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Fluid and electrolytes Fluid and electrolytes in childrenin children
Also see Also see http:/paedstudent.uwcm.ac.uhttp:/paedstudent.uwcm.ac.u
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Composition of Body FluidsComposition 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:
Water (L) Protein (kg)
Na+ (mmol)
K+ (mmol)
Total body 45 6 2550 4560
ICF 15 0.3 2250 60
ECF 30 5.7 300 4500
Composition of ECFComposition of ECF
Water (L)
Other constituents
Total ECF 15Contains 230g of albumin and 2250 mmol of Na+
Interstitial compartment
12Contains 1/4 of the conc. of albumin in plasma (110g of albumin)
Plasma volume 3 Contains 120g of albumin
Composition of Fluid in Composition of Fluid in CompartmentsCompartments
► 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:
Determinants of water Determinants of water distributiondistribution
ECF ICF (skeletal muscle)
Na+ (mmol/l) 141 10
K+ (mmol/l) 4.1 120-150
Cl- (mmol/l) 113 3
HCO3- (mmol/l) 26 10
Phosphate (mmol/l) 2.0140 (organic phosphates)
Movement of water across cell Movement of water across cell membranesmembranes
► 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.
Distribution of ECFDistribution of ECF
► 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:
Hydrostatic pressure difference
Colloid osmotic pressure difference
ULTRAFILTRATE
Lymphatics
Any questions?Any questions?
Water physiologyWater physiologyIn order to maintain the tonicity of body fluids, the body must In order to maintain the tonicity of body fluids, the body must be able to sense changes in body water and then excrete or be able to sense changes in body water and then excrete or conserve electrolyte-free water (EFW). conserve electrolyte-free water (EFW).
► SensorSensorAddition of EFW leads to dilution of solutes. Dilution of the ECF Addition of EFW leads to dilution of solutes. Dilution of the ECF leads to hyponatraemia. However in the ICF, it leads to leads to hyponatraemia. However in the ICF, it leads to swelling of cells. Cells in the CNS are sensitive to volume swelling of cells. Cells in the CNS are sensitive to volume changes and act as a "tonicity receptor". These cells are linked changes and act as a "tonicity receptor". These cells are linked to cells producing antidiuretic hormone (ADH) and to the to cells producing antidiuretic hormone (ADH) and to the "thirst" centre."thirst" centre.
► EffectsEffectsSwelling of these cells tells the "thirst" centre to reduce water Swelling of these cells tells the "thirst" centre to reduce water intake and stops ADH production, thereby causing the kidneys intake and stops ADH production, thereby causing the kidneys to produce dilute urine.to produce dilute urine.Thirst is stimulated by an increase in tonicity. Contraction of Thirst is stimulated by an increase in tonicity. Contraction of the ECF volume also stimulates thirst. At the same time the the ECF volume also stimulates thirst. At the same time the shrinkage of cells in the "tonicity" receptor stimulates the shrinkage of cells in the "tonicity" receptor stimulates the production of ADH by the posterior pituitary.production of ADH by the posterior pituitary.
ADH+ H2O
H2O
Collecting duct
AQP-2
AquaporinsAquaporins
Any questions?Any questions?
Sodium physiologySodium physiology
► The content of sodium determines the ECF The content of sodium determines the ECF volume, as Navolume, as Na++ and its accompanying anions and its accompanying anions account for 90% of the ECF osmoles. As a result, account for 90% of the ECF osmoles. As a result, the kidney, through its ability to control the the kidney, through its ability to control the excretion of sodium, is responsible for excretion of sodium, is responsible for maintaining ECF volume. maintaining ECF volume.
► To maintain the body sodium content there must To maintain the body sodium content there must be a balance between intake and excretion of be a balance between intake and excretion of sodium. This is achieved through: sodium. This is achieved through:
1. monitoring of effective arterial volume
2. signalling to the kidney
3. control of sodium excretion
Monitoring of effective arterial Monitoring of effective arterial volumevolume
► 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.
MessagesMessages
HormoneHormone StimulusStimulus Site of actionSite of action EffectEffect
Angiotensin II or ß adrenergics via renin release
Low ECF volume
Proximal convoluted tubule
Increased reabsorption of NaHCO3 & NaCl
Aldosterone Angiotensin II Hyperkalaemia
Cortical distal nephron
Reabsorption of NaClSecretion of K+
Atrial natriuretic peptide
Vascular volume expansion
GFRMedullary collecting duct
Increased GFRReduced reabsorption of NaCl
Control of sodium excretionControl 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".
Any questions?Any questions?
HyponatraemiaHyponatraemia
► 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
Acute hyponatraemiaAcute hyponatraemia
► Duration of less than 48 hours.Duration of less than 48 hours.► Need to identify the source of EFW.Need to identify the source of EFW.► Main concern is brain swelling and resultant Main concern is brain swelling and resultant
herniation.herniation.► Treatment should be prompt and aim at reducing Treatment should be prompt and aim at reducing
ICF volume using hypertonic saline for the ICF volume using hypertonic saline for the symptomatic patient with a plasma sodium < 125 symptomatic patient with a plasma sodium < 125 mmol/l. Aim to raise plasma sodium to 130 mmol/l.mmol/l. Aim to raise plasma sodium to 130 mmol/l.
► When calculating sodium deficit assume that the When calculating sodium deficit assume that the volume behaves as if the sodium is dissolved in volume behaves as if the sodium is dissolved in total body water as the cell membrane is total body water as the cell membrane is permeable to water and not sodium.permeable to water and not sodium.
Clinical problemClinical problem
How much 5% saline (856 mmol NaHow much 5% saline (856 mmol Na++/L) /L) should be given to a 35kg patient to should be given to a 35kg patient to raise the plasma sodium by 10 raise the plasma sodium by 10 mmol/l? mmol/l?
How much 5% saline (856 mmol NaHow much 5% saline (856 mmol Na++/L) /L) should be given to a 35kg patient to should be given to a 35kg patient to raise the plasma sodium by 10 mmol/l? raise the plasma sodium by 10 mmol/l?
Total body water = 35 x 0.6 = 21LTotal body water = 35 x 0.6 = 21L
21L x 10 mmol/l = 210 mmol21L x 10 mmol/l = 210 mmol
Amount of 5% saline needed = 210 / Amount of 5% saline needed = 210 / 856 = 0.245 L856 = 0.245 L
Plus any ongoing renal losses of Plus any ongoing renal losses of sodium.sodium.
Preventing hyponatraemiaPreventing 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.
To avoid hyponatraemiaTo 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
Hyponatraemia in an infantHyponatraemia in an infant
► The most common cause of hyponatraemia The most common cause of hyponatraemia in young children is loss of sodium in in young children is loss of sodium in conditions such as acute gastroenteritis. conditions such as acute gastroenteritis. Loss of fluid leads to a decrease in ECF Loss of fluid leads to a decrease in ECF volume and production of ADH. Commonly volume and production of ADH. Commonly hypo-osmolar fluids are given orally and this hypo-osmolar fluids are given orally and this leads to retention of EFW.leads to retention of EFW.
► Treatment of the hyponatraemia depends Treatment of the hyponatraemia depends on rapid reexpansion of the ECF volume and on rapid reexpansion of the ECF volume and a more gradual restoration of ICF volume.a more gradual restoration of ICF volume.
Chronic hyponatraemiaChronic hyponatraemia
► Commonly seen in hospitalized patients.Commonly seen in hospitalized patients.► Picked up on routine electrolyte measurement.Picked up on routine electrolyte measurement.► Must recognise that adaptive responses have taken Must recognise that adaptive responses have taken
place in order to maintain normal ICF volume: place in order to maintain normal ICF volume: Initially pumping out of KInitially pumping out of K++ and Cl and Cl-- from cells. from cells. Later, loss of organic molecules such as myo-inositol, Later, loss of organic molecules such as myo-inositol,
amino acids. amino acids. ► Therefore if the sodium concentration rises too Therefore if the sodium concentration rises too
quickly in the ECF and time is not allowed for these quickly in the ECF and time is not allowed for these intracellular osmoles to return, then cells will intracellular osmoles to return, then cells will shrink. In the CNS this may result in osmotic shrink. In the CNS this may result in osmotic demyelination syndrome (ODS).demyelination syndrome (ODS).
Causes of chronic Causes of chronic hyponatraemiahyponatraemia
►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?
Causes of chronic Causes of chronic hyponatraemiahyponatraemia
► 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
Treatment of chronic Treatment of chronic hyponatraemiahyponatraemia
► 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.
Gradual correctionGradual 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.
Any questions?Any questions?
HypernatraemiaHypernatraemia
► 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.
Hypernatraemia - AetiologyHypernatraemia - 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.
To assess the cause of hypernatraemia To assess the cause of hypernatraemia ask: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?
What is the ECF volume?What is the ECF volume?
Gain of sodium leads to ECF expansion.
All other causes of hypernatraemia are due primarily to water loss.
Has the body weight Has the body weight changed?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.
Is the thirst response Is the thirst response normal?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.
Is the renal response normal?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.
Causes of hypernatraemiaCauses 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
SymptomsSymptoms
►Mild confusion Mild confusion ►Thirst Thirst ►CNS dysfunctionCNS dysfunction►PolyuriaPolyuria
PolyuriaPolyuria►Polyuria is the excretion of too much
water for a given physiological state.
►When assessing polyuria consider: Urine volume Osmole excretion Urine osmolality
Causes of polyuriaCauses of polyuria
►Look at urine osmolality: Hyperosmolar Isosmolar Hypo-osmolar
Hyperosmolar urineHyperosmolar urine
If a large volume of hyperosmolar urine is If a large volume of hyperosmolar urine is excreted there must be the same number of excreted there must be the same number of osmoles being taken in. Normally adults osmoles being taken in. Normally adults excrete approx. 900 mOsm/day. If amounts excrete approx. 900 mOsm/day. If amounts greater than this are being excreted, an greater than this are being excreted, an osmotic diuretic such as urea or glucose must osmotic diuretic such as urea or glucose must be present. be present. During an osmotic diuresis, NaDuring an osmotic diuresis, Na++ (50 mmol/l) (50 mmol/l) and Kand K++ (25-50 mmol/l) will also be found in (25-50 mmol/l) will also be found in the urine. This can lead to a depletion of the urine. This can lead to a depletion of these ions and ECF contraction. these ions and ECF contraction.
Isosmolar urineIsosmolar 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.
Hypo-osmolar urineHypo-osmolar urine
Most of these patients with very dilute urine will have central diabetes insipidus.
Treatment of a water deficitTreatment of a water deficit
1. Stop any ongoing water loss
2. Replace the deficit slowly, if possible by the oral route
Stop any ongoing water lossStop 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.
Replace the deficit slowlyReplace 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.
Any questions?Any questions?
Potassium physiologyPotassium 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.
Factors influencing potassium shift from ICF to Factors influencing potassium shift from ICF to ECFECF
►Hormones►Acid-Base changes►Intracellular anions►Ion channels
HormonesHormones
► 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.
Acid-Base changesAcid-Base changes
► Metabolic acidosis, caused by a loss of bicarbonate or gain of HCl causes movement of potassium out of cells the K+ being displaced from the cell by the entry of H+.
► If the kidneys are working normally, via the action of aldosterone, the acidosis will be corrected and the plasma [K+] return to normal.
► In contrast, accumulation of organic acids does not lead to hyperkalaemia, as the associated anions (lactate in the case of lactic acid) move into the cells alongside the H+ and K+ ions are not displaced out.
► Respiratory acid-base problems do not produce any significant change in plasma potassium.
Intracellular anionsIntracellular anions
► Within the cell there is a balance between negative and positive charges.
► Most of the anions are large molecules (organic phosphates such as DNA and RNA).
► The number of these molecules remains relatively constant except in specific diseases such as diabetic ketoacidosis.
► If there is a lack of insulin, organic phosphates are degraded to maintain protein synthesis and this fall in anions leads to a parallel loss of K+.
Ion channelsIon channels► Some problems of potassium homeostasis are the result
of abnormalities of ion channels. Such a disease is periodic paralysis.
► Normally the voltage-gated Na+ channel in muscle cells is inactive, because of the negative potential within the cell. Nerve stimulation leads to opening of these Na+ channels allowing Na+ to rapidly enter the cell. This results in a transient depolarization of the muscle cell. There is a rapid restoration of the resting potential because: the fall in the negative charge within the cell switches off the Na+
channels voltage-gated K+ channels open, causing K+ to exit cells down its
concentration gradient restoring the negative charge within the cell
► Increased activity of the Na+K+ ATPase pumps Na+ out of the cell.
Periodic ParalysisPeriodic Paralysis
► Hyperkalaemic periodic paralysis – This condition is caused by an abnormality of the
skeletal muscle Na+ channel. When the [K+] in the ECF is raised, some of these voltage-gated Na+ channels remain active and the cell becomes inexcitable.
► Hypokalaemic periodic paralysis – This is an autosomal dominant condition
presenting as hypokalaemia and weakness in the second decade of life. The resting membrane potential is less negative than normal. The precise molecular abnormality is unknown.
The Kidneys and PotassiumThe Kidneys and Potassium
►In order to maintain potassium content in the body, the kidneys must excrete the 1-2 mmol/kg of potassium ingested each day.
►Control of potassium excretion takes place primarily in the cortical collecting duct. This is carried out by the principal cell.
The Principal cellThe Principal cell
Cl- Na+
LumenK+
Na+
K+
Na+K+ ATPaseENaC
Aldosterone stimulates the activity of the epithelial sodium channel (ENaC). This effect is blocked by amiloride. If reabsorption of sodium is in excess of that of chloride this leads to creation of an electrical gradient which augments the net secretion of potassium. Increased sodium in the principal cell also enhances the activity of the Na+K+ATPase, bringing more potassium into the cell. The result is a [K+] in the luminal fluid 10 times higher than in the ECF.
Any questions?Any questions?
HypokalaemiaHypokalaemia
► Hypokalaemia is a plasma [K+] of < 3.5 mmol/l.► The main concerns are cardiac arrythmias,
respiratory failure and hepatic encephalopathy.► The main effect is on the resting membrane potential
of cells, which become hyperpolarized as the ratio between the ICF and ECF [K+] increases: The ECF potassium comprises only 2% of the total potassium
in the body and this is reflected in the relative [K+] of the ICF and ECF.
If the ECF [K+] falls from 4 to 3 a comparable fall in ICF [K+] would take it from 150 to 112.5 mmol/l.
The actual change is less than half this and as a result the ratio of ICF to ECF [K+] must rise.
The resting membrane potential therefore rises as more K+ diffuses out of cells down an exaggerated concentration gradient.
AetiologyAetiology
Three possible causes:1. Inadequate intake2. Shift of potassium into cells3. Loss of potassium from the body
1. Inadequate intake1. Inadequate intake
►Hypokalaemia is rarely due to Hypokalaemia is rarely due to inadequate intake alone.inadequate intake alone.
►The kidneys are able to greatly reduce The kidneys are able to greatly reduce potassium excretion in the face of low potassium excretion in the face of low intake.intake.
►However low intake may exacerbate However low intake may exacerbate the problem in the presence of excess the problem in the presence of excess loss of potassium.loss of potassium.
2. Shift of potassium into 2. Shift of potassium into cellscells
►Metabolic alkalosis►Action of hormones
Metabolic alkalosisMetabolic alkalosis
HB B- + H+ H+
CO2
x
Na+
K+
Na+K+ATPase
HCO3-
Additional HCO3-
As H+ moves out of cells, K+ moves in:
Respiratory alkalosis does not produce the same effect.
Actions of hormonesActions of hormones► The hormones which cause potassium to move into cells are
insulin and beta-2-adrenergics. Insulin
► Insulin makes the resting membrane potential more negative. This is the result of increased activity of NHE-1. The extra sodium in the ICF is pumped out by the Na+K+ ATPase which exchanges 3Na+ for 2K+ i.e. a net loss of positive charge from the ICF.
► This effect of insulin is important to recognise when treating poorly controlled diabetes mellitus.
Beta-2-adrenergic actions► Increased beta-2-adrenergic activity leads to movement of potassium
into cells. This is seen in conditions associated with stress, hypoglycaemia etc. Also seen in association with beta-2-agonists used for the treatment of asthma.
Aldosterone► The main action of aldosterone is on the kidney, leading to potassium
wasting. However, in patients who have lacked aldosterone, there appears to be a reduced ability to retain potassium within cells, such that when aldosterone is administered there may be a sudden drop in plasma [K+].
3. Loss of potassium from the 3. Loss of potassium from the bodybody
►The main route for potassium to be lost from the body is via the kidneys.
►However certain diarrhoeal illnesses can lead to loss of potassium from the GI tract.
Non-renal loss of potassiumNon-renal loss of potassium
► Gastric secretions only contain approx. 15 mmol/L of potassium. Therefore relatively little potassium is lost directly through vomiting.
► However colonic losses can amount to 50 mmol/L of potassium in diarrhoea, increased further if the losses are from the distal colon eg villous adenoma of the rectum. This can lead directly to hypokalaemia.
► In addition, any excessive loss of fluid, leading to ECF volume contraction will cause renal potassium wasting.
Urinary potassium lossUrinary potassium loss
► Causes Causes DiureticsDiuretics Hypermineralocorticoid actionHypermineralocorticoid action
► Potassium excretion is high when there is a Potassium excretion is high when there is a
high [Khigh [K++] in the cortical collecting duct or a ] in the cortical collecting duct or a high volume of urine passing through the high volume of urine passing through the cortical collecting duct.cortical collecting duct.
► In virtually all cases of chronic In virtually all cases of chronic hypokalaemia, the underlying cause is renal hypokalaemia, the underlying cause is renal loss of potassium.loss of potassium.
Assessment of potassium Assessment of potassium excretionexcretion
► Potassium excretion is controlled primarily by events in the cortical collecting duct.K+ excretion = Urine [K+] x Urine volume
► Transtubular potassium gradient (TTKG) Assesses the driving force behind potassium secretion. TTKG = (Urine [K+] / Plasma [K+]) x (Plasma osmolality / Urine osmolality)
► This calculation makes a number of assumptions: That the quantity of water reabsorbed in the medullary collecting duct can
be estimated by comparing the rise in the osmolality of the fluid in the terminal cortical collecting duct (which is equal to that of the plasma) with that of the final urine.
Potassium is not reabsorbed or secreted in the meduallary collecting duct. The osmolality of the fluid in the terminal cortical collecting duct is
known. This is only true if the urine osmolality is greater than the plasma osmolality.
► The TTKG will be high (> 7) if mineralocorticoids are acting and will be low (< 2) if they are not.
Causes of excess Causes of excess mineralocorticoid activity (1)mineralocorticoid activity (1)
High levels of mineralocoticoid:
Angiotensin IIACE
Renal artery stenosis, or a tumour
Angiotensin
Renin
+
Low ECF volume
K+
+
+ACTH
Aldosterone
Causes of excess Causes of excess mineralocorticoid activity (2)mineralocorticoid activity (2)
Low levels of mineralocoticoid:
CCD
Na+
Principal cell
Abnormal ENaC (Liddle’s syndrome)Channel always open.
Insertion of an artificial Na+ channel eg amphotericin
Blockage of breakdown of cortisol by inhibition of 11β- hydroxysteroid dehydrogenase eg liquorice
TreatmentTreatmentThe treatment will depend on the cause.Indications for initiating therapy:
Absolute Digoxin therapyTherapy for diabetic acidosis, because of effect of insulin Presence of symptoms eg. respiratory muscle weaknessSevere hypokalaemia (< 2 mmol/l)
Strong Myocardial diseaseAnticipated hepatic encephalopathy Anticipated increase in another factor that causes a shift of potassium intracellularly eg. salbutamol
Modest Development of glucose intoleranceMild hypokalaemiaNeed for better antihypertensive control
How much potassium?How much potassium?
Aim: Acute correction to avoid serious complications. Then more gradual correction.Emergency administration should be via a large vein and with the patient on a cardiac monitor.As most of the body's potassium is intracellular, it is difficult to assess the potassium deficit from plasma potassium levels. Specific amounts are therefore difficult to calculate and it is therefore important to follow the replacement with regular measurements.
Methods of potassium Methods of potassium administrationadministration
►The safest route of administration is orally.
►Intravenous therapy is indicated if: GI problems limit absorption or intake there is severe hypokalaemia with either
respiratory muscle weakness or cardiac arrhythmias
therapy is likely to cause a shift of potassium into cells e.g. treatment of DKA
Rate of administrationRate of administration
► The concentration of potassium in fluids given through a peripheral intravenous cannula should not exceed 60 mmol/l because of local irritation to veins.
► The rate of infusion should not normally exceed 0.2 mmol/kg/hr although rates up to 0.5 mmol/kg/hr may sometimes be justified. This does not apply to acute episodes which may require larger doses to be administered via a central line.
PreparationsPreparations► KCl
This is the commonest form of potassium given.Important to replace chloride in cases of hypokalaemia associated with ECF volume contraction. Available in various forms.
► KHCO3Use if the patient also needs bicarbonate e.g. certain diarrhoeal states.Note that bicarbonaturia may promote the renal excretion of potassium.
► Potassium phosphateIf the potasium loss is accompanied by loss of intracellular anions (phosphate), the potassium deficit will only be corrected when phosphate is given. Examples:- Anabolism associated with total parenteral
nutritionRecovery from diabetic ketoacidosis
► Dietary potassiumThis is the ideal way to replace potassium.Foods rich in potassium include meats and fresh fruit.
