There’s No Such Thing As A SpO2 Simulator 6001759A There Is Sp O2 W

: Pdf 6001759A There-Is-No-Such-Thing-As-A-Spo2-Simulator W 6001759A_There-is-no-such-thing-as-a-SpO2-simulator_w 04 2015 x7mag uploads wp-content

Open the PDF directly: View PDF .
Page Count: 3

There’s no such thing

as a SpO2 simulator

Why a primary calibration reference does not exist

and how to properly measure SpO2 accuracy

White Paper

One of the most essential elements in medical

equipment maintenance is functional testing, or in

other words, verifying the competence of a device

by applying it to another device that mimics the

physiological signal of interest.

For the majority of devices, this simulation is

a direct analog of the clinical signal. Simulators

for the electrocardiograph (ECG) or for an electro-

encephalogram (EEG) provide voltage waveforms

that mimic those parameters. Simulators for

non-invasive blood pressure provide a pneumatic

pressure signal identical to the blood pressure

waveform found in an artery. Invasive blood pres-

sure, temperature, and cardiac output simulators

produce varying patterns of resistance or imped-

ance to mimic the output of a strain gauge or a

thermistor.

Simulation of oxygen saturation in humans,

however, presents a challenge. A pulse oxim-

eter uses two wavelengths of light: a red signal,

which is partially absorbed by non-oxygenated

hemoglobin, and an infra-red (IR) signal, which is

partially absorbed by oxygenated hemoglobin in

the patient’s arterial blood.

At any level of oxygen saturation, the ratio of the

pulsatile and non-pulsatile signals is derived for

both wavelengths. The red ratio is then divided

by the IR ratio (a “ratio of ratios”) to produce a

value (R) which correlates with the known oxygen

saturation value to produce an “R-curve” unique

to each manufacturer. At any value of R, the moni-

tor’s firmware “looks-up” and displays the percent

oxygen saturation. Since the absorption waveform

is pulsatile, the monitor also derives and displays

the pulse rate.

Where does the “R-curve” come from?

Before market introduction, all prototype oxygen

saturation monitors must be validated using in

vivo testing as specified in Standard 80601-2-61

(2011) of the International Standards Organization

(ISO). In a controlled desaturation study, volunteer

subjects breathe a sequence of gas mixtures of

decreasing oxygen content while connected to the

prototype monitor. Arterial blood samples are taken

from the subjects, and the saturation is measured

by a co-oximeter in a clinical laboratory. As shown

in Figure 1, the R-value derived in the monitor is

plotted against each saturation value.

Dennis J. McMahon, CBET-R

Figure 1. Example of an R-curve, correlating O2 saturation with the R value.

The resulting R-curve is then integrated into

the firmware of the monitor. The ISO standard

prescribes the number of subjects, the saturation

range, the laboratory analysis method, motion

conditions, and varied perfusion levels. This vali-

dation method is also stipulated in the pre-market

protocols of many regulatory agencies, such as

the U.S. Food and Drug Administration’s 510(k)

guidelines.

2 Fluke Biomedical There’s no such thing as a SpO2 simulator

Why a primary calibration reference

does not exist

Because pulse oximeters use the manufacturer-

specific R-curve to derive and display saturation,

we will never find a calibration pot or any other

means to adjust the device against a primary refer-

ence. The only currently accepted primary reference

is the in vivo study described above. Theoretically,

a true simulator of oxygen saturation would require

a pulsatile fluid loop which contains a hemoglobin-

like substance transporting oxygen in various

proportions. This theoretical simulator would need

to vary the rate and volume of the fluid pulsations

to mimic the pulse rates and perfusion levels, as

well as provide several levels of optical transmis-

sion to represent various tissue densities. Motion

artifact and interference from ambient lighting

would also be needed. And to be marketable, the

simulator would have to deal with the differences in

the various manufacturer R-curves. This hypotheti-

cal test device would be impractical as a bench-top,

production model. There are no simulators that in-

dependently verify the accuracy of a pulse oximeter.

How to properly measure SpO2

accuracy

Annex FF of ISO Standard 80601-2-61 makes clear

distinctions between the terms “simulator”, “cali-

brator”, and “functional tester”. A calibrator would

be a high-accuracy simulator, capable of electronic

signals or optical responses identical to a human

subject. By definition, a calibrator would also have

to have accuracy much greater than the device

under test, which is already typically 2 percent.

Another annex of the standard requires the

instruction manuals of pulse oximeter equipment

to state that functional testers cannot in general

be used to measure SpO2 accuracy. Rather than

being “primary standards” (or “gold standards”)

against which all monitors are calibrated, current

pulse oximeter testers are “transfer standards”

i.e. they are a reference of comparison validated

against a another standard established previously.

The U.S. FDA carefully defines this as “substantial

equivalence” in its pre-market guidelines.

Since the introduction of the first commercial

pulse oximeter in 1977, the objective performance

verification of these monitors has been elusive.

Healthcare technology personnel should bear in

mind that a SpO2 tester is a secondary standard

which transfers equivalence from prior devices

that have been validated before. The verb “simu-

late” means “to present a false appearance of”,

while the verb “emulate” means “to try to equal or

excel”. Other biomedical equipment testers may be

simulators, but SpO2 testers are, at best, emulators;

they only approximate the physiology of a human

subject.

All pulse oximeter testers currently on the

market require the user to select the monitor

manufacturer, or groups of manufacturers, in order

to accommodate the manufacturer’s R-curves. In

addition to saturation and pulse rate, most testers

offer user-selectable values of pulse amplitude,

tissue transmittance, arrhythmias, motion artifact,

and interference from power line frequencies.

Many models also have pre-set combinations of

clinically normal and abnormal values, and may

allow the user to define custom values. All testers

use either an electronic or optical interface with

the unit under test. Some models offer both modes.

Electronic testers apply an electrical signal to the

monitor through its sensor cable, without inclusion

of the sensor. The user-selected electrical signal

mimics various values of saturation and other vari-

ables. These units offer the option of testing the

sensor (presumed to be a finger sensor) by verify-

ing the continuity of the red and infrared LED’s

and that of the photodiode. Some models also test

to confirm the photodiode’s correct response to the

two light signals.

In contrast to the electronic interface, optical

testers provide a physical digit or “artificial finger”

that includes a mechanical and/or opto-electronic

element which allows variable transmission of the

two light signals. (Figure 2.)

This type tests the entire monitor system (sensor,

cable, and monitor) at once, which can save time

when performance testing many units. As with the

electronic types, these enable the user to select a

range of values of saturation and other variables

3 Fluke Biomedical There’s no such thing as a SpO2 simulator

and pre-sets, depending on the manufacturer.

One vendor offers a set of calibrated fingers, or

artificial digits with integrated dyes to allow

transmission of light corresponding to a specific

saturation.

Currently marketed SpO2 testers offer a variety

of functions, and the equipment technician must

decide how thoroughly to test. Most SpO2 moni-

tors will be accurate at clinically “normal” values,

but equipment technicians must detect when the

monitor gives inaccurate values in the abnormal

range, where clinicians must decide on clinical

corrective action.

One widely promoted tester provides five pre-

set combinations of saturation value, pulse rate,

and pulse amplitude, where a unit like the SPOT

Light SpO2 functional tester by Fluke Biomedical

enables independent selection of eight differ-

ent saturations and pulse rates, three levels of

light transmission, and the option of artifact from

respiration and line frequencies. The SPOT Light

also offers a selection of R-curves for eight differ-

ent manufacturers, where another unit offers only

three R-curves to cover all makers. With optical

testers, precise positioning of the sensor probe

on the digit can be problematic. Most units give

an indication if the probe is placed incorrectly

on the digit, but there is no indication of maxi-

mized signal strength. The SPOT Light includes

an on-screen signal quality indicator to enable

optimal placement of the probe, assuring consis-

tent readings. The SPOT Light sets up in seconds

to send SpO2 saturation, heart rate, perfusion,

transmission, artifact noise, and eight different

manufactures custom R-curves to a pulse oximeter

or patient monitor.

Figure 2. Testing finger using an LED and photodiode to interact with an

SpO2 sensor.

About the ProSim SPOT Light

SpO2 functional tester

Featuring an exclusive ergonomic

design, the ProSim SPOT Light is the first

comprehensive SpO2 functional tester

to come in a handheld and easy-to-use

device. SPOT Light is lightweight and

flexible with three custom presets spe-

cially-designed to make it the fastest and

easiest-to-use device on the market today

for pulse oximeter functional testing.

A helpful LCD display and three simple buttons

make it effortless to rapidly change parameters

and view each signal output sent to the pulse

oximeter at a glance. An interchangeable, long-life

battery ensures uninterrupted all-day operation

without need to connect to a power supply.

To learn more about the SPOT Light SpO2 Func-

tional Tester Pulse Oximeter Analyzer, click here

or visit www.flukebiomedical.com.

Fluke Biomedical.

Better products. More choices. One company.

Fluke Biomedical

6045 Cochran Road

Cleveland, OH 44139-3303 U.S.A.

For more information, contact us at:

(800) 850-4608 or Fax (440) 349-2307

Email: sales@flukebiomedical.com

Web access: www.flukebiomedical.com

©2013 Fluke Biomedical. Specifications subject

to change without notice. Printed in U.S.A.

12/2013 6001759A_EN

Modification of this document is not permitted

without written permission from Fluke Corporation.

About the author

Dennis J. McMahon, CBET-R, is a biomedical equipment technician with

over forty years of medical technology experience. After earning a degree

in Chemistry in the late ‘60s, he worked as an anesthesia technician at

Harborview Medical Center until 1977. He certified as a biomedical

technician in 1983, and attended service schools for a variety of clinical

technology over the following three decades. Dennis currently instructs at

North Seattle Community College and serves as the Education Chair for the

Washington State Biomedical Association (WSBA).

References

• Chan, A: Biomedical Device Technology 2008. C Thomas - Publisher

• FDA Guidance for Industry and FDA Staff: Pulse Oximeters – Premarket

Notification Submissions [510k]

• International Standards Organization, Standard 80601-2-61 (2011): Medical

electrical equipment - Particular requirements for the basic safety and

essential performance of pulse oximeter equipment

• Severinghaus, J: Takuo Aoyagi: Discovery of Pulse Oximetry

Anesth Analg 2007; 105 S1-4