Investigating Acid Base Disorders

INVESTIGATING ACID–BASE DISORDERS

An investigation of an acid–base disorder involves three stages. The first stage involves identifying whether the patient has an acidosis or an alkalosis. The second stage is to determine whether the acid–base disorder is metabolic or respiratory in nature while the third stage involves determining the degree of compensation.

Acid–base disorders are investigated as outlined in Figure 9.9. An arterial blood specimen is collected and its H⁺ concentration (pH) and PCO₂ measured. The blood must be collected from an artery into a syringe containing an

anticoagulant, such as heparin, and transported to the hospital laboratory at 4oC. Care must be taken that air does not enter any blood samples during collection. The concentration of H⁺ and PCO₂ level are measured directly using an automated analyzer that is also programmed to calculate the corresponding concentration of HCO₃^– (Figure 9.10) according to the Henderson-Hasselbalch equation. An acidosis or alkalosis can be identified from the pH, that is the concentration of H⁺, in the blood. An examination of the levels of PCO₂ and HCO₃^– shows whether the identified disorder is metabolic or respiratory in origin and indicates to what extent compensation is occurring.