Water & Electrolyte Lec 5 - PDF

Water &Electrolyte Disturbances Water & electrolytes Balance The disturbances could be either external or internal External: the external balance matches the input with output so that the total amounts of Na, K, & water in the body are kept constant.  The 1st loss starts in the ECF...

Water &Electrolyte Disturbances Water & electrolytes Balance The disturbances could be either external or internal External: the external balance matches the input with output so that the total amounts of Na, K, & water in the body are kept constant.  The 1st loss starts in the ECF Notice that the total body water represent a high percentage of the infant weight so in diarrhea the infant internal balance will be severely impaired  Each day these secretions will be reabsorbed into the body , but in severe illness there will be rapid &large losses of water and electrolytes as the volumes of these fluids secreted daily into the gut are large. E.g. Na & Cl: 1. In vomiting there will be large loss of Cl while the Na remains high 2. In diarrhea there will be loss in relative amounts  Water & Na Balance Sodium: the total Na in the body is about 4200 mmol, distributed 50% in ECF, 40% in bone & 10% in ICF, thus Na is mainly extra cellular whereas K is mainly intra cellular ( so in sampling of K we avoid haemolysis bc. K escape from RBC to ECF –serum or plasma) Internal balance The distribution of fluid bt. ICF and intravascular and extravascular compartments of ECF is affected by: 1. Osmolality: no. of particles of solutes per KG of plasma which is normally 285-295 mmol/Kg ( this affects movement of water across membranes ) 2. Colloid Osmotic Pressure: this affects movements within the ECF bt. The intra and extravascular compartments. Osmolality Measured by : A. Osmometer : an instrument used to estimate the osmolality , its principle is the freezing point depression –if we add solute to the water the freezing point will be lower B. Calculated Osmolality: 2[Na] + 2[K] + Glucose /18 + Blood Urea /6 *Sometimes osmolality = 2 [Na] Urine osmolality / serum osmolality RATIO Normally 1-3 After fluid restriction it becomes 3 or above Renal tubular deficiency ratio 1.5 ? Renal failure ratio < 1.1  External balance Water: controlled by: 1. Thirst : increase water intake 2. Arginine vasopressin (AVP) also known as ADH. its secretion by the posterior pituitary gland causes renal water retension ( reabsorption of water by the renal tububle) Increase of plasma osmolality increase sensation of thirst Decreased volume of ECF by 10% Sodium: The most important regulators of Na excretion: 1. GFR (glomerular filtration rate ) 2. The rennin-angiotensin-aldosterone system 3. Atrial Natriuritic peptides (ANP) 4. Dopamine release in the kidney  ADH leads to water retention( reabsorbtion) in the ECF volume (mediated by the osmoreceptors in the hypothalamus  Disturbance of electrolytes is either intake disturbance or loss disturbance ( through GIT, skin , lung) or may combine both intake & loss disturbances Depletion & excess of water Usually the NA + water loss is the common case and water loss only is uncommon Na Depletion & excess Oedema Increase the Na in the blood which will increase the retention of H2O by the following mechanisms : 1. hypoproteinaemia : this usually mainly involve plasma albumin , the reduced plasma albumin cause reduced plasma oncotic pressure and therefore reduced reabsorbtion of interstitial fluid at the venous end of the capillaries. Consequently, plasma vol. is reduced. This mech. Operates in the liver disease , the nephritic syndrome, and in severe malnutrition. 2. hydrostatic factors : increased venous pressure associated with other abnormal haemodynamic effects. Hypernatraemia : (aemia means in the blood Hyponatraemia: low ECF vol. Depletion of H2O and Na , meaning the water coming from the ICF to ECF , the thirst will the response to it. Depletion of water in infants by vomiting and diarrhea lead to decrease ICF and ECF severe dehydration Potassium balance K is the main IC cation. About 98% of total body K is contained in cells , the remaining 2% (about 50 mmol/l being present in the ECF. (150 mmol/l ICF, 4 mmol/l ECF) So if a specimen of hemolysed blood is analyzed the K result is the 1st to be cancelled bc. of the effusion giving false high result. Fluid balance: post operative management Patients admitted for major elective surgery and who might be liable to develop disturbances of water and electrolyte balance post- operatively, require pre-operative clinical assessment and determination of a set of baseline values for NA, K, Cl, blood urea , creatinine , liver function test. Patients who present for surgery with disturbances of water and electrolytes metabolism require to have the severity of the disturbances assessed and corrective measure instituted Correction of fluids 1.Calculate the fluid requirement. basic replacement of insensible loss which is usually 600 ml previous daily urine vol. other losses (pyrexia-1C 250 ml- , sweating 300-1000ml (if the patient shows clear symptoms of sweating u should replace by 1000) 2.calculate the Na requirement and provide it as isotonic solution of normal saline 0.9% (g/dl) the bottle size is 500 ml so.9 is 154 mmol NaCl 3.provide additional water and dextrose 5% and K as potassium chloride 4. cover the essential requirement 30-80 mmol/l K is the only one that is given with a great precaution because k>7.5 is fatal so if its given it should be accurate and gradually not a large dose In case of low urine output you should increase the flow rate of fluid bc. The correction is not enough In case of polyuira you should stop the fluid and start the diuretic and ALWAYS monitor the output which should equal the input IN ACUTE fluid loss Isotonic NaCl in pyrexia and sweating 5% dextrose in water depletion Isotonic NaCl + KCl in GIT disrurbances Normal values Na 132-150 m mol/l plasma in urine 75-200 m mol/24hr K 3.5-5.5 m mol/l plasma in urine 40-80 m mol/24hr Cl 96-107 m mol/l Intravenous preparations normal saline NaCl 0.9% gm/dl (weight/vol) dextrose 5 %10% 20 % (dextrose in infants 0.3% NaCl, 3.33% dextrose) (in adults 4% dextrose ) Ringer solution: Na, K, Ca & ringer ……..  How to calculate these values? to convert mass unit to SI unit use 0.9 % Calculation of the conc. In mol/l Mwt of NaCl 23+35.5 = 58.5 𝑤𝑡 𝑜𝑓 𝑠𝑜𝑙. 1000 0.9 1000 X = X = 0.154 mol / L 𝑀 𝑤𝑡 𝑜𝑓 𝑠𝑜𝑙 𝑣𝑜𝑙.𝑜𝑓 𝑠𝑜𝑙. 58.5 100