Pulse Oximetry
A pulse oximeter is a medical device that provides noninvasive and continuous information about the percent of oxygen that is combined with haemoglobin.
A pulse oximeter is often referred to as a hypoxaemia monitor because it can continuously reflect changes in a patient's arterial oxygen saturation. Monitors are electronic devices intended to keep track of certain situations. Because hypoxaemia can occur at any time and under any clinical circumstance, pulse oximetry is a valuable tool for patient safety and clinical management. A pulse oximeter used to take intermittent measurements of oxygen saturation is more correctly referred to as a "meter," or measuring device. If pulse oximetry is used intermittently, hypoxaemic episodes may be missed.
PULSE OXIMETRY TECHNOLOGY
Pulse oximetry works by applying a sensor to a pulsating arteriolar vascular-bed. The sensor contains a dual light source and photodetector which are used to measure the amount of oxygen that is combined with haemoglobin. The dual light source has a red and an infrared light. These light sources are used because each is absorbed differently by oxyhaemoglobin and deoxyhaemoglobin.
Bone, tissue, pigmentation and venous vessels normally absorb a constant amount of light over time. The arteriolar bed, however pulsates and absorbs variable amounts of light during systole and diastole, as blood volume increases and decreases. The ratio of the amount of each light source absorbed at systole and diastole is translated into an oxygen saturation measurement. An oxygen saturation measurement provided by a pulse oximeter is commonly referred to as SpO2.
ACCURACY
It is important to understand the accuracy level of pulse oximetry measurements. In general, accuracy specifications for pulse oximeters are determined by comparing a saturation obtained from SaO2 and measured by a laboratory co-oximeter (not an arterial blood gas analyzer) with an SpO2 measurement. The SpO2 measurement is taken at the same time arterial blood gas is drawn. This baseline testing is usually performed with healthy adult subjects.
The accuracy specifications for Nellcor® pulse oximeters are usually expressed as "± 2 from 70% to 100% at 1 standard deviation." This means that when the patient's true SaO2 falls within the 70% to 100% range, the Nellcor pulse oximeter will report a saturation that is within 2% of the true saturation about 68% of the time and 4% of the true saturation about 96% of the time.
Below are certain factors that may cause a greater difference between the SpO2 and the SaO2 measured directly from an arterial blood gas.
| Factors | Possible Causes/Rationale | Recommendations |
| Blood Gas Factors | Blood gas is drawn at a different time than the SpO2 measurement is taken. Inaccurate blood gas sampling technique. Blood gas machine is not calibrated accurately. SaO2 is calculated from PaO2 using arterial blood gas analyzer, and not directly measured with laboratory co-oximeter. |
Draw ABG at same time oxygen saturation is measured. Follow proper ABG techniques. Ensure ABG equipment is calibrated. Understand whether SaO2 reports represent measured or calculated Sa02 values. If SaO2 is calculated, do not expect SpO2 value to compare, especially if conditions that cause shifting of the oxyhaemoglobin dissociation curve (such as altered temperature, pH, PaCO2 and 2,3-DPG) are present. |
| Presence of Dysfunctional Haemoglobins | High levels of carboxyhaemoglobin and/or methaemoglobin will cause SaO2 to differ from SpO2 | Suspect elevated dysfunctional haemoglobins if a measured SaO2 differs from SpO2.
Assess oxygenation using a measured SaO2 whenever dysfunctional haemoglobins are suspected. |
| Intracardiac Shunting | Because of abnormal circulatory conditions, such as some forms of congenital heart disease, different oxygen saturation levels may exist in different parts of the body. | If such conditions are present, be aware that SpO2 may differ from SaO2 if measurements are made from different locations. |
| Intravascular Dyes | The injection of intravascular dyes may result in temporary aberration of the SpO2 reading. | Be aware that SaO2 and SpO2 may differ if measurements are made immediately after injection of a dye. |
INNOVATIONS IN TECHNOLOGY: OXISMART AND OXISMART XL
Traditional Pulse oximetry is reliable, especially for immobile and well-perfused patients. However, active patients or those with poor blood flow to a sensor site create challenges for monitoring. Because of motion artifact or because they are weak, pulse signals may be compromised. These conditions may lead to frequent nuisance alarms which can be distracting and time-consuming for clinicians. Because there may be a greater ratio of nuisance alarms to "true" alarms, staff may not respond to every alarm.
Oxismart and Oxismart XL Advanced Signal Processing and Alarm Management are the new generation of pulse oximetry technologies which were developed to address the problem of nuisance alarms common to monitoring conditions of patient motion and low perfusion.
Both Oxismart and Oxismart XL employ signal conditioning to clean the data from the patient. With the newer Oxismart XL, the data is then normailised and whitened to selectively minimalise noise while boosting ture physiologic signals. This whitening process is similar to a stereo systems'sgraphic equalizer, which accentuates some signals and minimizes others. Conditioning enables the Oximeter to perform in low perfusion and high noise environments.
Pattern Match - To determine the pulse rate, Oxismart technolosy utilizes a technology called Pattern Matching, which evaluates the shape of each potential pulse. If the pulse is qualified (i.e. has the right shape) data is sent to the beep tone engine as well as the pulse rate display, and is used to calculate saturation. In this case, pattern matching acts as a gatekeeper for qualifying pulses and passing data through the system. In Oxismart XL technology, pattern matching still drives the beep engine.
However, it is no longer the gatekeeper for the signal datato be sent to the pulse rate or saturation engines, as advanced signal processing technologies for rate and saturation no longer require beat-by-beat recognition. Both Oxismart and Oxismart XL technologies use pattern matching to find the pulse and drive the beep tone. It is the algorithm behind our 'no pulse, no beep' philosophy. The pattern match approach is also well suited to calculating arrhythmic pulses or sudden changes in heart rate, ensuring that during an arrhythmia the Oximeter beeps at the arrhythmic rate. This is the expected behaviour of pulse oximeters in today's market. The Pattern match rate is the proven Oxismart technology which is used to find the pulse, to reliably beep, and to capture arrhythmias and other irregular heart conditions.
Adaptive Comb Filter (ACF) While the strength of pattern matching is to find the pulse, the power of adaptive comb filtering is to reliably track the pulse rate. Similar in concept to digital signal processing use din submarine identification, the ACF cuts through an ocean of non-specific noise to 'lock onto' and follow the pulse at its slowly varying frequency. The algorithm is one of the most important enhancements of Oxismart XL technology.
Best Rate Arbitrator - Oxismart XL technology calculates two pulse rates, one by pattern matching and one by adaptive comb filtering. The best rate arbitrator algorithm chooses the best rate to display, based on proprietary signal quality metrics.
Because a spontaneously moving patient can be assumed to have a pulse, the monitor software continues to search for a pulse as long as continuous motion artifact is detected. If the pulse oximeter fails to detect at least one qualified pulse in a ten-second period, the display will alternate between data and dashes, and a data evaluation period is entered. During this period, if the patient is not moving and has no qualified pulse for six seconds, an audible alarm is triggered and the display flashes zeros. If the patient is constantly moving, the monitor will search for qualified pulses for up to 50 seconds and updates the display each time one is detected. If a qualified pulse signal cannot be detected during this time, an audible alarm sounds and zeros are displayed in the data windows. If an adequate number of qualified pulses are detected, the monitor returns to its normal operating mode and displays updated data on a beat-to-beat basis.
The development of Oxismart technology makes safety monitoring for hypoxemia more reliable and reduces the incidence of nuisance alarms. Clinicians are now better able to identify and manage hypoxemia in any setting where pulse oximetry is used.
In Oxismart XL, three measures are used to ensure that the signal cleaning is accurate and safe. The Kalman C-Lock calculates the saturation value occurring at the pulse rate which is tracked by the ACF. Up to 80 samples per second are used to calculate the ratio of pulsatile red and IR signals through the Least Squares Sat. Finally, the Best Rate Arbitrator chooses the best saturation to display based on the best signal quality.
OXISMART XL
Kalman C-Lock As with the pulse rate engine, saturation is also determined by redundant algorithms. The Kalman C-Lock algorithm calculates the saturation value occurring at the pulse rate that is tracked by the adaptive comb filter. Kalman C-Lock emphasizes signals that are synchronous with the normally rhythmic nature of the pulse signal. This algorithm is a uniquie feature of Oxismart XL technology. Kalman C-Lock can be thought of as a pulse-by-pulse ensemble average of the red and infrared (IR) signals, where the definition of the beginning and end of the pulse comes from the rate (ACF) as opposed to any particular features of the pulse waveform itself. This is similar to the pulse average for some of the earlier generations of Nellcor technology, when ECG triggers are used to define the beginning and the end of a pulse.
Least Squares Sat All pulse oximeters derive their saturation values from a ratio of pulsatile red and IR light signals. Earlier oximeters calculated saturation values using only two data points at the minimum and maximum points on the pulse waveform. With the introduction of Oxsmart technology, up to 80 samples per second are used to calculate the ratio of pulsatile red and IR signals. The oversampling of the waveform requires a calculation method to arrive at the best saturation value. The Least Squares method, which is widely used in statistical calculations, supports the calculation of saturation values in situations of nonrhythmic or rapidly changing pulse rates.
Best Sat Arbitrator Like the arbitrator for pulse rate, the best saturation arbitrator chooses the best saturation to display based on proprietary signal quality metrics.
SAT SECONDS ALARM MANAGEMENT
Nuisance alarms can be caused by two factors: Motion Artefact and Transient Saturation Changes (Episodic Hypoxemia) The exceptional performance of Oxismart XL technology is designed to accurately read saturation through motion. The introduction of SatSeconds Revolutionary Alarm Management gives clinicians for the first time a tool to manage nuisance alarms caused by transient desaturations.
A SatSecond can be thought of as the severity and duration of a Desaturation. It is the product of time and magnitude the patient is outside saturation alarm limits. For example: 1 point either side of the alarm limit for 10 seconds = 10 SatSeconds; 5 points for 20 seconds = 100 SatSeconds.
To manage the nuisance alarms, the clinician may employ the SatSecond feature to trigger an alarm only when the SatSeconds 'clock' reaches a user defined limit of 10, 25, 50 or 100 Sat Seconds. The factory default for this feature is set to 'off'. When three or more violations occur in 60 seconds, an alarm will sound, even if the SatSeconds clock setting has not been reached. This is the SatSeconds 'safety net'.
OPTIMIZING PULSE OXIMETRY
Certain conditions may result in pulse oximetry readings that are unreliable, incorrect or less informative. These considerations and associated recommendations for more reliable monitoring are listed below.
| Consideration | Recommendation |
| Motion | Move sensor to less active site, or replace adhesive. A reflectance sensor may be placed on the forehead, if the patient is not on a ventilator, or is not placed in a Trendelenburg or supine position. Adjust averaging time on pulse oximeter, if possible. Use Oxismart technology to enhance the reliability of measurements during motion. |
| Poor Perfusion | Use an adhesive digit sensor or, if the situations, an ear sensor may be appropriate. Protect sensor site from heat loss or rewarm sensor site as permitted by your clinical policies. Use Oxismart technology to improve the reliability of measurements during poor perfusion. |
| Venous Pulsation | Position digit sensor at heart level. Avoid restrictive taping. Use care when interpreting SpO2 values in patients with elevated venous pressure. |
| Dysfunctional | Dysfunctional haemoglobins, such as carboxy-haemoglobins, Haemoglobins or methaemoglobin, are unable to carry oxygen. However, SpO2 values only report functional saturation - oxygenated haemoglobin as a percentage of functional haemoglobin. Therefore, 5p02 values reported by a pulse oximeter may appear normal when dysfunctional haemoglobins are elevated, although total oxygen content may be compromised due to decreased oxygen carriers. A more complete assessment of oxygenation beyond pulse oximetry is recommended whenever dysfunctional haemoglobins are suspected. |
| Anaemia | Anaemia causes decreased arterial oxygen content by reducing the number of haemoglobin molecules that are available to carry oxygen. Although SpO2 percentages may be in the "normal" range, an anaemic patient may be hypoxic due to reduced haemoglobin levels and therefore reduced total oxygen content. Correcting anaemia can improve arterial oxygen content. The pulse oximeter may fail to provide an SpO2 reading if haemoglobin levels fall below 5 gm/dl. |
| Nail Polish | Remove nail polish (especially brown, blue, green) or apply sensor to unpolished site. |
| Intravascular Dyes | Use care when interpreting SpO2 values after injection of intravascular dyes, which may temporarily affect the reading. |
| Edema | Light from the sensor's light sources may scatter through edematous tissue, although the degree to which this may affect the SpO2 reading is unknown. Position the sensor on non-edematous sites. If peripheral edema is extensive, try the RS-1 5 Nasal Sensor, the Adult Reflectance Sensor, or the D-YSE Ear Clip. |
| Optical Shunt | Optical shunting occurs when some light from the sensor's light sources reaches the photodetector without first passing through the vascular beds. Choose an appropriate sensor for the patient's size, and ensure the sensor remains securely in position with the light sources opposite the photodetector. Replace the sensor when its adhesive is no longer effective. |
| Light Interference | Light interference may result in erratic or inaccurate SpO2 measurements. Cover the sensor with an opaque material in the presence of bright light sources, including direct sunlight, surgical lamps, infrared warming lamps and phototherapy lights. |
| Electrical Interference | Any electrical device, including wall outlets, electrical instruments (such as an electrocautery device), ECG monitors and ventilators, release electrical impulses that may interfere with signal acquisition at the sensor site. This interference can inhibit the pulse oximeter's ability to track the true pulse and result in inaccurate or erratic measurements. Plug the pulse oximeter into a wall outlet that is separate from other devices. Run the sensor cable away from, and perpendicular to, other electrical cables. Shield the sensor site. Newer generation pulse oximetry technology may help minimize electrical interference. |
DETECTION OF HYPOXAEMIA USING PULSE OXIMETRY
Pulse oximetry is a tool that measures arterial oxygen saturation, an important indicator of total arterial oxygen content. Its use as both a safety monitor and clinical management tool has become so significant to patient care that SpO2 is often referred to as the 5th vital sign.
To provide early recognition of hypoxaemia, monitoring with pulse oximetry should be continuous. Spot checks of SpO2 may be used in low-risk patients to verify clinical status and define the potential need for continuous monitoring. Telemetry systems for pulse oximetry allow communication of SpO2 information from the bedside or other remote setting to the caregiver, providing faster identification of changes in oxygen status.
Specific clinical applications for pulse oximetry across the continuum of care include:
- To improve patient safety, clinical management and lower the total cost of care by providing continuous safety monitoring for high-risk patients in any care setting.
- To safely monitor the patient during medical procedures.
- To provide continuous safety monitoring during sedation or pain management.
- When used with telemetry to allow for care of patients in less expensive care settings, especially if they do not require intensive interventions.
- To measure the 5th vital sign in vital signs assessment for patients in any location, including inpatient areas, home and physician offices.
- To determine the need to wean from oxygen therapy, which may result in lowered care costs
- To determine effectiveness of treatments, such as bronchodilators, positioning and suctioning.
- To determine the need for further treatment, such as intubation.
- To assess patient response and tolerance to activities, such as stress testing and activities of daily life.
- To monitor rehabilitation progress.
- To triage patients in the emergency department or clinic.
- To assess admission/transfer/discharge potential of patients.
- To spot check patients for intermittent assessment of oxygenation
Sensor Selection and Use
One of the most critical factors for ensuring reliable pulse oximetry readings is proper sensor selection and application. No single sensor is capable of monitoring all patients under all monitoring conditions. Consider the following when choosing a sensor for your patient:
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Tyco healthcare offers a wide range of Nellcor adhesive and reusable sensors. The cleaning and care instructions for these are always present in the individual boxes.
Sensor Application
Always apply a sensor according to the Directions for Use. Transmittance sensors must have the light source properly aligned with the photodetector. Reflectance sensors require proper alignment of the sensor against the surface of the skin. Tape is provided with the sensor. Do not apply additional tape to the sensor.
Sensor Site Change
Nellcor reusable sensors should be moved to another site at least every 4 hours to preserve skin integrity. Nellcor adhesive sensor sites should be checked at least every 8 hours. The clinician should document sensor site checks and changes. To protect circulation at the sensor site, use only the adhesive that comes with the sensor. Do not wrap additional tape or other material around the sensor.
Infection Control
Sterile, patient-dedicated sensors offer an infection control advantage over reusable sensors. Reusable sensors require cleaning between patients with 70% alcohol to minimize the risk of cross-contamination. Consider sterile, patient-dedicated sensors for infected patients or those at increased risk for infection, such as neonates or immunosuppressed patients.
Summary of Tips
- Ensure the light sources and photodetector of the sensor are properly aligned, as outlined in the Directions for Use.
- Check adhesive sensor site at least every 8 hours and move to a new site, if necessary. Move reusable sensors to a new site at least every 4 hours.
- Adhesive digit sensors may be reused on the same patient, if the adhesive tape adheres without slipping. Replace the sensor whenever the adhesive quality is depleted. Do not apply additional tape.
- When selecting a sensor site, priority should be given to an extremity that is free from an arterial catheter, blood pressure cuff, or intravascular infusion line.
- Reusable sensors should be thoroughly cleaned between patients. Refer to Directions for Use.
Maintenance and Care
Cleaning the monitor
It is recommended that the following checks be performed every 24 months.
- Inspect the equipment for mechanical and functional damage.
- Inspect the safety relevant labels for legibility.
Caution: Do not spray, pour, or spill any liquid on the monitors, their accessories, switches or openings in the chassis.
For surface-cleaning and disinfecting the monitor, follow your instructions procedures or:
- Surface clean using a soft cloth dampened with a commercial, nonabrasive cleaner or a solution of 70% alcohol in water, and lightly wiping the surfaces of the monitor.
- Monitors may also be disinfected using a soft cloth saturated with a 10% chlorine bleach in tap water solution.
Before attempting to clean an SpO2 sensor, read the directions for use enclosed with the sensor. Each sensor model has cleaning instructions specific to that sensor.
Follow the sensor cleaning and disinfecting procedures in the particular sensor's directions for use.
Returning Monitors
Contact the Nellcor Technical Services Department on 01869 328000, for shipping instructions. It is not necessary to return the sensor or any other accessory items with the monitor, unless instructed by the Technical Services Department. Ensure that the monitor is safely wrapped in a carton for protection during shipment.
Warning: The covers should only be removed by qualified personnel. There are no user-servicable parts inside.
Refer to the operation manual for battery changing procedure. If a service is necessary, contact your local Nellcor representative.
