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TAS Nephrology > Fluid And Electrolyte Homeostasis ✅ > Flashcards

Fluid And Electrolyte Homeostasis ✅ Flashcards

1
Q

How is fluidr distributed in the body?

A

Between intracellular fluid and extracellular fluid

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2
Q

How are the fluids in the intracellular and extracellular compartments different?

A

The volume composition differs

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3
Q

What maintains the solute composition of the intracellular and extracellular compartments?

A
  • Cell membrane pump activity

- Solute size and electrical charge

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4
Q

What effect will a 10% decrease in plasma water have on sodium?

A

Will lead to plasma sodium decrease from 140 to 154mmol/l

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5
Q

What effect will dilution of the plasma by 10% have on sodium?

A

Will decrease sodium to 126

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6
Q

What is the most important stimulus for the kidney?

A

Volume preservation (rather than serum sodium concentration )

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7
Q

How can the body detect serum sodium concentration?

A

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

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8
Q

What do changes in plasma tonicity detected by osmoreceptorsin the brain affect?

A

Renal water handling

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9
Q

How do changes in plasma tonicity cause changes in renal water handling?

A

Via anti-diuretic hormone (vasopressin)

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10
Q

What happens when there is conflicting information regarding volume status and plasma tonicity?

A

The most important principle is preservation or restoration of a normal plasma volume, rather than sodium concentration

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11
Q

What is explained by the concept that plasma volume status is prioritised over plasma sodium concentration?

A
  • 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
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12
Q

What can biochemical parameters such as fractional excretion of sodium be useful for?

A

Assisting in determining plasma volume status

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13
Q

What is hyponatraemia defined as?

A

Plasma Na <135mmol/l

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14
Q

When does hyponatraemia occur?

A

When there is either water gain in excess of sodium gain, or sodium loss in excess of water loss

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15
Q

What can cause factitious hyponatraemia?

A

The presence of abnormal solutes in the ECF, eg: mannitol, sorbitol, or excessive glucose

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16
Q

How does the presence of abnormal solutes in the ECF lead to fictitious hyponatraamia?

A

The extra molecules result in a fluid shift which alters the sodium measurement

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17
Q

How can factitious hyponatraemia due to the presence of abnormal solutes in the ECF be detected?

A

High measured osmolality in contrast to calculated osmolality despite low serum Na

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18
Q

How is calculated osmolality determined?

A

2 x (Na + K) + rea + glucose

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19
Q

What are the principles for managing hyponatraemia?

A
  • Rapid correction only if symptomatic, and should stop once symptoms improve
  • Fluid restriction often helpful
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20
Q

What symptoms indicate a need for rapid correction in hyponatraemia?

A
  • Coma

- Seizures

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21
Q

Who can initiate rapid correction of hyponatraemia?

A

Only specialists

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22
Q

How can rapid correction of hyponatraemia be achieved?

A

2-3ml/kg of 3% NaCl

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23
Q

What is the maximum rate of correction of hyponatraemia?

A

8-12mmol/24 hours

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24
Q

Why is fluid restriction often helpful in the management of hyponatraemia?

A

Most cases are due to excess water

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25
What might also be helpful in the management of hyponatraemia caused by excess water?
Furosemide
26
Why might furosemide be helpful in cases of hyponatraemia?
It increases free water clearance
27
In what condition are fluid restriction and furosemide often used in?
SIADH
28
What drug is being explored for use in refractory hyponatraemia?
Tolvaptan
29
What is tolvaptan?
A vasopressin receptor 2 antagonist
30
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
31
What is hypernatraemia defined as?
Plasma Na >145mmol/L
32
What can cause hypernatraemia?
- Sodium gain in excess of water gain | - Water loss in excess of sodium loss
33
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
34
What can severe hypernatraemia be associated with?
Brain damage
35
Why might severe hypernatraemia be associated with brain damage?
Brain tissue shrinks as a result of intracellular dehydration and blood becomes hypercoaguable
36
What brain pathologies can result from hypernatraemia?
- Encephalopathy - Cerebral haemorrhage - Thrombosis
37
What does the management of hypernatraemic dehydration include?
- Avoidance of rapid correction | - Sodium chloride 0.45% or 0.9%
38
Why should rapid correction be avoided in hypernatraemia?
Can result in cerebral oedema
39
Are boluses of normal saline given in hypernatraemic dehydration?
Only if there is shock
40
How quickly can acute hypernatraemia be corrected?
Over 24-48 hours
41
How does the speed of correction of chronic hypernatraemia compare to acute?
It should be slower in chronic
42
How can cerebral oedema result from rapid correction of hypernatraemia?
Fluid can pass rapidly into cells
43
At what rate can serum sodium be reduced in hypernatraemia?
No more than 0.5mmol/hour
44
How quickly should normal hydration be achieved in hypernatraemia?
36-48 hours
45
When might you want to delay normal hydration to over 72 hours in hypernatraemia?
If serum sodium >170
46
What % of potassium is intracellular?
98%
47
What is the result of potassium being primarily intracellular?
Plasma K is a poor representation of total body K
48
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
49
What is the relationship between potassium and ECGs?
The pattern of the T wave on ECG reflects K concentration
50
What is found on ECG in hypokalaemia?
- ST depression - Flat T wave - Emergence of U wave
51
What is found on ECG in hyperkalaemia?
Peaked T waves
52
What effect does acidosis have on potassium?
It causes hyperkalaemia
53
Why does acidosis cause hyperkalaemia?
As hydrogen ions displace potassium as the intracellular cation, and potassium shifts from the intracellular to extracellular compartment
54
What controls the distribution of potassium between ICF and ECF?
Na/K-ATPase channels
55
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
56
What drugs enhance the activity of the Na/K-ATPase channel?
- Insulin | - Adrenergics (salbutamol)
57
Where is most important K regulation done?
At the collecting duct
58
What happens to K at the collecting duct?
It is exchanged for Na
59
What controls the exchange of K for Na?
Aldosterone
60
What will impair K excretion at the collecting duct?
- Absence of aldosterone activity | - Insufficient sodium delivery
61
What can be used to assess aldosterone activity?
The transtubular potassium gradient (TTKG)
62
How is TTKG calculated?
(K in urine x serum osmolality) / (K in blood x urine osmolality)
63
What will the TTKG be in the presence of normal aldosterone activity?
>5
64
What will the TTKG be in the absence of aldosterone?
<3
65
What is required to be able to interpret TTKG?
- Urinary Na is >20mmol/L | - Urine osmolality is equal to or greater than plasma
66
Why do you need to ensure urinary Na is >20mmol/L when interpreting TTKG?
Confirms sodium delivery
67
What is hyperkalaemia defined as?
K >5.5mmol/L
68
What can cause artefactually high K?
- Haemolysed blood sample - Improper collection - Delay in processing blood sample - Markedly raised platelets, leukocytes, or erythrocytes
69
Give 2 ways in which improper collection can cause a falsely raised K?
- EDTA contamination | - Squeezed sample
70
Why is it essential that treatment for hyperkalaemia is started as soon as possible?
Severe hyperkalaemia can precipitate cardiac arrhythmias
71
How is hyperkalaemia treated?
- Measures to internalise potassium from extracellular to intracellular - Potassium binders - Stabilise myocardium
72
What measures can internalise potassium from extracellular to intracellular?
- Correction of acidosis - Insulin glucose infusion - Beta 2 agonist e.g. salbutamol
73
How is acidosis corrected in the treatment of hyperkalaemia?
Bicarbonate
74
Give an example of a potassium binder?
Calcium resonium resin
75
How do potassium binders work in hyperkalaemia?
They decrease the body potassium store
76
How is the myocardium stabilised in hyperkalaemia?
IV calcium gluconate
77
What is the first step in identifying the underlying aetiology of hyperkalaemia?
Assessment of renal function
78
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
79
What can aggravate hyperkalaemia caused by renal failure?
Other coincidental factors, e.g.; - Use of drugs - Increased K load
80
What drugs can contribute to hyperkalaemia?
- ACEi - ARBs - Beta blockers - Trimethoprim
81
What can cause an increased K load?
- Tumour lysis syndrome - Intravascular haemolysis - Rhabdomyolysis
82
What are the causes of hyperaemia?
- Renal failure - Acidosis - Adrenal insufficiency - Cell lysis - Excessive potassium intake
83
What needs to be excluded as a cause of hyperkalaemia in the critically ill neonate?
Inadequate cardiac output
84
What is hyperkalaemia in the presence of normal GFR usually due to?
- Failure of delivery of sodium to distal tubules | - Aldosterone deficiency/resistance
85
What can cause failure of delivery of sodium to the distal tubules?
Hypovolaemia
86
Give 2 causes of aldosterone deficiency/resistance?
- Congenital adrenal hyperplasia | - Primary hypoaldosteronism
87
What is hypokalaemia defined as?
K <3.5mmol/L
88
How can hypokalaemia present?
- Lethargy - Confusion - Muscle weakness - Intestinal ileus
89
How can muscle weakness caused by hypokalaemia progress in severe cases?
To paralysis
90
What are the potential pathological processes causing hypokalaemia?
- Decrease in total body potassium | - Shift of potassium to the intracellular space
91
What are the causes of hypokalaemia?
- Diarrhoea - Alkalosis - Volume depletion - Primary hyperaldosteronism - Diuretic abuse
92
What suggests renal loss in hypokalaemia?
A fractional excretion of >10%
93
What can cause hypokalaemia with volume excess?
- Aldosterone excess | - Renal artery stenosis
94
What is hypokalaemia and volume excess due to hyperaldosteronism known as?
Conn's syndrome
95
What causes hypokalaemia with acidosis?
Renal tubular acidosis
96
What causes hypokalaemia with low urinary potassium?
Extrarenal loss of potassium, e.g. diarrhoea
97
When is IV potassium treatment required in hypokalaemia?
If cardiac arrhythmias or respiratory insufficiency secondary to paralysis
98
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
99
What is considered to be a 'very concentrated' potassium solution?
>40mmol/L
100
What safety measures should be taken when emergency IV potassium replacement is required?
- Under ECG monitoring | - In PICU
101
What other electrolyte should be checked in hypokalaemia?
Magnesium
102
Why is it important to treat low serum magnesium in hypokalaemia?
It can increase the risk of cardiac arrhythmias

TAS Nephrology (16 decks)

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