Oxygen delivery and haemoglobin
are sometimes used to assess the adequacy of ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})
on the assumption that if ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})
is inadequate ({dot{V}}_{mbox{textsc{mathrm{o}}}_{2}})
becomes supply-dependent (see below). ({dot{V}}_{mbox{textsc{mathrm{o}}}_{2}})
can be measured directly by analysis of respiratory gases or derived from cardiac output and arterial and venous oxygen contents. Gas analysis techniques require specialized equipment that accurately measures gas volumes and concentrations adjusting for temperature and pressure changes and other sources of inaccuracy. Calculation from cardiac output and arterial-mixed venous oxygen content difference is simpler and can be done using a pulmonary artery catheter. The reverse/inverse Fick principle is used:
Oxygen extraction ratio
The normal o2ER is 0.2 to 0.3, indicating that only 20–30% of the delivered oxygen is utilized. This spare capacity enables the body to cope with a fall in ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})
without initially compromising aerobic respiration and ({dot{V}}_{mbox{textsc{mathrm{o}}}_{2}}.mbox{textsc{mathrm{o}}}_{2}mathrm{ER})
varies between organs; the heart has a high o2ER (∼0.6) so it is particularly sensitive to reductions in coronary artery ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})
.
Hypoxia
Hypoxia is a deficiency of oxygen at the tissue level. It follows that the chance of hypoxia is influenced by the demand of the tissues for oxygen (Table 2[7]). Under anaesthesia, metabolic rate decreases so a lower ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})