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How are ventilation and oxygenation monitored noninvasively during surgery, and how do these monitors work?
The purpose of ventilation is to remove carbon dioxide (CO2) from the lungs. The average 70-kg man produces approximately 220 mL/min of CO2 and normally maintains an arterial CO2 (PaCO2) of 40 mmHg. If removal of CO2 is impaired, or if production increases, and ventilation does not increase, the PaCO2 rises. Arterial blood gas analysis provides the ultimate monitor of ventilation: PaCO2. In the lungs, CO2 diffuses into alveoli, and in those areas where ventilation and perfusion are well matched, alveolar CO2 (PACO2) approximates PaCO2. PACO2 may be sampled as end-tidal (PE′CO2) and analyzed by various technologies to provide a noninvasive estimation of PACO2 and thereby, PaCO2. Overall, PE′CO2 is normally 4–6 mmHg less than PaCO2. Increases in dead space (VD/VT) or a decrease in alveolar ventilation result in increased PaCO2.
Capnography is the science whereby CO2 is measured and displayed as a concentration plotted against time (the capnogram). CO2 in gas mixtures may be measured using the following technologies: mass spectrometry, infrared light spectroscopy, infrared acoustic spectroscopy, and Raman scattering. The principle of mass spectrometry is ionization of molecules in a high vacuum chamber and separation on the basis of mass-to-charge ratio. CO2 has a mass/charge ratio of 44 (m.w. 44/charge of +1) and is indistinguishable from N2O. Mass spectrometers therefore usually measure the presence of carbon ions from fragmented CO2 mole-cules. Carbon ions are present in fixed ratio to CO2. The mass spectrometer is a proportioning system and provides true readings of percentage composition. If the computer is programmed such that 100% is equivalent to 713 mmHg, percentages can be expressed in mmHg.
Infrared (IR) light spectroscopy is based on 4.3 μm (4300 nm) wavelength IR radiation absorbance by inter-molecular bonds in the CO2 molecule. The greater the number of CO2 molecules present, the greater the radiation absorbance. Gas is drawn through a sample cell (cuvette). IR light of 4.3 μm is passed through the cell. Transmission of light through the gas sample to a photodetector is inversely proportional to the PCO2 in the gas sample. Such systems are in widespread use and sampling may be of the sidestream or mainstream (in the airway) design.
In photoacoustic IR spectrometry units, IR beams are pulsed through the cuvette. Pulsed beams cause heating and pulsatile expansion of the gas in proportion to the number of CO2 molecules present. The pulsatile expansions are detected as sound waves by a sensitive microphone. The amplitude of the waves (“volume”) is proportional to PCO2.
In Raman spectroscopy (Rascal II, Datex-Ohmeda, Boulder, CO), a helium–neon laser generates monochro-matic light of wavelength 633 nm, which is transmitted through a sampling cuvette. CO2 molecules absorb this light and re-emit it at a different and characteristic wave-length. The light re-emitted at the new wavelength is spe-cific for CO2, and its intensity is related to PCO2.
By continuously following the percentage (mass spec-trometer) or tension (other methods) of CO2 against time, the highest value is designated end-tidal and defines exhala-tion, whereas the lowest value is designated inspiratory and defines inspiration. Continuous capnography is considered the standard of care because it provides a continuous mon-itor of ventilation and can, with certain limitations, provide a noninvasive estimate of PaCO2.
Oxygenation is monitored best by sampling arterial blood and analyzing it for tension (PaO2), hemoglobin saturation with oxygen (fractional saturation, i.e., O2Hb/ total Hb), and total hemoglobin. The oxygen content of arterial blood (CaO2) is then given by the equation:
CaO2 = [(Hb × 1.34 × SaO2) + (PaO2 × 0.003)]
Normal arterial oxygen content can be calculated by substituting variables as shown:
CaO2 = [(15 × 1.34 × 0.95) + (95 × 0.003)]
=19.1 + 0.29
=20 mL oxygen/100 mL blood
This, however, requires invasion of an artery for blood sampling.
Noninvasive monitoring of oxygenation is most com-monly achieved using a pulse oximeter, which has also become the standard of care. The principle of operation of the pulse oximeter is based on two technologies: (1) spec-trophotometry of oxygenated and deoxygenated Hb and (2) optical plethysmography. The latter detects pulsatile com-ponents of changes in light transmitted through a fingertip. Each pulse oximeter probe has two light-emitting diodes (LEDs), which emit light at 660 nm and 940 nm, and one photodetector. The ratio of the pulse-added absorbance of light at 660 nm to that at 940 nm is related through an empirically derived algorithm, to a saturation reading which is designated SpO2. Pulse oximeter readings have a standard deviation of approximately 2% in the saturation range of 70–100%. At lower SpO2 levels, their accuracy decreases. Nevertheless, the pulse oximeter is valuable as a saturation trend monitor. Pulse oximeters are inaccurate or may fail in the presence of poor perfusion (vasoconstriction from low cardiac output or cold extremities), venous pulsa-tions, severe peripheral vascular disease, intravascular dyes, dyshemoglobins, certain nail polishes, and certain pigmen-tations. If doubt exists as to the validity of a pulse oximeter reading, an arterial blood gas sample should be drawn for blood gas tension analysis and saturation analysis in a labo-ratory co-oximeter.
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