# Question 20.3

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)

a)

Metabolic alkalosis

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: