Fluid and Electrolyte Balance Flashcards
Describe TBW in different groups of people
- 60% ideal weight of man
- 50-55% ideal weight of woman
- 70% of weight in infants
- 65% of weight in children
Why are infants more vulnerable to dehydration?
Bc they don’t have as much protective non-water areas
What causes worse outcomes, fluid overload or dehydration?
Fluid overload
What is the relationship between fat and water?
The more fat you have, the less water you have
Which is the only area of fluid we can access and therefore directly affect?
Plasma
What is the normal blood volume?
2.5L (65ml/kg in females and 75ml/kg in males)
Describe water loss
- Generate fluid through metabolism
- Normally 500ml/day loss (but no absolute amount)
- Skin and lungs = insensible losses
- Urine is a measure of water loss however skin, lungs and faeces is not easily measurable
Describe fluid loss in most people?
Most people are euvolaemic → in balance, generally controlled by the amount of urine passed
Describe net charge in body fluid
- In any fluid, the total cations = total anions bc we do not have a net charge
- The anion gap is just based on what we choose to measure
Describe how different ions are kept in balance
- Sodium and potassium gradient is maintained by Na-K ATPase
- Calcium → intracellular calcium has peaks and troughs all the time, but is high locally where it is having an effect
- Chloride is driven by sodium → high sodium levels = high chloride levels
What is the normal range for plasma sodium?
135-145 mmol/L (reflects body water content)
Describe sodium in the body
- 25% non-exchangeable in tissues e.g. bones (slow turnover)
- 75% exchangeable in solution in ECF
- Sodium consumption variable 110-220 mmol/day
- ~ 5-10mmol loss in sweat and faeces
- Sodium drives water reabsorption
- Total body exchangeable sodium content reflects TBW
What is sodium excretion regulated by?
1) RAA
2) Natriuretic peptides → increase sodium excretion in the kidneys
3) Intrinsic renal mechanisms
What is natriuresis?
Excretion of sodium in the urine
If you’re dehydrated or hypovolaemic what are your sodium levels like?
Low
Where are the juxtaglomerular cells?
In macula densa on the DCT but close to the glomerulus
Where is ACE?
In lungs and endothelial cells
What is the effect of aldosterone?
Increased sodium reabsorption in the DCT
What happens to osmolarity when you are water deficient?
It increases
What is the action of ADH?
Acts on the collecting duct to reabsorb water
When do natriuretic peptides act?
When there is fluid overload
What is fluid overload caused by?
1) Kidney failure
2) Heart failure → LH = lung, RH = periphery
3) Liver failure → fluid build up in abdomen
What does increased GFR lead to?
Less sodium reabsorption and more sodium secretion → natriuresis
What causes the heart to release ANP and BNP?
Cardiac distension, sympathetic stimulation and angiotensin II
What are the actions of ANP and BNP?
Vasodilation, decrease renin and decrease GFR
What is involved in a fluid balance examination
Limbs, head and neck, chest, abdomen, fluid balance chart, weight chart
What are you looking for in the limbs part of a fluid balance examination?
1) Temperature, pulse (volume and rate), BP sitting and standing
2) Skin turgor → if dehydrated will be decreased (but look at context of age), pitting oedema is newly acquired
What are you looking for in the head and neck part of a fluid balance examination?
1) Sunken eyes → if losing fluid from eyes, quite significant dehydration
2) Dry mucous membranes → not necessarily dehydrated but def not fluid overloaded (can’t be dehydrated without dry mucous membranes)
3) JVP and carotid pulse
What are you looking for in the chest part of a fluid balance examination?
1) Capillary refill
2) Lung auscultation for pulmonary oedema
What are you looking for in the abdomen part of a fluid balance examination?
Ascites → might be fluid overloaded by TBW still isn’t high, just fluid in wrong place
What are 6 symptoms/history in people with hypovolaemia?
1) Thirst
2) Lethargy
3) Postural dizziness
4) Confusion
5) GI losses
6) Reduced urine volume
What 5 things might you see in an examination of someone with hypovolaemia?
1) Fast and weak pulse
2) BP postural drop of > 20 mmHg OR low BP
3) Loss of skin turgor
4) Sunken eyes
5) Dry mucous membranes
What are 5 symptoms/history in people with hypervolaemia?
1) Breathlessness
2) Confusion
3) Abdominal bloating
4) Peripheral oedema
5) Weight gain
What 6 things might you see in an examination of someone with hypervolaemia?
1) Fast and bounding pulse
2) BP can be high or low
3) Skin turgor generally maintained
4) Peripheral oedema
5) Elevated JVP
6) Ascites
What is hyponatraemia?
Na < 135 mmol/L
What does hyponatraemia result from?
1) The intake and subsequent retention of water → excess of water in relation to sodium
2) The depletion of total body sodium in excess of concurrent body water losses
What is the key feature of hyponatraemia?
Volume status of the patient
What are the 3 classifications of hyponatraemia?
1) Hypovolaemia
2) Euvolaemia
3) Hypervolaemia
What are the two types of causes of hypovolaemia leading to hyponatraemia?
1) Extra-renal losses
2) Renal losses
What are causes of extra-renal losses leading to hypovolaemia and hyponatraemia?
1) Dermal losses → burns
2) GI losses → vomiting, diarrhoea
3) 3rd space → pancreatitis
What is urine sodium like in extra-renal losses leading to hypovolaemia and hyponatraemia?
< 25 mmol/L
What is urine sodium like in renal losses leading to hypovolaemia and hyponatraemia?
> 30mmol/L
What are causes of renal losses leading to hypovolaemia and hyponatraemia?
1) Diuretic therapy
2) Cerebral salt wasting
3) Primary adrenal insufficiency → lack of aldosterone
What is urine sodium like in euvolaemia causing hyponatraemia?
> 30 mmol/L
What are causes of euvolaemia leading to hyponatraemia?
1) Water intoxication → primary polydipsia, excess IV hypotonic fluids
2) Hypothyroidism
3) Hypopituitarism → glucocorticoid deficiency
4) Pregnancy
5) SIADH (evaluate underlying aetiology)
Generally what causes hypervolaemia leading to hyponatraemia?
The failures
What causes hypervolaemia leading to hyponatraemia with urine sodium < 25?
1) Congestive cardiac dysfunction
2) Cirrhosis with ascites
3) Nephrotic syndrome
What causes hypervolaemia leading to hyponatraemia with urine sodium >30?
CKD
What is SIADH?
Syndrome of inappropriate ADH secretion
What are the types of causes of SIADH?
1) Neoplastic
2) Pulmonary
3) CNS
4) Drugs
5) Other
What are neoplastic (cancer) causes of SIADH?
1) Lung → small cell, mesothelioma
2) GI → stomach, pancreas
3) GU → bladder, prostate, endometrium
4) Thymoma
5) Leukaemia
6) Lymphoma
7) Sarcoma
What are pulmonary causes of SIADH?
1) Infection → pneumonia, abscess, TB, aspergillosis
2) Asthma
3) CF
4) PPV (positive pressure ventilation)
What are CNS causes of SIADH?
1) Infection → abscess, meningitis, AIDS
2) Bleeds → subdural, SAH
3) CVA
4) Head trauma
5) MS, GBS
6) Shy-Drager
What are drugs causes of SIADH?
1) AVP analogues → desmopressin, oxytocin, vasopressin
2) Stimulate AVP release/action → SSRIs, antipsychotics, anti-epileptics, NSAIDs, MDMA
What are other causes of SIADH?
1) Idiopathic
2) Hereditary (V2 receptor)
When are you more likely to have symptoms in hyponatraemia?
If it comes on more rapidly
What do the symptoms of hyponatraemia depend on?
Severity of hyponatraemia
What are symptoms in mild hyponatraemia (130-135)?
Asymptomatic
What are symptoms in moderate hyponatraemia (121-129)?
Cramps, weakness, nausea
What are symptoms in severe hyponatraemia (<120)?
Lethargy, headache, confusion
What are symptoms in severe and rapidly evolving hyponatraemia?
Seizures, coma, respiratory arest
Why can hyponatraemia cause neurological symptoms?
1) Water moves into cells bc of the osmotic gradient between ECF and ICF within the brain
2) This leads to raised ICP due to oedema → neurological symptoms
What is hypernatraemia?
Na > 145 mmol/L
How does hypernatraemia occur?
- Net water loss or hypertonic sodium gain
- The increase in plasma tonicity pulls water out the cells, resulting in a decrease in intracellular volume
What classifies as severe hypernatraemia?
If Na > 158 mmol/L
What are symptoms of hypernatraemia?
Thirst, anorexia, weakness, stupor, seizures, coma
What are the two types of hypernatraemia?
1) Unreplaced water loss
2) Sodium overload
What causes unreplaced water loss leading to hypernatraemia?
1) Insensible and sweat losses
2) GI losses
3) Central diabetes insipidus (lack of ADH)
4) Nephrogenic diabetes insipidus
5) Osmotic diuresis → high glucose in uncontrolled diabetes mellitus
6) Poor water intake → elderly, impaired thirst or osmoregulation (hypothalamus)
What causes sodium overload leading hypernatraemia?
1) Administration of high salt load → hypertonic sodium solutions
2) Hyperaldosteronism
What is hypokalaemia?
K < 3.5 mmol/L
Describe potassium in the body
Potassium enters the body via oral intake or IV, is largely stored in the cells, then excreted in the urine
What is severity proportionate to?
Rate of onset
What are symptoms of hypokalaemia (proportionate to severity)?
1) If K < 2.5 → muscle weakness which progresses from lower extremities
2) ECG changes and arrhythmias
3) If chronic → renal abnormalities e.g. impaired concentrating ability, raised BP
What are the 4 causes of hypokalaemia?
1) Increased entry into cells
2) Increased GI losses
3) Increased urine losses
4) Other
What causes increased entry into cells, leading to hypokalaemia?
1) Extracellular pH rise
2) Increased insulin
3) Elevated beta-adrenergic activity
4) Hypothermia
5) Drugs e.g. antipsychotics
What causes increase GI losses, leading to hypokalaemia?
1) Vomiting
2) Diarrhoea
3) Laxative abuse
What causes increased urine losses, leading to hypokalaemia?
1) Diuretics
2) Primary mineralocorticoid excess
3) Renal tubular acidosis
4) Drugs e.g. amphotericin B
5) Salt-wasting nephropathies e.g. Bartter’s, Gitelman’s
6) Hypomagnasaemia
What are other causes leading to hypokalaemia?
1) Reduced intake (rare)
2) Increased sweat losses
What will urine potassium be in the case of increased GI losses?
< 20
What will urine potassium be in the case of increased urine losses?
> 40
What ECG changes do you see with hypokalaemia?
Opposite to hyperkalaemia
1) Flat T wave
2) U waves (in between T an P wave)
3) ST depression
4) PR interval prolonged
5) Prolonged QT interval
What is hyperkalaemia?
K > 5.5 mmol/L
What are the 2 types of causes of hyperkalaemia?
1) Increased release from cells
2) Reduced urinary excretion
What causes increased release of K from cells leading to hyperkalaemia?
1) Metabolic acidosis
2) Pseudohyperkalaemia (K movement out of cells after blood is drawn)
3) Insulin deficiency, hyperglycaemia and hyperosmolarity
4) Increased tissue catabolism → trauma, cytotoxic drugs
5) Beta blockers
6) Exercise
7) Hyperkalaemic periodic paralysis
8) Other e.g. digoxin toxicity, blood transfusions
What causes reduced urinary excretion leading to hyperkalaemia?
1) Acute and chronic kidney disease
2) Reduced aldosterone secretion
3) Reduced response to aldosterone
4) Drugs e.g. K sparing diuretics, ACE inhibitors,
5) Type IV renal tubular acidosis
What are symptoms of hyperkalaemia?
1) Paresthesia
2) Muscle weakness → paralysis
3) Arrhythmias
What ECG changes occur in hyperkalaemia?
1) Tall peaked T waves
2) Shortened QT interval
3) PR interval lengthening
4) QRS widening
5) P waves disappear → sine wave
