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End-tidal CO2 Interpretation

End-tidal carbon dioxide measurement allows non-invasive measurement of the ability of the respiratory system to excrete carbon dioxide (usually indicative of respiratory function). Most anesthetics are respiratory depressants and end-tidal CO2 allows early detection of respiratory impairment so appropriate intervention can occur before the problem becomes life threatening.


Examples of some common capnograms are:

Variants of normal ETCO2 tracings from normal anesthetized animals.

Common narrow plateaus seen during anesthetic administration with a non-rebreathing system or also representative of a leak around the cuff of the endotracheal tube. Other abnormalities which allow entrainment of air would also tend to resemble this waveform.

Notched plateau sometimes seen with diaphragmatic movement during positive pressure ventilation. This is also often representative of the curare cleft seen when spontaneous ventilation returns after neuromuscular junction blocking drug administration.

Increasing inspired CO2 and expired CO2. This is representative of a capnogram seen when a unidirectional valve is stuck or when the carbon dioxide absorbent is consumed. Increased inspired CO2 should be treated as an anesthetic emergency.

Elevated expired carbon dioxide is indicative of respiratory depression.

Beating of the heart can occasionally cause small stairsteps to appear after the plateau. This is not usually of any clinical significance except that it may result in inaccurate ETCO2 measurements.

Rapidly decreasing expired CO2 may indicate cardiac arrest, especially during controlled ventilation. This pattern should be treated as an anesthetic emergency and cardiac function should be immediately evaluated.

Irregular baseline patterns are often the result of a mechanical influence on the thorax resulting in tiny oscillations of gas at the end of the endotracheal tube. This can result from manipulation of abdominal organs, or from surgeons leaning on the thorax of an anesthetized animal.

The appearance of a secondary hump following the main plateau can indicate return on spontaneous ventilation during intermittent positive pressure ventilation. It can also represent other causes of baseline irregularity outlined above.

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