Nearly all oxygen is carried to the tissues by haemoglobin (Hb). Each gram/dl of Hb carries 13 ml oxygen when fully saturated. A negligible amount is dissolved in plasma. The oxygen content of blood can therefore be calculated:
Hb (g/dl) x oxygen saturation of Hb x 13 and x 10 to convert to litres
The delivery of oxygen to the tissues depends on the cardiac output. From this we derive the oxygen delivery equation:
Understanding that oxygen delivery depends on other factors as well as oxygen therapy will help to optimise oxygenation in any patient who is unwell. In a 70 kg man a normal Hb is 14 g/dl, normal saturation is above 95% and normal CO is 5 litres per minute. Oxygen delivery is therefore:
Patients with pneumonia or severe asthma can be extremely dehydrated. If a patient has an Hb of 14, an SaO2 of 93%, and a reduced cardiac output (4 litres per minute) because of dehydration, his oxygen delivery is 14 x 0 93 x 1 3 x 10 x 4 = 677 ml O2 per minute. By increasing his oxygen so that his saturations are now 98%, his oxygen delivery can be increased to 14 x 0 98 x 13 x 10 x 4 = 713 ml O2 per minute, but if a fluid challenge is given to increase his cardiac output to normal (5 litres per minute) and his oxygen is kept the same, his oxygen delivery would be 14 x 0 93 x 1 3 x 10 x 5 = 846 ml O2 per minute. Oxygen delivery has been increased more by giving fluid than by giving oxygen. The oxygen delivery equation also illustrates that an SaO2 of 95% with severe anaemia is worse than an SaO2 of 80% with a haemoglobin of
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.