K+ enter the cell. • K+ diffuses out of cells, down the concentration gradient.  Potassium ions diffuse through cell membranes more rapidly than sodium ions.  The majority of the intracellular anions are large macromolecules and therefore cannot diffuse out of the cells.

49. Factors influencing potassium shift from ICF to ECF • Hormones • Acid-Base changes • Intracellular anions • Ion channels

50. Hormones • Hormones can affect the activity of the Na+K+ ATPase by: • Enhancing the electroneutral entry of sodium into cells by activating the Na+/H+ antiporter. • Stimulating existing Na+K+ ATPase enzymes directly. • Stimulating the production of more Na+K+ ATPase. • The main hormones involved are insulin and catecholamines.  • Insulin and ß-adrenergics lead to a fall in plasma [K+] • Alpha-adrenergics lead to movement of potassium out of cells.