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.