Fluid And Electrolyte Homeostasis ✅ Flashcards
How is fluidr distributed in the body?
Between intracellular fluid and extracellular fluid
How are the fluids in the intracellular and extracellular compartments different?
The volume composition differs
What maintains the solute composition of the intracellular and extracellular compartments?
- Cell membrane pump activity
- Solute size and electrical charge
What effect will a 10% decrease in plasma water have on sodium?
Will lead to plasma sodium decrease from 140 to 154mmol/l
What effect will dilution of the plasma by 10% have on sodium?
Will decrease sodium to 126
What is the most important stimulus for the kidney?
Volume preservation (rather than serum sodium concentration )
How can the body detect serum sodium concentration?
There are no bodily receptors that can detect serum sodium levels directly, but changes in plasma tonicity can be detected by osmoreceptors in the brain
What do changes in plasma tonicity detected by osmoreceptorsin the brain affect?
Renal water handling
How do changes in plasma tonicity cause changes in renal water handling?
Via anti-diuretic hormone (vasopressin)
What happens when there is conflicting information regarding volume status and plasma tonicity?
The most important principle is preservation or restoration of a normal plasma volume, rather than sodium concentration
What is explained by the concept that plasma volume status is prioritised over plasma sodium concentration?
- Even in the presence of hypernatraemic dehydration, urine sodium may be low
- Urine sodium is usually elevated in SIADH or in acute water intoxication despite profound hyponatraemia
What can biochemical parameters such as fractional excretion of sodium be useful for?
Assisting in determining plasma volume status
What is hyponatraemia defined as?
Plasma Na <135mmol/l
When does hyponatraemia occur?
When there is either water gain in excess of sodium gain, or sodium loss in excess of water loss
What can cause factitious hyponatraemia?
The presence of abnormal solutes in the ECF, eg: mannitol, sorbitol, or excessive glucose
How does the presence of abnormal solutes in the ECF lead to fictitious hyponatraamia?
The extra molecules result in a fluid shift which alters the sodium measurement
How can factitious hyponatraemia due to the presence of abnormal solutes in the ECF be detected?
High measured osmolality in contrast to calculated osmolality despite low serum Na
How is calculated osmolality determined?
2 x (Na + K) + rea + glucose
What are the principles for managing hyponatraemia?
- Rapid correction only if symptomatic, and should stop once symptoms improve
- Fluid restriction often helpful
What symptoms indicate a need for rapid correction in hyponatraemia?
- Coma
- Seizures
Who can initiate rapid correction of hyponatraemia?
Only specialists
How can rapid correction of hyponatraemia be achieved?
2-3ml/kg of 3% NaCl
What is the maximum rate of correction of hyponatraemia?
8-12mmol/24 hours
Why is fluid restriction often helpful in the management of hyponatraemia?
Most cases are due to excess water
What might also be helpful in the management of hyponatraemia caused by excess water?
Furosemide
Why might furosemide be helpful in cases of hyponatraemia?
It increases free water clearance
In what condition are fluid restriction and furosemide often used in?
SIADH
What drug is being explored for use in refractory hyponatraemia?
Tolvaptan
What is tolvaptan?
A vasopressin receptor 2 antagonist
What is required in the management of hyponatraemia where salt loss is in excess of water loss?
Replacement of volume deficit and ongoing losses with normal saline
What is hypernatraemia defined as?
Plasma Na >145mmol/L
What can cause hypernatraemia?
- Sodium gain in excess of water gain
- Water loss in excess of sodium loss
Why can the assessment of the degree of dehydration in hypernatraemic dehydration be difficult?
As sodium is the principle ECF osmole, the ECF volume is relatively well maintained and signs of dehydration/hypovolaemia are less apparent
What can severe hypernatraemia be associated with?
Brain damage
Why might severe hypernatraemia be associated with brain damage?
Brain tissue shrinks as a result of intracellular dehydration and blood becomes hypercoaguable
What brain pathologies can result from hypernatraemia?
- Encephalopathy
- Cerebral haemorrhage
- Thrombosis
What does the management of hypernatraemic dehydration include?
- Avoidance of rapid correction
- Sodium chloride 0.45% or 0.9%
Why should rapid correction be avoided in hypernatraemia?
Can result in cerebral oedema
Are boluses of normal saline given in hypernatraemic dehydration?
Only if there is shock
How quickly can acute hypernatraemia be corrected?
Over 24-48 hours
How does the speed of correction of chronic hypernatraemia compare to acute?
It should be slower in chronic
How can cerebral oedema result from rapid correction of hypernatraemia?
Fluid can pass rapidly into cells
At what rate can serum sodium be reduced in hypernatraemia?
No more than 0.5mmol/hour
How quickly should normal hydration be achieved in hypernatraemia?
36-48 hours
When might you want to delay normal hydration to over 72 hours in hypernatraemia?
If serum sodium >170
What % of potassium is intracellular?
98%
What is the result of potassium being primarily intracellular?
Plasma K is a poor representation of total body K
Why are acute changes in potassium life threatening?
The ratio of intra- and extracellular K is a major determinant for the membrane potential of excitable cells, e.g. in the heart and nervous system
What is the relationship between potassium and ECGs?
The pattern of the T wave on ECG reflects K concentration
What is found on ECG in hypokalaemia?
- ST depression
- Flat T wave
- Emergence of U wave
What is found on ECG in hyperkalaemia?
Peaked T waves
What effect does acidosis have on potassium?
It causes hyperkalaemia
Why does acidosis cause hyperkalaemia?
As hydrogen ions displace potassium as the intracellular cation, and potassium shifts from the intracellular to extracellular compartment
What controls the distribution of potassium between ICF and ECF?
Na/K-ATPase channels
What is the result of the Na/K-ATPase channel controlling the distribution of potassium?
Compounds that enhance the activity of this pump can be used for the treatment of hyperkalaemia
What drugs enhance the activity of the Na/K-ATPase channel?
- Insulin
- Adrenergics (salbutamol)
Where is most important K regulation done?
At the collecting duct
What happens to K at the collecting duct?
It is exchanged for Na
What controls the exchange of K for Na?
Aldosterone
What will impair K excretion at the collecting duct?
- Absence of aldosterone activity
- Insufficient sodium delivery
What can be used to assess aldosterone activity?
The transtubular potassium gradient (TTKG)
How is TTKG calculated?
(K in urine x serum osmolality) / (K in blood x urine osmolality)
What will the TTKG be in the presence of normal aldosterone activity?
> 5
What will the TTKG be in the absence of aldosterone?
<3
What is required to be able to interpret TTKG?
- Urinary Na is >20mmol/L
- Urine osmolality is equal to or greater than plasma
Why do you need to ensure urinary Na is >20mmol/L when interpreting TTKG?
Confirms sodium delivery
What is hyperkalaemia defined as?
K >5.5mmol/L
What can cause artefactually high K?
- Haemolysed blood sample
- Improper collection
- Delay in processing blood sample
- Markedly raised platelets, leukocytes, or erythrocytes
Give 2 ways in which improper collection can cause a falsely raised K?
- EDTA contamination
- Squeezed sample
Why is it essential that treatment for hyperkalaemia is started as soon as possible?
Severe hyperkalaemia can precipitate cardiac arrhythmias
How is hyperkalaemia treated?
- Measures to internalise potassium from extracellular to intracellular
- Potassium binders
- Stabilise myocardium
What measures can internalise potassium from extracellular to intracellular?
- Correction of acidosis
- Insulin glucose infusion
- Beta 2 agonist e.g. salbutamol
How is acidosis corrected in the treatment of hyperkalaemia?
Bicarbonate
Give an example of a potassium binder?
Calcium resonium resin
How do potassium binders work in hyperkalaemia?
They decrease the body potassium store
How is the myocardium stabilised in hyperkalaemia?
IV calcium gluconate
What is the first step in identifying the underlying aetiology of hyperkalaemia?
Assessment of renal function
Why is assessment of renal function the first step in the identified of the cause of hyperkalaemia?
Renal failure from any cause will lead to hyperkalaemia
What can aggravate hyperkalaemia caused by renal failure?
Other coincidental factors, e.g.;
- Use of drugs
- Increased K load
What drugs can contribute to hyperkalaemia?
- ACEi
- ARBs
- Beta blockers
- Trimethoprim
What can cause an increased K load?
- Tumour lysis syndrome
- Intravascular haemolysis
- Rhabdomyolysis
What are the causes of hyperaemia?
- Renal failure
- Acidosis
- Adrenal insufficiency
- Cell lysis
- Excessive potassium intake
What needs to be excluded as a cause of hyperkalaemia in the critically ill neonate?
Inadequate cardiac output
What is hyperkalaemia in the presence of normal GFR usually due to?
- Failure of delivery of sodium to distal tubules
- Aldosterone deficiency/resistance
What can cause failure of delivery of sodium to the distal tubules?
Hypovolaemia
Give 2 causes of aldosterone deficiency/resistance?
- Congenital adrenal hyperplasia
- Primary hypoaldosteronism
What is hypokalaemia defined as?
K <3.5mmol/L
How can hypokalaemia present?
- Lethargy
- Confusion
- Muscle weakness
- Intestinal ileus
How can muscle weakness caused by hypokalaemia progress in severe cases?
To paralysis
What are the potential pathological processes causing hypokalaemia?
- Decrease in total body potassium
- Shift of potassium to the intracellular space
What are the causes of hypokalaemia?
- Diarrhoea
- Alkalosis
- Volume depletion
- Primary hyperaldosteronism
- Diuretic abuse
What suggests renal loss in hypokalaemia?
A fractional excretion of >10%
What can cause hypokalaemia with volume excess?
- Aldosterone excess
- Renal artery stenosis
What is hypokalaemia and volume excess due to hyperaldosteronism known as?
Conn’s syndrome
What causes hypokalaemia with acidosis?
Renal tubular acidosis
What causes hypokalaemia with low urinary potassium?
Extrarenal loss of potassium, e.g. diarrhoea
When is IV potassium treatment required in hypokalaemia?
If cardiac arrhythmias or respiratory insufficiency secondary to paralysis
Why should very rapid infusion of potassium be avoided in hypokalaemia?
- Potential to cause major adverse effects
- Very concentrated potassium solutions damaging to peripheral veins
What is considered to be a ‘very concentrated’ potassium solution?
> 40mmol/L
What safety measures should be taken when emergency IV potassium replacement is required?
- Under ECG monitoring
- In PICU
What other electrolyte should be checked in hypokalaemia?
Magnesium
Why is it important to treat low serum magnesium in hypokalaemia?
It can increase the risk of cardiac arrhythmias
