Specification of acid-base status

This point is taken up again later in the chapter because the haemoglobin concentration appears on the Siggaard-Andersen nomogram. 

In previous chapters, respiratory and non-respiratory disorders of acid-base physiology have been described. In this chapter, methods of measuring the respiratory and non-respiratory components of a disturbance of acid-base physiology have been considered. We now consider these two groups of ideas together. 

In the initial stages of a disturbance of acid-base physiology, the condition is uncompensated, which essentially means that chemical buffering alone is operating. At this stage therefore, there is only one component to the disorder. This component is respiratory in respiratory disorders and metabolic in metabolic disorders. In the respiratory disorder the subject moves along the normal blood line. The partial pressure of carbon dioxide is abnormal but the base excess, the measure of metabolic component, is zero. This is shown in Table 4.4A. For an uncompensated metabolic disorder (Table 4.4C), it is the 

Metabolic alkalosis Uncompensated Zero change High 

Let us now consider what happens when the effect which was secondary in the primary respiratory disorder is instead the primary disorder. This would then be a primary metabolic alkalosis, as in the vomiting of gastric contents. In the uncompensated condition, there is a positive base excess with a normal partial pressure of carbon dioxide already noted (Table 4.4C). The respiratory compensation is hypoventilation, brought about by the partial withdrawal of the normal stimulus of hydrogen ions to the peripheral chemoreceptors. The partial pressure of carbon dioxide rises, adding a respiratory component to the acid-base disorder (Table 4.4D). 

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