Question 6.1

The following arterial blood gas was taken from a female hospitalised for recurrent urinary tract infections. She was transferred to the ICU because of nosocomial pneumonia.



Normal Adult Range

Barometric Pressure

760 mmHg (100kPa)





7.35 – 7.45


31 mmHg (4 kPa)

35– 45 (4.6 – 5.9)


83 mmHg (11 kPa)


25 mmol/L

24– 32

Standard Base Excess*

3.3 mmol/L

-2.0 – +2.0

  • Comment on the acid-base status.
  • List two likely causes of the acid-base abnormality in this patient.

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College Answer

6.1 a)

Mixed respiratory and metabolic alkalosis

6.1 b)

Respiratory alkalosis from the hyperventilation due to the pneumonia.

Metabolic alkalosis from vomiting or diuretic use.


Let us dissect these results systematically.

  1. The A-a gradient is high: 
    PAO2 = (0.3 × 713) - (31 × 1.25) = 175
    Thus, A-a = ( 175 - 83) = 92mmHg.
  2. There is alkalaemia
  3. The PaCO2 is contributing to the alkalosis
  4. The SBE is 3.3, suggesting a mild metabolic alkalosis
  5. There is no respiratory compensation.This disorder ends up a mixed metabolic and respiratory alkalosis whichever way you calculate it.
    The expected PaCO2 is (0.7 × 25) + 20 = 37.5mmHg, and thus there is also a respiratory alkalosis. 
    If you were to assume that the respiratory disorder is acute and the primary cause of the problem, you would calculate a 2mmol/L decrease in HCO3- for every 10mmHg drop in PaCO2. That would suggest that the expected HCO3- for this patient would be somewhere in the vicinity of 22mmol/L.

When offering a "likely diagnosis" to explain this gas, one can resort to mix-and-matching any cause of metabolic alkalosis (eg.vomiting, diuretic use) with any cause of respiratory alkalosis (eg. anxiety, pain, etc). For all we know, she maybe prone to panic attacks and has been binge-eating licorice.


McCurdy, Donna Kern. "Mixed metabolic and respiratory acid-base disturbances: diagnosis and treatment." CHEST Journal 62.2_Supplement (1972): 35S-44S.

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

Barker, E. S., et al. "The renal response in man to acute experimental respiratory alkalosis and acidosis." Journal of Clinical Investigation 36.4 (1957): 515.