Mainstream Capnometers
In a mainstream capnometer, with the measuring head placed close
to the endotracheal tube, the measuring chamber is usually heated to about 40°C
to prevent water condensation on the chamber window. The heating sensing head must
be kept away from direct contact with the patient's skin; it is relatively heavy
and must be supported to prevent endotracheal tube kinking. The sensor's window
must be kept clean of mucus and particles to prevent false readings; calibration
may be problematic. Despite all these problems, the response time is faster because
no gas is subtracted from the breathing circuit, no sampling pumps or other suction
devices add complexities to the mechanical system, and there is no uncertainty caused
by the rate of gas sampling.
Capnographs must be calibrated periodically, at different intervals
in various models, but usually at least daily. Equipment drift is most often caused
by accumulation of
Figure 36-16
Monitoring a spontaneously breathing patient with a sidestream
capnograph. A, Attachment of the sampling line to
a non-rebreathing mask usually provides waveforms adequate for monitoring the respiratory
rate, but end-tidal CO2
tension is low because of mixing within the mask.
B, Placement of a sample probe close to a nostril
increases the accuracy of the PETCO2
measurement.
C, A specially designed probe (Oridion, Needham,
MA) samples exhaled gas from the nose and mouth for assessing exhaled gas from mouth-breathers.
A second port in the same device can be used to administer supplemental oxygen.
saliva or other extraneous materials in the light path of the analyzer. Calibration
can usually be performed by periodic use of calibrated gases. Mainstream capnometers
are often equipped with calibration sample cells sealed with mixtures of CO2
and N2
. In some instruments, room air is sampled in the mainstream cuvette
to "zero" the CO2
level automatically.
A useful analysis range varies from 0% to 10% (76 mm Hg) for end-tidal
CO2
to an extended range up to 100 mm Hg, which may be useful in rare
cases of hypoventilation or malignant hyperthermia. The inspired CO2
range must include 0 and extend up to 15 mm Hg for cases in which rebreathing must
be detected and peak values determined.
