This chapter discusses the reduced and increased CO2 pressures and outlines some of the pathophysiology of CO2 production and transport, with special reference to capnography. Capnography has a special use in the assessment of patients for brain death with regard to the apnea test, wherein measurement of CO2 tensions are a component of assessing the viability of the brainstem. Any disruption in normal mitochondrial function manifests as diminished energy reserves and CO2 production, and is most rapidly evident in organs with high-energy requirements such as the brain, heart, kidney, and, especially, the eye. Carbonic anhydrase comprises a family name for a group of zinc-dependent enzymes that catalyze the reaction of bicarbonate and a proton to form water and CO2. Sodium bicarbonate is sometimes intravenously injected to treat acidemia in patients suffering from metabolic acidosis.

The brain death criteria include assessment of the following: normothermia; cause of brain death; unresponsiveness; absence of brainstem reflexes; and apnea. The presence of withdrawal reflexes, typically seen in the lower extremities, do not preclude the diagnosis of brain death given that withdrawal reflexes can be spinally mediated. The absence of brainstem reflexes includes pupillary responses, eye movements, and lower brainstem reflexes such as response to tracheal stimulation. Apnea testing is performed to demonstrate lack of respiratory effort, even in the presence of elevated pCO2 levels. When no respiratory effort is made during the apnea test and the other criteria have been met, the patient may be pronounced dead. The diagnosis of brain death is not without controversy. For instance, thermoregulation is regarded as a brainstem function; therefore, some argue that those patients who maintain their body temperature cannot be declared brain dead.