Weakness? Look at the gas…

Labs and Lytes 031

Author: Xiuxian Pham
Peer Reviewer: Chris Nickson

A 24 year-old woman presents to the emergency department (ED) after waking with generalised weakness. She has no past medical history, no significant family history and has not taken any new medications.

On examination:

  • Vitals: RR 17/min, SaO2 100% RA, BP 110/65 mmHg, HR 80/min, temp 36C
  • Global weakness, including 3/5power in all limb muscle groups
  • No other altered neurology (sensation, reflexes NAD)
  • Remainder of examination unremarkable

A venous blood gas (VBG) is taken in ED:

pH 7.14
pCO2 (mmHg) 40
pO2 (mmHg) 20
Bicarbonate (mmol/L) 13
Hb (g/L) 157
Na (mmol/L) 141
K (mmol/L) 1.4
Cl (mmol/L) 116
Glucose (mmol/L) 7.0
Lactate (mmol/L) 1.2
iCa (mmol/L) 1.20

Assume that albumin is normal (40 g/L)

Q1. What the key findings on the VBG?

Key findings:

  • Severe hypokalaemia (likely cause of weakness)
  • Mild hyperchloraemia
  • Normal anion gap metabolic acidosis (NAGMA)
    • Anion gap = Na – (Cl + HCO3)
    • 141 – (116 + 13) = 12 mmol/L (8-12 mmol/L is normal)
    • Remember that anion gap needs to be corrected in the presence of hypoalbuminaemia: normal AG = 0.2 x [albumin] (g/L) + 1.5 x [phosphate] (mmol/L)
  • Co-existent respiratory acidosis (may be due to global weakness)
    • PCO2 = 40 mmHg
    • With complete respiratory compensation, according to Winter’s formula, expected pCO2 = 1.5 (Actual [HCO3]) + 8 mmHg = (1.5 x 13) + 8 = 27.5 mmHg
    • Caveat: PvCO2 and PaCO2 do not strictly correlate, and PvCO2 may be higher

Q2. What urine investigation(s) may help identify the underlying cause?

Urinary pH and electrolytes to allow calculation of the urinary anion gap, which helps distinguish between renal and gastrointestinal causes of hyperchloraemic metabolic acidosis.

The results for this patient are:

Urine pH and electrolytes
pH 7
Na (mmol/L) 20
K (mmol/L) 18
Cl (mmol/L) 35
Ca (mmol/L) 1.46

Q3. What your interpretation of the findings in Q2?

The findings are:

  • Abnormal urinary pH – in an acidotic state, the urine should acidify in an attempt to remove H+
  • Urinary anion gap:
    • Urinary Na + K – Cl
    • 20 + 18 – 35 = +3
    • Positive urinary anion gap
  • A positive urinary anion gap means that there is a lack of production of NH4+
    • H+ is excreted via NH4+ in the urine in response to acidaemia
    • NH4+ is an unmeasured cation
    • In a non-renal cause of acidosis, there would be a negative anion gap due to the presence of NH4+

In summary, the lack of an acidotic urine and a positive urinary anion gap is suggestive of a renal cause of the normal anion gap metabolic acidosis.

Q4. What is the likely diagnosis?

Type 1 renal tubular acidosis (RTA)

There are 3 main types of RTA:

  • Type 1: unable to excrete H+ from distal defect
  • Type 2: failure of proximal tubular absorption of HCO3-
  • Type 4: aldosterone deficiency or impairment leading to distal tubule resistance to aldosterone

In proximal RTA (i.e. type 2) the production of NH4+ (i.e. the excretion of H+) is maintained as this occurs in the collecting ducts of the renal tubule

In distal RTA (i.e. types 1 and 4) the loss of H+ excretion leads to a positive urinary anion gap.

The presence of hypokalaemia suggests Type 1 RTA, rather than Type 4.

  • Inability to excrete H+ results in excretion of K+ to compensate for an electronegative lumen in Type 1 RTA
  • Type 4 RTA presents with hyperkalaemia

Q5. What is the primary treatment for this patient?

Bicarbonate, which corrects metabolic acidosis and hypokalaemia

  • In particular, potassium citrate provides both potassium and the citrate will convert to bicarbonate via hepatic metabolism, as well as citrate in the urine inhibits stone formation (patients with type 1 RTA are often hypocitraturic and can present with multiple renal calculi)
  • However, in this case, given the patient presented with hypokalaemic paralysis, the priority is to replace potassium first instead of correcting the metabolic acidosis

Q6. What are common underlying causes of this condition?

Primary (paediatrics):

  • genetic mutation in transporters

Secondary:

  • autoimmune (Sjogren’s, SLE, RA)
  • calcium disorders
  • thyroid disorders
  • drugs (amphotericin B, lithium, NSAIDs)

In an adult presenting with Type 1 RTA, investigations must workup an underlying secondary cause.

All case-based scenarios on INTENSIVE are fictional. They may include realistic non-identifiable clinical data and are derived from learning points taken from clinical practice. Clinical details are not those of any particular person; they are created to add educational value to the scenarios.

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