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The ABCs of Pulse Oximetry
An anesthesiologist explains what SP02 technology does - and doesn't do.
Julian Goldman
Publish Date: October 10, 2007   |  Tags:   Anesthesia

The pulse oximeter is one of the most important, yet least understood, patient monitoring advances of the last 25 years. The technology provides a means to easily, quickly, continuously, accurately and non-invasively estimate a patient"s arterial blood saturation level and helps the clinician determine if the patient"s blood is adequately oxygenated. Some clinicians have even said that if, for some reason, they could only have one monitor, SPO2 would be the most important one.

But why is this so? What does oximetry actually do? What does it not tell us? And why has there been so much concern over the many ways that the technology can get "thrown off"? With so many oximeters on the market, a little knowledge about the technology"s principles and performance issues goes a long way in helping you incorporate pulse oximetry to make the most clinical and economic sense in your facility.

What's the value of pulse oximetry?
It was shown years ago that modest reductions in SPO2 cannot be detected by examining the patient. Yet, early small reductions in SPO2 can lead to life-threatening hypoxemia if left untreated. Therefore, the most widespread use of pulse oximetry (PO) is to help detect hypoxia before the patient becomes clinically cyanotic (blue). .Moreover, the oximeter alerts the clinician to potentially significant problems by activating alarms. Sometimes PO are used to take a quick "spot" reading of the SPO2 to assess the patient"s oxygenation in lieu of an arterial blood gas sample. While not as accurate or complete a test as a blood gas, the PO can improve patient comfort and save time when it replaces a blood gas.

What does pulse oximetry do?
All pulse oximeters do not operate exactly alike but they function under the same scientific principle. They use the principle that the color of blood is primarily a function of oxyhemoglobin saturation. The oximeter sensor or probe contains light emitting diodes (LEDs) to generate light and a photodetector. The LEDs produce light at red (approximately 650 nm) and infrared (approximately 905 nm) wavelengths. The light is transmitted through tissue and measured by a photodetector. A wide range of sensors have been designed to permit placement on a finger, earlobe, forehead, toe or other sites.

Why the "pulse" in pulse oximetry? The PO shines light through tissue that contains many light-absorbing components (such as muscle, skin, arterial blood, venous blood, etc.) However, the clinically important value is the color, or saturation, of the arterial blood. It"s the pulsatility of arterial blood that permits arterial blood absorption to be separated from the non-pulsatile absorption of light by tissue, muscle, venous blood and anything else photons pass through on their voyage from the LED emitters to photodetector. Ideally, the oximeter filters out everything but the photoplethymographic variations produced by arterial blood flow and analyzes only these variations to display the SPO2 measurement.

Where Should You Place the Sensor?

Pulse oximetry sensor placement is subject to ongoing debate among clinicians and is one of the ways that device manufacturers distinguish their products from their competitors". For example, some clinicians prefer forehead or earlobe placement of the sensor especially during peripheral tissue hypoperfusion because these sites usually maintain adequate perfusion to obtain reliable signals, the SPO2 at these sites tracks changes in central arterial oxygenation faster than the fingers, and there is less motion at these sites. Another approach is to utilize a perfusion index (PI) to determine the best sensor site for the case. Sometimes, out of desperation to obtain a measurable signal, clinicians place sensors on the cheek, nose, or tongue. The catch is that placing sensors on sites that they are not designed for will reduce accuracy.

- Julian M. Goldman, MD

What doesn't pulse oximetry do?
There are a variety of factors that may impair the performance of a pulse oximeter. We"ll look at these momentarily.

Even under ideal operating conditions, pulse oximeters aren"t perfect instruments. It"s crucial to keep in mind that the clinician, not the machine, is the "real" SP02 monitor. By this, I mean that a pulse oximeter estimates the oxygen saturation of hemoglobin, but the accuracy of the readings may be reduced by a number of clinical and environmental factors. Therefore, when in doubt about the veracity of an SP02 reading (and if it"s clinically indicated), the clinician should measure an arterial sample. And, like any other clinical data, SPO2 should be interpreted in association with the patient"s clinical condition.

Additionally, oximeters give us no information about carbon dioxide levels or acid-base (pH) status. They"re of limited help by themselves in helping the clinician detect respiratory failure and associated CO2 retention. A elevated aterial carbon dioxide level (PaCO2)- and consequent respiratory acidosis- can develop even at normal SPO2 levels (especially when the patient is breathing supplemental oxygen). That"s why adequacy of ventilation must be monitored separately than oxygenation.

New Technology Can Create New Problems

While new pulse oximetry technologies help minimize existing equipment limitations, they sometimes create new problems of their own. For example, SPO2 calculations of "noisy" data (information that competes with signals generated by motion, electrosurgical or other equipment) require the oximeter to analyze larger epochs of signal to extract the arterial saturation from all the clutter. This precludes beat-to-beat calculation of SPO2 and, under some circumstances, may hinder overall oximeter performance. Furthermore, some oximeter technologies must acquire an accurate pulse rate measurement in order to produce a reliable SPO2 calculation.

What are the factors that impair pulse oximeter performance?
When manufacturers introduced the first commercially viable pulse oximeters in the 1980s, consumers tolerated their performance flaws. Motion, low perfusion, tissue distortion and signal interference from other medical equipment could make their SP02 estimates significantly less reliable and more prone to generating false alarms.

Today, our clinical expectations are much higher. Through improved technology (better hardware and advanced algorithm-derived readings), pulse oximeters are now remarkably effective instruments. There are many good oximeters on the market-dedicated SPO2 monitors and ones that measure SPO2 along with other functions, such as ECG, non-invasive blood pressure and temperature.

Nevertheless, some performance limitations exist with all oximeters and performance varies from one make and model to the next. When you shop for a "new generation" oximeter, it"s helpful to assess how reliably the device addresses each of the following problems:

  • Motion. Motion is a notorious pulse oximetry artifact-inducer. Motion produces photoplethysmographic signals that the oximeter can "mistake" for a signal generated by arterial blood flow and calculate a spurious SPO2 value and pulse rate. Even in the complete absence of blood flow, movement of normally non-pulsatile venuous blood can generate a photoplethysmographic signal. It"s easy to demonstrate how this phenomenon occurs by occuluding upper extremity blood flow with a blood pressure cuff and wiggling the fingers to generate a signal. Today"s oximeters do a much better job of reducing the significance of this factor. A good oximeter can identify arterial-induced pulsations buried in artifact-riddled signals but it still assumes the signal of interest is being generated by arterial pulsations.
  • Low perfusion. The oximeter may not receive a pulsatile signal of high enough quality to interpret SPO2.Hpotension or vasoconstriction from cold exposure, can reduce overall blood flow and reduce the amplitude of the pulsatile signal. Low-perfusion states can produce a signal that is 100 times smaller than one measured in a well-perfused patient. Oximeters vary in their ability to measure these low-amplitude signals. The challenge is intensified if motion produces noise signals that are much larger than the arterial signal. There can also be large finger-to-finger variations in perfusion, so moving the sensor to a better perfused finger or to a non-moving hand may provide a better signal.

    Some oximeters now incorporate a so-called "perfusion index" (PI) which displays the signal strength of the pulsatile signal. The PI can help clinicians select the best sensor site and position and obtain insight into vascular activity. While PI is a useful tool, it isn"t absolutely essential for effective oximetry.

  • False alarms. In the past, the poor motion and low perfusion performance of oximeters generated so many false alarms that the audible alarms were more of a detriment than a help in many environments. Oximeters also generate "nuisance alarms," which are true alarms of questionable clinical significance (for example, a transient dip in SPO2 below the alarm threshold may not require immediate intervention with some patients).

Manufacturers and facilities alike face ongoing challenge of crafting alarm management strategies that heighten alarm specificity without sacrificing alarm sensitivity. When you consider this issue, it"s beneficial to huddle with your anesthesia personnel and focus on evaluating the appropriateness of both the oximeter performance settings-such as signal averaging time-and alarm settings that match the clinical context of the cases you perform. This provides a mechanism for tailoring the oximeter to your facility needs. Secondly, formulating a facility-specific alarm management strategy helps you meet JCAHO"s 2004 National Patient Safety Goal of improving the effectiveness of clinical alarms.

An evolving technology
It"s amazing how far pulse oximetry has come since its introduction. It"s not overstating the case to say that the technology has revolutionized patient care. Nevertheless, there"s no such thing as the "perfect oximeter." Find one that"s right for your facility.

What"s New in Pulse Oximeters


Nellcor
N-595
(800) 635-5267
www.nellcor.com
List price: $4,500

The N-595 incorporates Nellcor's proprietary Oximax digital signal processing technology. The monitor offers exceptionally accurate performance even when faced with motion and low perfusion at the same time, according to the company. A variable-pitch beep tone enables clinicians to hear point-by-point SPO2 changes.


Respironics Novametrix
Model 2001 (MARSPO2)
(800) 345-6443
www.novametrix.com
List price: $3,400

The MARS system is a third-generation oximetry technology with an SpO2 algorithm designed to reject motion artifacts and remove extraneous noise from patient signals, even at low perfusion. The plethysmograph and signal-strength indicator assist the provider in locating the optimal place for the sensor. Many applicators, including an ear clip, are available for customizing the sensor to several sites.


Masimo
Radical
(949) 250-9688
www.masimo.com
List price: $4,995

The Radical incorporates Masimo's Signal Extraction Technology (SET), which can detect arterial pulsations even among artifact-riddled signals and low-perfusion conditions. Masimo says the Radical series has the highest sensitivity in detecting true alarms and specificity in rejecting false alarms of any pulse oximeter.


Nonin
Avant9600
(763) 553-9968
www.nonin.com
List price: $1,225

This tabletop/portable oximeter features three user-adjustable audible and visual signal quality displays. A qualitative tri-color LED pulse-strength indicator coincides with pulses identified for assessment. A pulse-quality indicator appears with subtle to dramatic changes in the patient's pulse quality; an amber-colored indicator alerts the provider to perfusion inconsistencies.


Datex-Ohmeda
3800 and 3900
(800) 345-2700
www.us.datex-ohmeda.com
List price: 3800 is $1,307; 3900 is $1,418

These monitors from Datex-Ohmeda (now part of GE Medical) feature proprietary "TruTrak " technology, which the company says is a five-step process that provides high-speed data sampling, SpO2 calculation and correction during clinical patient motion (such as clenching, flexing or kicking) and provides fewer false alarms. The monitors feature a proprietary PI to help the provider determine signal quality and locate the best sensor site.


Dolphin Medical
2100 Pulse Oximeter
www.dolphinmedical.com
(866) 588-9539
List price: Not provided

The 2100 features a 4- to 12-second SpO2 response averaging mode, depending on the signal-to-noise ratio, and an automatically scaled plethysmographic waveform. The oximeter also has user-selected high- and low-alarm limits to adjust to low perfusion and motion, including a temporary alarm silence feature and an all-mute alarm to silence audible alarms until deactivated. The oximeter performs an automatic self-test upon start-up.


Criticare
504DX
(262) 798-8282
www.csiusa.com
List price: Not provided

This portable monitor incorporates Criticare's proprietary DOX digital pulse oximetry and heart-rate monitoring technology, which the company says greatly improve pulse recognition and artifact rejection. Audible and visual alarms provide instant notification during periodic spot-check monitoring and extended, continuous monitoring. The system is integratable with an optional printer to produce user-defined graphical and tubular SpO2 and heart-rate documentation.

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