Current pulse oximetry technology provides significant advances in performance and alarm reduction in patient situations involving low perfusion.

Pulse oximetry is a useful method of monitoring patients in many circumstances, and in the face of limited resources, the pulse oximeter is a wise choice for monitoring and assessing several different patient parameters. The pulse oximeter is now a standard monitor that provides clinicians with a noninvasive indication of patients’ cardiopulmonary status. Having been successfully used in critical care, anesthesia and postanesthesia care unit, and home care, pulse oximeters have been introduced in other areas of health care. The technique of pulse oximetry does have limitations and these limitations are enhanced with improper use, such as incorrect sensor selection, poor sensor placement, or a limited understanding of the capabilities of pulse oximetry.

A respiratory therapist performs pulse oximetry testing on an ICU patient.

The overall limitation to a pulse oximeter’s ability to accurately read and track the state of the true arterial circulation is the signal-to-noise ratio. Signal represents the patient information and noise is extraneous information at the detector within the sensor. This is commonly experienced as difficulty in monitoring due to low perfusion or high motion. Signal is generated from arterial blood flow coming from cardiac pulses, while noise comes from a variety of electrical sources as well as motion artifact. When the amplitude of pulses is very low during states of low perfusion, the limiting factors are the inherent noise in the pulse oximeter, external sources of electromagnetic interference such as patient care equipment through the sensor, and motion artifact.¹

Pulse oximeters measure the arterial oxygen saturation of hemoglobin. The technology involves two basic physical principles. First, the absorption of light at two different wavelengths by hemoglobin differs depending on the degree of oxygenation of hemoglobin. Second, the light signal following transmission or reflectance through the tissues has a pulsatile or arterial component, resulting from the changing volume of arterial blood with each pulse beat. This arterial or pulsatile component is differentiated from the nonpulsatile component by the pulse oximeter if perfusion and blood flow at the sensor site are adequate.

The function of a pulse oximeter is affected by many variables, including the calibration of the oximeter against a blood gas device, ambient light, shivering, presence of abnormal hemoglobins, pulse rate and rhythm, vasoconstriction, and cardiac function. A pulse oximeter gives no indication of a patient’s ventilation, only of the oxygenation status, and, thus, can give a false sense of security related to ventilation status with the administration of supplemental oxygen. In addition, there may be a delay between the occurrence of a hypoxic event such as respiratory obstruction and detection of low oxygen saturation by a pulse oximeter due to the transport time and signal averaging times. A pulse oximeter is a valuable noninvasive monitor of a patient’s cardiopulmonary system, which has undoubtedly improved patient safety in many circumstances.

Principles of Current Pulse Oximetry
The pulse oximeter measures the saturation of hemoglobin in arterial blood, which is a measure of the average amount of oxygen bound to each hemoglobin molecule plus any dyshemoglobins. The percentage saturation is given as a digital readout together with an audible signal varying in pitch depending on the oxygen saturation. The pulse oximeter also displays the pulse rate in beats per minute, averaged over 5 to 20 seconds, or the digital value is modified with each beat-to-beat measurement. An additional value is available on some pulse oximeters—the perfusion index (PI) value, which is an indication of relative perfusion at the sensor site.

Oxygen is carried in the bloodstream primarily bound to hemoglobin. One molecule of hemoglobin can carry up to four molecules of oxygen, and it is then 100% saturated with oxygen. The average percentage saturation of a population of hemoglobin molecules in a blood sample is the oxygen saturation of the blood. In addition, a very small quantity of oxygen is carried dissolved in the plasma. Oxygen transported with the plasma is not measured by pulse oximetry.

A pulse oximeter consists of a sensor, together with an oximeter unit, displaying a waveform, the oxygen saturation, and the pulse rate, and/or PI value. The sensor is placed on a peripheral tissue bed such as a digit, ear lobe, or nose. Proper sensor selection and placement are critical to accurate and continuous measurements in the presence of low perfusion. Within the sensor are two or three light emitting diodes (LEDs). The diodes emit light, which is both visible and invisible to the human eye and passes through the tissues to a photodetector.

The pulse oximeter identifies the absorbancy of the pulsatile fraction of blood—defined as the arterial component—from the absorbancy due to nonpulsatile venous or capillary blood and other tissue pigments. Identification and isolation of the pulsatile component are difficult in the presence of low perfusion in the patient. Recent advances in pulse oximetry technology have reduced the effects of low perfusion on pulse oximeter function.

Clinical Uses of Pulse Oximetry
While pulse oximetry appears to be a simple display of a physiological parameter, the actual measurement, analysis, and display of a Spo2 value are a complicated process involving the transfer of energy, the absorbancy of light, signal amplification, and use of sophisticated algorithms to reject artifacts. Pulse oximetry is a safe, noninvasive monitor of the cardiopulmonary status of critically ill patients in the emergency department, during anesthesia, postoperatively, and intensive care.

The Low Perfusion Breakthrough
Low perfusion monitoring is a breakthrough in the measurement and analysis of oxygen saturation in low perfusion states and in quantification of the perfusion at the sensor site. The attributes of Spo2 measurement that make it reliable are its accuracy and its repeatability. These attributes are determined by the interaction of hardware—the sensor and pulse oximeter electronics—and software—the algorithm, used to measure the electrical signal at the sensor and convert the signal into clinically relevant information. Older pulse oximeters have demonstrated issues with noise when amplifying weak signals in low perfusion. Performance enhancements in low perfusion empower RCPs to manage nuisance alarms caused by transient and persistent low perfusion.

Sensors with brighter LEDs may offer a significant advantage over other sensors when monitoring patients with low perfusion or in situations where rapid detection of hypoxemia is critical. Low perfusion is the product of reduced peripheral blood flow and subsequent reduction in the detectable signal at the sensor site. The plethysmographic waveform is valuable in tracking low perfusion states, but the clinician must understand the high level of amplification involved in displaying a plethysmographic waveform. The accuracy of the oximetry readings available from the new technologies improves performance and provides consistent clinical information superior to other pulse oximetry devices when used on severely ill patients with extremely low perfusion.

Algorithms in Low Perfusion
Pulse oximetry is based on the fact that oxygenated blood absorbs red light differently than infrared light. The ratio of the two absorptions is used to calculate the arterial blood oxygenation. The older algorithms examine the changes in the signal from the sensor during arterial blood pulsation and identify the minimum and maximum of the red and infrared signals that correspond to the pulsation. The ratio of the values of both red and infrared signals is then calculated as the saturation value. Noise present in the signal affects the correct detection of these ratios and may cause inaccurate Spo2 values. If the pulse pressure—systolic/diastolic difference—is small as often occurs in low perfusion, the noise element of the signal strength is stronger and the signal element is weaker.²⁰

Key Technology Differences
Conventional pulse oximetry assumes that arterial blood is the only blood moving (pulsating) in the measurement site. During low perfusion states, the venous blood pulsations are difficult to characterize and eliminate, which causes older pulse oximetry to display incorrect values. Current pulse oximetry technology identifies the venous blood signal, isolates it, and, using mathematical models, cancels the noise and extracts the arterial signal. The current technology works accurately where conventional pulse oximetry tends to fail.

The new mathematical models used in current pulse oximetry technology have a different approach to displaying a reliable and accurate signal. Multiple algorithms are used to identify and isolate the noise produced by motion and amplify the signal in a low perfusion state.

The result is an increase in measurement accuracy during low perfusion, which is particularly noticeable in the presence of motion artifacts, but also means a significant reduction in the number of false alarms—an achievement that has been proven in recent laboratory results. State-of-the-art low noise hardware contributes to further improved measurement accuracy with extremely weak signals, for instance, if the patient has very low perfusion.

Improved Oximeter Performance with Patients in Shock and Low Perfusion
The first priority in establishing accurate Spo2 measurements during periods of low perfusion is sensor selection and placement. Select a sensor that is appropriate for the patient’s size and site. Sensor placement is critical to proper functioning in low perfusion states. Select an appropriate location for the sensor—one that is least subject to motion and that has adequate circulation. Use an alcohol wipe to make sure the monitoring site is clean and dry. Align the LED and photodetector markings on the sensor so they are exactly opposite each other. Evaluate the plethysmographic waveform and signal strength to assess the accuracy of data. While the new technologies substantially eliminate the low perfusion problems associated with conventional pulse oximeters, the design of the sensors can reduce the problems associated with noise inherent in low perfusion, producing a cleaner and artifact-free waveform and digital values on the display. Current technologies identify and isolate the noise from the arterial signal, which eliminates problems related to low peripheral perfusion and weak signal-to-noise ratios during measurement.

With the introduction of these new technologies designed for use with all patient populations in all clinical environments, RCPs can address the challenges of consistently tracking patients’ saturation and pulse rate in a low perfusion state. The new technologies produce accurate pulse rate and saturation values on patients during low perfusion and can reduce nuisance alarms. The latest technologies respond to the clinical needs in many patient care areas. The pulse oximeters available today help busy clinicians accurately assess a patient’s oxygen saturation and pulse rate values and prevent them from spending valuable time on nuisance alarms during low perfusion.

Building on the foundation of new technology, which is at the heart of today’s pulse oximeters, manufacturers bring clinicians the next level of sophisticated technology. RCPs can select from a full spectrum of pulse oximetry monitoring options and sensors that match technology with patient size, acuity, and cost concerns based on clinical condition and environment. No single algorithm can accurately track through every patient condition. Multiple algorithms, working in parallel more accurately, track both saturation and pulse rate in low perfusion.

New Technologies
Advanced mathematical algorithms added to the already proven technology make new technologies a powerful force that filters out corrupted signals to deliver accurate plethysmographic waveforms, pulse rate calculation, and oxygen saturation values. New technologies use multiple algorithms within a single monitor. Within a few pulsatile signals identified at the sensor site, the pulse oximeter locks onto a pulse signal, and then multiple algorithms work together to track the pulse rate and saturation. This means clinicians receive accurate patient readings—even during low perfusion, environmental noise, and motion conditions.

Practical Pulse Oximetry in Low Perfusion States
Turn the pulse oximeter on and wait for it to go through its calibration and check tests. Select the sensor with particular attention to correct sizing. Position the sensor with proper alignment of the emitter and detector. Position the sensor on the optimal digit or other sensor site, avoiding excess pressure to reduce the risks of pressure necrosis. Monitor the plethysmographic waveform and confirm the presence of the dicrotic notch. Monitor and document the PI value. Changes in the PI value may indicate changes in perfusion at the sensor site and act as an early indicator of peripheral perfusion of altered hemodynamics. Pulse oximetry is a sensitive index of peripheral perfusion.³

Artifact in pulse oximetry arises when the pulse oximeter is overloaded with signals from the sensor, caused by patient movement or misalignment of the emitter and detector at the sensor site. One of the most common patient movements resulting in high motion and/or low perfusion is finger flexion. This patient activity results in significant artifacts in the signal. These artifacts can limit severely the applications of pulse oximetry in monitoring the conscious subject. The major manufacturers of this technology have introduced practical solutions to this problem that involve identification and isolation of the artifacts. Current mathematical algorithms cancel the artifact signals using information partly established during periods of low artifact. The method is successful in characterizing the artifacts present in low perfusion situations. Utilization of these artifact reduction methods simplifies the derivation of oxygen saturation from the pulsatile signals.

Perfusion Index
The PI is an indication of the quality of patients’ perfusion at the sensor site. Though the value is not a direct measure of patients’ perfusion, RCPs can use the trend in the PI value to monitor changes in local tissue perfusion. This may be used to trigger certain clinical actions. For example, a decreasing perfusion value indicates deterioration in the patient’s perfusion, which could be a good sign to change the site of the Spo2 sensor.

PI is used to assess the viability of limbs after vascular, aesthetic (plastic), limb re-implantation, and orthopedic surgery, and where there is soft tissue swelling or aortic dissection.

Limitations in Low Perfusion
Pulse oximetry is not a monitor of ventilation. Pulse oximetry provides an analysis of adequate oxygenation, but no direct information about ventilation. Critically ill patients pose a challenge for pulse oximetry with reduced tissue perfusion combined with motion. Carboxyhemoglobin caused by carbon monoxide poisoning induces displayed saturation values to move toward 100%. A pulse oximeter is extremely misleading in cases of carbon monoxide poisoning for this reason and should not be used. Co-oximetry is the only available method of estimating the severity of carbon monoxide poisoning. Vasoconstriction and hypothermia cause reduced tissue perfusion and failure to register any pulsatile signal.⁴ The sensor must be correctly sized by the RCP and should not exert excessive pressure. Special sensors are now available for pediatric use.

The penumbra effect reemphasizes the importance of correct sensor positioning. This effect causes falsely low readings and occurs when the sensor is not symmetrically placed, such that the path length between the two LEDs and the photodetector is unequal, causing one wavelength to be overused in the calculations of saturation. Repositioning of the sensor often leads to sudden improvement in saturation readings. The penumbra effect may be compounded by the presence of variable blood flow through the tissue underlying the sensor.

Conclusion
Despite respiratory care clinicians’ acceptance of this technology, pulse oximetry has proven unreliable in certain circumstances in the past. Patient motion, low peripheral perfusion, intense ambient light, and electrical interference often result in inaccurate readings and false alarms. Manufacturers have continuously improved their technology to address these potentially dangerous and costly issues. Current pulse oximetry technology provides significant advances in performance and alarm reduction in patient situations involving low perfusion.

Pulse oximeters provide noninvasive analysis of the arterial hemoglobin oxygen saturation. Two principles are involved:

1. Differential light absorption by hemoglobin and oxyhemoglobin; and
2. Identifying pulsatile component of signal.

Pulse oximetry does not provide a direct indication of patients’ ventilation, only of oxygenation. A time delay does exist between a potentially hypoxic event such as respiratory obstruction and a pulse oximeter detecting and displaying a low oxygen saturation.

Inaccuracies in the displayed value may be due to different calibration factors, ambient light, shivering and vasoconstriction present in the hypothermic patient,⁵ abnormal hemoglobins such as carboxyhemoglobin, and changes in pulse rate and rhythm. Advances in pulse oximetry technology have led to improved performance with multiple algorithms for varying types of motion and low perfusion. Recent studies have evaluated new technologies that use new methods for amplification of pulse oximetry data, characterization of noise and patient motion, and identification of patients’ plethysmographic waveform and numeric values. These technologies appear to offer significant advances in patient situations involving low signal-to-high noise, such as low perfusion. In low perfusion, the signal (patient information) is extremely low and must be amplified by the pulse oximeter. The noise produced during this amplification can obscure the patient information and must be filtered to produce a clear and accurate measurement. Older technologies cannot differentiate between arterial and venous pulsations during periods of low perfusion and/or motion. New technologies identify the venous blood signal and isolate it from the arterial signal. Using mathematical filtration, the new technologies cancel the noise, which allows the measurement to reflect arterial oxygen saturation.

The impact of artifact reduction on the displayed Spo2 value has been analyzed over a range of low perfusion. Severe artifacts, such as motion or light interference, are amplified in the presence of low perfusion. Improvements are seen with the new pulse oximetry technologies, even when the artifacts are produced by high levels of pulse oximetry amplification to detect the pulsatile component of the light sensed at the detector.

Dan Hatlestad is an author, speaker, and trainer in Littleton, Colo; prioritymed@mindspring.com.

References
1. Gehring H, Hornberger C, Matz H, et al. The effects of motion artifact and low perfusion on the performance of a new generation of pulse oximeters in volunteers undergoing hypoxemia. Respir Care. 2002;47:48-60.
2. Palve H, Vuori A. Minimum pulse pressure and peripheral temperature needed for pulse oximetry during cardiac surgery with cardiopulmonary bypass. J Cardiothorac Vasc Anesth. 1991;5:327-330.
3. Joyce WP, Walsh K, Gough DB, et al. Pulse oximetry: a new noninvasive assessment of peripheral arterial occlusive disease. Br J Surg. 1991;78:889.
4. Schramm WM, Bartunek A, Gilly H. Effect of local limb temperature on pulse oximetry and the plethysmographic wave. Int J Clin Monit Comput. 1997;14:17-22.
5. Villanueva R, Bell C, Kain ZN, Colingo KA. Effect of peripheral perfusion on accuracy of pulse oximetry in children. J Clin Anesth. 1999;11:317-322.