18. IV fluids and Hyponatraemia

Question 1

What are the steps to prescribing IV fluids

Answer 1

1. Assess volume status: euvolaemic, dehydrated or fluid overload.
2. Is drinking enough, enteral feeding or already fluid overloaded- then no. If not drinking, lost or ongoing fluid loss then YES
3. Choose an IV fluid based on fluid status, serum Na and urine osmolality- generally isotonic

Question 2

How is volume status assessed/ signs

Answer 2

Dehydrated: thirsty, excessive vomiting, diarrhoea, quick weight loss, low BP, decreased JVP-2, decreased tissue turgor when young.

Fluid overload: weight gain, high JVP +2 , swelling in ankles, eyes, breathless

Question 3

What are the reasons for needing IV fluid and the differences between them in application

Answer 3

1. Needed for Maintenance of fluid loss through breathing, skin, excretion- small amount for longer- 2/3 L/day
2. Replacement of losses- eg after operation
3. Rescuscitation: lots of bags, quickly, continue monitoring.

Question 4

What are the three types of IV fluid, eg, what do and when used

Answer 4

Hypotonic eg. 5% dextrose: moves water from ECF into ICF. Used for cardiac care when high Na+ and HF risk.

Hypertonic: eg. 3% saline, moves water from ICF to ECF (not v used)

Isotonic: eg. 0.9% sod-chlr, plasmalyte. keep same; just add ECF volume.

Question 5

What is hyponatraemia, causes

Answer 5

<120 mmol/L serum Na+ mostly due to
-Excess water: Cirrhosis, HF, Nephrotic syndrome- hypertension. SIADH and polydipsia. However some ppl still present as euvolemic

-Sodium Loss:
Vomiting, diarrhoea, Diuretics
hypo-aldosteronism : Less Na aborbed- lost in urine (along with water) using Na/K xchanger so Hyperkalaemia, hyperpigmentation is expected.
-Pseudohyponatraemia: lab anomaly for normal Na+ caused by hypertriglyceridemia or hyperproteinemia.

Question 6

What is the expected K+ and Na+ in ICF vs ECF

Answer 6

Na+ is the dominant cation in the ECF which is the driver of serum osmolality while K+ is dominant in ICF with concentrations of the other low viceversa.

Question 7

How is Hyponatraemia cause narrowed down so that it can be treated by addressing the underlying cause

Warning no rapid correction - can lead to osmotic demyelination, quadraparesis, loc.

Answer 7

1. History: vomiting, diarrhoea, dehydration, and medication
   - if due to Na+ loss: saline replacement
2. Examination fluid status and signs of addisons disease
3. Check that serum osmolality and Na+ match- if normal could be pseudohyponatremia bc osmolality not affected by hyp3glyc or hypprotein.
4. If presenting as excess fluid then look at history for causes- treat with fluid restriction
   If presenting as Euvolaemia could be
   -SIADH - more ADH despite not dry or hypotensive
   -Polydipsia: low urine osmolarity
   -Over hydration with low Na IV fluids- eg. dextrose - hypotonic fluids
   -Diuretics: Thiazide, causes Na+ loss but also activates baro-receptors and osmoreceptors to increase ADH- shifting the ADH curve lower set point so ADH released in high amounts at even lower osmolarities