Pulse Oximetry - Department of Critical Care Medicine

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Pulse Oximetry
Rafael Ortega, M.D., Christopher J. Hansen, M.A., Kelly Elterman, M.D.,
and Albert Woo, M.D.
Overview
Pulse oximetry has become the standard of care in operating rooms, intensive care
units (ICUs), and hospital wards in the United States and many other nations. 1
Before pulse oximetry was available, physicians relied on invasive procedures, such
as arterial puncture for blood gas analysis, to identify the presence of hypoxemia.
Unlike arterial blood gas analysis, pulse oximetry allows for noninvasive and continuous
monitoring of arterial blood oxygen saturation. Although the pulse oximeter
is easy to use, the clinician must understand the principles of pulse oximetry
and must know how the equipment works in order to interpret the information it
provides.
From the Department of Anesthesiology,
Boston Medical Center, Boston University
School of Medicine, Boston. Address reprint
requests to Dr. Ortega at the Department
of Anesthesiology, Boston Medical
Center, Boston University School of Medicine,
88 E. Newton St., Boston, MA, 02118,
or at rortega@bu.edu.
N Engl J Med 2011;364:e33.
Copyright © 2011 Massachusetts Medical Society.
Definitions
Oxygen is present in two forms in the blood: dissolved and bound to hemoglobin.
Hemoglobin can be functional or nonfunctional in terms of oxygen binding and
transport. Functional hemoglobin binds and transports oxygen and is present as
oxyhemoglobin, which is hemoglobin that contains bound oxygen, and deoxyhemoglobin,
which is reduced hemoglobin, without bound oxygen. Nonfunctional
hemoglobin is unable to bind or transport oxygen and is present as carboxyhemoglobin
and methemoglobin. Carboxyhemoglobin is hemoglobin bound to carbon
monoxide. Methemoglobin is hemoglobin that contains ferric iron, Fe 3+ , an oxidized
form of the oxygen-carrying ferrous iron, Fe 2+ .
The partial pressure of oxygen dissolved in arterial blood is termed PaO 2 . The
percent saturation of oxygen bound to hemoglobin in arterial blood is termed
SaO 2 . When measured by a pulse oximeter, this value is called SpO 2 .
Indications
Pulse oximetry is indicated in all clinical settings in which hypoxemia may occur,
such as operating rooms, ICUs, postanesthesia care units, emergency departments
and ambulances, endoscopy suites, sleep laboratories, cardiac catheterization laboratories,
delivery suites, and wards. The use of pulse oximetry in these settings may
reduce the need for arterial blood gas analyses and may allow titration of the fraction
of inspired oxygen (FiO 2 ) in patients requiring oxygen or mechanical ventilation.
Pulse oximetry can also be used to screen for cardiopulmonary disease.
Contraindications
There are no contraindications to pulse oximetry. It is generally safe to use in
monitoring all patients.
Principles of Operation
Pulse oximeters consist of a peripheral probe and a microprocessor unit. Traditionally,
the peripheral probe contains a photodetector and two light-emitting diodes.
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Figure 1. Emitted Red Light Reaching
the Photodetector.
Figure 2. Pulse-Oximeter Waveform.
Figure 3. Pulse-Oximeter Probe
on Earlobe.
The light-emitting diodes each emit light of a different wavelength. The light emitted
by the diodes is absorbed by tissues, and the amount of absorption is determined
by the photodetector (Fig. 1). Using this information, the microprocessor
determines the concentration of oxyhemoglobin and deoxyhemoglobin. It then calculates
the percentage of oxyhemoglobin and displays the hemoglobin oxygen saturation
in arterial blood, a waveform corresponding to the pulsatile flow in arterial
vessels, and the heart rate.
Pulse oximeters function on the principle that oxygenated hemoglobin and deoxygenated
hemoglobin absorb red and infrared light differently. One light-emitting
diode emits light in the red spectrum, at a wavelength of 660 nm, at which the light
absorption of deoxyhemoglobin is greater than that of oxyhemoglobin. The other
diode emits light in the infrared spectrum, at a wavelength of 940 nm, at which
oxyhemoglobin absorbs more light than deoxyhemoglobin. The microprocessor
analyzes the light absorption of the tissues at each wavelength to determine the
concentrations of oxyhemoglobin and deoxyhemoglobin, respectively. It then divides
the concentration of oxyhemoglobin by the concentration of both oxyhemoglobin
and deoxyhemoglobin to yield the SpO 2 .
The probe is positioned so that the photodetector and light-emitting diodes face
each other, with layers of tissue between them. The photodiodes turn on and off
several hundred times per second to record the light absorption during pulsatile
and nonpulsatile flow. During pulsatile flow, the light absorption of arterial blood,
background tissues, and venous blood is detected. During nonpulsatile flow, only
the light absorption of background tissues and venous blood is detected. The microprocessing
unit compares the light absorption during both pulsatile and nonpulsatile
flow to isolate the light absorption of arterial blood and thus determine
the SpO 2 . 2
Data Interpretation
Pulse oximetry provides both qualitative and quantitative data. The qualitative data
are obtained through the sounds emitted by the pulse oximeter; these sounds correlate
with the level of oxygen saturation. A higher pitch indicates higher oxygen
saturation, and a lower pitch warns of decreasing oxygen saturation. The quantitative
data are obtained through the display of a pulsatile waveform that corresponds
to the flow of arterial blood (Fig. 2), and the display of numbers indicates the SpO 2
and the heart rate.
Proper Use of Pulse Oximetry
The ideal site for placement of the pulse-oximeter probe is one that is well perfused,
relatively immobile, comfortable for the patient, and easily accessible. The earlobes
(Fig. 3) and the fingers are commonly used sites. However, other sites, including
toes, cheeks, nose, and tongue, may be used in cases of low peripheral perfusion.
In adults, the probe can be placed on either side of the body. However, in newborns,
in which SpO 2 measurements can aid in the diagnosis of congenital cardiac disease,
placement on the right upper arm is preferable. This is because some newborns
may have a patent ductus arteriosus, in which blood flowing to the right
upper arm is least diluted by the shunt and is therefore the most oxygenated. 3
It is important to choose a probe that is the proper size for the patient. If the
probe is not the right size, the light-emitting diodes may not line up correctly with
the photodetector and may produce inaccurate data. For example, a probe that is
too large for a patient’s finger may slip, and the light may not pass completely
across the finger. Venous pulsations may occur if a probe is too small or is placed
too tightly on the finger (Fig. 4). These venous pulsations will produce interfer-
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Pulse Oximetry
ence, resulting in a falsely low pulse-oximeter reading. Pulse-oximeter probes with
adhesive sensors can minimize motion and may yield more accurate results than
nonadhesive sensors.
Common Problems and Limitations
Although easy to use, the pulse oximeter can produce erroneous data in certain
circumstances. One common problem is the occurrence of movement artifacts if
there is movement at the probe placement site. This may interfere with proper function,
and most often occurs if a patient is shivering, has seizures, or is being transported
by ambulance or helicopter. The waveform of the pulse oximeter will be
distorted in such circumstances, and the clinician should recognize that the measurement
displayed is inaccurate. Similarly, it is important to recognize that the
accuracy of pulse oximeters deteriorates when the SpO 2 falls below 80%.
A bright light, such as an operating room lamp, may cause light interference,
leading to an erroneous reading. Electromagnetic radiation, such as that emitted
by magnetic resonance imaging (MRI), may also interfere with pulse oximetry.
Only MRI-compatible pulse oximeters should be used for patients undergoing
MRI. If the probe is poorly placed or is the wrong size, light from only one lightemitting
diode may pass through tissues or the light may not reach the detectors,
either of which may result in falsely elevated or depressed results. The presence of
intravascular dyes, such as methylene blue or indigo carmine, may alter the red
and infrared light-absorption properties of tissues, which can also lead to an inaccurate
reading. Nail polish that absorbs light at 660 nm or 940 nm may interfere
with the ability of the pulse oximeter to interpret the SaO 2 . This problem can be
avoided by removal of the nail polish.
Nonfunctional hemoglobin can also result in inaccurate data. At 940 nm, the
light absorption of carboxyhemoglobin is minimal and does not affect the SpO 2 .
However, at 660 nm, the light absorption of carboxyhemoglobin is similar to that
of oxyhemoglobin. The pulse oximeter cannot differentiate between oxyhemoglobin
and carboxyhemoglobin in this instance and may erroneously display a normal
or near-normal measurement of SpO 2 when the actual SaO 2 is low.
Methemoglobin absorbs more light than either deoxyhemoglobin or oxidized
hemoglobin at 940 nm, but at 660 nm, the light absorption of methemoglobin is
similar to that of deoxyhemoglobin. In this instance, the pulse oximeter cannot
differentiate between methemoglobin and deoxyhemoglobin and may incorrectly
perceive an elevated concentration of deoxyhemoglobin, which would result in a
falsely low interpretation of SaO 2 . Carboxyhemoglobin may cause a falsely high
SpO 2 , whereas methemoglobin may cause a falsely high or falsely low reading
depending on the actual concentration of oxyhemoglobin. Newer pulse oximeters
are able to emit light at up to eight different wavelengths, which makes it possible
for the oximeter to measure carboxyhemoglobin and methemoglobin.
The presence of peripheral vasoconstriction, hypoperfusion, hypothermia, and
shock may yield an inaccurate measurement of SpO 2 . When peripheral blood flow
is decreased, as it is in these conditions, the pulse oximeter may not properly differentiate
between pulsatile and nonpulsatile flow, resulting in an erroneous
measurement. It is important to remember that pulse oximeters function normally
in anemic patients, who have reduced numbers of red cells. In an extremely
anemic patient, the oxygen saturation may be normal, but there may be insufficient
hemoglobin to carry an adequate amount of oxygen to the tissues. Finally,
pulse oximetry is an indirect method of monitoring the patient’s breathing. A
normal SpO 2 may be falsely reassuring in patients receiving supplemental oxygen
because a drop in SpO 2 may occur only after the patient is severely hypercarbic.
Figure 4. Overly Tight Placement
of Adhesive Pulse-Oximeter Probe.
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Pulse Oximetry
References
1. Haynes AB, Weiser TG, Berry WR, et
al. A surgical safety checklist to reduce
morbidity and mortality in a global population.
N Engl J Med 2009;360:491-9.
2. Miller RD, ed. Miller’s anesthesia. 7th
ed. Philadelphia: Churchill Livingstone,
2010.
3. Mahle WT, Newburger JW, Matherne
GP, et al. Role of pulse oximetry in examining
newborns for congenital heart disease:
a scientific statement from the
American Heart Association and American
Academy of Pediatrics. Circulation
2009;120:447-58.
Copyright © 2011 Massachusetts Medical Society.
New-generation pulse oximeters are manufactured with improved algorithms
that minimize motion-related erroneous data by filtering out body motions. In
addition, because they use multiple wavelengths, these new pulse oximeters are
capable of measuring the concentration of hemoglobin, carboxyhemoglobin, and
methemoglobin.
Troubleshooting
Artifacts should be the last explanation for changes in critical values. Therefore,
when a clinician obtains a low SpO 2 reading, it is imperative to do a physical assessment
of the patient before assuming that the value represents equipment malfunction.
If the patient appears to be clinically stable, consider the possibility of a
mechanical malfunction. Make sure the cable connecting the probe to the microprocessor
is intact and is in fact connected to the microprocessor. The cable may
become loose or detached, making it seem as though the pulse oximeter cannot
determine the SaO 2 . If the cable is loose, adjust the connection as necessary. Make
sure that the red light on the inside of the probe is evident. If you do not see the red
light, check the power cable. If you can see the red light and have made sure the
cable connecting the probe to the microprocessor is intact, clean the probe and
place it on your own finger to see whether the issue is specific to the patient. If this
is not the case, it is possible that either the probe or the microprocessor is not working
properly. Replace the probe, the microprocessor, or both, as necessary.
Complications
Although the pulse oximeter is generally a safe device, its use still carries some risk
of adverse events. Burning or blistering at the placement site may occur if the lightemitting
diode becomes overheated. Ischemic pressure necrosis may result if the
probe is placed too tightly on the patient. Perioperative corneal abrasion may occur
if a patient with a probe on a finger rubs his or her eyes when awakening from
anesthesia and in doing so scrapes the cornea. Prolonged placement of a pulseoximeter
probe, which can occur in patients in the ICU, may lead to mechanical
injury, such as finger stiffness, making it difficult for the patient to flex the finger
after the probe has been removed. Although these complications have been documented,
they remain uncommon.
Summary
When used correctly, pulse oximetry is a potentially lifesaving tool. Health care
providers need to be aware of the indications, benefits, and disadvantages of pulse
oximetry. More important, clinicians must be able to interpret the information
pulse oximetry provides. With proper training and instruction, clinicians will find
pulse oximetry an invaluable monitoring tool.
No potential conflict of interest relevant to this article was reported.
Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.
n engl j med 364;16 nejm.org april 21, 2011
The New England Journal of Medicine
Downloaded from nejm.org by MICHAEL PINSKY on May 2, 2011. For personal use only. No other uses without permission.
Copyright © 2011 Massachusetts Medical Society. All rights reserved.