A small unit dedicated to the measurement of carbon dioxide (CO2) content of expired gas obtained through a sampling tube connected to a main respiratory channel (mainstream) or a side-channel (sidestream) of a breathing circuit. It is designed to operate as part of a patient monitoring system enhancing the function of this (the parent device). The module automatically plugs into the parent device when the user places it into a standardized slot in the parent device or in a connected module rack. The parent device operates as a mainframe computer displaying the CO2 parameter/information measured and provided by this module.