Practical Matters: An abnormal capnograph reading may also signal anesthetic system malfunction
Capnographs are noninvasive monitoring tools used to estimate arterial carbon dioxide tension in anesthetized patients by sampling the patients' expired air. In normal and awake animals, arterial carbon dioxide tension is 35 to 45 mm Hg. Increased end-tidal carbon dioxide tension indicates hypoventilation, a common finding in anesthetized patients that should be corrected with assisted ventilation. However, capnographs can also be used to detect anesthetic machine problems. For example, exhausted carbon dioxide absorbent or malfunctioning one-way valves can cause increased end-tidal carbon dioxide readings in circle anesthetic systems, and insufficient oxygen flow can result in high end-tidal carbon dioxide readings with nonrebreathing systems. But unlike true hypoventilation, these machine-related problems will also increase patients' inspiratory carbon dioxide values because the machines allow patients to rebreathe exhaled carbon dioxide. So be suspicious of increased carbon dioxide readings during inspiration; they may indicate an anesthetic machine problem.
John R. Dodam, DVM, MS, PhD, DACVA