The following arterial blood gas result was obtained from a 65-year-old lady with exacerbation of chronic obstructive pulmonary disease (COPD), day 7 in ICU following intubation and ventilation for respiratory failure.

Parameter

Patient Value

Normal Adult Range

FiO2

0.3

pH

7.48*

7.35

– 7.45

PCO2

42 mmHg (5.5 kPa)

35

45 (4.6 – 6.0)

PO2

104 mmHg (13.7 kPa)

Total haemoglobin

122 g/L

115 – 165

SpO2

98%

95

100

Base Excess

7.0 mmol/L*

-3.0 – +3.0

Bicarbonate

31 mmol/L

22

32

a)    Interpret the arterial blood gas.    (10% marks)

b)    Give four possible reasons for the acid-base disturbance seen.    (10% marks)

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College Answer

a)

Metabolic alkalosis

Raised A-a gradient

b)

Diuretics

Steroids

NG losses

Post hypercapnia

Discussion

A systematic approach to this problem would resemble the following:

  1. The A-a gradient is high:
    The alveolar oxygen tension is (0.3 × 713) - (42 × 1.25) = 161.4
    thus the A-a gradient is 57.4
  2. There is alkalaemia.
  3. The PaCO2 is on the high end of normal, which is a move in the appropriate direction given the presence of alkalaemia
  4. The SBE is 7.0 mmol/L, suggesting a mild metabolic alkalosis
    If the SBE was not available, the bicarbonate of 31 would have been enough to call this a metabolic alkalosis
  5. The respiratory compensation is adequate.
    According to the Boston rules, the expected PaCO2 is (31 × 0.7) + 20 = 41.7mmHg, and so there is totally appropriate respiratory compensation.
    With the Copenhagen rules, we can expect a PaCO2 of 44.2mmHg, which is close enough.
  6. The anion gap cannot be calculated, and it is in any case irrelevant.

In summary:

  • Raised A-a gradient
  • Metabolic alkalosis

Why would this be? One may refer to the list of causes for metabolic alkalosis. In a ventilated COPD patient with a normal-looking PaCO2 one could very easily point the blame at a posthypercapneic state.In fact, that is the only thing you can conclude from the bare minimum history the college has given. However, the question asks for four possible reasons. The college model answer offers diuretics, steroids and NG losses as potential contributors. Because of how little history is given, rhese differentials are at least as likely as the use of β-lactam antibiotics, Liddle's syndrome or clay ingestion.

References

References

Banga, Amit, and G. C. Khilnani. "Post-hypercapnic alkalosis is associated with ventilator dependence and increased ICU stay." COPD: Journal of Chronic Obstructive Pulmonary Disease 6.6 (2009): 437-440.