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.
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.
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
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.
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.
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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