I decided to do a tutorial on end tidal CO2 as there has been a lot of discussion about it’s merits and limitations in our practice. It is fairly long and can be broken into sections at 20 minutes and 37 minutes if you have a short attention span (I will split it up into smaller segments at some stage in the future).
The content is absolutely essential for doctors and nurses working in anesthesiology and intensive care. In my opinion measuring expiratory CO2 from the ventilator circuit is the most useful clinical measurement tool that we have. It gives us information about cellular metabolic activity, blood flow, venous return, lung unit perfusion, gas exchange and alveolar ventilation. The tutorial commences with a discussion of CO2 as a gas and discusses Henry’s and Daltons’ laws. I then discuss the various different CO2 moieties, particularly bicarbonate. Subsequently I go on to discuss the impact of alveolar ventilation on PaCO2. After 20 minutes I move on to discuss capnometry – the measurement of the presence and quantity of CO2 emerging from the lung at end expiration. I discuss why the etCO2 may rise of fall. I then look at a specific clinical scenario where the etCO2 falls precipitously. After 37 minutes I discuss capnography – initially the normal capnograph and then a series of different capnography traces that you should be able to recognize. As a final thought I mention that CO2 is not the only waste produce or metabolic intermediary that we measure, routinely, in clinical practice.