The single-breath-diagram for CO2 (SBD-CO2) ist the graphical presentation of the plot of the
expired carbon dioxide concentration versus expired volume and provides valuable information
about lung function parameters, such as physiological deadspace, airway deadspace and alveolar
deadspace. FOWLER (1948) first divided the CO2 expirogram into three phases. Each phase is
related to an anatomic compartment of the respiratory system. Phase I respresents the relatively
carbon dioxide free exhaled air of the upper airways. Phase II shows the transition from the expired
gas from the proximal conducting airways to the alveolar gas and therefore is characterised by a
rapid upswing. Phase III represents the carbon dioxide rich gas expired from the alveoli and is also
known as the alveolar plateau.
The analysis of the SBD-CO2 is well investigated in humans, but rarely described in veterinary
anaesthesia. Especially in horses, whose lung function is due to the animal´s size and weight severly
impaired during anaesthesia, it might be a useful tool for continuous monitoring of deadspace. The
required signals for creating a SBD-CO2 (Volume and CO2) are measured with spirometry and
capnometry. To calculate physiological and alveolar deadspace it is necessary to draw an arterial
blood sample and analyse it for PCO2.
The aim of this study was to find out, if it is possible to generate a single-breath-diagram for CO2
with the softwareprogram SBCO2Calc (Oberli Engineering, Switzerland) by using the CO2- and
volume-signals measured during anaesthesia of a horse with the sidestream Horse-lite-spirometer
and the Datex-Ohmeda S/5-anaesthesia-monitor. The results have been compared to the findings
collected with the mainstream revolver-spirometer and the NICO-monitor (Respironics) and with
analysis-plus-software (Novametrix Medical Systems Inc.) evaluated signals.