Oxygen delivery and haemoglobin
Abstract
Oxygen must be transported effectively from the atmosphere to the tissues in order to sustain normal metabolism. An understanding of oxygen delivery is therefore central to the management of patients during anaesthesia, resuscitation and during critical illness. This review deals specifically with the transport of oxygen from the lungs to non-pulmonary tissues.
Key points
Oxygen is carried in arterial blood dissolved in solution (<2% when breathing air) and combined with haemoglobin (>98% in air).
Tissue hypoxia is a deficiency of oxygen at the tissue level; it may be caused by increased demand, decreased supply or abnormal cellular utilization.
The main physiological responses to acute normovolaemic anaemia are increased cardiac output and increased oxygen extraction ratio.
The ‘critical’ haemoglobin concentration is that below which tissue hypoxia occurs; it is ∼50 g litre−1 in healthy humans.
Most stable perioperative and critically ill patients can be managed with a haemoglobin transfusion threshold of 70 g litre−1.
Definitions
Oxygen delivery
) is the amount of oxygen delivered to the whole body from the lungs. It is the product of total blood flow or cardiac output (CO) and the oxygen content of arterial blood (Cao2) and is usually expressed in ml min−1:The oxygen content of arterial blood (Cao2) is described using the equation:
Where Hb is the haemoglobin concentration (g litre−1), Sao2 is the arterial Hb oxygen saturation and Pao2 is arterial oxygen partial pressure. Arterial oxygen content is the sum of the two forms in which oxygen is carried. In health >98% of oxygen is bound to Hb (Table 1[1]). The oxygen combining capacity of Hb is represented by the constant k1 above and is sometimes termed Hüfner’s constant. The exact value for this constant is controversial and differs between authors. In theory, each gram of Hb binds 1.39 ml of oxygen. However, in practice, the presence of abnormal forms of Hb, such as carboxyhaemoglobin and methaemoglobin, reduces the oxygen combining capacity of Hb to 1.31 ml g−1. Most modern co-oximeters measure the proportion of these Hb types. Dissolved oxygen in plasma is determined by the solubility coefficient of oxygen at body temperature (k2 above; 0.23 ml litre−1 kPa−1) and the Pao2. Even at high Fio2, this oxygen is insignificant (Table 1[2]) at normal atmospheric pressure.
The relative influence of anaemia on oxygen delivery
| Parameter | Normal | Anaemic | Anaemic + oxygen therapy |
|---|---|---|---|
| Inspired oxygen (%) | 21 | 21 | 100 |
| Pao2 (kPa) | 12 | 12 | 85 |
| Sao2 (%) | 98 | 98 | 98 |
| Hb concentration (g litre−1) | 150 | 75 | 75 |
| Dissolved oxygen (ml litre−1) | 3 | 3 | 19 |
| Hb-bound oxygen (ml litre−1) | 197 | 98 | 98 |
| Total Cao2 (ml litre−1) | 200 | 101 | 117 |
| ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}}) |