grams must always be interpreted in conjunction with
other physiological variables, and clinical judgment is
important. Loss of the capnographic waveform may occur
in several circumstances. These include apnea, the
disconnection of a ventilator circuit, the occlusion or
dislodgment of an endotracheal tube, or the occlusion or
disconnection of the sampling catheter. Waveforms that
do not return to zero during inspiration indicate rebreathing
of carbon dioxide, which can occur if the carbon
dioxide absorber in an anesthesia machine is chemically
exhausted, if a valve in the ventilator circuit is not
functioning properly, or if the flow of fresh gas is insufficient.
An elevated baseline can also be seen if the device
is calibrated incorrectly. Decreases in ETCO2 can
result from hyperventilation, pulmonary embolism, cardiac
arrest, sudden hypotension, hypovolemia, hypothermia,
leaks in the sampling system, or a partial airway
obstruction [4]. Increases in ETCO2 may be caused by
hypoventilation, rising body temperature, bronchospasm,
adrenergic discharge, release of a tourniquet on an arm or
leg, or ventilation of a previously unventilated lung. Shallow
breathing causes transient lowering of ETCO2 because
of dead-space ventilation, but the ETCO2 rises
again after a deep inspiration when full gas exchange
occurs