3b Critical haemoglobin or haematocrit levels

The basic aspects of the delivery, consumption and deficits of oxygen are briefly recalled. As well as haemoglobin (Hb) or haematocrit (Hct) levels, several ‘non-Hb’ variables (notably O₂ demand, cardiac output and the arterial saturation of the available Hb) are important for adequate whole-body oxygenation. Their interaction with Hb can be analysed by computer simulation, which shows that the ‘critical’ level of Hb or Hct, sometimes called the ‘transfusion trigger’, is an individual and not a generally valid figure. This conclusion is borne out by clinical experience with Hb or Hct levels ranging approximately from 11 to <8 g/dl or from 33% to <24%, respectively. For the myocardium, whose performance is decisive for the compensation of low Hb or Hct levels, 7–8 g/dl for Hb or 21–24% Hct may be the limit in otherwise ideal circumstances, but in patients with overt or silent episodes of myocardial ischaemia, a level of less than 10 g/dl (30%) carries risks that should be avoided.