Patient Monitoring Technical Library

SpO2 Monitoring

Performance Digest

0 6 1 4 - 907736
Printed :2018/xx/xx

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Contents
1. Introduction ...................................................................................................................................1
2. APPLICATION AND MEASUREMENT PRINCIPLE OF PULSE OXIMETRY .........................................2
2.1. SpO2 Applications ...........................................................................................................2
2.2. Principle of Measurement..............................................................................................3
Oxygen Saturation and Hemoglobin.......................................................................3
Pulse Oximeter and Light Wavelengths ...................................................................3
3. INTERNATIONAL PULSE OXIMETER STANDARD .............................................................................5
3.1. SpO2 Accuracy ...............................................................................................................5
Measurement Criteria ................................................................................................5Definition of Accuracy and Acceptable Range....................................................5
Accuracy Validation Method...................................................................................5
3.2. Skin Temperature at the Pulse Oximeter Probe ..........................................................6
3.3. Probes and Connected Monitors .................................................................................6
3.4. Notification and Alarm of Measurement Condition...................................................6
Data Update Period ..................................................................................................6
Detection of Fault in Pulse Oximeter Probe or Probe Cable Extender ................6
SpO2 Calculation in the Presence of Inadequate Input Signal ............................6
4. CLINICAL CONSIDERATIONS.........................................................................................................7
4.1. Difference between Different Models and Manufacturers .......................................7
4.2. Probe Placement............................................................................................................8
4.3. Interference from Light and Movement ......................................................................84.4. Peripheral Circulatory Failure ........................................................................................84.5. Skin Injuries Caused by Probes ......................................................................................84.6. Effects of Abnormal Hemoglobin .................................................................................94.7. Nail Polish .........................................................................................................................9
5. NIHON KOHDEN SpO2 TECHNOLOGY ........................................................................................10
5.1 Introduction of NPi Algorithm .......................................................................................10
6. NIHON KOHDEN SpO2 MEASUREMENT PERFORMANCE ............................................................14
6.1. SpO2 Measurement Accuracy ....................................................................................14
Measurement Accuracy: SpO2 Validation on Adult Volunteer Subjects ..........14
Measurement Accuracy: SpO2 Validation in Neonate Patients with Cyanotic
Heart Disease ............................................................................................................15
6.2. Magnitude of Input Signal and Waveform................................................................16
6.3. Sensitivity Setting ...........................................................................................................176.4. Responsiveness .............................................................................................................18
6.5. Temperature Increase at the Probe Site ....................................................................19
7. MESSAGES AND ALARMS ............................................................................................................217.1. Messages and Alarms Related to SpO2 Monitoring .................................................21
7.2. SQI ..................................................................................................................................22
8. CONCLUSION ...............................................................................................................................23
9. REFERENCES ..................................................................................................................................24

SpO2 Monitoring Performance Digest

i1. Introduction
The pulse oximeter is a non-invasive device to percutaneously measure oxygen saturation in the blood with a finger
probe. The principle of pulse oximetry was developed by Takuo Aoyagi, a researcher at Nihon Kohden Corporation, andfirst presented at the Japanese Society for Medical and Biological Engineering (formerly The Japan Society of Medical
Electronics & Biological Engineering) in 1974.1
Before pulse oximeters were developed, the common method of measuring oxygen saturation in arterial blood wasspectroscopic analysis. However, this method is time consuming and not suitable for continuous measurement so it
was difficult to detect sudden changes in patient’s condition. The pulse oximeter provides non-invasive, continuous andreal-time measurement. In the 1980s, pulse oximetry attracted much attention as an effective way to prevent medical
accidents during surgery. Its advantages in respiratory and safety management became widely recognized and use of
the pulse oximeter has spreaded rapidly in intensive care units (ICU), neonatal intensive care units (NICU), emergency
rooms (ER) and other areas. Now, pulse oximetry is a standard measurement technique in patient monitoring.
The development and widespread use of pulse oximeters has created a demand for more advanced technology inclinical practices. This demand has accelerated the development and improvement of the technology by pulse oximeter
manufacturers.
However, even the best technology can have unexpected negative consequences if it is used inappropriately. Since
the pulse oximeter is easy to use and provides important vital information, there is the danger that it might be used
inappropriately or in a way that is beyond its performance capabilities. This can be prevented by using the device
properly and understanding its measurement principle and performance limitations.
This document describes Nihon Kohden’s pulse oximeter technology and performance as well as its measurement
principle and related international standards in order to promote appropriate use and better understanding of the pulse
oximeter.

SpO2 Monitoring Performance Digest

12. APPLICATION AND MEASUREMENT PRINCIPLE OF PULSE OXIMETRY
2.1. SpO2 Applications
The pulse oximeter is commonly used for patient care including management of anesthesia andrespiration in ICU patients. With the development of smaller devices and transmitters, it has become astandard for both inpatients and outpatients. With the trend toward downsizing and price reductions,
it is also widely used in homecare.
As a device for continuously monitoring SpO2, the pulse oximeter has the important functions ofdetecting hypoxia and alerting the medical staff with an alarm, especially during monitoring of severely
ill patients. For neonatal patients in the NICU, SpO2 monitoring plays an important role in preventing
hyperoxia which could cause retinopathy in the premature infant.
The pulse oximeter is used for different types of patients in different areas, so several types of pulse
oximeters and probes have been developed for different applications.
Table 1. Pulse oximeter applications
Operating Room Recovery Unit

ICU

NICU

Ward

2

● Oxygenation evaluation during and after anesthesia
induction● Respiratory management during ventilator use● Criteria for ventilator weaning● Respiratory management for patients taking sedatives or
painkillers● Detection of hypoxemia● Oxygen-level management to prevent retinopathy ofprematurity (ROP)● Vital sign monitoring with SpO2 and pulse rate
● Respiratory management for patients taking sedatives or
painkillers● Spot check of oxygen level during medical rounds

Emergency Patient Transport● Oxygenation management

Endoscopy● Detection of hypoxemia by bronchoscopy or endoscopy

Homecare● Oxygen therapy● Respiratory management● Screening of sleep apnea syndrome (SAS)SpO2 Monitoring Performance Digest

2.2. Principle of Measurement
Oxygen Saturation and Hemoglobin
The pulse oximeter measures oxygen saturation in arterial blood with a non-invasive probe. Oxygen
saturation measured by spectroscopic analysis of sampled blood is called SaO2 and requires a blooddraw to obtain a sample for analysis. Oxygen saturation measured by a pulse oximeter is called SpO2.
The “p” stands for pulse because the principle of SpO2 makes use of the change in blood with eachpulse wave.
Hemoglobin in the blood has a property that only allows red light to easily pass through. Hemoglobin
which has oxygen bound to it is called oxygenated hemoglobin or oxyhemoglobin. Hemoglobin which
does not have oxygen bound to it is called deoxyhemoglobin. Oxygen saturation in the blood is theratio of oxygenated hemoglobin to deoxyhemoglobin.
Arterial blood has a high concentration of oxygenated hemoglobin and appears bright red. Venous
blood appears darker as it has discharged oxygen into the body. This difference in color is because
oxyhemoglobin and reduced hemoglobin have optical absorption properties. Therefore, from the colorof the blood, we can measure the amount of oxygen which is bound to the hemoglobin. In other words,
we can measure the oxygen saturation.

Pulse Oximeter and Light Wavelengths
The pulse oximeter shines light through a fingertip or alternate relatively thin tissue and measures theintensity of light which is transmitted through to the other side. The waveform which is obtained fromthe change in the intensity of transmitted light over time is called the photoplethysmographic wave, or
the pulse wave.
In Nihon Kohden’s SpO2 measurement, the optical absorbance of the arterial blood can be extracted
and used to determine oxygen saturation by using the pulse waves which are obtained from shining
two light wavelengths (660 nm red light and 940 mm infrared light). The amplitudes of the pulse
waves indicate the change in blood volume with each pulse. The differing amplitudes of the pulse
waves for each wavelength of transmitted light indicate the change in optical absorption. The opticallight wavelength and its optical absorption by the blood are related as shown in Figure 1. Even if thevolume of blood doesn’t change, the amount of light absorbed by the blood is different at different
wavelengths. The amplitudes of the pulse waves at the two light wavelengths are different because theoptical absorbance changes according to the degree of oxygen saturation. The pulse oximeter obtains theratio, φ, of the pulse wave amplitudes at the red and infrared light wavelengths and oxygen saturation
is calculated from φ (Figure 2).

SpO2 Monitoring Performance Digest

3

200

H igh

660nm

940nm

Absorbanceof l igh t(mmol / ) - 1

100

H igh
absorbanceof red
1

H b

500

L ow

600 700 800 900 1, 000
w avel ength (nm : 10 - 9 m)

red

400

purpl ebl ue
green
y el l ow
orangeul traviol et2

L ow
absorbanceof red

0.1
0.05
300

H bO

660nm

940nm

P ul se w ave at both w avel ength s
w h en SpO 2 is 100%

intrared

Absorbanceind ex (cm) - 1

10

V isibl e l igh t

Figure 1. Optical absorption of oxyhemoglobin (HbO2, pink line) and reduced hemoglobin (Hb)2
2.0

1.5

1.0

Φ

0.5

50

60

70

SaO

80
2

90

0
100

(% )

Figure 2. Ratio (φ) of the pulse wave amplitudes at 660 and 940 nm as SpO2 changes3

4

SpO2 Monitoring Performance Digest

3. INTERNATIONAL PULSE OXIMETER STANDARD
ISO 9199, the international standard for pulse oximeter equipment, was issued in 1992. It defined basic requirements
for medical devices; safety, environmental conditions protection against different hazards and labeling. Thereafter, it
was revised and published as ISO 80601-2-61:2011 “Medical electrical equipment – Particular requirements for thebasic safety and essential performance of pulse oximeter equipment”. In addition to the basic requirements, this edition
further defines specific and essential features of pulse oximetry, including measurement accuracy and temperature at thesensor/skin interface. The development process of this requirement is included in the appendix.
These requirements will be explained in this section so you can understand the necessary conditions for the use of apulse oximeter.

3.1. SpO2 Accuracy
Measurement Criteria
ISO 80601-2-61:2011 specifies that accuracy of SpO2 measured by a pulse oximeter should be evaluatedby comparing the SpO2 reading of the pulse oximeter equipment to values of SaO2 determined by aCO-oximeter. The CO-oximeter is considered the gold standard.

Definition of Accuracy and Acceptable Range
ISO 80601-2-61:2011 specifies that SpO2 accuracy should be stated in terms of the root-mean-square(RMS) difference between the SaO2 (reference) value in operation manuals. It also defines the acceptable
range of SpO2 accuracy to be an RMS difference of less than or equal to 4% SpO2 over the range of
70% to 100%. This RMS difference means that about two-thirds of the SpO2 values measured can be
expected to fall within the range of RMS. In other words, about one-third of SpO2 values could be outof range. After active discussions at international standards meetings, this range, at the current level oftechnology, was defined as the minimum acceptable range in consideration of various factors that cause
errors such as color of skin, sex, age, and pathosis.

Accuracy Validation Method
Accuracy Validation of Controlled Desaturation Study on Adult Volunteers
It is important to eliminate error causing factors to properly compare SpO2 and SaO2. For example,
SpO2 and SaO2 could be different due to different body parts used for drawing a blood sample andprobe application. Also, the values may vary depending on the timing of the blood analysis, and thedifference between the values might be recognized as an error. To precisely perform an accuracy
validation, it is necessary to eliminate these error causing factors. One of the most effective ways tovalidate the accuracy is a controlled desaturation test on adult volunteers since it can be performed
under well controlled conditions. The desaturation test on adult volunteers has some advantages: 1)
equally distributed data can be obtained without deflection in specific SaO2 range because SaO2
values are under control, 2) easy to perform on various subjects of different races, sexes, ages, etc., 3)
statistically reliable data can be obtained.
ISO 80601-2-61:2011 also accepts validation by comparing the readings of pulse oximeter equipment
under test to “secondary standard” pulse oximeter equipment that has previously been directly
validated against a CO-oximeter.

SpO2 Monitoring Performance Digest

5Accuracy Validation Applicable to Various Clinical Situations
Not every feature of pulse oximeter equipment is theoretically clarified. Therefore, the impact ofvarious error factors on accuracy need to be demonstratively validated in consideration of eachapplication purpose. This is mentioned in the ISO 80601-2-61:2011 SpO2 standard, but thevalidation method is not specified by any international standard. Traditionally, clinical validations
have been conducted in a manner that is consistent with the primary purpose of medical treatments.
You may refer to these methods as useful references.
Performance Validation under Conditions of Motion and Low Perfusion
For performance validation under conditions of motion artifacts or weak signals due to low perfusion,
there is no internationally defined method. These validations are conducted in consideration of eachclinical condition.

3.2. Skin Temperature at the Pulse Oximeter Probe
The maximum limit for skin temperature at the probe is defined to not exceed 41°C for skin temperatureof 35°C in use with normal conditions or a single fault-condition. The pulse oximeter probe generates asmall amount of heat due to the light-emitting diode (LED). When the pulse oximeter probe is applied
to the patient’s skin where heat loss is prevented due to poor circulation, the temperature of the skin incontact with the probe might increase and this could increase the risk of low temperature burn. In therevision of ISO 80601-2-61 :2011, thorough discussion and reviews of literature on this matter led tothe industry standard conclusion that it is appropriate and safe to retain the 41°C limit for infants under
the age of 1 year and apply the adult limit (42°C for 8 hours, 43°C for 4 hours) for all older patients.

3.3. Probes and Connected Monitors
ISO 80601-2-61:2011 requires manufacturers to conduct tests to ensure that all pulse oximeter probe
requirements are met by each pulse oximeter monitor with which the probe is intended to be used. All
pulse oximeter monitors with which compatibility is claimed should be listed in the accompanying
documents including the operation manual. Also, ISO 80601-2-61:2011 is applicable not only to newprobes, but to reprocessed ones. Therefore, manufacturers are responsible to validate their processes toensure that any new or reprocessed probe complies with the requirements in regard to safety and basic
performance.

3.4. Notification and Alarm of Measurement Condition
Data Update Period

It is required that at least a low priority alarm should be generated when SpO2 or pulse rate (PR) data
update period exceeds 30 seconds. When the data update period is less than 30 seconds, it should be
disclosed in the instructions for use.

Detection of Fault in Pulse Oximeter Probe or Probe Cable Extender

When any fault occurs in the probe or probe cable extender such as disconnection or short-circuit, it
should create a notification. If the pulse oximeter equipment has an alarm function, it should generate
an alarm. If it does not have any alarm function it should provide indication of abnormal operation.

SpO2 Calculation in the Presence of Inadequate Input Signal

6

The pulse oximeter is required to provide an indicator of signal inadequacy to the operator if thedisplayed SpO2 or PR values may be incorrect due to low quality input signal caused by artifacts. The
pulse waveform displayed together with the signal strength may help the user judge whether the signal
is adequate for measurement.
SpO2 Monitoring Performance Digest

4. CLINICAL CONSIDERATIONSSpO2 is a parameter that provides important information for respiratory management and the pulse oximeter has
the great advantage of being easy to use. However, various factors can cause measurement errors such as the patient’s
hemodynamics, the condition of the skin at the probe, probe attachment, and measurement algorithm. This section
describes instructions and directions for clinically proper and safe use of the pulse oximeter.

4.1. Difference between Different Models and Manufacturers
Not every feature of the pulse oximeter has been validated or defined, so proper caution is necessary
when using a pulse oximeter.
• The measurement accuracy and safety of pulse oximeter is validated by a combination of a
specific probe and measurement circuit. Therefore, if you use a different probe, it is necessary
to validate the performance of the combination. Some manufacturers specify a specific probe towork with their pulse oximeter equipment.
• Each manufacturer has different specifications, including wavelength errors, correction constantand signal processing methods. Even if the performance based on these specifications is within
acceptable range, it is still recommended to refer to the technical information provided by eachmanufacturer for proper use of the equipment.
Figure 3 compares SpO2 readings provided probes from Nihon Kohden (blue line) and anothermanufacturer (purple line) which is not validated for use with the monitor (BSS-9800, Nihon Kohden),to which these probes are connected.4 Compared to the reference value provided by the Nihon Kohden
probe, probe which is not validated shows incorrect values at all the measurement points throughout
the measurement for one and a half hours. As you can see in this example, any probe not validated foruse with the SpO2 equipment should be avoided. And, care must be taken as some manufacturers may
indicate that their probes are made to “work with” or have a “compatible connector”. This does notmean the probe has been tested by Nihon Kohden, nor does it mean the probe will perform as expected.
SpO

2

(% )

100

P robe w h ich
is notval id ated

95

N K probe9085

20 minTime

Figure 3. SpO2 trend with probes from Nihon Kohden and another company,
connected to a BSS-9800 bedside monitor

SpO2 Monitoring Performance Digest

74.2. Probe Placement
The light-emitting part and light-receiving part should be placed so they are facing each other. Whenusing a clip finger probe, ensure that the probe covers the finger properly. Otherwise, some of thelight will not pass through the tissue to reach the light-receiving part and this will lead to incorrect
measurement results.
The more distant the light-emitting and light-receiving parts, the weaker the transmitted light intensity
becomes. Also, a thinner body part tends to have less blood volume and a weaker pulse signal. The mostsuitable thickness of the probe site is about 10 mm.

4.3. Interference from Light and Movement
The pulse oximeter can be affected and interfered with by many factors such as ambient light andmotion artifact.
In order to reduce such interference, it is advisable to avoid any strong ambient light, use lighter andsmaller probes, and keep the cable from moving by taping it to the finger.
Although there have been numerous technical advancement to improve the performance of the pulse
oximeter, such as filters to reject motion artifacts, it is still necessary to pay attention to light and motion
interference to ensure accurate measurement.

4.4. Peripheral Circulatory Failure
In patients with decreased blood flow due to peripheral circulatory failure, the pulse at the probe site
(e.g. fingertip) may be too weak to be measured. In these cases, alternate measurement sites must be
used and whenever the probe is attached, it must be attached in a way that does not block or restrict
the blood flow.

4.5. Skin Injuries Caused by Probes
The site where the probe is applied gets pressure regardless of what type of probe is used. Clip type probes
can give high pressure on the finger depending on the structure of the probe or size and shape of thefinger. Skin injuries such as erythema and marking or impressions can be caused by high local pressure
from the hard component parts of the probe, including light-emitting diode (LED) and photodetector(PD). On the other hand, self-adhesive type disposable probes can block the blood flow if the probe is
attached too tightly or supplemental tape is applied. We recommend not to use elastic wrap to hold thesensor in place because pressure from the tape can increase over time.
The light-emitting part of the probe can increase the skin temperature by about 2°C. Because thetemperature increase is slight, low temperature burns are not common (Refer to 6.5 for more information
about temperature increase at the probe site). But the skin at the attachment site should be checked at
specified intervals.

8

SpO2 Monitoring Performance Digest

The body naturally eliminates heat by increasing blood flow in the skin. If the probe applies excessive
pressure, it could block the blood flow so that metabolic heat can’t be released and the local skintemperature at the probe site may increase.5 Skin burns from the probe seem to be caused by a combinationof pressure from the probe and heat from the light-emitting part. There are also reports that the LED
temperature can rise when using the pulse oximeter with probes from other manufacturers. Therefore,
to prevent skin injury it is recommended to avoid using probes which have not been validated in regard
to the synergistic combination of burn and high local pressure from the probe.
It is recommended that the probe not be continuously applied at the same site for a long period of time
and that the probe should not be attached too tightly. The measurement site should be changed every 8hours for disposable probes and every 4 hours for reusable probes.

4.6. Effects of Abnormal Hemoglobin
In addition to oxyhemoglobin and deoxyhemoglobin, there are abnormal forms of hemoglobin,carboxyhemoglobin (COHb) and methemoglobin (MetHb). COHb attaches firmly to carbon
monoxide instead of oxygen so it has no capability of delivering oxygen. MetHb contains Fe3+ insteadof Fe2+ and cannot carry oxygen effectively.
The two-wavelength pulse oximeter cannot distinguish normal and abnormal hemoglobin, so if theabnormal hemoglobin level increases, it can cause considerable error in the measurement. Normal
COHb level is about 1%, and in smokers up to about 5%. COHb level within this range rarely has
impact on SpO2. However, extra attention is needed when hemoglobin is increased due to carbon
monoxide poisoning or methemoglobinemia.

4.7. Nail Polish
A probe site that is very soiled or covered with nail polish could block light transmission and interfere withthe measurement. Figure 4 shows how the effects of nail polish on the accuracy of SpO2 measurement.
If the probe site gets dirty with blood or the patient wears nail polish, clean the probe site or remove nailpolish before measurement. For the case where nail polish cannot be removed, it has been reported thatreorienting the probe in the lateral direction on the fingertip could diminish the effect of nail polish on
the accuracy of SpO2 measurement.6, 7

1 0
5
0
-5

Color of nail polishG reenW hiteT ransparent

B rownRed

B lue

B lack

-1 5

S ilver

-1 0
G old

D ifference from the unmanicured ( % )1 5Figure 4. Effect of nail polish8
SpO2 Monitoring Performance Digest

95. NIHON KOHDEN SpO2 TECHNOLOGYOver the course of many years of improving SpO2 measurement technology — whose full scope has not yet been
uncovered — Nihon Kohden has given primary consideration to optimizing the SpO2 measurement system in regard
to the different uses and applications in various clinical practices.
Quality SpO2 signals can be obtained using various combinations of probes, sensor performances, signal detection
circuits, and signal processing algorithms. Proper attachment and handling of the probe are also important. It isnecessary to consider all possible elements related to SpO2 measurement in order to provide optimum performance.
Nihon Kohden has a wide range of SpO2 sensor technologies including BluPro series reusable probes and disposable
probes in different sizes and shapes with connection cables that focus on continuous monitoring of SpO2.
Nihon Kohden also offers a wide variety of SpO2 monitoring devices for different applications. All devices have different
power consumptions and shapes and it is necessary to consider all these differences in the process of developing andimproving SpO2 technology.
Nihon Kohden has also developed an original measurement circuit and algorithm (NPi algorithm) that works withevery Nihon Kohden device.
Any component other than the arterial pulse which is superimposed on the input signal can become artifact in SpO2
measurement. Artifacts such as light interference by surgical light and sunlight, improper attachment of probe, andbody movement can be mistaken as pulse waves or can interfere with pulse wave detection. This causes incorrect SpO2
measurement. Nihon Kohden SpO2 technology has been designed to extract the pulse wave derived information andcalculate the ratio of pulse wave amplitude, ϕ, as accurately as possible.

5.1 Introduction of NPi Algorithm
The basic processing of Nihon Kohden pulse oximeters includes filtering of the A/D converted input
signal, calculation of the ratio of pulse wave amplitude ϕ, weighted moving average, and calculation of
the SpO2 value.
The characteristic features of the NPi algorithm include a filtering function which removes artifacts
from the pulse waves. The artifact removal is performed using narrow-band filtering based on thefundamental frequency that is extracted by frequency analysis (Figure 5). This narrow-band filtering
is especially effective when there are relatively large artifacts, especially in the ICU and NICU, such as
restless patients with peripheral circulation insufficiency neonatal patients presenting with respiratory
variation.

10

SpO2 Monitoring Performance Digest

L E D

W aveformd ispl ay

P D

A/ D
conversion

W id e- band
fil ter

P ul se w aved etection

R atio ofpul se w ave
ampl itud e Φ(regression
meth od )W eigh ted
movingaverage

SpO 2
cal cul ation
P R
cal cul ation
F und amental
freq uencyof pul sew ave

N umeric
val ued ispl ay

N arrow band fil ter

Measurement
cond ition
al armU pperl ow er l imit
al armFigure 5. Construction of NPi algorithm

Pulse waves contain a fundamental frequency of the pulse rate and constant multiples of the frequency(Figure 6a). When performing frequency analysis in the presence of large artifacts superimposed on
pulse waves, it is difficult to identify the pulse wave frequency because the pulse signal gets lost in
the artifact (Figure 6b). In such cases, Nihon Kohden’s original coordinate-transform method is used
to enhance the discrimination between pulse wave and artifact (Figure 7). By performing frequency
analysis on the pulse signals obtained by the coordinate-transform method, the fundamental frequencyof the pulse wave can be identified (Figure 6c).
Pulse wave components are selectively passed through a narrow-band filter of the fundamental
frequency so that SpO2 values can be calculated using pulse wave components without artifacts. This
Nihon Kohden original technology, which is based on a well-established signal processing technology,
has been patented.9

SpO2 Monitoring Performance Digest

11

P ul se w ave at rest

F req uency of
pul se w avea.

0: 00: 13 0: 00: 15 0: 00: 16 0: 00: 18 0: 00: 20 0: 00: 22

0

1

2

I nfrared l igh t signal

4

5

6

7

9 10

8

F req uency of pul sew ave cannot be
id entified d ue to
noise

b.

0: 00: 23 0: 00: 25 0: 00: 26 0: 00: 28 0: 00: 30 0: 00: 32

3

0

1

2

3

O utput signal

4

5

6

7

8

9 10

F req uency of
pul se w avec.

0: 00: 23 0: 00: 25 0: 00: 26 0: 00: 28 0: 00: 30 0: 00: 32

0

Time

1

2

3

4

5

6

7

8

9 10

F req uency

Figure 6. Frequency component of pulse wave
Red light signal
FilterExtract pulse
wave
component
Calculate Φ
Coordinate-transform

Filterinfrared light signal

Figure 7. Identification of fundamental frequency and filtering using coordinate-transform method

12

SpO2 Monitoring Performance Digest

The ratio of pulse wave amplitude, ϕ, is calculated using the filtered pulse waves. The pulse oximeter
calculates the SpO2 value based on ϕ which can be obtained from two LEDs of red and infrared light.
Correct calculation of ϕ leads to accurate SpO2 measurement. In Nihon Kohden’s SpO2 measurement,
data obtained at every sampling interval are plotted. The x-axis is the pulse signal of the red light and they-axis is the pulse signal of the infrared light (Figure 8). ϕ is derived based on the slope of the regression
line obtained by the least squares method.10
By using the entire waveform instead of using only the maximal and minimal amplitude, accurate
calculation of ϕ can be achieved even in conditions such as peripheral circulation insufficiency withlower amplitude pulse waves.
Furthermore, a weighted moving average is applied according to the degree of variation in the ϕobtained every beat to limit the influence of SpO2 variation due to artifact.

P ul se w ave obtained from red l igh t

TimeP ul se w ave obtained from infrared l igh t

TimeFigure 8. Calculation of φ using regression method

SpO2 Monitoring Performance Digest

136. NIHON KOHDEN SpO2 MEASUREMENT PERFORMANCEThis chapter describes the basic performance of Nihon Kohden’s SpO2 measurement including measurement accuracy,
measurement range of input signals, responsiveness, temperature increase at probe site, as well as how the basic
performance was verified. In addition, measurement performance under noisy conditions due to artifact is shown withwaveform examples.

6.1. SpO2 Measurement Accuracy
The SpO2 measurement accuracy has been evaluated in comparison with CO-oximeter readings (SaO2)
in adult volunteers and pediatric patients with cyanotic heart disease.

Measurement Accuracy: SpO2 Validation on Adult Volunteer Subjects
The result of accuracy validation using a Nihon Kohden OLV-3100 pulse oximeter and four types ofprobes is presented here. Hypoxia was induced to different levels of arterial oxyhemoglobin saturation
between 70% and 100% by controlling the inspired oxygen concentration. The SpO2 readings of thepulse oximeters were recorded simultaneously with arterial blood sampling. Two CO-oximeters (OSM3®,Radiometer Inc.) were used to determine the reference SaO2 values by analysis of arterial blood samples
from the radial artery. Statistical analysis was performed to compare the SpO2 readings and the reference
SaO2. Exclusion criteria for the statistical analysis were predefined. Data was excluded if:• There was considerable change in SpO2 value (4% or more during a 10-second period).
• Co-oximeter values disagreed by 2% or more.
Table 2 shows the result of the accuracy analysis of the validated probes, along with the measurement
accuracy standard of Nihon Kohden pulse oximeters and the ISO 80601-2-61:2011 requirement.
All pulse oximeters are designed and manufactured to meet Nihon Kohden’s SpO2 accuracy standard
which is stricter than the ISO 80601-2-61:2011 requirements of 4%. The results in Table 2 show thatall validated probes have met the accuracy requirements.
Table 2. SpO2 accuracy of hypoxia test on adult volunteers (RMS)
SaO2 Range

1470–80%

80–100%Finger probe

TL-201T

1.62%

1.07%

Multi-site Y probe

TL-260T

1.83%

1.56%

Multi-site Y probe with clip adapter for ear lobe

TL-260T

2.14%

1.62%

Finger probe

TL-631T

2.79%

1.45%

Disposable SpO2 probe

TL-271T

2.05%

1.39%

Disposable SpO2 probe

TL-051S

1.93%

1.59%

Disposable SpO2 probe

TL-535U

2.01%

1.37%

Nihon Kohden pulse oximeter SpO2 accuracy

3% or less

2% or less

ISO80601-2-61 SpO2 accuracy

4% or less

4% or lessSpO2 Monitoring Performance Digest

Figure 9 shows the data plot obtained from the validation of TL-201T. The left figure shows goodcorrelation between SpO2 and SaO2. The right figure shows the difference between SpO2 and SaO2.
SaO2 values in the range of 70–80% tend to vary more widely than the range of 80–100%. A previous
study has reported that SaO2 values are more likely to be variable at low SaO2.11 In consideration ofthis characteristic, the accuracy of Nihon Kohden pulse oximeter was evaluated in two different SaO2
ranges.
SaO

2

- SpO

2

pl ot

B l and and Al tman pl ot

100

10

95

8
6

90
(% )
2

- SaO

75

2
0

2

80

SpO

SpO

2

(% )

4

85

-2
-4

70
-6

65

-8

60

- 10

60

65

70

75

80
SaO

2

85

90

95

100

60

65

70

75

80
SaO

(% )

85
2

90

95

100

(% )

Pulse oximeter: OLV-3100, Probe: TL-201T
CO-oximeter: OSM3® (Radiometer)
Figure 9. TL-201T SpO2 and SaO2 data comparison

For more details on the validation study, refer to SpO2 Monitoring Pulse Oximeter Accuracy Study (extracts
from a test report from the Hypoxia Research Laboratory, University of California, San Francisco) (No.
0604-905574A).

Measurement Accuracy: SpO2 Validation in Neonate Patients with Cyanotic
Heart Disease
Pulse oximeter probes come in a variety of types for patients of different ages and different probe sites.
The overall performance of all pulse oximeters and probes should be evaluated in consideration of eachapplication and usage.
However, the same kind of studies that are carried out in adult volunteers as well as induced hypoxia
studies cannot be conducted in children.
Clinical functionality of a Nihon Kohden TL-535U has been demonstrated in comparison with the
SaO2 of a CO-oximeter on a population of hospitalized neonate patients. The data presented here wasobtained from 19 neonatal patients below the age of 1 month.
The results are presented in Table 3 and Figure 10. The results show that the TL-535U probe met thestandard of the SpO2 accuracy criteria.
Table 3. SpO2 accuracy in neonatal patients at the NICU
CO-oximeter SaO2 range

70–100%

Number of subjects

19 (6 subjects weighing ≤1000 g, 13 subjects weighing≥1000 g and ≤2000 g)

Number of data

422

SpO2-SaO2 average-0.25%

SpO2-SaO2 standard deviation

2.43%

SpO2-SaO2 RMS

2.25%
SpO2 Monitoring Performance Digest

15

SaO

2

vs SpO2pl ot

B l and and Al tman pl ot
20.0

90

10.0

80

0.0

2

SpO

SpO

5.0

2

85

(% )

15.0

- SaO

95

2

(% )

100

75

- 5.0

70

- 10.065- 15.060- 20.0

60

65

70

75

80
SaO

2

85

90

95

100

(% )

60

65

70

75
SaO

80
)
2 (%

85

90

95

100

Pulse oximeter: Nihon Kohden OLV-3100, Probe: Nihon Kohden TL-535U
CO-oximeter: OSM3® (Radiometer)
Figure 10. Comparison of SpO2 and SaO2 data for TL-535U probe

6.2. Magnitude of Input Signal and Waveform
The transmitted light intensity becomes weaker when the probe is attached to a thick body part
because more LED light is absorbed by the tissue. Also, pulsations in the artery become weak when the
peripheral circulation at the probe applied part is poor. This leads to smaller pulse components of thetransmitted light signals from the probe.
Figure 11 shows data of the thickness of the finger and the strength of the input signal from the probe.
The vertical axis shows the percentage of pulsatile signal in the entire transmitted infrared radiation(IR) signal, This value is called the Pulse-amplitude Index (PI) and can be displayed on the monitor.
The area within the blue lines is the measurable range of Nihon Kohden pulse oximeters. When thefinger thickness is about 10 mm, PI of 0.02% is the limit of the pulse wave detection. On a monitoring
screen, the pulse wave amplitude increases in accordance with higher ratio of the pulse component. An
amplitude of about 10 mm in the ×1 sensitivity setting means a PI of 1%.
In figure 11, the measurement condition becomes more challenging as the distance from the lower left
corner decreases. In challenging measurement conditions, the input signal becomes smaller and theartifact becomes relatively higher so the input signals tend to be susceptible to artifacts. For optimum
measurement, avoid attaching the probe to a body part that is too thick and observe the monitoring
screen to make sure that proper pulse wave amplitude is obtained.

16

SpO2 Monitoring Performance Digest

I nput- referred current val ue (A)
1n

10n

100n

1µ

10µ

100µ

10%

1%
P l
0.1%

0.01%

0.001%
21

Th ick15

93

Th ick nessat th e probe site (mm)

0

Th in

Input signal of red light
Input signal of infrared light

Figure 11. Thickness of probe site and strength of input signal from probe

6.3. Sensitivity Setting
PI of 0.02% used to be the limit of pulse wave detection. However, the NPi algorithm can detect evenweak pulses buried in noise. The limit of pulse wave detection can be extended by setting the SpO2
measurement sensitivity mode.
Normally, select NORMAL mode. The NORMAL setting enhances the performance of the pulse
oximeter in identifying improper probe attachment and in distinguishing physiological pulses from
non-physiological pulses.
Consider using the MAX setting when it is difficult to detect the pulse such as when monitoring thefollowing patients.
• Patients with low perfusion• Patients undergoing intra-aortic balloon pumping (IABP) where non-physiological pulses orsignals exist
The limit of pulse wave detection is extended up to about 0.01% when MAX is selected. Therefore,
even non-physiological pulses (e.g. pulse wave of patients undergoing IABP), which is normally notmeasured, is detected so that SpO2 can be displayed. The MAX setting prioritizes detection of weakpulses over identifying probe disconnection and non-physiological pulses.

SpO2 Monitoring Performance Digest

17

It is important to set the appropriate sensitivity setting according to the performance characteristics ofeach sensitivity mode (see Table 4).
Table 4. Performance of each sensitivity mode in different algorithms
Sensitivity Mode

NORMALMAX

Detection of improper probe attachment

Good

Fair

Measurement in the presence of distorted R, IR waveform

Good

Good

Measurement in the presence of respiratory waveform superposition

Good

Good

Detection of pulse wave with low amplitude

Fair

Good

Algorithm

6.4. ResponsivenessThe responsiveness of Nihon Kohden pulse oximeters can be set by the user (response setting). The
response setting is the setting for SpO2 averaging time, and the responsiveness relies on this setting. As
specified in ISO 80601-2-61:2011, the responsiveness of the Nihon Kohden pulse oximeter is presented
as a graph as shown in Figure 12. Signals in which the SpO2 values changed by 0.6% every second were
input in a simulated hypoxia to obtain the response time of different responsiveness settings (Figure 12).
The signals were input at the rate of 140 per minute which is equivalent to a typical neonatal heart rate.
The response setting can be set in three stages: fast, normal and slow, and the averaging time can beselected from 4, 8 and 16 seconds. The longer the averaging time, the more stable SpO2 values become.
But, longer averaging time leads to lower responsiveness and decreased fidelity to changes in SpO2. As
an example, Figure 13 shows the graph which is in ISO 80601-2-61 :2011.
The default setting of the response setting is “normal”. “Slow” is appropriate to suppress the numberof alarms triggered by the decrease in SpO2 values. If it is necessary to accurately record the frequency
and severity of apnea, select “fast”. For neurology applications and products from Nihon Kohden, it isrequired to use “extra fast” and averaging should be performed every 3 beats.
100
95

2

85

SpO

(% )

90

80
75
70
65
60
0

10

20

30

40

C h ange in SpO 2 val ue
R esponse N ormal

50

60

70 80
Time (s)90

100 110 120 130 140

R esponse E x tra F ast
R esponse Sl ow

R esponse F ast

Figure 12. Responsiveness of different settings (PR 140/min)

18

SpO2 Monitoring Performance Digest

1: raw SpO2
2: SpO2 avg (fast)
3: SpO2 avg (normal)
4: SpO2 avg (slow)
5: Alarm threshold

95

85

SpO

2

(% )

90

4

1

80

2

3

5

75
0

10

20

30
40
Time (s)

50

60

70

Figure 13. Effect of each response setting at low oxygen saturation126.5. Temperature Increase at the Probe Site
The probe generates heat due to the LED light. But a cooling effect of blood flow limits the increase insurface temperature of the pulse oximeter probe-tissue interface to around 2°C. Temperature changes
at the probe attachment site were evaluated in 5 adult volunteers at an ambient temperature of 35°C
(Fig 14). The temperature at the probe attachment site was 36.8±0.22°C five minutes after starting
SpO2 monitoring, and after that the temperature remained around 37°C. This meets the ISO 806012-61:2011 temperature requirement of 41°C or less.
However, it should be noted that if the blood flow at the probe attachment site decreases due to poor
peripheral circulation or disruption of blood flow, the blood flow cooling effect does not work sufficiently
so further temperature increase could be expected.
Therefore, the following should be noted when using the probe:• Change the measurement site every 4 hours for reusable probes and every 8 hours for disposable
probes and reusable finger-tip type probes (TL-631T/T3).
• Do not wrap the probe tape too tightly to avoid poor circulation.
• Take extreme care to change the measurement site more frequently in patients with fever or poor
peripheral circulation, and neonates or low birth weight infants with delicate skin.

SpO2 Monitoring Performance Digest

19Temperature ( )41.0Ambient temperature(average)

40.0

Subject #1

39.0

Subject #2
Subject #3

38.0

Subject #4
Subject #5

37.0

Start of monitoring

36.0

5 min after start ofmonitoring

35.0
34.0

0

100

200

300

400

500

600

Measurement time (s)Subjects
Five adult male volunteers
Pulse oximeter Nihon Kohden OLV-3100
Probe
TL-260T (placed on fourth finger)
Temperature 35.3–36.6°C (36.0±0.3°C) (first finger)
Environment
35°C ambient temperature, 80% humidity
Figure 14. Temperature increase at attachment site of TL-260T SpO2 probe

20

SpO2 Monitoring Performance Digest

7. MESSAGES AND ALARMS
7.1. Messages and Alarms Related to SpO2 Monitoring
Messages are displayed on the monitor screen in different situations. Some messages are output asalarms. When SpO2 values cannot be calculated for any reason, the measured value remains unchanged
for 30 seconds and is then displayed as “---”. SpO2 cannot be calculated when the measurement has
interference from artifact or when the pulse wave is too small to detect. SpO2 measurement can beaffected by both external factors and physiological factors.
Table 5. SpO2 monitoring related messages
StatusSpO2/PR
Display

CANNOT DETECT
PULSE

Pulse wave cannot be detected from signals.
This message appears in conditions such as no physiologicalpulse wave, probe is too tight or too loose, or too muchnoise due to body movement.

---

CHECK PROBEorCHECK PROBE SITE

Appropriate signal cannot be obtained. Probe may bedetached or not attached properly. This message also
appears when measurement cannot be performed for 30
seconds.

---

DETECTING PULSE

Searching for the patient pulse. This message is displayed
until pulses are detected after the probe is attached to thepatient. If this status continues for 30 seconds, a “CHECKPROBE” message appears.

---

LIGHT INTERFERENCE

Intense light such as surgical light or a phototherapy device
is interfering with measurement. Invisible light can also affectthe measurement. In most cases, measurement becomes
possible by covering the attachment site with a blanket
to block any light interference, but care must be taken to
ensure proper circulation and probe should continue to be
moved at proper intervals.

---

LOW QUALITY
SIGNAL

Pulse wave is unstable and signal quality is low. Measurement
continues as long as there is an appropriate input signal.
But when the quality of the signal is too low to obtain SpO2
value, the measured value remains unchanged for 30
seconds and is displayed as “---”.

SpO2
value/---WEAK PULSE

Pulse-amplitude Index (PI) is very small. This message
appears when peripheral circulation is poor or when theprobe is too tight or too loose.

SpO2
value

Message

SpO2 Monitoring Performance Digest

21

7.2. SQI
The signal quality index (SQI) bar graph shows the quality of the pulse waveform signal which is used
to calculate SpO2 (Table 6). When the signal quality is low, update of the measured SpO2 value canbecome intermittently unavailable.
When the messages frequently occur or the signal quality remains low, SpO2 monitoring is not being
properly performed and critical patient conditions such as low oxygen saturation may not be correctly
detected.
Take action to address the possible causes of the displayed messages. Also, appropriate use of the probe
is essential for proper SpO2 monitoring. Make sure to choose the right probe for the measurement siteand check that the probe and all cables are properly connected.
The SQI bar graph shows the signal quality of the pulse waveform in 5 levels as shown in Table 6.
Table 6. SQI bar graph
SQI Bar Graph

Quality of Pulse Waveform

ActionThe signal quality is high.

-There may be some artifacts.

-

The signal quality is reduced due tolarge artifact. If this state continues for along time, check the patient and probe
attachment.

If this status continues, checkthe condition of the patient andprobe and reattach the probe ifnecessary.

The signal quality is very low. Check thepatient and probe attachment.

If the problem persists, SaO2
measurement using a blood
sample is recommended.

The signal cannot be measured
properly.

Follow the other messages thatappear on the screen.

4 green bars
3 green bars

2 yellow bars

1 red bar

No bars

22

SpO2 Monitoring Performance Digest

8. CONCLUSION
Since the invention of pulse oximetry by Nihon Kohden researcher Dr. Takuo Aoyagi, the pulse oximeter is one of
the most widely used medical devices. Continued industry advances have improved SpO2 measurement accuracy, butclinicians need to be aware of proper placement and care relative to the patient status. Nihon Kohden is committed
to continued development and improvement of SpO2 technology to meet the needs of healthcare professionals andcontribute to the advancement of medical technology.

SpO2 Monitoring Performance Digest

23

9. REFERENCES
1

Aoyagi T. Improvement of the Earpiece Oximeter. Proc 13th Conf Jpn Soc Med ElectroBiol Eng 12:90-91[published in Japanese]

2

Ukawa T. Pulse oximeter. Jpn J Med Instrum 2004;74:366-371

3

Aoyagi T. Invention of pulse oximeter and its theory. J Jpn Soc Clin Anesth 1990 Jan;10(1):1-11. [published inJapanese]

4

ME center of Kitasato University Hospital (Tojo K. et al.). Pulse oximeter gokan probe no seido ni kansuru kento (Theaccuracy of pulse oximeter compatible SpO2 probes). 15 February 2008, The 35th Annual Meeting of the JapaneseSociety of Intensive Care Medicine, Tokyo [presented in Japanese]5

Yamada Y. Teion yakedo ni tsuite, seihin to anzen (Low temperature burn—products and safety) 1999;72:2-8[published in Japanese]

6

White PF, Boyle WA. Nail polish and oximetry. Anesth Analg. 1989;68:546-548

7

Sakuma A, Kurosawa A, Asano Y, Muto R, Terada T, Ochiai R. (2017, June). Examination about the optical
validity of the side-to-side method avoiding error of measurement of the pulse oximeter with the manicure. Poster
session presented at the 64th Annual Meeting of the Japanese Society of Anesthesiologists. Kobe (in Japanese)

8

Ukawa T. Current State and Problems of Pulse Oximetry. Jpn J Med Instrum 2007;77(2):52-59

9

Ito K et al. (2006). Method for reducing noise, and pulse photometer using the method. US7025728 B2

10 Aoyagi T et al. (1997). Apparatus and method for measuring oxygen saturation in blood and apparatus and method
for measuring concentration of light-absorbing materials in blood. US5690104 A
11 John R. Feiner, MD, John W. Severinghaus, MD, Philip E. Bickler, MD, PhD: Dark skin decreases the accuracyof pulse oximeters at low oxygen saturation, Anesthesia & Analgesia Vol.105, No.6, 2007.
12 ISO80601-2-61:2011(E). Medical electrical equipment — Part 2-61: Particular requirements for basic safety andessential performance of pulse oximeter equipment

24

SpO2 Monitoring Performance Digest

First Edition

7 March 2018

SpO2 Monitoring
Performance Digest
Patient Monitoring Technical Library

NIHON KOHDEN CORPORATION
1-31-4 Nishiochiai, Shinjuku-ku, Tokyo
161-8560, Japan
Phone +81 3-5996-8041

SpO2 Monitoring

Performance Digest

NIHON KOHDEN CORPORATION

1-31-4 Nishiochiai, Shinjuku-ku, Tokyo 161-8560, Japan
Phone +81 3-5996-8041 Fax +81 3-5996-8100

http://www.nihonkohden.com