Abstract
Pulse oximetry is a useful noninvasive tool in the assessment of a patient suspected of hypoxia. On literature review, few medical and nursing articles explain how to properly use this tool. This article reviews the basic physics of the pulse oximeter machine and proper use of the oximeter. Limitations and sources of error in pulse oximeter technology are also delineated. Finally, correct interpretation and application of information obtained by pulse oximetry are explained.
Keywords
The human eye is not very trustworthy when it comes to detecting hypoxemia. The traditional sign of circumoral cyanosis is a late indicator of decreased oxygenation and is subjective, depending on the experience and eyesight of the observer, as well as the ambient lighting and the patient's skin pigmentation. Pulse oximetry is an objective measurement of oxygenation and is simple, reliable, and accurate when used appropriately.
Pulse oximetry is a useful tool in the evaluation of a patient's oxygenation status and may be used routinely in many areas of clinical practice. Through use of pulse oximetry, oxygenation can be monitored easily and noninvasively. Advances in microprocessor technology, along with improvements in light-emitting diodes and photoelectric sensors, have improved the accuracy and reliability of pulse oximetry.
1
However, because of the inherent limitations of noninvasive technology, it is important to know how to interpret the information received from oximetry.Pulse oximetry has gained wide clinical acceptance in many areas. Small portable systems are available for use virtually anywhere. Almost every patient that has oxygen or mechanical ventilation requirements would benefit from clinical monitoring of their oxygen status by pulse oximetry. This may be in the form of continuous monitoring or by intermittent testing.
2
The oxygen saturation as calculated by pulse oximetry has a 95% confidence rate of ±4%, so oximetry is considered to be reliable at readings that range between 70% and 100% SpO2.
3
This means that, although pulse oximetry is not a replacement for blood gas testing, it can be used as a screening tool when poor oxygen saturation is suspected (Table 1).Table 1Common Areas for Use of Pulse Oximetry
| 1 | During anesthesia and postanesthesia care, including both general and conscious sedation |
| 2 | Intensive care units |
| 3 | Neonatal care units, including delivery, nursery, and neonatal intensive care unit |
| 4 | Hospital medical units |
| 5 | Transportation within the hospital and during ambulance or air ambulance transportation |
| 6 | Diagnostic testing, such as pulmonary function testing, exercise testing, and during sleep studies |
| 7 | Subacute care centers, such as nursing homes and rehabilitation centers |
| 8 | Home care patients |
Physics
The pulse oximeter measures and displays the pulse rate and the saturation of hemoglobin in arterial blood. The oximeter uses a sensor device consisting of two light sources (red and infrared) and a photo detector to measure absorption of visible light. The photo detector then measures light absorption as it moves through the tissue. The amount of dissolved oxygen in the blood dictates the number of oxygen molecules bound to hemoglobin, or how saturated the hemoglobin is with oxygen. The ratio between the amplitude of the red and infrared wavelength is used to determine oxygen saturation by the pulse oximeter.
2
Pulse oximeters measure the difference in light absorption of oxygenated and reduced hemoglobin, and then calculate the percentage of hemoglobin that is saturated with oxygen. Oxygen saturation is defined as the ratio of oxyhemoglobin (HbO2) to the total concentration of hemoglobin in the blood.
4
When an arterial blood gas (ABG) measures arterial oxyhemoglobin saturation, it is referred to as SaO2 [SaO2 = HbO2/(HbO2 + Hb)].4
When arterial oxyhemoglobin saturation is measured noninvasively by pulse oximetry, it is referred to as SpO2.The pulse oximeter uses two sensors and a light source to determine the percentage of oxygen saturation in the blood. The two sensors measure the color of both oxygenated blood and unoxygenated blood by detecting different shades of light transmitted through the tissue from the light source. The oxygenated blood is a brighter shade of red than the unoxygenated blood. The pulse oximeter calculates the difference between the two measurements to determine the percentage of oxygen saturation.
5
The pulse oximeter uses empirical calibration curves developed from studies of healthy volunteers to calculate SpO2.
2
The partial pressure of oxygen dissolved in the plasma is measured as the PaO2. The oxygen dissociation curve shows the relation between SpO2 and PaO2. An SpO2 greater than 95% correlates to the normal range of PaO2, which is 80 to 100 mm Hg. A PaO2 of 60 mm Hg or less correlates to a SpO2 of less than 90% per the dissociation curve.6
Changes in temperature and pH cause a shift in this relation. As pH increases (alkalosis) or temperature decreases (hypothermia), the shift is to the left as hemoglobin binds more tightly with oxygen delaying its release to tissues. Acidosis (low pH) and fever shift the curve to the right, as the hemoglobin molecule loosens its affinity for oxygen, making it easier for oxygen to be released to the tissues.7
The pulse oximeter readings lag behind the patient's condition because there is a response delay as a result of signal averaging in the monitor. The length of signal averaging can be set on some machines for 4, 8, or 16 seconds, with most oximeters having a common default setting of 8 seconds. This means there is an interval after the actual arterial blood oxygen saturation starts to fall before it is detected by oximetry because the signal is averaged over several seconds. The PaO2 could potentially decrease to a critical level before the decreased SpO is displayed by the oximeter. The clinician needs to be aware of this time delay between a potentially hypoxic event such as a respiratory obstruction and the pulse oximeter registering a low oxygen saturation.
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Usage
The pulse oximeter consists of a peripheral probe, together with a microprocessor unit, which displays a waveform, the calculated oxygen saturation percentage, and the averaged pulse rate. Alarms are available which sound when a low SpO2 level is detected, or when the pulse rate is either tachycardic or bradycardic.
The probe must be placed on a pulsing vascular bed. A sharp waveform with a clear dicrotic notch indicates a good signal. The machine then reads and averages the values that the waveform receives from the vascular bed, which are read over 5 to 20 seconds, depending on the machine's internal setting. Averaging the signal reduces erroneous readings and distinguishes artifact from true signal. A pulse rate and percentage of oxygen saturation are interpreted from these averaged values.
Correct use of the pulse oximeter includes proper placement of the probe. The emitter and detectors must be opposite each other, and light must not reach the detector other than through the tissue. Care must be used to ensure that the digit is fully inserted into the probe.
The finger should be the first choice of site used for measurement. A nondisposable clip-type probe is adequate for most purposes, especially when a single or spot reading is being taken. Self-adhesive probes are more useful for long-term monitoring or when motion artifact is expected. Toes may be used instead of fingers, but poor signal because of decreased perfusion is more likely. The lobe or pinna of the ear can be used with a clip-type probe. Care must be taken when using the ear so that pressure from the clip does not impair perfusion. Forehead or nasal sensors may also be used but, depending on clinical area of use, may not be as readily available as finger probes.
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Limitations
Pulse oximeters do not require user calibration. Thus, it is important that users of the device are aware of the inherent limitations that may give false readings. The pulse oximeter will function properly only if it is able to detect a modulation in transmitted light. If perfusion is decreased and pulse amplitude is small, the signal will be decreased, and the device will be liable to error or be unable to obtain a reading.
2
Several conditions adversely affect pulse oximetry readings. Poor peripheral perfusion because of cold or hypotension is the principal cause for failure to obtain a satisfactory signal, mainly because of an inadequate pulse wave. Hypothermia, cold extremities, and poor peripheral perfusion may cause the oximeter to fail to register a signal. During cardiac arrest peripheral pulses may be so weak that the device cannot detect them, so monitoring SpO2 by pulse oximetry could be contraindicated.
10
Pulse oximetry has been found to be reliable with systolic blood pressure readings greater than 80 mm Hg. Hypotensive systolic blood pressure readings less than 80 mm Hg cause inaccurate and unreliable pulse oximetry readings.
10
Hypotension, low cardiac output, vasoconstriction, vasoactive drugs (dobutamine or dopamine), and hypothermia all reduce tissue blood flow.11
These low perfusion states produce a low signal-to-noise ratio and create a signal that can be altered by artifact.8
Lowered perfusion reduces the signal strength, and the oximeter may not be able to adequately differentiate between arterial pulsations and background noise.2
This causes inaccurate readings, because the sensor is unable to distinguish the true signal.Motion artifact can interfere with signal detection and interpretation of the signal by the device because of an unstable waveform. Improperly seated sensors, shivering, seizures, or parkinsonian tremors can cause movement, creating an inaccurate reading. Adjustment of the device to a longer signal averaging time may reduce the effects of motion artifact.
2
The pulse oximeter may also be inaccurate in bradycardia and irregular cardiac rhythms, because the device is not able to average the signal waveform in the set amount of time. Edema or venous congestion of the limb can also interfere with readings because of decreased signal.6
, 13
Because pulse oximeters use two-wavelength spectrophotometry, readings are inaccurate in the presence of abnormal hemoglobin levels. Thus, carbon monoxide poisoning will result in an erroneous SpO2 reading as a result of carboxyhemoglobin. Smokers will often have artificially high readings after smoking a cigarette because cigarette smoke contains carbon monoxide. The presence of methemoglobin will also give an unreliable oximeter reading
2
(Table 2).Table 2Troubleshooting the Pulse Oximeter
2
| Problem | Possible Cause | Possible Solution |
|---|---|---|
| No signal, SpO2 of zero |
|
|
| Erratic signal |
|
|
| SpO2 is much different than measured SaO2 | (COHb or METHb) | |
| Heart rate on the pulse oximeter is different fromthe apical heart rate |
|
|
ABGs indicate arterial blood gases; METHb, methemoglobin; COHb, carboxyhemoglobin.
High-intensity lighting, typically fluorescent lights, may lead to false readings. This can be corrected by turning off the bright light. Nail polish and artificial fingernails generally do not interfere with oximetry readings, except in the case of black, blue, or green nail polish.
3
, 14
, 15
Age, sex, anemia, jaundice, hyperbilirubinemia, and dark skin have not been shown to cause abnormal readings (Table 3).16
Table 3Factors That Affect Pulse Oximetry
| Factors | Effect on Pulse Oximetry | |
|---|---|---|
| Emergent | ||
| Cardiac arrest | Poor signal | |
| Respiratory arrest | Poor signal | |
| Shock | Poor signal | |
| Physiologic | ||
| Jaundice | No interference | |
| Anemia | No interference | |
| Carbon monoxide poisoning | False increase in reading, often reading 100% | |
| Cardiac arrhythmia | Poor signal | |
| Dark skin | No interference | |
| Peripheral vasoconstriction | Poor signal | |
| Edema | Poor signal | |
| Arteriovenous fistula | Poor signal | |
| Environmental | ||
| Nail polish | Poor signal with black, green, blue polish | |
| Bright light or sunshine | False increase in signal (overreading) | |
| Cold room | Poor signal | |
| Shivering, tremors, rigors, motion | Poor signal | |
| Electrical frequencies | Interference with signal | |
| Elevation of probe higher than heart | Poor signal | |
| Mechanical interference with circulation (blood pressure cuff, tourniquet, arterial line) | Poor signal | |
| Dirty sensor | Poor signal | |
Interpretation
Normal arterial oxygen saturation is considered to range between 97% and 99%. Some people, especially long-term smokers, may typically have an SpO2 between 93% and 95%. Readings of 90% or less may indicate that the patient needs supplemental oxygen and further tests as confirmation of hypoxia.
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It is important to remember that pulse oximeters measure and calculate the oxygen saturation of the hemoglobin in arterial blood, not the actual oxygen content of the blood; therefore, they do not provide a measure of actual tissue oxygenation or how well the patient is ventilated. Be cautious interpreting readings when there has been a sudden change in SpO2. One example would be a sudden decrease from 97% SpO2 to 85% SpO2; this is physiologically impossible. Evaluate this information in conjunction with the patient's clinical condition and the above-listed limitations.
Oxygen saturation values below 70% obtained by pulse oximetry are unreliable. Any time hypoxia is suspected, but not confirmed with pulse oximetry, ABGs should be performed.
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Even when the pulse oximeter reads the SpO2 as normal, the patient could have undetected carbon dioxide retention. Therefore, it is important not to rely on the information from pulse oximeters alone in the assessment and diagnosis of hypoxemia (Table 4, Table 5, Table 6).Table 4Pulse Oximetry: What Do the Numbers Mean?
7
| SpO2, % | PaO2, mm Hg | Oxygenation Status |
|---|---|---|
| 95–100 | 80–100 | Normal |
| 91–94 | 60–80 | Mild hypoxia |
| 86–90 | 50–60 | Moderate hypoxia |
| Less than 85 | Less than 50 | Severe hypoxia |
Table 5Patients Who Need Oxygen Regardless of Oxygen Saturation Measurement
| 1 | Patients in cardiac or respiratory arrest |
| 2 | Patients with chest pain suspected to be of cardiac origin |
| 3 | Patients with multisystem trauma |
| 4 | Patients who are apneic or who require assisted ventilation |
| 5 | Patients with suspected or confirmed carbon monoxide poisoning or smoke inhalation |
| 6 | Neonatal patients in distress |
| 7 | Patients with suspected sickle cell crisis |
| 8 | Hypotensive patients (SBP < 80 mm Hg) |
| 9 | Near drowning patients |
Table 6Case Study
| A 62-year-old patient with pneumonia has an SpO2 of 96% with a heart rate of 90 beats per minute (bpm) at the start of a shift. After a few hours, the SpO2 decreases to 91%, and the patient seems lethargic. The apical heart rate and oximeter pulse rate correlate at 98 bpm. What does this mean? |
| Answer: This is a true desaturation. With this decrease in oxygen saturation as read by the oximeter, the PaO2 has decreased from approximately 90 mm Hg to 60 mm Hg. Making sure the apical heart rate correlates to the oximeter pulse rate is a check on the accuracy of the oximeter reading. |
Summary
The pulse oximeter, like any clinical monitoring tool, must be used correctly and the results interpreted properly. The oximeter is used throughout health care as a screening tool through intermittent and continuous monitoring of a patient's SpO2. It is a noninvasive, easy-to-use tool that provides useful information about the oxygenation of a patient's blood.
Clinicians must recognize the limitations of pulse oximetry. The pulse oximeter is an additional assessment tool, not a substitute for other elements of the assessment. Pulse oximeters can increase patient safety by alerting the clinician to hypoxia. However, the reading should always be interpreted in association with the patient's clinical condition.
References
- Mechanical ventilation: physiological and clinical applications. Mosby, St Louis, MO1998
- Respiratory care equipment. Lippincott Williams & Wilkins, Philadelphia, PA1999
- What is the effect of fingernail polish on pulse oximetry? [letter].Chest. 2003; 123: 2163-2164
- “Motion-resistant” pulse oximetry: a comparison of new and old models.Anesth Analg. 2002; 95: 967-972
- Enamel nail polish does not interfere with pulse oximetry among normoxic volunteers.J Clin Monit Comput. 2002; 17: 93-96
- Limitations of pulse oximetry: respiratory insufficiency—a failure of detection [case report].BMJ. 1993; 307: 372-373
- The University of Texas Medical Branch.(Accessed.)
- Pulse oximetry. Principles and practice series (Hahn C, Adams A, series editors). BMJ Publishing, London, United Kingdom1994
- Pulse oximetry.Update Anaesth. 1995; 5
- Understanding pulse oximetry, advantages, and limitations.Home Health Care Manag Pract. 2005; 17: 416-418
- The accuracy of the Biox 3700 pulse oximeter in patients receiving vasoactive therapy.Intensive Care Med. 1991; 17: 484-486
- Practical applications of pulse oximetry.Update Anaesth. 2000; 11
- Pulse oximetry for hypoxemia: a warning to users and manufacturers.Intensive Care Med. 1989; 15: 540-542
- The benefits and pitfalls of pulse oximetry [editorial].BMJ. 1993; 307: 457-458
- Nail polish color can affect pulse oximeter saturation [letter].Anesthesiology. 1988; 68: 825
- Accuracy of pulse oximetry in the intensive care unit.Intensive Care Med. 2001; 27: 1606-1613
Article info
Footnotes
Author has no relationship with business or industry to disclose.
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Copyright
© 2007 American College of Nurse Practitioners. Published by Elsevier Inc. All rights reserved.
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