Mon22November0200PM 2

Analysis of a sample of arterial blood provides the following data:

pH 7.5

PaCO2 7 kPa (52 mmHg)

Base excess +9.5 mmol/l-1

PaO2 12 kPa (90 mmHg)

The blood could be from a patient who:

True / False

Has had pyloric obstruction for several days Correct

Was given 500 mmol 8.4% bicarbonate after resuscitation from cardiac arrest Incorrect answer selected

Is receiving large doses of steroids in the treatment of severe asthma Correct

Has been ventilated for 36 hours with an excessive added dead space Correct

Has acute respiratory failure Correct

Explanation

The patient from whom the blood is sampled has an alkalosis. The PaCO2 is slightly elevated. The causes of a metabolic alkalosis include:

Increased alkali in the extracellular fluid:

Exogenous alkali - sodium bicarbonate infusion

Endogenous alkali - from metabolism of ketoanions

Loss of potassium from the extracellular fluid:

From the gut - prolonged vomiting of pyloric obstruction or nasogastric tube suction

From renal losses - diuretic therapy

Excess corticosteroids have some mineralocorticoid effects and because of this can produce a metabolic alkalosis. The alkalosis is most severe with the syndrome of ectopic ACTH production.

Longstanding chronic respiratory deficiency and increased deadspace ventilation are examples of a chronic respiratory acidosis. The physiological responses to a respiratory acidosis are both acute (buffering) and chronic (renal retention of bicarbonate ions). The latter response commences at about 6 hours and maximal by 4 days. For each 10 mmHg (1.33 kPa) change in PaCO2, there will be an increase of plasma bicarbonate of about 4 mmol/L (so-called "rule of four"). This maximum effect will not be sufficient to return the pH to normal and certainly not sufficient to cause an alkalosis.

A mild metabolic alkalosis can occur in patients with stable chronic respiratory disease but pH values will still be low . If the pH is in the normal range (upper limit of normal then one should look for evidence of an additional contributing factor (e.g. use of corticosteroids or diuretics) which might tie-in with this diagnosis.

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