Capnography is monitoring modality that measures end tidal carbon dioxide (ETCO2) and displays a waveform of exhaled carbon dioxide during the respiratory cycle called a capnogram (shown below)
ETCO2 is measured at point D on the capnogram and represents the CO2 removed by gas exchange across the alveolar membrane. In the normal patient it is equivalent to the arterial partial pressure of CO2 and is therefore used as a non-invasive surrogate. End tidal CO2 is used as a measure of ventilation and therefore as a guide to efficiency of patient ventilation. The normal reference range in a spontaneously breathing awake patient is 35-45mmHg, although during anaesthesia up to 55-60mmHg may be tolerated prior to intervention. Concentrations above this range indicate hypo ventilation and intervention may consist of assessing depth of anaesthesia followed by manual and/or mechanical ventilation. The most common reasons for hypo ventilation during anaesthesia are excessive depth of anaesthesia and the use of mu opioid agonists. Concentrations below this range may indicate hyperventilation or inadequate tidal volume movement for example if the patient is panting. Intervention should consist of assessment of depth of anaesthesia, administration of additional analgesia if appropriate and supplementary ventilatory support if required.
The capnogram is a waveform display of CO2 concentration plotted against time. In addition to giving the respiratory rate the waveform can be used to detect leaks within the breathing system, changes in gas flow resistance for example due to kinking of or airway secretions in the endotracheal tube as well as exhausted carbon dioxide absorbent, low gas flows and faulty breathing systems.