DISORDERS OF FLUIDS AND ELECTROLYTES BALANCE Flashcards
sodium makes up about_______% of total plasma solutes.
92%
Sodium constitutes about_______mmol of extra cellular fluid
3000 mmol
Daily intake of water and sodium are approximately____ and _____respectively
1.5L-2L
60-150mmol
Water and sodium are lost through what?
urine (kidneys), feaces (gastrointestinal tract), sweat (skin) as well as expired air (lungs).
The roles of Antidiuretic Hormone (ADH) in water balance
•It improves water reabsorption in excess of solutes at the collecting ducts, hence its stimulation by plasma osmolality and dehydration
Aldosterone
•This is a mineralocorticord hormone secreted from adrenal cortex (zona glomerulosa)
•It prevents loss of sodium mainly in renal tubular cells and to some extent in feaces and sweat
•It affects the control of sodium-potassiun and sodium-hydrogen ion exchange across all cell membranes.
•It stimulates the reabsorption of sodium in distal tubular cell in exchange for potassium or hydrogen ion.
•One can conclude that the actions of Aldosterone lead to retention of sodium than water with net loss of potassium and hydrogen ion in a normal functioning system
Angiotensin II
•It has vasoconstriction effect by its direct action on capillary walls.
•It helps in the production and secretion of aldosterone by its action on adrenal cortex zonal glomerulosa
•It stimulates taste centre to promote oral fluid intake
•With the functions of angiotensin II above, one can conclude that sodium secretion is not entirely dependent on plasma sodium level but also on effect of angiotensin II on aldosterone secretion.
Atrial Natriuretic peptide
•This hormone is secreted in the right atrial wall and it is sensitive to stretch receptors.
•It stimulates excretion of sodium in the tubule due to it suppressive action on renin, aldosterone and increased GFR.
Haemodilution & Hemoconcentration
Decrease in the concentration of plasma proteins and hemoglobin due to increase in plasma volume
Increase in the concentration of plasma proteins and hemoglobin due to a decrease in plasma volume
Osmotic pressure
Osmotic pressure is the pressure exerted by a solution to prevent the flow of water molecules into it through a semipermeable membrane.
It’s a measure of the concentration of solutes in a solution and is responsible for regulating the balance of fluids within living cells.
Measurement/calculation of plasma osmolarity
(Plasma osmolarity=2[Na+ + K+] + [Urea]+[glucose] in mmol/l)
How is Sodium Status assessed?
Why isn’t it enough to measure plasma sodium only when assessing sodium balance?
Assessment of Sodium Status
fractional excretion of sodium (FENa%)
Urine [Na] Plasma [creatinine]
FENa% = ----------- X ----------------
Plasma [Na] Urine [creatinine] •Assessing only plasma sodium may not completely show disturbance in sodium balance. •Measurement of urinary sodium shows whether the renal blood flow is adequate or not.
A value of a FENa% > 1% means what and <1% means what?
A value of less than 1% means poor renal perfusion and value more than 1% means intrinsic renal damage.
Hypovolaemia
•This can be simply defined as depletion of intravascular volume with subsequent reduction in functional ECF and ICF.
•It is as a result of reduced rate of sodium and water intake which is less than renal and extra renal loss.
Clinical features of hypovolemia
Depending on the degree of loss, weakness and tachycardia
With moderate loss patients may experience hypotension and reduced urine volume
With severe loss, there is severe volume contraction with increased sympathetic activity, very low blood pressure, cold extremities and oliguria
It should be noted that these features above are worse in patients with decreased cardiovascular reserve
Diagnosis of hypovolemia
Diagnosis
*Identify the cause (s) of fluid loss from the history
*Urine output measurement
*Urine specific gravity and osmolarity measurement
*Plasma and urine sodium measurements as well as plasma osmolarity
*Fractional excretion of sodium will also help to differentiate extra renal and intrinsic renal loss
Treatment principle of hypovolaemia
- Treat identified cause(s) of fluid loss
- Rate of fluid loss should be calculated and replaced orally or intravenousely as the case maybe.
- Blood loss is replaced with blood, extracellular fluid loss is replaced with normal saline ( 0.9% saline) and dextrose saline is used as maintenance fluids
What is Hypervolaemia?
•This can be simply defined as excess of body fluid with associated increase in ECF and ICF
Causes of hypervolemia
•Excessive intake of fluid
•Excessive transfusion of blood
•Inadequate sodium excretion as seen in renal and heart failures
•Mineralocorticord excess
Clinical features, Investigations & Treatment principle of hypervolemia
Clinical features
Increased blood pressure as a result of sodium retention and increased intravascular volume
Oedema as a result of increased hydrostatic pressure
Dyspnoea on exertion as a result of accumulation of fluid in pulmonary instertitium
Investigations
*Plasma sodium may be normal because of compensation or hyponatreamia as a result of increased volume
*Plasma urea and creatinine are deranged if renal failure is the cause.
Treatment principle
*Primary cause should be treated
*The use of diuretics
Hyponatraemia and types
This is defined as plasma concentration of sodium lower than lower reference limit of what is expected for population.
Hyponatraemia could be true or pseudo hyponatraemia
•True hyponatraemia: there are three major types of true hyponatraemia
•Hyponatraemia with hypovolaemia.
•Hyponatraemia with euvolaemia
•Hyponatraemia with hypervolaemia
What is Hyponatraemia with hypovolemia?
Total body water decreases but total body sodium decreases to a larger extent. This is seen in patients with extra renal loss like vomiting, diarrhoea, excessive sweating; renal loss like in the use of diuretics and aldosterone deficiency.
What is Hyponatraemia with euvolaemia?
-Hyponatraemia with euvolaemia: in this total body sodium is normal but there is increase in total body water. This is seen in patients with syndrome of inappropriate ADH secretion like in malignant neoplasm, drugs like chlorpropamide, post operative hyponatraemia
What is Hyponatraemia with hypervolaemia?
Hyponatraemia with hypervolaemia:
in this the total body sodium is increased but total body water is increased to a larger extent. This is seen in oedematous state as seen in congestive cardiac failure, renal failure and nephrotic syndrome.
What is Pseudohyponatraemia?
Pseudohyponatraemia: : plasma sodium is normal. Hyponatraemia is confused because of gross hyperlipidaemia or hyperoteinaemia
Investigation of hyponatraemia
•Rule out artefactual cause
•Request for lipid profile and plasma protein as well as plasma glucose to rule out pseudohyponatraemia
•Do serum aldosterone level to exclude adrenal insufficiency . In the case of Addison’s disease, assess adrenal function
• Thyroid hormone profile to rule out hypothyroidism
•Renal and cardiac status are checked
•Cancer survey in case of SIADH
Principle of treatment hypovolemia
*Look for underlined cause and treat
*Hypertonicity is treated by administration of intravenous fluid. This should be done slowly to prevent cerebral oedema
*Hypovolaemia is treated with fluid replacement mainly done with isotonic saline
*Total requirement is calculated thus: TBW X ( desired Na+ - current Na+)
*TBW = body weight in kg X % water (children=0.6, Adult= 0.55, Elderly= 0.5)
Hypernatraemia
•Plasma sodium is higher than upper reference limit for the population.
•It always reflects hyperosmolality if all artefactual causes are eliminated because it is the major solute that is responsible for plasma osmolality.
•It should be noted that it is usually due to predominant loss of water than sodium
Causes and Investigations of hypernatremia
Causes
*Artefactual causes
*Poor intake of water and excessive intake of sodium
*Increased renal loss of sodium and water
*Increased extra renal loss like insensible loss
*Administration of drugs like diuretics and drug containing sodium like carbenicillin
Investigation
*Remove identified cause
*Hydration status of the patient should be assessed clinically
*If there is hypernatraemai and hypokalaemia, consider mineralocorticord excess
Treatment of hypernatremia
Treatment
*If it is due to water depletion, oral water replacement is advised or if impossible intravenous fluid with low sodium is used
* The aim is to lower plasma sodium at no more than 1-2mmol/l per hour
*Treatment of primary hyperadosteronism and cushing’s symdrome are instituted if identified as causes
What is the daily intake of potassium & the distribution and regualtion of it
The daily intake of potassium has been estimated to be between 40-200mmol.
Distribution and regulation
*Potassium when ingested is almost completely absorbed in the intestine
*Functional extracellular potassium is a function of total body potassium and distribution in extracellular fluid and intracellular fluid.
*The total body potassium balance is difference between potassium intake and its excretion.
*Potassium is more abundant in the cells and this is kept intact with the help of Na+/K+ ATP pump using Na+/K+ ATPase.
Internal potassium balance
•This is referred to balance of potassium between ECF and ICF compartments.
• This may be under the influence of physiological or pathological factors
Physiological factors of Internal potassium balance
*Plasma potassium concentration in which the increase leads to movement of potassium into the cells and vise versa
*Insulin : this hormone influences movement of potassium into the cell along with glucose
*Catecholamine mainly beta2 adrenergic agent promotes inflow of potassium into the cell
*Na+/K+ ATPase. It keeps potassium and sodium in their concentrations both in the cell and outside the cell respectively.
*Exercise : this may cause hyperkalaemia as a result of potassium release from the muscular cells.
Pathological factors of Internal potassium balance
*Trauma : this leads to breaking down of cells with subsequent release of potassium.
*Hypertonicity : Movement of fluid from ICF to ECF with the aim of diluting tonicity of plasma, potassium may be dragged along
*Acid/Base balance : with acidosis (fall in plasma PH) potassium moves into plasma from the cell resulting into hyperkalaemia and the reverse is the case with alkalosis
Functions of potassium
•Regulation of protein and glycogen synthesis
•It helps in normal responsiveness of renal tubular cells to ADH
•It also involves in cell depolarisation
Hypokalaemia, causes & investigations
Hypokalaemia
This can be simply defined as plasma potassium lower than lower reference limit for the population
Causes
*Taking sample for the laboratory analysis from the site of intravenous fluid
*Reduce intake as seen in starvation
*Increase cell entry as seen in insulin therapy, alkalosis, catecholamines
*Excessive loss through vomiting, diarrhoea, sweat, burns, primary hyperaldosteronism, Cushing’s syndrome or high dose corticosteroid therapy, diuretic therapy, renal tubular acidosis
Investigations
Plasma electrolytes, urea and creatinine
Urine potassium. If less than 20mmol/l it indicates extra renal loss
If urine potassium is abnormally high.
Further investigations to exclude renal loss are indicated. Plasma renin and aldosterone are determined
Electrocardiograme (ECG) is done . This gives idea of effect of hypokalaemia on heart
Investigations of hypokalemia
•Plasma electrolytes, urea and creatinine
•Urine potassium. If less than 20mmol/l it indicates extra renal loss
•If urine potassium is abnormally high. Further investigations to exclude renal loss are indicated. Plasma renin and aldosterone are determined
•Electrocardiograme (ECG) is done . This gives idea of effect of hypokalaemia on heart
What is Hyperkalaemia and the causes?
This can be simply defined as plasma potassium concentration more than upper reference limit for the population
Causes
*Artefactual causes
*Excessive oral supplement and blood transfusion
*Insulin deficiency, metabolic acidosis, use of beta-2 adrenergic blocker, tissue damage
*Decrease urine excretion
Clinical features and investigations of hyperkalemia
Clinical features
*Problem with neuromuscular function resulting into weakness and paralysis; Diarrhoea and abdominal distension
*Cardiac arrhythmias
Investigations
*Any abnormal result requires immediate repeat with well taken fresh sample
*Plasma electrolytes, urea and creatinine
*Plasma renin and aldosterone
*ECG2.5
Investigations of hypovolaemia
•Plasma sodium may be normal because of compensation or hyponatreamia as a result of increased volume
•Plasma urea and creatinine are deranged if renal failure is the cause.
What is Appropriate hyponatraemia?
Appropriate hyponatraemia: this can also called redistributive hyponatraemia: This is hyponatraemia in the phase of hyperglycaemia, acute uraemia, infusion of amino acids and monitor. These are all osmotically active agents that have dilution effect
What is Artefactual hyponatraemia?
•Artefactual hyponatraemia: this is as a result of blood sample taking from the site of infusion if fluid being given is free of sodium.
