Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 92

Most blood gas analysers provide two different measurements of bicarbonate – ‘actual bicarbonate’ and ‘standard bicarbonate’ – in addition to another value, ‘base excess’. This can cause confusion, although it needn’t. The analyser measures the bicarbonate level in the blood sample. This actual measurement is (conveniently) known as the actual bicarbonate (aHCO₃ ^–). As can be seen in Fig. 3.9, the actual level is directly dependent on the PCO₂ (for a given pH: the higher the PCO₂, the higher the aHCO₃ ^–; the lower the PCO₂, the lower the aHCO₃ ^–). What we’d like to know is what the bicarbonate would have been if the PCO₂ had been normal (5.3 kPa) because that gives us a direct handle on ‘what the metabolic system is doing’. If we know the actual bicarbonate (aHCO₃ ^–) and the PCO₂ then we can calculate that value. However, we don’t need to, the machine will do it for us. This calculated bicarbonate value for a ‘standard’ PCO₂ is (conveniently) known as the standard bicarbonate (sHCO₃ ^–). Either aHCO₃ ^– or sHCO₃ ^– can be used in the interpretation of blood gases, but the sHCO₃ ^– takes out the immediate effect of CO₂ on the bicarbonate level and can loosely be regarded as giving a more direct indication of the metabolic activity influencing acid/base balance, (I think it’s easier). The base excess takes into account the fact that there are other buffers apart from bicarbonate in the blood. It tells a similar story to the bicarbonate level in terms of acid/base disturbance. Its principal advantage is the ease with which its normal range can be remembered. As one might anticipate from the name, the ‘excess’ should be zero (normal range is 0±2 mmol/L). There aren’t many numbers easier to remember than zero.

Figure 3.9 Bicarbonate isopleths (diagonal lines; the bicarbonate level is constant along the lines). It can be seen that, if the bicarbonate level and PCO₂ are known, the pH can be calculated. Indeed, if any two of the three values of bicarbonate, pH and PCO₂ are known then the other value can be calculated. These values are yoked together. A change in ventilation will move the arterial point up or down an isopleth as shown, changing pH (bicarbonate level does not change). A pure metabolic disturbance (before any respiratory response) changes the bicarbonate level, moving from one bicarbonate isopleth to another and changing pH.