Question 26.1

Created on Tue, 05/12/2015 - 17:53
Last updated on Wed, 07/15/2015 - 15:23
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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.

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College Answer

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