Current pulse oximetry has served clinicians well in the years since it became clinically useful, delivering important information on oxygenation in respiratory care patients
Pulse oximetry is a widely accepted noninvasive method of monitoring the percentage of saturated hemoglobin (Hb). It consists of a sensor attached to the patients tissue site, cabling between the sensor and the pulse oximeter, and the oximeter, which may be integrated into a multiparameter monitor. The pulse oximeter, or the monitor into which it is integrated, displays the percentage of saturated Hb oxygen together with an audible signal for each pulse beat, pulse rate, and, in some models, a graphical display, the plethysmograph, and quantification of perfusion at the sensor site. Problems associated with patient-induced motion and low peripheral perfusion have been common limitations of pulse oximetry since it was introduced. Recent developments in technology detect episodes of desaturation even in the presence of significant motion and low perfusion.
Pulse oximetry has become the standard technique for monitoring oxygenation during procedural sedation, anesthesia, therapy in the intensive care unit and neonatal intensive care unit, and recovery from anesthesia in the postanesthesia care unit. A pulse oximeter monitors, calculates, and displays the measured hemoglobin saturation as a percentage of SpO2. This technology has been rapidly adopted throughout the health care arena due to the ease of use and the value of the information provided continuously and noninvasively during monitoring.
Certain therapies increase the risk for hypoventilation and hypoxemia and the patients should be considered candidates for continuous monitoring with pulse oximetry. These therapies include anesthesia, recovery from anesthesia, pain management, obstructive sleep apnea assessments, and invasive and noninvasive ventilation. Specific groups are also at risk including those who are obese or have significant cardiopulmonary disease, and obstetric patients.
Pulse oximeters may be used in a variety of situations but are of particular value for monitoring oxygenation, peripheral perfusion, and pulse rate during procedural sedation. In the critical care environment, pulse oximeters are utilized in patients requiring mechanical ventilation and may detect oxygenation problems prior to the development of clinical signs of hypoxia.
Although pulse oximeters are quite valuable in monitoring oxygenation, the technology provides little clinical information on the adequacy of ventilation. Carbon dioxide levels can be modified significantly with no change in SpO2 levels, particularly in patients with chronic obstructive pulmonary disease. These limitations in the assessment of patients developing respiratory failure due to carbon dioxide retention may indicate a need for end-tidal carbon dioxide (Etco2) monitoring in these patients. The value must always be utilized in association and compared with the patients clinical status. When responding to a pulse oximetry-associated alarm, the RCP must always initially evaluate the patient and subsequently respond to the monitor.
The pulse oximeter uses light emitting diodes (LEDs) within the sensor to generate two wavelengths of lightone at 650 nm and the other at 805 nm. Hemoglobin and other tissues such as bone, fat, skin, and fingernails between the LEDs and the detector absorb these red and infrared light wavelengths. The amount of light absorbed varies, depending on the pulsating arterial flow in the tissues and the saturation level of the hemoglobin in the erythrocytes. The pulse oximeter calculates the absorption based on two physical principles: the two wavelengths of light are absorbed differently by oxygenated hemoglobin and deoxygenated hemoglobin; and tissues such as bone and fat and venous blood absorb a relatively constant amount of light, producing a relatively constant rate of light absorption.
The ratio of absorption at these two wavelengths varies with the oxygen saturation. This ratio is converted to a numerical SpO2 value through the use of calibration curves derived from the manufacturers volunteer desaturation studies.
All pulse oximeters require a pulsatile signal at the sensor site to generate a digital value on the display. The pulsatile quality of the signal is produced by the fluctuating volume of arterial blood between the emitter and detector in the sensor. In situations involving decreased blood flow and/or high peripheral vascular resistance, the pulse oximeter may be unable to display an accurate measurement.
The audible tone produced by a pulse oximeter varies with the measured saturation. The tone is valuable while visually monitoring the patient. Consider reducing or eliminating the audible tone once treatment is complete to allow the patient to rest without disturbance.
Several manufacturers provide a plethysmographic waveform correlating to the flow of blood (pulsatile signal) at the sensor site. Specific models automatically increase the gain or size (autoscaling) of the plethysmographic waveform in situations involving reduced blood flow at the sensor site. This autoscaling of the waveform may prove misleading, as the waveform does not change in height to reflect changes in perfusion. The use of a perfusion index scale quantifies the perfusion at the sensor site and may prove to be of value when correlating the plethysmographic waveform to blood flow (see Table 1, page 46).
The Revolution in Technology
The revolution in pulse oximetry continues due to recognition of problems associated with the current technology. While oximeters have quickly became a standard clinical tool in health care, certain patients and situations produce erratic or absent numerical values. In addition to detecting pulsatile light absorbance, pulse oximeters are prone to detect multiple forms of energy at the detector. Clinicians who care for patients within these groups are, at times, frustrated by the loss of data. Current pulse oximeters are manufactured by dozens of firms globally and found in the majority of health care facilities.
Once they became widely used, it became clear that pulse oximeters did not work in all situations. The most common causes of inaccuracies and absent data are low patient perfusion at the sensor site and patient motion. Other causes include interference by ambient light and electrosurgery. Failures can result in false alarms, inaccurate readings, and the loss of continuous pulse oximetry numerical display.
Consider the issue of motion. All clinicians recognize the artifact produced in an electrocardiogram (ECG) tracing due to patient motion. This artifact is produced by the addition of energy to the sensors. In ECG monitoring, the sensors are the electrodes applied to the patient skin. Patient motion, electrical energy, and clinical interventions all produce artifact in the ECG tracing. Various filters are used in ECG monitors to reduce or eliminate the presence of artifact on the display.
In pulse oximetry, the sensor is the set of LED emitters and detectors in the probe. The probe acquires both pulsatile signals from the patient and extraneous artifacts from a variety of sources. Both are forms of energy. Any source of energy applied to the sensor may produce artifact. Reducing or filtering the artifact is critical to the quality of the signal and the data displayed on the monitor.
Motion results in significant oxygen saturation reading failures in all patient groups. To reduce the motion artifact, several manufacturers have coupled improvements in both software algorithms and hardware enhancements to the pulse oximeter and patient sensor. Reduction in motion artifact is significant when using such devices. Problems with motion artifact as well as the need for enhanced performance in low perfusion have created a clinical need for the advanced technologies.
A common method for reducing the impact of artifact on numerical values is to modify the signal averaging time. A longer signal averaging time will average out low spurious SpO2 values, but loses the ability to quickly detect and display rapid desaturations. A shorter signal averaging time may respond rapidly to changes in saturation but may produce more false alarms.
Fluorescent and especially bright procedural lights can cause both false-normal and high readings, even when there is no patient connected to the pulse oximeter. Covering the probe with opaque material has been shown to minimize these effects of ambient fluorescent light. Other potential sources of inaccuracy include the presence of finger nail polish, intravenous dyes, and dyshemoglobins such as carboxyhemoglobin due to smoking (see Table 1).
|1. Reduced pulsatile blood flow at the sensor site. An indicator of perfusion at the sensor site is valuable in monitoring changes in local peripheral perfusion. Clinical situations that may produce diminished pulsatile flow include:
peripheral vasoconstriction hypovolemia
severe hypotension hypothermia
reduced cardiac output peripheral vascular disease
2. Venous congestion may produce venous pulsations. Venous congestion of the limb may affect readings as can a badly positioned probe. When readings are outside the expected range, the operator should attempt repositioning after checking the patients status. The plethysmographic waveform is valuable in confirming an accurate measurement. Clinical causes of venous congestion include:
3. Artifact due to any form of energy reaching the detector portion of the sensor. Just as patient movement such as shivering causes significant artifact on an ECG monitor, any form of artifact will cause artifact in a pulse oximeter. Common causes of artifact include:
4. Pulse oximeters cannot differentiate between the different forms of saturated hemoglobin. Common forms of saturated hemoglobin that interfere with accurate pulse oximetry readings include:
5. The diagnostic use of intravenous methylene blue may produce an increase in methemoglobin levels with a corresponding reduction in the displayed saturation.
|Table 1. Situations that may produce erroneous pulse oximeter measurements.|
Failures related to pulse oximetry monitoring are relatively common. The duration of failures may be prolonged at the point at which the patients physiological information is required most, such as periods of hypotension. The clinician may experience failures frequently throughout care, assessment, and intervention. The failure rate may increase with prolonged periods of hypotension and hypothermia. Critical care monitor alarms are a major burden on both clinicians and patients.2 The relatively high incidence of false alarms can result in potential delays in response or ignoring alarms completely. Desensitization of clinical staff to alarms carries the risk of late intervention in a severe hypoxemic event.3
The low-perfusion limits of pulse oximetry have recently been extensively investigated. Comparisons of current pulse oximeters displayed values with advanced technologies demonstrate significant clinical superiority in advanced technologies. These technologies also demonstrate the lowest dropout rate or loss of digital data.
Over the past 4 years, manufacturers have been developing advanced instruments to improve the reliability of pulse oximetry. These advanced technologies include signal processing and alarm management technologies.4 Continuous pulse oximetry in the population of patients at risk for hypoxemia demonstrates that advances in signal processing reduce the frequency of false alarms. Advanced pulse oximetry technology identifies and rejects artifacts that could otherwise be mistaken for a pulse. It distinguishes between the actual loss of a pulse and one that is obscured by low perfusion, motion artifact, and electronic or optical noise. This technology is currently available in several pulse oximetry products including those integrated into critical care and home care monitors.
The enhanced pulse oximeters may offer an advantage compared with oximeters without advanced technology because of the reduced frequency of alarms and total time in alarm status when monitoring patients in the hospital and home health care environments.
Advanced pulse oximetry technology has made safety monitoring for hypoxemia more reliable and has reduced nuisance alarms. Because of this advancement in technology, clinicians can identify and rapidly intervene during periods of hypoxemia in all patient populations in all clinical environments.
Pulse oximetry alarm management algorithms are designed to identify and reject artifacts that could be calculated into a pulsatile signal. Technological advances in pulse oximetry use a variety of signal filters and alarm management algorithms to reduce the incidence of lost data.5
Several manufacturers have developed advanced algorithms for processing the red and infrared signals. These products use advanced digital signal processing, sophisticated low-noise monitor designs, and advanced sensor technology to reduce the effect of ambient noise due to light and the influence of nonarterial physiologic noise produced during motion.6 These technologies do not use alarm management algorithms but provide the clinician with continuously updated information during the period(s) of low perfusion and motion.7
Research indicates desaturations due to artifact during motion and low perfusion are usually caused by the incorrect detection of pulsatile flow at the sensor site caused by motion. Pulsating venous blood along with the pulsating arterial signal may confuse the oximeter. Current technologies cannot consistently differentiate between pulsating arterial and pulsating venous blood at the sensor site. The advanced algorithms identify the pulsating arterial blood components in the pulsating red and infrared signal and define the actual hemoglobin saturation. The measurement error produced by current pulse oximeters may not be very significant for normothermic and adequately perfused patients, but in the presence of poor peripheral perfusion, the venous O2 saturation may be significantly decreased and erroneous detection of pulsating venous blood can incorrectly decrease the displayed SpO2. The impact of hypothermia, reduced cardiac output, and reduced arterial blood pressure combined with an increase in peripheral vascular resistance all play interactive roles in pulse oximeter performance.
Advanced pulse oximeters with hardware and software improvements reduce the number of alarms due to desaturation episodes, and loss of clinical data. The frequency and nature of superfluous pulse oximetry alarms in current technologies may create an increased demand on RCPs time in responding to alarms as compared to the advanced technologies.
The correct use of the oximeter is to measure only arterial oxygen saturation (SpO2), pulse rate, and (in some models) relative perfusion index pulsatile value. A pulse oximeter does not measure respiration and under no circumstances should one be used as a substitute for an apnea monitor. It must not be used as the primary monitor for patients monitored for apnea in the hospital or the home. A pulse oximeter is often used during sleep studies for adults, but must be used only to gather information regarding SpO2, pulse rate, and perfusion index values. Specific models of pulse oximeters are available for use in a magnetic resonance imaging environment.
Prolonged monitoring or patient condition may require periodically rotating the sensor site. To reduce the risk of blistering, skin erosion, or ischemic skin necrosis, the sensor should be rotated between sites as specified in the user instructions from the manufacturer. If any evidence of these skin conditions (discoloration or reddening) appears before the specified time period, the sensor site should be changed immediately. To prevent patient injury or equipment damage, only the sensor specified by the manufacturer should be used. Sensors cannot be used interchangeably between different oximeters, and if the sensor is damaged or does not function properly, it should be discontinued immediately.
Current pulse oximetry has served clinicians well since it became clinically useful, delivering important information on oxygenation in respiratory care patients. It is subject to the loss of data in certain situations, particularly those of low perfusion and motion. Advanced pulse oximetry technology has evolved rapidly in the recent past. There have been promising clinical results from some of the latest generation of devices, achieving low rates of missed events and false alarms even with conditions of high patient motion and/or low perfusion. All of the advanced technologies use differing algorithms, software, and hardware to track the patient information. Thus, it is more important than ever for RCPs to understand the differences and know which technology is used in the monitors.
Dan Hatlestad is a trainer, speaker, and author from Littleton, Colo, and can be contacted at [email protected].
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