For each case of disturbed acid–base balance A–E, select the most appropriate option from the following list of results of arterial blood analysis

A. Option 3 This patient has features suggesting respiratory failure – drowsiness and cyanosis in someone with chronic obstructive airways disease. So we are looking for signs of a respiratory acidosis – low pH due to high carbon dioxide levels and a reduced oxygen level to account for the cyanosis. Only Option 3 has these three features. In someone with a long-standing respiratory acidosis the bicarbonate is usually raised as in this case (for comparison, results in Option 2 are all average normal).

B. Option 6 This patient is very similar to the one above except that he has been receiving oxygen therapy for his hypoxic hypoxia. Deterioration on oxygen suggests the possibility that complete relief of the hypoxia has resulted in respiratory depression with a rising carbon dioxide level and worsening respiratory acidosis. Results in Option 6 confirm this with the very high oxygen pressure which can be produced by breathing oxygen together with a high carbon dioxide level and a dangerously low pH. Correct therapy is to give controlled oxygen at, for example, 24–28 per cent and monitor the blood gases so that the oxygen level is above dangerous levels but the carbon dioxide does not rise dangerously.

C. Option 1 This patient has the symptoms of severe renal failure, a condition which leads to a non-respiratory (or metabolic) acidosis. This is confirmed by the very low bicarbonate level and the very low pH. Such a condition leads to respiratory compensation by hyperventilation to lower the carbon dioxide level as shown. The hyperventilation also raises the oxygen level towards that in the atmosphere.

D. Option 4 High altitudes lead to hyperventilation triggered by the carotid bodies in response to hypoxic hypoxia. The hyperventilation improves the oxygen level (which is still below that at sea level) but produces a respiratory alkalosis due to washout of carbon dioxide. With acclimatization the kidney responds by lowering the bicarbonate level by reducing tubular secretion of the now scarce hydrogen ions.

E. Option 5 This is now a rather rare cause of metabolic alkalosis – ingestion of large amounts of sodium bicarbonate which relieves ulcer pain by temporarily buffering the gastric acid. However the bicarbonate is absorbed and can lead to a metabolic alkalosis. Alkalosis increases the binding of available calcium ions in the blood by plasma proteins and can lead to tetany, which usually starts in adults with ‘carpo-pedal’ spasm. Metabolic alkalosis is compensated by depression of respiration, allowing the carbon dioxide level to rise and balance the increased bicarbonate level. The oxygen pressure tends to fall with the hypoventilation.