ABSTRACT
Despite the multiple mechanisms involved in trying to maintain oxygen transport to the tissues, PO2 is not tightly regulated. This is illustrated by the finding that the normal range for arterial PO2 (PaO2) is wide (10-13 kPa) and that ventilation is not stimulated much below a PaO2 of 8 kPa. During the normal fluctuations of everyday life, it is carbon dioxide production which is most coupled to ventilation. It is perhaps for this reason that hypoxaemic respiratory failure is defined not as a PaO2 below the normal range, but as a PaO2 of less than 8 kPa (type I respiratory failure).1 By contrast, as soon as PaCO2 is above the normal range (>6 kPa) then type I respiratory failure becomes type II respiratory failure.1
