In this module we will discuss CO2 monitoring and invasive haemodynamic monitors.
Capnography (CO2 monitor)
This is a device that measures the partial pressure of CO2 in a gas sample. The sample is normally taken from the breathing circuit through a sampling line. It is the recommendation of the Royal College of Anaesthetists and the Intercollegiate Board for Training in Intensive Care Medicine that a Capnograph is used every time a trainee inserts an endotracheal tube in a patient.
The reason for this recommendation is that capnography is considered the gold standard monitor for intubation and allows quick distinction between an endotracheal and an oesophageal intubation, see fig 1 below.
Capnography can be kept on during ventilation to give a continuous indication of the PaCO2. This is desirable in some conditions, such as head injured patients. You should be aware, however, that the CO2 level measured by capnography (end tidal CO2 or etCO2) will always be lower than the PaCO2. The amount by which it is lower depends on the dead space of the patient’s respiratory system, which may change. Dead space increases in certain lung conditions e.g. infection and fluid. Thus an ABG should be done to confirm actual level of PaCO2.
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Figure 1. Capnography
Typical capnography waveform during IPPV ventilation. Exp = expiratory phase, and Insp = inspiratory phase of respiration.