Monitoring of the inspired and end-tidal oxygen, carbon dioxide, and nitrous oxide concentrations: Clinical applications during anesthesia and recovery

Abstract

Respiratory oxygen, carbon dioxide, and nitrous oxide concentrations were recorded in 20 patients breath-by-breath during general anesthesia and early recovery, using the Cardiocap multiparameter monitor. Several approved maneuvers were performed to demonstrate the usefulness of endtidal oxygen measurement. "Oxygrams" provided by the fast paramagnetic oxygen sensor confirmed the capnometric information in the diagnosis of hypoventilation, apnea, and disconnections. In one patient, the alarm for inspiratory oxygen concentration, set at 18%, appeared to prevent alveolar hypoxia and low arterial saturation from occurring when oxygen instead of nitrous oxide was turned off. Low end-tidal oxygen levels revealed inadequate fresh gas oxygen supplementation while low flow circuits were closed. During manual hypoventilation at the end of anesthesia, the inspiratory-expiratory oxygen difference increased almost twofold while end-tidal carbon dioxide increased by only 30%. Changes in nitrous oxide concentration often complemented oxygen-related information obtained in our observations. In the recovery room, a decrease in end-tidal oxygen concentration preceded low pulse oximetry readings. Therefore, it is suggested that all three gases should be monitored continuously to prevent mishaps related to insufficient ventilation and inappropriate gas concentrations during anesthesia and immediate recovery.