It should be apparent that the accuracy of a pulse oximeter reading is dependent on the identification of an arterial pulse. In cases where perfusion is low, such as hypovolemia, the pulse oximeter may not be able to accurately identify a pulsatile signal, resulting in either an intermittent or absent SpO₂ reading. Other situations that may contribute to this problem include administering peripheral vasoconstrictors, hypothermia, and heart-lung bypass (i.e., extracorporeal membrane oxygenation).9 Although some oximeters are better than others in dealing with low perfusion states, compensation for the weak signal associated with low perfusion states is limited. The reason for this limitation is that a low perfusion state produces a low signal-to-noise ratio and, thus, a signal that can be potentially altered by motion artifacts.

The Masimo Signal Extraction Technology (SET) (Masimo Corp, Irvine, Calif.) pulse oximeter is a relatively new processing system that uses special algorithms to minimize interference from motion artifacts. Conventional pulse oximetry assumes that arterial blood accounts for the pulsations or AC component of the pulse oximetry signal, while venous blood produces a non-pulsatile or DC component of the signal. During patient motion, the venous blood may also contribute to the pulsatile signal and cause the pulse oximeter to underestimate the SpO₂ because it cannot distinguish between the arterial and venous blood. Masimo SET signal processing identifies the venous blood signal, isolates it, and using adaptive filters, cancels the noise and extracts the arterial signal. When tested on healthy individuals simulating various motion artifacts, the Masimo SET oximeter exhibited a much lower error rate compared with conventional pulse oximeters. Studies with critically ill patients also demonstrated fewer false alarms of hypoxemia when compared to conventional pulse oximeters¹⁰

Dysfunctional Hemoglobins and Dyes

Adult blood typically contains four types of hemoglobin: reduced or deoxygenated hemoglobin (HHb), oxyhemoglobin (O₂Hb), carboxyhemoglobin (COHb), and methemoglobin (MetHb). Two terms that are often used when describing oxyhemoglobin saturation are fractional and functional saturations. Fractional hemoglobin saturation is calculated by dividing the amount of oxyhemoglobin measured by the sum of concentrations all four types of hemoglobin present, or

Fractional O2Hb=O2Hb/[HHb+O2Hb+COHb+MetHb].

Functional hemoglobin saturation is calculated by dividing the oxyhemoglobin concentration by the concentration of hemoglobin capable of carrying oxygen, or

Functional O2Hb=O2Hb/[HHb+O2Hb].

Laboratory CO-oximeters measure all four types of hemoglobin by using separate wavelengths of light to identify each species, whereas pulse oximeters use only two wavelengths to quantify the amount of O₂Hb and HHb present. Laboratory CO-oximeters are therefore capable of reporting fractional oxyhemoglobin saturations, and pulse oximeters are typically described as evaluating functional hemoglobins.

This description of a pulse oximeter capability may be somewhat misleading because, as Fig. 10-2 shows, O₂Hb and COHb have similar absorption coefficients for red light (660 nm), whereas COHb is relatively transparent to infrared light (940 nm). Additionally, MetHb and HHb have the same absorption coefficients for red light; however, MetHb demonstrates a greater absorbency for infrared light (940 nm) than does oxyhemoglobin. Accordingly, the presence of significant levels of COHb, as occurs in carbon monoxide poisoning, will lead to an overestimation of SpO₂ (Key Point 10-1).⁸ Methemoglobinemia, a complication of administering benzocaine (topical anesthetic) and dapsone (an antibiotic used to treat malaria and Pneumocystis carinii), can cause erroneous SpO₂ values because MetHb absorbs both red and IR light.¹¹ If enough MetHb is present to dominate all pulsatile absorption, the pulse oximeter will measure a red-to-IR ratio of 1 : 1, corresponding to an SpO₂ of about 85%. Consequently, the pulse oximeter reading will overestimate or underestimate the true oxyhemoglobin saturation, depending on whether the actual SaO₂ is less than or more than 85%.^9,11

It is well documented that intravascular dyes can adversely affect SpO₂ values by absorbing a portion of the incident light emitted by the pulse oximeter diodes. Scheller and colleagues¹² demonstrated that injection of methylene blue and indigo carmine into human volunteers caused a false drop in SpO₂, whereas indocyanine green had little effect on pulse oximeter values.

Nail polish (particularly blue and black nail polish) can severely affect SpO₂ readings. Some believe that nail polish causes light to be shunted around the finger periphery (called optical shunting).13,14 The transmitted light never comes in contact with the vascular bed; consequently, SpO₂ values can be erroneously high or low, depending on whether this light takes on a pulsatile character. This problem can largely be alleviated by placing the device over the lateral aspects of the digit rather than over the nail.

Theoretically, skin pigmentation should have no effect on pulse oximeter readings. In practice, however, SpO₂ readings are typically higher for patients with dark skin pigmentation. For example, an SpO₂ of 95% in an African American patient may actually represent an SaO₂ of only 92%.15–17 Many clinicians, therefore, use higher threshold values (i.e., SpO₂ >92%) for initiating oxygen therapy in African American patients. Although using higher target SpO₂ values does not lead to untreated hypoxemia in most of these patients, some will have arterial oxygen pressure (PaO₂) values as high as 200 mm Hg when therapy is based on measured SpO₂.

Bilirubin, a breakdown product of heme metabolism, is the pigment responsible for the yellow discoloration seen in jaundiced patients. Although an elevated bilirubin level (>20 mg/dL) has been shown to affect O₂Hb values recorded with CO-oximetry, pulse oximetry measurements do not appear to be affected by hyperbilirubinemia.^18,19

Direct sunlight and external light sources (e.g., fluorescent lights, heat lamps, fiberoptic light sources, and surgical lamps) have been shown to affect SpO₂ readings adversely.20–22 Most commercially available pulse oximeters attempt to compensate for this interference by continually cycling the transmitted red and IR light on and off at a rate of about 480 cycles per second.

Chemical capnometers are handheld devices composed of specially treated filter paper in a plastic casing that can be attached to the patient’s endotracheal tube (ET) (Fig. 10-5). The amount of CO₂ present in the patient’s inspired and exhaled gas can be estimated by noting the color changes in the filter paper. Each device uses a characteristic series of colors to indicate the approximate CO₂ concentrations. For example, the Portex CO₂ Clip device, which is manufactured by Smiths Medical Canada (St. Paul, Minn.), appears blue when 0 to less than 1% CO₂ in the respired gas, green when the exhaled gas contains 1% to 2%, and green-yellow to indicate a CO₂ concentration of 2% to 5%; and yellow for CO₂ concentrations over 5.0%.

Chemical capnometers are particularly useful in emergency situations in assessing airway placement. It is important to understand that changes in the color of the filter paper are the result of a chemical reaction that affects the pH of the filter paper. (Note that acidic liquids like regurgitated gastric contents contact the filter paper; an irreversible color change will occur and render the device unusable.28) Proper placement of an ET can be determined because the color of the paper will change continually as inhaled and exhaled CO₂ levels vary while the patient breathes into and out of the device. In some cases, ET placement in the trachea rather than the stomach may be difficult to determine because the patient’s gastric PCO₂ may be elevated after receiving mouth-to-mouth breathing or if the patient has recently ingested a carbonated beverage.

Infrared spectroscopy is based on the principle that molecules containing more than one element absorb infrared light in a characteristic manner.29 CO₂ absorbs IR radiation maximally at 4.26 µm. The concentration of CO₂ in a gas sample can be estimated because its concentration is directly related to the amount of IR light absorbed.³⁰ The presence of other gases (e.g., water [H₂O] and nitrous oxide [N₂O]) can adversely affect the accuracy of CO₂ measurements by causing a phenomenon called pressure broadening. This phenomenon occurs because the peak absorbance of IR radiation by CO₂ lies between the peak absorbencies of H₂O and N₂O. The presence of these gases increases the absorption of infrared radiation and result in erroneously high CO₂ readings. Pressure broadening can be minimized by removing water vapor from the gas sample before it is analyzed and by using electronic filters to subtract the IR absorption by gases other than CO₂.²¹

Figure 10-6 is a schematic of a double beam, positive-filter capnograph. The gas is drawn into a cuvette inside the sample chamber. IR radiation is beamed through the cuvette and through a reference chamber containing CO₂-free gas. The CO₂ in the sample chamber absorbs some of the radiation, reducing the amount of radiation that reaches the detector. The difference between the radiation transmitted through the sample cell and the radiation transmitted through the reference causes movement of a diaphragm in the detector chamber. This movement is converted into an electrical signal, which is amplified and displayed. The displayed value can be displayed in millimeters of mercury (mm Hg) representing the partial pressure, or as percent CO₂ (% CO₂).

In clinical practice, IR analyzers are typically classified according to the method of sampling of respired gases. Fig. 10-7 illustrates two methods: sidestream sampling and mainstream sampling. In sidestream devices, gas from the airway is extracted through a narrow plastic tube to the sample measuring chamber, which is located in a separate console. In mainstream devices, the sampling chamber attaches directly to the ET and analysis is performed at the airway.

Phase 3 (i.e., alveolar plateau) becomes indistinguishable when physiological dead space increases (as occurs in COPD) (Fig. 10-10, A). Hyperventilation is characterized by a reduction in PaCO₂ and, therefore, PetCO₂ (Fig. 10-10, B). Conversely, hypoventilation is associated with elevated PaCO₂ and PetCO₂ (see Fig. 10-10, B). Figure 10-10, C, is a capnogram of a patient with Cheyne-Stokes breathing. During bradypnea, phase 4 will occasionally show cardiac oscillations resulting from the motion of the beating heart transferred to the conducting airways (Fig. 10-10, D). Failure of the capnogram to return to baseline is an indicator of rebreathing of exhaled gas (Fig. 10-10, E). Figure 10-10, F, shows a capnogram for a paralyzed patient, demonstrating the characteristic “curare cleft” during phase 3. This is a positive sign that the paralyzed patient is receiving insufficient neuromuscular blockade; however, other factors can contribute to this capnographic finding, such as patient inspiratory effort during controlled ventilation (e.g., patient-ventilator asynchrony).

As mentioned, capnography can also be used to detect accidental esophageal intubation. The gastric PCO₂ generally is equal to room air; therefore failure to detect the characteristic changes in CO₂ concentration during ventilation possibly indicates esophageal intubation. However, low perfusion of the lungs is associated with low PetCO₂ and should not be confused with esophageal intubation. Also, the gastric PCO₂ may be elevated after mouth-to-mouth breathing or if the patient recently ingested a carbonated beverage. Case Study 10-2 gives an example situation of the use of capnographic data (Key Point 10-2).

The P(a-et)CO₂ for tidal breathing should be approximately 4 to 6 mm Hg. It is elevated in patients with COPD, left-sided heart failure, and pulmonary embolism caused by an increase in physiological dead space.31 Another technique that can be used to further evaluate the severity of the disease is to compare arterial PCO₂ measurements with maximum expired PCO₂ measurements (the arterial to maximum expiratory PCO₂ gradient). With this technique, the expired PCO₂ recorded at the end of a maximum exhalation is compared with the PaCO₂. The difference normally is minimal. Patients with COPD and left-sided heart failure do not show an arterial to maximum expired PCO₂ difference, whereas patients with pulmonary embolism do show an increased gradient. Fig. 10-11 shows the capnographic appearance of these differences.

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