# Question 26.1

Created on Tue, 05/12/2015 - 17:53
Last updated on Wed, 07/15/2015 - 15:23
Pass rate: ?
Highest mark: ?

## Other SAQs on this topic

The following results are from the arterial blood gas analysis of a 46-year-old male ventilated in ICU for three weeks with severe community-acquired pneumonia and ARDS:

 Parameter Patient Value Normal Adult Range FiO2 0.6 pH 7.5* 7.35 – 7.45 PO2 79.0 mmHg (10.5 kPa) PCO2 45.0 mmHg (6.0 kPa) 35 – 45 (4.6 – 6.0) Bicarbonate 36 mmol/L* 22 – 27 Base Excess 12 mmol/L* -2.0 – +2.0 Sodium 138 mmol/L 135 – 145 Potassium 5.0 mmol/L 3.5 – 5.0 Chloride 97 mmol/L 95 – 105

a) Describe the abnormalities.

b) Give one likely cause.

a)

Metabolic alkalosis (PCO2 appropriate using 0.7 x [HCO3] + 20 +/- 5) A-a DO2 = 295 (P/F 130 “moderate” ARDS)

b)

• Resolution of primary respiratory acidosis with delayed correction of metabolic compensation
• Diuretic therapy

## Discussion

Let us dissect these results systematically.

1. The A-a gradient is high:
PAO2 = (0.6 x 713) - (45 x 1.25) = 371.55
Thus, A-a = 292.55mmHg
2. There is alkalaemia
3. The PaCO2 is compensatory (increased, though still within the normal range)
4. The SBE is 12, suggesting a metabolic alkalosis
5. The respiratory compensation is adequate - the expected PaCO2(36× 0.7) + 20 = 45.2mmHg
6. The anion gap is normal:
(138 + 5.0) - (97 + 36) = 10
7. The urinary electrolytes and pH are irrelevant.

Thus, this patient has a metabolic alkalosis with respiratory compensation.

This is either a recovery from chronic respiratory acidosis, or the evidence of loop diuretic treatment.

Either is equally likely given this ARDS story.

## References

Khanna, Apurv, and Neil A. Kurtzman. "Metabolic alkalosis." J NEPHROL 2006; 19 (suppl 9): S86-S96