Question 20.3

Created on Tue, 06/06/2017 - 04:16
Last updated on Sat, 08/12/2017 - 16:10
Pass rate: 72.5%
Highest mark: 7.3

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The following arterial blood gas result was obtained from a 70-year-old female with type 2 diabetes, presenting with acute exacerbation of asthma.

Parameter

Measured Value

Adult Normal Range

Fi02

0.21

pH

7.21'

7.35 - 7.45

PaCO2

60 mmHg (8.0 kPa)*

35 -45 (4.6 - 6.0)

PaO2

55 mmHg (7 kPa)

Bicarbonate

23 mmol/L

22 - 27

Base Excess

-4 mmol/L*

-2 - +2

Sodium

135 mmol/L

135 - 145

Potassium

5.3 mmol/L*

3.5 - 5.0

Chloride

100 mmol/L

100 - 110

Glucose

9.2 mmol/L*

3.5 - 6.0

Urea

8.3 mmol/L*

3.5 -7.2

Creatinine

120 umol/L*

50 - 100

Lactate

4.8 mmol/L*

< 2.0

HbA1c

11.0 mmol/mol*

50 -60

Describe the abnormalities in the above results, giving likely explanations             (30% marks)

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College answer

  • Normal A-a gradient (20) for 70 year old – hypoventilation
  • Respiratory acidosis – acute type 2 respiratory failure tiring from acute asthmatic attack
  • Normal anion gap metabolic acidosis  – underlying type IV RTA secondary to diabetic  nephropathy (or any reasonable cause)
  • Hyperlactataemia – beta-2 agonist induced
  • Hyperkalaemia and raised creatinine- renal impairment
  • Poor diabetic control (high BSL and HbA1C) 

Discussion

Let us dissect this systematically;

  1. The patient is hypoxic (PaO2 of 55 mmHg) but the A-a gradient is minimally raised (~ 19) which suggests that alveolar hypoventilation is the main cause of the hypoxia.
  2. There is acidaemia
  3. The CO2 is contributing to the acidaemia
  4. The SBE is -4, suggesting that there is also a mild metabolic acidosis
  5. The anion gap is (135+5.3 - 100 - 23) = 17.3; assuming the albumin is normal we would expect an anion gap of 12, which means it has risen by 5.3. The drop in bicarbonate is smaller, which suggests that this is a high anion gap metabolic acidosis in the presence of a pre-existing metabolic alkalosis. Judging by the SBE
    Interestingly, if you omit the use of potassium in the anion gap equation, you get an anion gap of 12, i.e. a normal anion gap which would agree with the college answer. This demonstrates that this alternative method is a valid shortcut for CICM exam work.
  6.  The urea and creatinine are slightly raised, reflecting the possibility that in this asthma episode the patient has been too breathless to eat and drink normally.
  7. The lactate is raised, suggesting that salbutamol is being administered (or else, that hypovolaemia is quite severe)
  8. The glucose is slightly elevated and the HbA1c is raised, suggesting that this patient is a diabetic and their attention to their BSl control is patchy and haphazard.

Thus, there is a respiratory acidosis and mild metabolic acidosis with hypoxia due to alveolar hypoventilation.

References