ANESTHETIC GAS ANALYSIS
Analysis of anesthetic gases is
essential during any procedure requiring inhalation anesthesia. There are no
contraindications to analyzing these gases.
Techniques for analyzing multiple
anesthetic gases involve mass spectrometry, Raman spectroscopy, infrared
spectrophotometry, or piezoelectric crystal (quartz) oscillation. Mass
spectrometry and Raman spectroscopy are primarily of historical interest, as
most anesthetic gases are now measured by infrared absorption analysis.
Infrared units use a variety of
techniques similar to that described for capnography. These devices are all
based on the Beer–Lambert law, which provides a formula for measuring an
unknown gas within inspired gas because the absorption of infrared light
passing through a solvent (inspired or expired gas) is proportional to the
amount of the unknown gas. Oxygen and nitrogen do not absorb infrared light.
There are a number of commercially available devices that use a single- or
dual-beam infrared light source and positive or negative filtering. Because
oxygen molecules do not absorb infrared light, their concentration cannot be
measured with monitors that rely on infrared technology and, hence, it must be
measured by other means .
The piezoelectric method uses
oscillating quartz crystals, one of which is covered with lipid. Volatile
anesthetics dissolve in the lipid layer and change the frequency of
oscillation, which, when compared with the frequency of oscillation of an
uncovered crystal, allows the concentration of the volatile anesthetic to be
calculated. Neither these devices nor infrared photoacoustic analysis allow
different anesthetic agents to be distinguished. New dual-beam infrared optical
analyzers do allow gases to be separated and an improperly filled vaporizer to
be detected.
To measure the Fio2 of inhaled gas, manufactur-ers of anesthesia
machines have relied on various technologies.
Galvanic cell (fuel cell) contains a lead anode and gold cathode bathed in potassium chloride. At the gold terminal, hydroxyl ions are formed that react with the lead electrode (thereby gradually con-suming it) to produce lead oxide, causing current, which is proportional to the amount of oxygen being measured, to flow. Because the lead electrode is con-sumed, monitor life can be prolonged by exposing it to room air when not in use. These are the oxygen monitors used on many anesthesia machines in the inspiratory limb.
Oxygen is a nonpolar gas, but it is
paramagnetic, and when placed in a magnetic field, the gas will expand, contracting
when the magnet is turned off. By switching the field on and off and comparing
the resulting change in volume (or pressure or flow) to a known standard, the
amount of oxygen can be measured.
A polarographic electrode has a gold (or
platinum) cathode and a silver anode, both bathed in an elec-trolyte, separated
from the gas to be measured by a semipermeable membrane. Unlike the galvanic
cell, a polarographic electrode works only if a small voltage is applied to two
electrodes. When voltage is applied to the cathode, electrons combine with
oxygen to form hydroxide ions. The amount of cur-rent that flows between the
anode and the cathode is proportional to the amount of oxygen present.
Newer anesthesia machines can measure
(and therefore manage) airway pressures, volume, and flow to calculate
resistance and compliance and to display the relationship of these variables as
flow (ie, volume or pressure–volume loops). Measurements of flow and volume are
made by mechanical devices that are usually fairly lightweight and are often
placed in the inspiratory limb of the anesthesia circuit.
The most fundamental measurements
include low peak inspiratory pressure and high peak inspi-ratory pressure,
which indicate either a ventilator or circuit disconnect, or an airway
obstruction, respec-tively. By measuring Vt and breathing frequency (f ), exhaled minute ventilation (Ve) can
be calculated, providing some sense of security that ventilation requirements
are being met.
Spirometric loops and waveforms are
charac-teristically altered by certain disease processes and events. If a
normal loop is observed shortly after induction of anesthesia and a subsequent
loop is dif-ferent, the observant anesthesiologist is alerted to the fact that
pulmonary and/or airway compliance may have changed. Spirometric loops are
usually displayed as flow versus volume and volume ver-sus pressure (Figure 6–6).
There are characteristic changes with obstruction, bronchial intubation,
reactive airways disease, and so forth.
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