Calculation of Oxygen in Blood

The majority of oxygen in the blood is bound to haemoglobin, with the exact amount varying based on the oxygen saturation and haemoglobin level. To calculate the amount of oxygen per litre of blood, the following formula can be used: (13.9 × Hb × sats/100) + (PaO2 × 0.03). For example, an ordinary man with an Hb of 14, sats of 98% on room air, and a PaO2 of 12 would have 191 ml of oxygen per litre of blood. It is important to note that only 0.36 ml of this oxygen is dissolved in the blood.

Acid-Base Imbalances and Their Causes

Poorly controlled diabetes can result in the breakdown of fatty acids, leading to the production of ketones as an alternative energy source. Since ketones are acidic, an excess of them can cause metabolic acidosis. On the other hand, COPD and suffocation can cause retention of CO2, leading to respiratory acidosis. In COPD, there may be a chronic compensatory metabolic alkalosis. Voluntary hyperventilation can cause a reduction in CO2, resulting in respiratory alkalosis. Lastly, vomiting can lead to metabolic alkalosis.

In summary, acid-base imbalances can occur due to various factors such as diabetes, respiratory disorders, and vomiting. It is important to identify the underlying cause of the imbalance to provide appropriate treatment.

Interpreting Oxygen Saturation in Anemic Patients

Interpreting oxygen saturation levels in anemic patients can be misleading. While a patient may have a saturation level of 98%, it does not necessarily mean that they have enough oxygen in their blood. This is because the patient may not have enough hemoglobin to carry the oxygen. To calculate the amount of oxygen per liter of blood, the equation (13.9 × Hb × sats/100) + (PaO2 × 0.03) ml O2/litre blood can be used. The latter part of the equation refers to the amount of dissolved oxygen in the blood and can be ignored for now.

Advising an anemic patient to breathe more rapidly may slightly increase their oxygen saturation levels, but it will cause them to blow off all their CO2 and experience tingling in their fingers and around their mouth. Increasing their saturation levels from 98% to 100% will not make a significant difference. Similarly, advising the patient to breathe more slowly is unlikely to have any significant effect. Giving the patient maximum oxygen through a facemask or artificial ventilation will also not be helpful since the limiting factor is the amount of hemoglobin.

In cases where anemic patients require oxygen therapy, it is important to consider their hemoglobin levels. One unit of blood transfusion can raise Hb by approximately 10 grams/L, which can make a significant difference in the amount of oxygen carried by the blood. Therefore, in such cases, administering two units of blood transfusion may be the most effective solution.