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.
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.
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.