Fluid Balance, Sodium and Potassium Flashcards
What % of the body is water?
60%
what is the ratio of Intracellular fluid to Extracellular fluid?
2:1
3 areas where extracellular fluid can be?
Intravascular
Interstitial (bathing cells- the largest component of ECF
Transcellular (within epithelial-lined spaces, e.g. CSF, joint fluid, bladder urine, aqueous humour)
What is Osmolality?
Total number of particles in solution - measured with an osmometer., units = mmol/kg
What is Osmolarity?
Calculated, measure of solute per liter of solution, units - mmol/l
=2(Na + K) +urea + glucose
Physiological and Pathological determinants of osmolality/osmolarity in serum/plasma..
Physiological - Na+K+Cl+HCO3+urea+glucose
Pathological = Endogenous (i.e. glucose), Exogenous (ethanol, mannitol)
What can osmolality be used to diagnose?
SIADH - the normal range for serum osmolality is 275-295mmol/kg
What is the difference between osmolality and osmolarity known as?
the osmolar gap, and can be useful in metabolic acidosis cases
osmolality and osmolarity should roughly equate
Normal ranges for Sodium?
135-145 mmol/l
How is Sodium distributed around the body?
70% is freely exchangeable, the rest complexed in bone
How are levels of sodium maintained?
Predominantly an extravellular cation, largely maintained by active pumping from ICF>ECF by NA/K ATPase
Hyponatraemia - Values at which symptoms occur
less than 136 = Nausea and vomiting
less than 131 = confusion
less than 125 = non cardiogenic pulmonary oedema
less than 117 = coma
Hyponatraemia - treatment?
Treat underlying cause not the hyponatreamia (unless severe
Hyponatraemia - symptoms?
Symptomatic hyponatraemia is a medical emergency
What is TURP syndrome?
Hyponatraemia from water absorbed through damaged prostate
In true hyponatraemia is Osmolality high or low?
Low
Causes of hypovolaemic hyponatraemia ?
Urinary Sodium greater than 20mmol/l= Renal
- Diuretics, Addison’s, Salt losing nephropathies
Urinary Sodium less than 20mmol/l = Non-Renal
- Vomiting, Diarrhoea, excess sweating, Third space losses (ascites, burns)
Causes of Euvolaemic hyponatraemia?
Urinary Sodium >20mmol/l - SIADH, Primary polydipsia, Severe hypothyroidism
Causes of Hypervolaemic hyponatraemia?
Urinary Sodium >20mmol/l = Renal
- ARF, CRF
Urinary Sodium less than 20mmol/l = Non-Renal
- Cardiac failure, Cirrhosis, Inappropriate IV fluid
Correction of Hyponatraemia, things to be aware of?
Rapid correction can lead to Central Pontine Myelinolysis (pseudobulbar palsy, paraparesis, locked-in syndrome) therefore aim to increase Na by 1mmol/l per hour
Causes of hyponatreamia post surgery?
Over hydration with hypotonic IV fluids
Transient increase in ADH due to stress of surgery
Lab criteria for SIADH
True Hyponatraemia (low serum osmolality) Clinically euvolaemic Inappropriately high urine osmolality and increased renal sodium excretion (>20mmol/l) due to decreasing aldosterone levels Normal renal, adrenal, thryroid and cardiac function
A diangosis of exclusion
Causes of SIADH
- Malignancy - small cell lung cancer, pancreas, prostate, lymphome (ectopic secretion)
- CNS disoders - meningoencephalitis, haemorrhage, abscess
- Chest Disease - TB, pneumonia, abscess
- Drugs - opiates, SSRIs, carbamazepine
Hypernatraemia - sodium level? symptoms? classification? what ward is it most likely to be seen on?
- Less common than hyponatraemia, but usually clinically significant (plasma NA> 148mmol/l)
- Symptoms = thirst–> confusion –> seizures + ataxia –> coma
- can be classifed based on hydration status
- In hospital often iatrogenic, common problem in ITU patients
RAPID CORRECTION CAN LEAD TO CEREBRAL OEDEMA!!!
What causes Hypovolaemic Hypernatraemia?
- GI loss - vomiting, diarrhoea
- Skin loss - excess sweating, burins
- Renal loss - loop diuretics, Renal disease (impaired concentrating ability), Osmotic diuresis (glucose, mannitol)
What causes Euvolaemic Hypernatraemia?
- Respiratory loss - Tachypnoea
- Skin loss - excessive sweating, Fever
- Renal loss - Diabetes Insipidus
- Misc - No water!
What causes Hypervolaemic Hypernatraemia?
- Mineralocorticoid excess (Conns Syndrome)
- Hypertonic saline
Clinical features of Diabetes Insipidus?
- Hypernatraemia (lethargy, thirst, irritability, confusion, coma, fits)
- Clincially euvolaemic
- Polyuria and polydipsia
- Urine: Plasma olsmolality is less than 2
What are the 2 types of Diabetes Insipidus?
1) Cranial
2) Nephrogenic
What is Cranial Diabetes Insipidus and what are the causes of it?
Lack of/ No ADH causes - Head trauma - Tumour - Surgery
What is Nephrogenic Diabetes Insipidus and what are the causes of it?
Receptor defect - insensitivity to ADH causes - Inherited - Lithium - Chronic renal failure
What test if used to diagnose Diabetes Insipidus?
8hr fluid deprication test
What is a normal result of a 8hr fluid deprication test?
Urine concentration greater than 600mOsmol/kg
What result of a 8hr fluid deprication test would lead you to diagnose Primary Polydipsia?
Urine concentrates 400-600mOsmol/kg
What result of a 8hr fluid deprication test would lead you to diagnose Cranial DI?
Urine concentrates only after giving desmopressin
What result of a 8hr fluid deprication test would lead you to diagnose Nephrogenic DI?
No concentraton of urine after desmopressin
Potassium normal range
3.5-5.5 mmol/l
Potassium - distribution in the body
Predominantly intracellular cation (only 2% is extracellular), maintained by active pumping from EC-> ICF by Na/K ATPase
90% freely exchangeable, the rest bound in RBCs, bone and brain tissue
Hypokalaemia - below what value? causes?
below 3.5mmol/L
1-GI loss- Diarrhea, excess sweat
2-Renal loss - Hyperaldosteronism (Conn’s) - due to aldosterones effect on Na/K ATPase in kidneys causing the loss of K via urine and reabsorption of Na and water
. - Excess Cortisol (Cushings) - cortisol acts like aldosterone
3-Redistribution into cells - Insulin (increases activity of Na/K ATPase shifting K into cells), Beta-agonists (salbutamol)
4) rare - Hypomagnesesaemia - Mg needed for K processing
Hyperkalaemia - above what value? causes?
> 5.5mmol/L
Less common than hypokalaemia, but more dangerous
Caused by excessive intake (almost always iatrogenic), movement out of cells or decreased excretion
Causes of Hyperkalaemia due to excessive intake?
Oral (fastin)
Parenteral (via IV)
Stored blood transfusion
Causes of Hyperkalaemia due to Transcellular Movement (ICF>ECF)?
Acidosis
Insulin shortage
Tissue damage/ catabolic state
Causes of Hyperkalaemia due to Decreased excretion?
Acute Renal Failure (oliguric phase) CRF (late) Potassium sparing diuretics (spironolactone) Mineralocorticoid deficiency (Addisons) NSAIDs, ACEi
FUN FACT ABOUT POTASSIUM AND H+ IONS…
… H+ and potassium are intimately linked as one moves into the cells one moves out.
for every drop in pH of 0.1 there is an increase in K+ of 0.7
