Chapter 5 Pulse Oximetry, Capnography, and Blood Gas Analysis
Pulse oximetry
1 What is pulse oximetry and how does it work?
Pulse oximetry is the continuous noninvasive estimation of arterial hemoglobin-oxygen saturation. It is used routinely to monitor oxygenation in diverse clinical settings, including the operating room, emergency department, and intensive care unit. Clinical use of pulse oximetry falls into two main categories:
1. It is used as a screening or warning for arterial hemoglobin-oxygen desaturation.
2. It is used as an end point for titration of therapeutic interventions. Because pulse oximeters detect pulsatile blood flow, they also monitor heart rate and can provide a relative perfusion index.
Pulse oximeters function by transmitting red light (660 nm, absorbed by oxyhemoglobin [O2Hb]) and infrared light (940 nm, absorbed by deoxyhemoglobin [deoxyHb]) from two light-emitting diodes (LEDs) through tissue containing pulsatile blood. The saturation of hemoglobin with oxygen is a function of the ratio of red to infrared light absorption from the pulsatile and nonpulsatile components of the signals. Thus the saturation (SpO2) is a function of the ratio of two ratios, cancelling out most differences caused by finger thickness, pigmentation, and other factors. A microprocessor algorithm is used to calculate the arterial saturation on the basis of calibration studies done by comparing true saturation measured on arterial blood with a CO-oximeter with the pulse oximeter reading. This calibration is factory set and is not adjustable.
Pulse oximeter probes can be applied to any site that allows orientation of the LED and photodetector opposite one another across a vascular bed. If the tissue is too thick, the signal is attenuated before reaching the detector and the oximeter cannot function. Oximeters can be applied to fingers, toes, earlobes, lips, cheeks, and the bridge of the nose. Esophageal and oral probes are also in development. Several manufacturers offer reflectance oximeter probes that can be applied to flat tissue surfaces such as the forehead or chest. Recently introduced earlobe-mounted sensors combine a pulse oximeter and a transcutaneous CO2 electrode. Many pulse oximeters now include noise and artifact rejection software. This refinement aids the determination of SpO2 in patients with low perfusion or motion (e.g., tremor).
2 How accurate are pulse oximeters?
Pulse oximetry is accurate within 2% to 3% of the true O2Hb levels as measured in vitro with multiwavelength oximeters. The U.S. Food and Drug Administration requires manufacturers to demonstrate that their instruments confirm to this degree of accuracy in human subjects at between 70% and 100% oxygen saturation. Because the principle of measurement is based on a ratio of absorbance ratios, no calibration of the instrument by the user is needed or possible.
3 What factors interfere with pulse oximetry?
Most errors in oximetry measurement are the result of poor signal quality (i.e., hypoperfusion, vasoconstriction) or excessive noise (i.e., motion artifact). Optical interference may be introduced by extraneous light from fluorescent sources or infrared surgical navigation systems. Intravenous dyes (methylene blue, indocyanine green) and nail polishes (especially green, blue, or black) absorb at the wavelengths used by the oximeter and can produce artificially low measurements. Contamination from venous pulsations caused by dependent venous pooling or valvular insufficiency may also cause low readings. For a similar reason, the simultaneous arterial and venous pulsation during cardiopulmonary resuscitation (CPR) make oximeter data unreliable. Extreme hyperbilirubinemia has been reported to have variable effects on SpO2 values. The presence of dysfunctional hemoglobin species can alter the ability of oximetry to reflect the true oxygen saturation. Studies show that darkly pigmented skin can falsely increase saturation estimates derived by some widely used pulse oximeters by up to 7% in the range of 70% to 80% saturation.
4 What effects does dyshemoglobinemia have on pulse oximetry?
Because pulse oximeters use two wavelengths of light, they are capable of differentiating only two species of hemoglobin: Hb and O2Hb. Given that abnormal hemoglobin species such as carboxyhemoglobin (CoHb) or methemoglobin (MetHb) also absorb red and infrared light, their presence affects the SpO2 measurement, and their quantitative contribution cannot be determined. The pulse oximeter assumes that only functional hemoglobin is present (O2Hb or Hb), and the oxygen saturation is calculated on the basis of these amounts.
For example, CoHb is read by the limited wavelength analysis of a pulse oximeter as O2Hb (CoHb is scarlet red), which will falsely elevate the SpO2 reading. The absorption pattern of MetHb is interpreted by the pulse oximeter as 85% saturation; thus, progressively higher levels of MetHb cause the SpO2 value to converge on 85% regardless of the actual SaO2. When the presence of significant amounts of dysfunctional hemoglobin is suspected, a CO-oximeter should be used to determine O2Hb saturation. A multiwavelength laboratory CO-oximeter determines SaO2 more accurately in the presence of dysfunctional hemoglobins because it possesses wavelengths of light that can be used to detect the presence of CoHb and MetHb.
The presence of fetal hemoglobin has not been shown to significantly affect the accuracy of SpO2 measurements because its light absorption properties are similar to those of adult hemoglobin.
Capnography
5 What is capnography, and how does it work?
Capnography is the continuous measurement and graphic display of exhaled carbon dioxide. It is a noninvasive method to assess both ventilation and cardiac output. Most commonly, infrared light absorption by CO2 is the method used to determine the CO2 concentration. Sampling usually occurs in one of two ways. In a mainstream capnograph, CO2 levels are measured with a sensor (light source and detector) placed directly in the patient’s breathing circuit. With sidestream capnography, a continuous sample of airway gas is diverted from the patient’s breathing circuit or airway to the capnograph for analysis and display. The mainstream method has a very rapid response time, but, because the sensor must be placed near the patient, long-term monitoring may be cumbersome. The sidestream method, because it uses a thin plastic sampling tube, is lighter and allows for greater flexibility, but, because transit time is unavoidable, a slower response time (approximately 3-5 seconds) results. Because of mixing of gases in the sample stream, the absolute values of the plateau and baseline may also be attenuated. The sidestream device can also be used with a modified nasal cannula or face mask to monitor CO2 concentrations in the breath of patients who do not have endotracheal tubes in place.
The most commonly used method for measuring carbon dioxide in expired gases is infrared light absorbance. In addition, technologies such as Raman spectrometry and mass spectrometry are reliable, accurate, and responsive but generally more expensive. However, these options also offer detection of a variety of other gases and anesthetic vapors. Colorimetric detectors that attach to endotracheal tubes are available to help assess endotracheal tube placement. The colorimetric detector uses a pH-sensitive indicator strip to semiquantitatively detect exhaled CO2. Although portable and convenient, these devices yield results that are often more difficult to interpret than conventional capnographs, and they do not provide continuous measurement of CO2.
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