| The measurement of blood gases
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| The so-called 'blood gas measurement' is an important first-line investigation performed whenever there is a suspicion of respiratory failure or acid-base disorders such as diabetic ketoacidosis (see Chapter 20). In respiratory failure, the results of such measurements are also an essential guide to oxygen therapy and assisted ventilation.
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| Figure 23.2 Intracellular buffers: proteins, phosphates, and potassium exchange. In contrast to blood, intracellular buffers are primarily proteins and phosphates. Hydrogen ion from the plasma enters cells in exchange for potassium. Therefore, an accumulation of the hydrogen ion in the plasma (acidemia) and the entry of large amounts of hydrogen ion into cells lead to increased plasma potassium concentration. Conversely, a deficit of hydrogen ion in plasma (alkalemia) may lead to a low plasma potassium concentration. Prot, protein. |
| These measurements are performed by taking a sample of arterial blood, usually from the radial artery in the forearm or, less commonly, from the femoral artery in the leg. The jargon term 'blood gases' means the measurements of pO2, pCO2, and pH (or hydrogen ion concentration) from which the concentration of bicarbonate is calculated using the Henderson-Hasselbalch equation. Several other indices are also computed: they include the total amount of buffers in the blood (so-called buffer base) and the difference between the desired (normal) amount of buffers in the blood and the actual amount (base excess). The acid-base analysers used in the hospitals are programmed to perform these calculations.
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| Figure 23.3 Components of the bicarbonate buffer. Blood pH is proportional to the ratio of plasma bicarbonate to the partial pressure of carbon dioxide in blood (pCO2). Carbon dioxide and bicarbonate are the components of the bicarbonate buffer. The pCO2 is called 'the respiratory component of the acid-base balance' because it depends on the rate of respiration. Bicarbonate is called 'the metabolic component of the acid base balance' because its concentration is affected by the nonvolatile acids produced in tissues. |
| The reference values for pH, pCO2, and O2 are given in Table 23.2.
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