Fluid Status

Question 1

Fluid Basics:

Total body weight components?

Different fluid compartments?

Answer 1

•	Total body weight = 70kg
o	40% mass = tissues = 28kg
o	60% mass = water = 42kg = 42L
	2/3 = intracellular = 28L
	1/3 = extracellular = 14L
•	Interstitial = 10L
•	Plasma = 3L
•	Transcellular (i.e. cerebrospinal, synovial, peritoneal, pleural fluids) = 1L

• ECF-ICF  Exchange occurs due to OSMOTIC (electrolytes) pressures

• ISF-plasma  Exchange occurs due to STARLING FORCES/GIBBS-DONNAN EFFECT
o Hydrostatic pressure
o Oncotic pressure

Question 2

Reasons for IV fluid administration?

Answer 2

o Resus  Intravascular or extra-cellular deficit
o Replacement  Losses
o Maintenance  Supply daily needs

Question 3

Types of fluids?

Answer 3

• Crystalloids

o	Dextrose (5, 10, 20, 50%)  ALL hypotonic once dextrose is metabolised
	5% (hypotonic)  Distributes equally in whole body (Plasma = 3/42 of whole body water = 70mL stays in vasculature)

o	Saline (0.5, 0.9, 3%)
	0.9% (isotonic): only 1/5 stays in blood
•	Distributes evenly in ECF (Plasma = 3/14 of ECF = 214mL stays in vasculature)

o Hartmann’s (NB: Slightly hypotonic as opposed to normal saline)

• Colloids (risk of allergic reaction)  NO EVIDENCE FOR ANY OVER N-SALINE

o Gelatin-based:
 Gelofusine
 Haemaccel

o	Hydroxyethyl Starches:
	Pentastarch
	Tetrastarch
•	Voluven
•	Volulyte

o Dextran (high weight dextrose)

o Human albumin solution

o Blood

Question 4

Output of fluids?

Answer 4

•	Fluid Output
o	Urine output – 0.5-1mL/kg/hr   ~750mL/day for a 60kg person
o	Insensible losses: ~1L
	Feces – 500mL/day
	Sweat – 500mL/day

o Total = 1.5-2L
 If unwell = 3L (e.g. fever)

• Electrolyte output:
o Loss of 100mM SODIUM
o Loss of 60mM POTASSIUM

Question 5

Fluid requirements to maintain fluids + daily loses replacement?

Answer 5

Maintenance fluid – 30mL/kg/day
Na – 1-2mmoL/kg/day
K – 1mmoL/kg/day

• Replacing these losses daily: 3L fluid, 150mM Na+, 60mM K+, sugars
o N-saline (1L; 150mM Na+) + 20mM KCl
o 5% dextrose (1L; 50g dextrose) + 20mM KCl
o 5% dextrose (1L; 50g dextrose) + 20mM KCl

o One salt, two sweet, three bananas

Question 6

Hyponatraemia: figures, different types?

Answer 6

Hyponatremia (Na <135; Significant when Na <125 or if rapid fall by >20 in 24hr)
• Hypervolemia (edematous states  ankle swelling, SOB on lying, etc)

o Heart failure, Liver failure, Renal failure

o Inappropriate IV fluids (too much D5W)

o Psuedo-hyponatremia
 ↑Glucose

• Euvolemia (absence of hyper/hypo volemia symptoms)

o Urine Na >40
 SIADH (HIGH urine osmolality)
• Cause
o Organic  CNS (any), pulmonary (any), neoplastic disease
o Inorganic  Drugs (SSRI, TCA, Haloperidol, Carbamazepine)

 Hypothyroidism (unknown mechanism)

o Urine Na <40
 Psychogenic polydipsia/thirst (LOW urine osmolality)

o Drugs  Diuretics, ACE-I (can also be hypovolaemic)

• Hypovolemia (tachycardia, ↓BP, ↓cap refill, dizziness)

o	Urine Na <20  retaining sodium
	Dehydration of any cause
•	Vomiting + Diarrhoea
•	Small bowel obstruction
•	Trauma, burns

o Urine Na >20  wasting sodium (should retain when hypovolaemic!)
 Renal pathology (proximal tubule)
 Addison’s disease (low aldo states)
 Drugs  Diuretics, ACE-I

Question 7

Symptoms of Hyponatraemia?

Answer 7

•	Symptoms  Neurological
o	Confusion
o	Decreasing GCS
o	Seizures
o	Coma
o	Death (respiratory arrest

Question 8

Mx of hyponatraemia?

Answer 8

• Treatment: If Na <125 or if symptomatic; Do NOT increase Na by >10mmol/L in 24hr

• Stop hyponatraemic causing drugs in ALL cases
• The actual value of sodium does NOT matter – symptoms matter!

o Hypovolaemic: 0.9% Sodium Chloride (to replace sodium)

o Euvolaemic:
 Restrict fluids to 0.5-1L/day
 Consider demeclocycline if no response to fluid restriction

o	Hypervolaemic:
	Restrict fluids to 0.5-1L/day
	Restrict oral sodium intake
	Treat underlying disorder
	Diuretics as necessary

o Severe/Seizure, Coma: Do 2-4 hourly U+E
 3% hypertonic saline
• 500mL boluses then re-evaluate immediately – UK
 5% hypertonic saline
• 100mL boluses then re-evaluate immediately

Question 9

complications of Hyponatraemia?

Answer 9

• Complication: Rapid correction leads to CENTRAL PONTINE MYELINOLYSIS (AKA osmotic demyelination syndrome [as can occur outside of the pons also])
o Pathophysiology:
 Rapid sodium shifts leads to rapid water shifts
 Rapidly increasing sodium causes rapid sodium and fluid shift INTO cells
• NB: In hypernatraemia, cells SHRINK rapidly when lowering sodium
 Oedema of myelin sheaths mainly of PONS and other parts of CNS occurs
 Myelin destruction

o Features:
 Quadriplegia (flaccid to start, spasticity may develop later)
 Dysarthria + dysphagia
 Sometimes “locked-in syndrome”

o Ix: MRI

o Rx:
 Prevent:
• Slow sodium correction (max 8-12mmol/L in 24hr)

Question 10

Hypernatraemia: definitionl, causes, symptoms, complications?

Answer 10

• Hypernatremia (Na >145, clinically significant if >155 or rapid rise >20mmol/L in <24hr)
o Cause: DEHYDRATION

o Symptoms: Neurological (confusion, seizures, coma)

o Complication: Rapid correction leads to Central Pontine Myelinolysis
 Correct by no more than 8mmol/L in 24hr
 Will need regular bloods

Question 11

Hypernatraemia: Mx?

Answer 11

 Acute (hours) water loss: 5% Dextrose ONLY (sugar in free water)
 Chronic (days) water loss: Dex (5%)-Saline (0.45%) (hypotonic saline)

Question 12

Mechanisms of potassium movement?

Answer 12

o Insulin (drives 1Glucose + 1K into cell)

o 3Na out of cell/2K in to cell ATPase (stimulated by beta-agonists; blocked by digoxin)

o Acid-Base balance (hydrogen exchanges for potassium)

o Rapid tissue destruction (95% of K is intracellular); Rapid tissue generation (B12/folate)

o Aldosterone diseases
 Na retention, K depletion if high [Conn’s]
 Na depletion, K retention if low [Addison’s]

o Magnesium depletion (Mg is necessary for K renal reabsorption)

Question 13

Mechanisms of hyperkalaemia?

Answer 13

Hyperkalemia (cellular shift or decreased excretion) Hypokalemia
↓Insulin ↑Insulin
Drugs: Beta-Blocker, Digoxin, NSAIDs, spironolactone, ACE-I Drugs: Beta-Agonist, Diuretics
Acidosis Alkalosis
Lysis (Tumour, Hemo, Rhabdo) B12/Folate replacement
Addison’s (↓Aldo) Conn’s + Cushing’s (↑Aldo)
Pseudohypokalemia
*cells lyse in tube or tourniquet on too long ↓Mg
Renal failure Vomiting, Diarrhoea

Question 14

Mechanisms of Hypokalaemia?

Answer 14

Hyperkalemia (cellular shift or decreased excretion) Hypokalemia
↓Insulin ↑Insulin
Drugs: Beta-Blocker, Digoxin, NSAIDs, spironolactone, ACE-I Drugs: Beta-Agonist, Diuretics
Acidosis Alkalosis
Lysis (Tumour, Hemo, Rhabdo) B12/Folate replacement
Addison’s (↓Aldo) Conn’s + Cushing’s (↑Aldo)
Pseudohypokalemia
*cells lyse in tube or tourniquet on too long ↓Mg
Renal failure Vomiting, Diarrhoea

Question 15

Presentation of hyperkalamia & Hypokalaemia?

Answer 15

• Presentation:
o Hyperkalemia
 Muscles and heart (weakness and arrhythmia [e.g. VF, asystole])

o Hypokalemia
 Muscles and heart (weakness and arrhythmia)

Question 16

Ix of hyper/hypokalaemia?

Answer 16

•	Investigation: U+E, ECG (telemetry if very high or low, or replacing large amounts)
o	Hyperkalemia (flat P, Wide QRS, Slurred ST, tented/peaked t-wave)

o Hypokalemia (t-wave flattening/inversion, U-wave, prolonged PR, prolonged QT, ST dep)

Question 17

Mx of hyperkalaemia?

Answer 17

• Treat: Continuous ECG monitoring is needed for hyperkalaemia, and also for hypokalaemia if >20mmol K+ is being infused rapidly IV – you MUST document the need for this in the notes
o Hyperkalemia  Stop all K+ retaining drugs
 >6.5, ECG changes OR marked muscle weakness: Ca Gluconate (10mL of 10% via slow IV [max 2mL/min])

 Shift potassium intracellulary:
• Glucose + insulin/novorapid
o 10u in 50mL of 50% dextrose in 30min
o Takes 30min to work (hence need for calcium gluconate first)

• Salbutamol 2.5mg 6 hourly (nebulised)
• 1.26% Sodium Bicarbonate

 Excrete potassium:
• IV saline

• Ca polystyrene sulfonate (Resonium) 15mg (PO or PR) TDS
o Takes up to 6hr to work

• Lactulose (10-20mL BD)
• Loop diuretics (Furosemide)
• Dialysis

 Notes:
• If patient has high potassium but does not produce urine OR is fluid overloaded, hard to manage hyperkalaemia without dialysis as:
o Treatments will cause fluid overload (e.g. dextrose, bicarbonate)
o Potassium will only shift intracellular, and since not excreted by kidneys will eventually leech out and rebound the hyperkalaemia

Question 18

Mx of hypokalaem ia?

Answer 18

• Hypokalemia
o Correct underlying cause
o Repletion of K+ (oral or IV)  HypoMg often co-exists, MUST correct this first!
 K+ 2.5-3.5: Oral K+ supplements

 K+ <2.5, intolerant of oral K, or symptomatic: IV K+
• In most cases 40mmol/L is max in 1L
• In emergency can up this to 80mmol/L (higher = risk of thrombophlebitis; but can do bit higher in central line)
• Do NOT infuse at higher rate than 20mmol/hr (i.e. 40mmol in 1L = 500mL max in one hour)  Risk of fatal arrhythmia

 K+ <2.2 or ECG changes: Contact ICU for central K+ therapy