Fluid and Electrolytes PDF

2022/12/06 Fluid and Electrolytes Dr B. Chale‐Matsau Department of Chemical Pathology Introduction Body water content – 60% of body weight in males and 55% in females 66% present in ICF and 33% in ECF Only 8% of this is in the plasma Not actively tra...

2022/12/06 Fluid and Electrolytes Dr B. Chale‐Matsau Department of Chemical Pathology Introduction Body water content – 60% of body weight in males and 55% in females 66% present in ICF and 33% in ECF Only 8% of this is in the plasma Not actively transported, moving freely across compartment Movement determined by the osmotic content Water is obtained from the diet and oxidative metabolism Lost through kidneys, gut, lungs and skin …Introduction Approximately 170L of water is filtered by the kidneys Minimum volume of urine 500mL/24hrs for adequate excretion of waste ICF and ECF have different compositions, with Na+ predominant in ECF and K+ in ICF Maintenance of their concentrations is by the energy dependent Na+/K+‐ATPase 1  2022/12/06 Schematic Presentation Osmolality and osmolarity Osmolality is measured ‐ osmometer (principle: freezing point depression) Osmolarity is calculated: 2[Na+]+ [urea]+ [glucose] Osmolal gap (OG) = osmolality – osmolarity OG increases with ↑ in other osmoles RF, DKA, lactic acidosis (OG>10) Ethanol, ethylene glycol (OG>15) Physiological response to water loss 2  2022/12/06 Water and sodium homoeostasis Water intake varies greatly influenced by several factors – availability, diet, habit, social factors etc. Kidneys are able to vary urine output in terms of both volume and concentration (obligatory loss of about 500mL/24hrs) Two major homeostatic systems that are involved in the control of salt and water balance Renin‐angiotensin‐aldosterone (RAA) system Antidiuretic hormone (ADH) Regulation of Na+ and H2O balance Hyponatraemia Hyponatraemia = plasma concentration below reference limit (~ 135mmol/L) First exclude pseudo hypoNa –,excess lipid, protein Common electrolyte abnormality, but clinical effects may not be apparent until plasma concentration reaches ≤ 125mmol/L Rate of decrease is important Chronic hypoNa may be present for several months or years and may be apparently asymptomatic – common in the elderly Whilst same degree of hypoNa may be fatal if occurring in rapidly – common in infants 3  2022/12/06 …Hyponatraemia In response to cellular volume, cells extrude organic and inorganic particles (osmolytes), thus preventing intracellular rise in volume. In the presence of rapid decline in sodium levels, these adaptive mechanisms are not achieved → cerebral oedema – Development of symptoms ranging from nausea and vomiting, confusion, coma and even death Acute hyponatraemia = a medical emergency Requires early and aggressive treatment Chronic hyponatraemia – dangerous if treated aggressively Approach to HypoNa Volume status provides approximation of TBNa Because the treatment of these patients will be different Hypervolaemia – ≈↑TBNa+ and ↑↑TBW – Lasix plus fluid restriction Euvolaemia – ≈↔TBNa+ and ↑TBW – Fluid restriction Hypovolaemia – ≈↓↓TBNa+ and ↓TBW – Saline rehydration Approach to hyponatraemia 4  2022/12/06 Syndrome of inappropriate ADH Excess antidiuretic hormone Measurement of ADH is not practical, thus not done by most clinical labs SIAD(H) = diagnosis of exclusion ⇒only considered when other conditions that may cause hyponatraemia are excluded Diagnostic Criteria: – Hyponatraemia and low serum osmolality – Urine must not be maximally diluted (osmolality >100 mOsm/kg) – Normal ECF volume – Normal kidney, adrenal and thyroid function – Patient must not be on any drug that may cause hyponatraemia Aetiology and Investigation Hypernatraemia Occurs less commonly than hypoNa Refers to serum conc > 145 mmol/L Inadequate H2O intake is prerequisite for hyper Na+ Hypernatraemia seldom occurs in: – alert patients with – intact thirst mechanism + – access to water + – ability to drink water 5  2022/12/06 Approach to hypernatraemia First asses volume status – Reflects potential cause – Management differ Hypervolaemia – ≈↑↑TBNa+ ↑TBW – Furosemide and 5% dextrose IVI Euvolaemia – ≈↔TBNa+ and ↓TBW – 5% dextrose IVI or H2O orally or n‐g tube Hypovolaemia – ≈↓TBNa+ and ↓↓TBW – Isotonic saline and 5% dextrose IVI …Hypernatraemia When water loss is the primary cause, ECF depletion occurs late as it is buffered by the much larger ICF volume When there is salt gain, ECF expansion occurs, and the symptoms occur quickly – E.g primary or secondary mineralocorticoid excess Hypernatraemia may also occur when ADH secretion is impaired or inability of the collecting tubules to concentrate urine – Diabetes insipidus (central or peripheral) …Hypernatraemia 6  2022/12/06 Cerebral adaptation Cerebral adaptation starts within hrs and is established by 2‐3 days Brain adapts to hyponatraemia – by secreting idiogenic molecules (osmolytes) out of brain cells to ECF Brain adapts to hypernatraemia – by accumulating intracellular idiogenic molecules Overzealous correction may be detrimental Effect of hypernatraemia on the brain Effect of hyponatraemia on the brain 7  2022/12/06 Potassium homeostasis In the kidneys, filtered potassium is mostly reabsorbed in the proximal tubules Some active secretion takes place in the distal part of the tubule. Thus urinary K excretion influenced by several factors: – Circulating aldosterone levels – Amount of Na arriving at the distal tubules – Relative availability hydrogen and potassium levels of the cells at the distal tubules and collecting ducts – Capacity of the cells to secrete hydrogen ions – Dietary potassium intake – Intravascular volume (reduction stimulate aldosterone secretion) Hypokalaemia – principal causes Transcellular K movement – Alkalosis – Insulin administration – Refeeding syndrome – Increased K excretion Renal causes – Diuretics – AKI (diuretic phase) – RTA 1 and 2 – Mineralocorticoid excess: Primary and secondary aldosteronism, Cushing syndrome – Tubular disorders: Batter syndrome, Liddle syndrome, Gitelman’s syndrome Extra renal causes – Diarrhoea – Vomiting Decreased intake Hyperkalaemia Spurious – Haemolysis – EDTA contamination – Old sample – Abnormal blood cells (leukaemia, thrombocytosis) Transcellular movement – Acidosis – Tissue damage (tumour lysis syndrome) – Vigorous exercise – Decreased K excretion – AKI and CKD – K‐sparing diuretics – Mineralocorticoid defiency: e.g Addison’s syndrome; adrenolectomy; hyporeninaemic hypoaldosteronism Excessive intake – E.g slow K – Parenteral infusion 8  2022/12/06 Summary/Conclusions Sodium, potassium and water homeostasis are interlinked Na and K are transported actively H2O follows passive transport Hyponatraemia – is very common in inpatients and requires proper investigation → ideal management Electrolyte disorders can lead to devastating outcome if not appropriately managed 9