What is a possible reason for a PETCO₂ reading of zero with an absent capnogram?

A PETCO₂ reading of zero alongside an absent capnogram typically indicates that there is no carbon dioxide being measured in the exhaled gases. This situation can occur if there is a disconnection in the sample line, preventing the gas from being properly collected and analyzed. When the sample line is disconnected, the capnometer can't obtain any air or gas samples, leading to a failure to detect carbon dioxide, which would be reflected as a reading of zero.

In a clinical context, it’s crucial to ensure that all connections in the respiratory equipment are secure and functioning properly. If the sampling line is intact and functioning correctly, other factors such as normal respiratory patterns or low tidal volume may still produce some level of PETCO₂, and hypercapnia would usually indicate elevated CO₂ levels rather than a reading of zero. Hence, the disconnection of the line stands out as the most plausible reason for the described scenario.