For each person described below A–E, select the most appropriate set of arterial blood gas results (pH and bicarbonate are included) from the following list

A. Option 2 PO2 reduced, others normal. At 1000 metres, atmospheric pressure and hence ambient oxygen pressure fall by about 10 per cent. This reduces alveolar and hence arterial PO2 by slightly more, since alveolar water vapour pressure and PCO2 do not change. However, due to the plateau of the oxygen dissociation curve, arterial saturation falls only slightly. Bodily function is unaffected, with no change in ventilation, so carbon dioxide, pH and bicarbonate do not change.

B. Option 6 All reduced, apart from Po2, which is increased. The ketoacidosis is due to abnormal accumulation of highly acidic metabolites. Hence the pH is reduced. Bicarbonate ions have buffered most of the surplus hydrogen ions to minimize the fall in pH, hence bicarbonate is reduced. The acidosis stimulates ventilation. This reduces PCO2 and also limits pH fall; the increased ventilation also increases PO2, though as in (A) the effect on saturation is minute.

C. Option 8 pH increased, others reduced. In this case, serious ‘hypoxic hypoxia’ leads to increased ventilation reflexly via the carotid and aortic bodies. Increased ventilation improves PO2, but it remains well below normal, causing general tissue hypoxia. The increased ventilation acts mainly by lowering PCO2, allowing oxygen to replace some of the carbon dioxide in the alveoli. However, lowered PCO2 causes a respiratory alkalosis so pH rises. To compensate for the respiratory alkalosis, the renal tubules reduce secretion of hydrogen ions and lower blood bicarbonate level. However, all these compensations merely reduce the abnormalities. The combination of tissue hypoxia, decreased cerebral blood flow (due to the low PCO2), and alkalosis seriously disturb bodily function. Some people tolerate this quite well, but others, like this person, feel wretched and are at risk of serious and even fatal complications.

D. Option 5 pH reduced, others increased. The 55-year-old has inadequate ventilation related to chronic respiratory disease. The inadequate ventilation is shown by the bluish colour (cyanosis) due to excessive desaturated haemoglobin. This is another variety of ‘hypoxic hypoxia’. The inadequate ventilation also raises the PCO2, causing a respiratory acidosis. As with (C), but in the opposite direction, the renal tubules raise the blood bicarbonate level. The increased PO2 can only be explained by the administration of oxygen. Before this the oxygen level would have been reduced. Such patients require careful monitoring of their blood gases and control of the administered oxygen.

E. Option 6 All reduced, apart from PO2, which is increased. Here the renal damage impairs hydrogen ion excretion in the urine and addition of bicarbonate to the blood. Just like (B) this patient has a metabolic acidosis. The respiratory compensation and its effects are explained in the same way.