Pathophysiology
P-II-20. Acid-base disorder, Case 3
酸塩基平衡障害 症例3
Case Presentation
A 29-year-old man presents for a routine check-up. He has a history of type I diabetes mellitus.
Physical examination: Male, well-developed, appears healthy.
Vital signs:
- Blood pressure 110/70 mmHg
- Pulse 76/min
- Respirations 26/min
- No fever
Examination findings:
- Heart/lung: no abnormalities
- No cyanosis or edema
Laboratory Values
Arterial blood gas:
- pH 7.33
- PCO₂ 30 mmHg
- PO₂ 105 mmHg
- Bicarbonate 14 mmol/l
Chemistry (value / normal range):
- Sodium 135 (135–146 mmol/l)
- Potassium 6.4 (3.5–5.1 mmol/l)
- Chloride 112 (98–107 mmol/l)
- Calcium 2.4 (2.2–2.65 mmol/l)
- BUN 8.5 (2.8–7.2 mmol/l)
- Creatinine 90 (45–84 µmol/l)
- Glucose 6 (3.9–5.8 mmol/l)
Urine: pH 5.5
Key Quotes & What They Tell Us
| Quote / Value | Interpretation |
|---|---|
| pH 7.33; HCO₃ 14 mmol/l (low) | Acidaemia with low bicarbonate → metabolic acidosis |
| PCO₂ 30 mmHg (low) | Appropriate respiratory compensation |
| Anion gap = 135 − (112 + 14) = 9; chloride 112 (high) | Normal anion gap, hyperchloraemic metabolic acidosis |
| Potassium 6.4 mmol/l (high) | Hyperkalaemia accompanying the acidosis → points to type 4 RTA |
| Type 1 diabetes mellitus; glucose normal (6) | Diabetes is the classic cause of hyporeninaemic hypoaldosteronism; normal glucose excludes ketoacidosis |
| Urine pH 5.5 | Kidney can still acidify urine (pH < 5.5) → distinguishes type 4 from type 1 (distal) RTA |
| BUN 8.5, creatinine 90 (mildly raised) | Mild renal impairment consistent with diabetic kidney involvement |
Key Points
- Diagnosis: Type 4 renal tubular acidosis (hyperkalaemic, hyperchloraemic, normal anion gap metabolic acidosis) in a diabetic.
- Compensation: Low PCO2 shows appropriate respiratory compensation.
- Mechanism: Hyporeninaemic hypoaldosteronism (common in diabetes) → reduced aldosterone effect → impaired H⁺ and K⁺ excretion → acidosis with hyperkalaemia.
- Key discriminator: Hyperkalaemia with a normal anion gap, and an acid urine pH, separates type 4 RTA from type 1 (distal) RTA.
- Contrast: Unlike DKA, the anion gap is normal and glucose is not elevated.
一問一答
▶How do you calculate the anion gap in this patient, and what is it?
AG = Na − (Cl + HCO3) = 135 − (112 + 14) = 9, which is normal.
▶What is the underlying mechanism of type 4 renal tubular acidosis?
Hyporeninaemic hypoaldosteronism (common in diabetes) reduces aldosterone effect, impairing renal H+ and K+ excretion.
▶Why does a normal glucose (6 mmol/L) help exclude diabetic ketoacidosis here?
DKA requires hyperglycaemia and ketosis; a normal glucose with a normal anion gap rules it out.
▶What is the acid-base diagnosis in a type 1 diabetic with normal-gap metabolic acidosis, hyperkalaemia, and an acid urine pH?
Type 4 renal tubular acidosis (hyperkalaemic, hyperchloraemic, normal anion gap metabolic acidosis).
▶What distinguishes type 4 RTA from type 1 (distal) RTA?
Type 4 has hyperkalaemia and the ability to acidify urine (pH <5.5), whereas type 1 has hypokalaemia and an inappropriately high urine pH.
▶What does the low PCO2 (30 mmHg) indicate in this metabolic acidosis?
Appropriate respiratory compensation through hyperventilation.
▶Why does aldosterone deficiency cause both acidosis and hyperkalaemia?
Reduced aldosterone impairs renal excretion of both H+ (causing acidosis) and K+ (causing hyperkalaemia).
▶What is the normal role of aldosterone in the distal nephron?
It promotes sodium reabsorption and the excretion of potassium and hydrogen ions.
▶Why is hyperkalaemia of 6.4 mmol/L clinically dangerous?
It can cause cardiac arrhythmias and characteristic ECG changes (e.g. peaked T waves).
▶How does type 4 RTA differ from type 2 (proximal) RTA?
Type 2 RTA is from impaired proximal bicarbonate reabsorption (usually hypokalaemic); type 4 is from aldosterone deficiency/resistance with hyperkalaemia.
▶Why is diabetes a classic cause of type 4 RTA?
Diabetic kidney damage causes hyporeninaemic hypoaldosteronism, reducing aldosterone action.
▶What does an acid urine pH (5.5) tell you about distal tubular function?
The distal tubule can still secrete H+ to acidify the urine, which excludes distal (type 1) RTA.
▶What treatments are used in type 4 RTA?
Dietary potassium restriction, loop diuretics, and sometimes fludrocortisone (mineralocorticoid replacement).
▶Why is the chloride elevated (112) in type 4 RTA?
It is a hyperchloraemic (normal anion gap) acidosis — chloride is retained as bicarbonate is lost.
▶Why does the respiratory rate rise to 26/min in this patient?
To compensate for the metabolic acidosis by blowing off CO2.
▶What do the mildly raised BUN and creatinine suggest in this diabetic?
Mild renal impairment consistent with early diabetic kidney involvement.
▶What medications can precipitate or worsen type 4 RTA?
ACE inhibitors/ARBs, potassium-sparing diuretics, NSAIDs, and heparin (all reduce aldosterone action or potassium excretion).
▶Why does hyperkalaemia itself worsen the acidosis in type 4 RTA?
High potassium impairs renal ammonia (ammonium) production, reducing the kidney's capacity to excrete acid.
▶How do the three main types of RTA differ in serum potassium?
Type 1 (distal) and type 2 (proximal) are typically hypokalaemic, whereas type 4 is hyperkalaemic.
▶Why is the patient asymptomatic despite the metabolic abnormalities?
The acidosis is mild and chronic with effective compensation, so it is often detected incidentally.