Fluid Assessment and Balance

Question 1

What % of body weight is total body water (TBW)? On average how many litres is this?

Answer 1

60%, ~42L

Question 2

What % of TBW is ECF?

Answer 2

1/3

Question 3

What % of TBW is ICF?

Answer 3

2/3

Question 4

How does ECF vary?

Answer 4

With osmolar (sodium and chloride) content, which is subject to intake less than excretion or vice versa

Question 5

What is the impact of sodium intake and the menstrual cycle on ECV?

Answer 5

On average ECV ~14L, +/- 1L due to impact of sodium and menstrual cycle

Question 6

When does ICV vary?

Answer 6

Fairly constant

Can vary with water intake and plasma sodium concentration

Question 7

How can ECV be assessed?

Answer 7

History
Abnormal intake or excretion
Examination of CVS (tissue turgor, mucous membranes, BP, JVP, oedema)
Urine concentration/sodium content

Question 8

What is NOT a good guide to ECV?

Answer 8

Plasma sodium concentration

Question 9

How can ICV be assessed?

Answer 9

History
Physical examination (limited value)
Plasma sodium or osmolality (parallels IC osmolality)

Question 10

What is NOT a good guide to ICV?

Answer 10

Physical examination is of limited value

Question 11

Which fluid compartment does the administration of saline affect?

Answer 11

Sodium remains in ECV and water follows osmols

Question 12

List 5 causes of hypovolaemia

Answer 12

Diarrhoea and vomiting
Disease: CV, DM, renal, intracranial, intrathoracic
Denial of oral intake: peri-operative, coma
anti-Diuretic hormone (ADH): osmotically inappropriate secretion (ectopic production, altered control - baroreceptors, intracranial, intrathoracic, CVD)
Drugs: Diuretics, anti-Diabetics, anti-Depressants/anti-psychotics

Question 13

Give 3 examples of anti-depressants which may cause hypovolaemia

Answer 13

Lithium
Carbamazepine
SSRIs

Question 14

Describe the ECV and ICV in terms of their composites

Answer 14

ECV: sodium, chloride (an environment supporting IC osmolality, cell membrane transport and delivery of nutrients and removal of waste - i.e. the circulation)
ICV: potassium, organic anions (an environment supporting IC function and cell membrane transport)

Question 15

TBW 42L + 3L 150mmol/L saline = 45L

How is this distributed into the ECV and ICV? What signs may be seen?

Answer 15

Water follows osmols, sodium remains in ECV, so 3L is added to ECV resulting in oedema, possible raised JVP, pulmonary oedema if existing CVD disease
Plasma sodium and osmolality unchanged
ICV unchanged

Question 16

TBW 42L + 3L water = 45L

How is this distributed in the ECV and ICV?

Answer 16

No osmolar capture of water
ECV gains 1/3 (1L): changes are hard to detect, there may be mild oedema, plasma sodium and osmolality decrease by ~7%
ICV gains 2/3 (2L): Sx and signs unlikely, assessed by plasma sodium or osmolality (if renal perfusion and ADH control normal, rapid excretion of excess water follows)

Question 17

TBW 42L - 3L fluid loss of sodium rich fluid (diarrhoea) = 39L
What are the effects on ECV and ICV? What signs may be seen?

Answer 17

ECV loses 3L: low BP, tachycardia, low JVP, axillary dryness, peripheral vasoconstriction, low urine sodium but concentrated urine
NB Plasma sodium normal
ICV unchanged

Question 18

“Dehydration”
TBW 42L - 3L from failure to drink (unconscious) and obligatory loss = 39L
What are the effects on ECV and ICV? What signs may be seen?

Answer 18

ECV loses 1/3 (1L): hard to detect O/E unless CVD, plasma sodium/osmolality increased by ~7%
ICV loses 2/3 (2L): dry tongue, axilla, low tissue turgor in young patients, plasma sodium/osmolality increased by 7%, concentrated urine

Question 19

21 year old female with 6/7 N+V, diarrhoea
Has noticed marked weight loss of 3kg and is drinking but not eating
O/E: cold fingers and feet (hypoperfused and vasoconstricted), HR 100, BP 120/80 lying and 100/60 standing (postural drop)
Plasma Na+ 138 mmol/L, creatinine 80 umol/L (both normal)
ICV, ECV, body sodium?

Answer 19

ICV normal
ECV low
Body sodium low

Question 20

64 year old female presents with 3/12 productive cough with haemoptysis and recent confusion
O/E: BP 160/90 lying and standing, HR 76, RR 20, JVP not raised, signs of consolidation in apex of R lung
Plasma Na+ 120 mmol/L (low), creatinine 70 umol/L (normal)
ICV, ECV, body sodium?

Answer 20

SiADH! Keeping on water
ICV high
ECV high
Body sodium normal

Question 21

77 year old man found by relatives comatose after 3/7 of no contact with him
O/E: evidence of hemiplegia (new), BP 100/70, HR 90, dry mucosa, cold periphery
Plasma Na+ 157 mmol/L (high), urine sodium 10 mmol/L (low)
ICV, ECV, body sodium?

Answer 21

Increased ADH secretion
ICV low
ECV low
Body sodium normal

Question 22

67 year old man, with long-standing IHD, presents SOB with obvious pulmonary oedema and with massive peripheral oedema; he has gained 15kg weight since last reviewed
O/E: BP 100/60, HR 120, JVP 15cm
Plasma Na+ 120mmol/L (low), creatinine 180 umol/L (high)
ICV, ECV, body sodium?

Answer 22

ICV high
ECV high
Total body sodium normal/high

Question 23

36 year old female presents with an exacerbation of UC, manifest by excessive diarrhoea; she feels cold and complains dizziness when she stands up, and is also thirsty and has been drinking a lot
O/E: BP 90/60, HR 120, JVP not visible
Plasma Na+ 120mmol/L (low), urine Na+ 3 mmol/L (low), plasma creatinine 110 umol/L (high)
ICV, ECV, body sodium?

Answer 23

ICV low?? (losing Na+)
ECV low
Total body sodium low

Question 24

62 year old woman presents with a few days regurgitation of food/vomiting on a background of difficulty in swallowing solids since a gastric stapling operation 15 years ago
She is on lithium for bipolar disorder and reports a weight loss of 7kg
O/E: looks unwell, BP 100/60, HR 120, JVP not visible, cold periphery, dry tongue
Plasma Na+ 165 mmol/L (high), osmolality 330 (high), creatinine 272 umol/L (high), urine Na+ 12 mmol/L (low), chloride 8 mmol/L (very low)
ICV, ECV, body sodium?

Answer 24

Lithium causing diabetes insipidus (losing water)
ICV low
ECV low
Body sodium normal

Question 25

60 year old female piano teacher presents with 4/7 Hx severe watery diarrhoea, and is now thirsty and also gets dizzy when standing up O/E: looks unwell, BP 90/60 (lying) and 70/40 (standing), HR 100 and JVP is not visible lying down Plasma Na+ 155 mmol/L (high), creatinine 80 umol/L (normal) Fluid and electrolyte status is most likely represented by: a) increased total body Na+ b) decreased ECV c) normal total body Na+ d) increased IC water content e) normal osmolality

Answer 25

b)

Question 26

You are on a camping trip in the Western Desert, when your 26 year old friend starts vomiting blood and continues to do so, while you drive to the nearest cattle station On arrival your friend looks pale and has a weak, rapid pulse While they call the air ambulance, you find they have an IV giving set and 2 sorts of IV fluid You decide to give him some IV fluid. Which of the following would you choose: a) 5% dextrose b) 150 mmolar NaCl

Answer 26

b)

Question 27

You are on a camping trip in the Western Desert, when your 26 year old friend starts vomiting blood and continues to do so, while you drive to the nearest cattle station On arrival your friend looks pale and has a weak, rapid pulse While they call the air ambulance, you find they have an IV giving set and 2 sorts of IV fluid How much fluid would you give him, while waiting for the air ambulance (ETA 2/24): a) 1L over 2hrs, reassessing every 15 mins b) 2L over 2hrs, reassessing every 30 mins c) 1L as rapidly as possible, then reassess

Answer 27

c) Pros: rapid resuscitation and prevention of cardiac arrest Cons: can precipitate HF if pre-existing CVD or cause APO

Question 28

List 3 causes of high plasma osmolality/sodium

Answer 28

Water intake insufficient due to: 1) Obligatory water loss (urine, sweat) 2) Access to water 3) Increased loss (diabetes insipidus, nephrogenic diabetes insipidus, osmotic diuresis)

Question 29

List 4 causes of low plasma osmolality/sodium

Answer 29

Water intake > ability to excrete (normal 20 L/day) reduced with: 1) Increased ADH (siADH, baroreceptor stimulation, lack of glucocorticoids) 2) Decreased distal tubule flow (decreased renal perfusion) 3) Other tubular factors limiting free water excretion 4) Drugs (e.g. diuretics, carbamazepine, antidepressants esp SSRIs), with variable contributions from above factors

Question 30

Why do changes in ECV usually occur without a change in plasma sodium concentration?

Answer 30

Because osmotic control frequently overrides volume control

Question 31

Do changes in ECV and blood always correlate?

Answer 31

Yes, unless there is hypoalbuminaemia or leaky capillaries

Question 32

What % of total body K+ does plasma K+ represent?

Answer 32

2%

Question 33

76 year old previously healthy woman presents for a check up and found to have HTN on repeated measurement (BP 170/100); she has been commenced on irbesartan BP now 155/90 but testing reveals: Plasma Na+ 138 mmol/L, K+ mmol/L (high), creatinine 150 umol/L (high) BSL, bicarb, other electrolytes normal ICV, ECV, Na+, K+?

Answer 33

``` ARB results in decreased eGFR, results in increased K+ ICV normal ECV normal Na+ normal/low K+ high ```

Question 34

76 year old previously healthy woman presents for a check up and found to have HTN on repeated measurement (BP 170/100); she has been commenced on irbesartan BP now 155/90 but testing reveals: Plasma Na+ 138 mmol/L, K+ mmol/L (high), creatinine 150 umol/L (high) HCT (Avopro) was added and BP now 125/70, but complains of bouts of dizziness on standing up Now plasma creatinine 160 umol/L (high), K+ 5.3 mmol/L (normal) and Na+ 125 mmol/L (low) ICV, ECV, Na+, K+?

Answer 34

ICV high? ECV normal? low? Na+ low K+ normal Would get rid of the thiazide and never prescribe again (has failed these)

Question 35

What are the complications of high ECV and low ECV?

Answer 35

High ECV: pulmonary oedema, mortality | Low ECV: poor perfusion (brain, kidney, etc)

Question 36

What are the complications of high vs low osmolality?

Answer 36

High: improvable mental function Low: improvable mental function, decreased falls (achieve plasma Na+ >135)

Question 37

Complications of high or low K+

Answer 37

Cardiac arrhythmia | Muscle weakness

Question 38

Principles of osmolality and plasma Na+/K+

Answer 38

Plasma Na+ does not tell you body sodium content, Hx and CV examination do (and ECV) Intracellular osmols are relatively constant and there are no osmolar gradients, so plasma osmolality (or sodium concentration) reflects ICV (inversely) An abnormal plasma K+ can reflect a shift or abnormal content

Question 39

23 year old insulin-dependent diabetic develops severe glandular fever, with fever, anorexia and a sore throat; presents feeling awful O/E: looks unwell, BP 110/60, HR 110, RR 24 Ix: BSL 20, pH 7.25, pCO2 24, plasma K+ 6.5 mmol/L, Na+ 140 mmol/L, HCO3- 10 ICV, ECV, Na+, K+?

Answer 39

ICV normal ECV low Na+ normal K+ low (due to N+V, diarrhoea seen in ketoacidosis; in Mx, insulin drives K+ into cells, fluids drives ketones into cells, UO increases and more K+ is lost)

Question 40

What does an abnormal plasma K+ reflect?

Answer 40

Shift between intra- and extra-cellular space or deficiency or excess If there is no evidence of shift, plasma K+ reflects total body K+

Question 41

Give 4 examples of agents which shifts K+ into cells

Answer 41

``` Insulin B agonists Aldosterone Alkalosis NB Their absence or opposites shift K+ out of cells ```

Question 42

What K+ level is of concern in a stable patient?

Answer 42

Less than 3.0 | >6.0