Biomedical Engineering Reference
In-Depth Information
Capnometry and agent analysis
molecules, so it is an absolute, and not a relative, mea-
surement. Carbon dioxide absorbs IR light at about 4.3
m
m, nitrous oxide (N
2
O) about 4.5
m
m, and volatile
anesthetic agents range from 9-12
m
m. Because CO
2
and
N
2
O absorb IR closely to one another, there is signal
bleeding between the two. As a result, some CO
2
mon-
itors need N
2
O compensation when calibrated. In the
monitor, the sampled patient gas runs through water-
permeable tubes to dry the moist sample prior to de-
livery to the measurement bench. Water vapor absorbs IR
at the same spectrum and therefore is a contaminant for
CO
2
measurements. To measure different gases, dual-
beam infrared spectrometers use a spinning wheel with
band pass filters to tune its spectrum for the particular
gas of interest for maximum signal and a reference. The
reference is a zero or minimal response point to which to
compare the measurement. Absorption is detected and
displayed as a waveform and number.
Volatile anesthetic agents can be measured using in-
frared spectroscopy and have individual signatures at
different wavelengths. Some monitors require simple
software updates to read newer agents, while others
require more extensive replacement of hardware. Unfor-
tunately, there also are models that are cost-prohibitive
to update.
End-user calibration of side-stream monitors involves
a zero and span setting. Newer-style monitors have au-
tomatic zeroing where room air sample is taken in-
ternally. Older monitors require the person completing
the calibration to remove the sampling line from the
breathing circuit, and then to complete a sequence of
actions (i.e., turn knobs or activate keys). Span requires
the person calibrating to use a known calibration gas that
can be specific to the model of monitor. The calibration
gas is sprayed into the sampling port, and the upper
measurement range is established either by software or
physically setting a potentiometer.
Existing mainstream monitors are more common for
use outside of anesthesia due to technology constraints as
they are limited to measuring only one gas. They are
frequently found as options on physiological monitors.
ASA standards originally stated that capnometers would
be used to verify the presence of carbon dioxide to
ensure tracheal, versus esophageal, intubation. The latest
revision specifies continuous CO
2
measurement. There
are clinical indications that can be detected using capn-
ometers. Apnea caused by disconnects, ventilator failure,
and complete obstruction in the breathing circuit or the
scavenging system can be detected, possibly early enough
before arterial blood-oxygen desaturation occurs. CO
2
waveforms during controlled ventilation display charac-
teristics that help the physician to identify potential
patient-management issues. An abnormally slow rise can
indicate a restricted airway or kinked endotracheal tube,
while a baseline drift can indicate a faulty expiratory limb
valve, consumed CO
2
absorbent, or channeling. BMETs
and CEs should learn typical and atypical capnograms
and their causes. This knowledge assists in field trou-
bleshooting and in communication, especially when
a physician states that the CO
2
waveform looks peculiar.
Monitors can be divided into two categories: side
stream or mainstreammeasurement devices. Side stream
monitors remove a small sample from the breathing-
circuit gases and delivers it to a measurement chamber.
The effluent can be either scavenged or returned to the
expiratory limb to be recirculated. Mainstream monitors
measure the patient's expired carbon dioxide concen-
tration through an optical window in a tube connected
to the breathing circuit, most often at the end of the
endotracheal tube.
There are a number of different technologies em-
ployed in side-stream monitors, including mass spec-
trometry, infrared (IR) analyzers (single and dual beam),
and Raman spectroscopy. The advantages of a side stream
monitor are the simplicity of items connecting to the
breathing circuit and the ability to read multiple gases.
The ancillary items do not need to be reprocessed, are
disposable (thus minimizing risk for cross-contamination),
do not need optical properties or cleanliness, and there-
fore are easier to use on a daily basis. The components
added to the breathing circuit are physically smaller
an
added advantage when working with limited access and
sterile drapes. Their largest drawback is in working with
expired water. The sampling tube can condense water
vapor, and if the monitor is unable to clear the droplets on
its own, it requires user interaction. Many monitors in-
corporate a water trap that must be emptied on occasion,
and hydrophobic filters that can occlude.
Molecular asymmetry is required for IR light absorp-
tion, resulting in vibration or rotation of dipole bonds.
Nonpolar molecules, such as oxygen and nitrogen, do not
absorb IR energy and cannot be measured with IR spec-
troscopy. Absorption correlates with the number of
d
Temperature
Temperature is the last physiological parameter men-
tioned in the ASA monitoring standard. Patients un-
dergoing anesthesia frequently experience hypothermia
caused by the mechanics of anesthesia and skin exposure
to the cold environment of an OR. Induction of anes-
thesia suppresses the body's ability to regulate core
temperature at the most fundamental state. An in-
dividual who is awake will make behavioral changes (e.g.,
in terms of dress or shivering) when sensing a change in
environmental temperature. The hypothalamus responds
