Challenging Cases in Nephrology

Question 1

Hydrochlorothiazide

Answer 1

Blocks sodium chloride co-transporters in DCT
Counterregulatory mechanism - reduces preload and cardiac output, increased sympathetic and RAAS activity, resultant in increased resorption in proximal sites
- thus does not overtly dehydrated or go into shock

Adverse effect: contraction alkalosis, hypokalaemia

Question 2

Potassium salt substitute (75% NaCl 25% KCl)
- Neal B, NEJM 2021 (China study)
- Dog study 1972

Answer 2

• Reduction in stroke, CVS event, all cause mortality
• Systolic BP lower than on sodium chloride salt
• Similar and low risk of hyperkalaemia
• Potassium salt reduces hypertensive changes in dogs

Question 3

Potassium salt substitution - aldosterone paradox

Answer 3

Volume depletion
- NaCl retention with minimal K secretion

In low K diet
- Drives K and Cl loss from DCT, removing Cl inactivation of WNK4 and augmenting NCC activitiy
- Low Na excretion

Hyperkalaemia
- Stimulates DCT via bradykinin and other systems
- Waste K, high Na excretion (thus without Na retention)

Question 4

Congestive heart failure with hyponatraemia, hypokalaemia, alkalosis

Answer 4

Inadequate CO activates baroreceptors and effector mechanisms
- Sympathetic outflow, RAAS -> ADH, thirst, salt hunger
> Increased HR, contractility
> Constrict peripheral vasculature to improve venous return, arteriolar resistance
> Renal retention of salt and water

Question 5

Different crystalloids on dehydrated patients
How much is enough?

Answer 5

Does not matter
No benefit for non-critically ill
1% benefit for balanced crystalloids in critically ill

Give enough to fix haemodynamics and restore adequate preload
Can check urine sodium and FeNa - improve in 1-2% indicates adequacy