Respiratory CO2 measurement (capnography) is an important diagnosis tool that lacks inexpensive and wearable sensors. This paper develops techniques to enable use of inexpensive but slow CO2 sensors for breath-by-breath tracking of CO2 concentration. This is achieved by mathematically modeling the dynamic response and using model-inversion techniques to predict input CO2 concentration from the slowly varying output. Experiments are designed to identify model-dynamics and extract relevant model-parameters for a solid-state room monitoring CO2 sensor. A second-order model that accounts for flow through the sensor's filter and casing is found to be accurate in describing the sensor's slow response. The corresponding model-inversion algorithm is however found to be susceptible to noise sources. Techniques to remove spurious noise, while retaining quality of estimate are developed. The resulting estimate is compared with a standard-of-care respiratory CO2 analyzer and shown to effectively track variation in breath-by-breath CO2 concentration. This methodology is potentially useful for measuring fast-varying inputs to any slow sensor.