Measurements of Respiratory Carbon Dioxide

Carbon dioxide plays a fundamental role in the metabolism of all living organisms. In man, the carbon dioxide concentration is mainly regulated through respiration. Consequently, measurements and monitoring of respiratory carbon dioxide is important in physiological applications, i.e. sports medicine as well as in clinical practice, i.e. in emergency care. This requires appropriate techniques for measurements. The aims of this thesis were to critically review present methods and devices used for respiratory measurements, to suggest and evaluate improvements, and to suggest and evaluate possible new applications of carbon dioxide measurements using an electro acoustic sensor. Carbon dioxide measurement in expired air has the advantages over other suggested methods for respiratory monitoring in providing extra information about conditions in arterial blood in subjects with healthy lungs. The suggested technique is based on an electro acoustic sensor sensing the carbon dioxide in respiratory air, with a filter to reduce humidity and temperature variations. The sensor consists of an ultrasound transmitter and a reflector placed in each end of a perforated tube. The sensor measures the molecular mass in the gas mixture inside its cavity. The molecular mass has a linear relationship to the carbon dioxide concentration. The fact that the partial pressure of end tidal carbon dioxide can be measured with the electro acoustic sensor system makes it useful in several clinical situations. The partial pressure of end tidal carbon dioxide has also been found to be a useful indicator of the lactate threshold. This indicates a new application for sports medicine. Whit this method it is possible to measure the variation in the lactate threshold for an athlete in a specific sport on a daily basis, a prerequisite for optimising the work intensity during training and competition. The electro acoustic sensor system has been shown to be an useful measuring equipment in this application, although it is not selective to carbon dioxide and thereby influenced by change in oxygen content in the expiratory air.