Question 26.1

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.

[Click here to toggle visibility of the answers]

College Answer

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

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

Discussion

Let us dissect these results systematically.

The A-a gradient is high:
PAO₂ = (0.6 x 713) - (45 x 1.25) = 371.55
Thus, A-a = 292.55mmHg
There is alkalaemia
The PaCO₂ is compensatory (increased, though still within the normal range)
The SBE is 12, suggesting a metabolic alkalosis
The respiratory compensation is adequate - the expected PaCO₂(36× 0.7) + 20 = 45.2mmHg
The anion gap is normal:
(138) - (97 + 36) = 5, or 10 when calculated with potassium
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

[Submit a comment or correction]