How One Technological Advance Changed Medicine
Pulse oximetry is a simple, non-invasive
method of monitoring the percentage of hemoglobin (Hb) that is
saturated with oxygen. The pulse oximeter consists of a probe attached
to the patient’s finger or earlobe, which is linked to a computerized
unit. The unit displays the percentage of Hb saturated with oxygen
together with an audible signal for each pulse beat, a calculated
heart rate in some models, and a graphical display of blood flow past
the probe. Audible alarms that can be programmed by the user become
clinically cyanosed.
Since the device is non-invasive and allows
immediate and real time monitoring, it use has expanded to include
other purposes such as screening, diagnosis, patient follow-up, and
self-monitoring.
History
In 1935, Matthes developed the first
2-wavelength ear O2 saturation meter with red and green filters, later
switched to red and infrared filters. This was the first device to
measure O2 saturation.
In 1949, Wood added a pressure capsule to
squeeze blood out of the ear to obtain zero setting in an effort to
obtain absolute saturation value when blood was remitted. This concept
is similar to today’s conventional pulse oximetry but suffered due to
unstable photocells and light sources.
This method was not used clinically. In 1964,
Shaw assembled the first absolute reading ear oximeter by using eight
wavelengths of light. Commercialized by Hewlett Packard, it use was
limited to pulmonary functions and sleep laboratories due to the cost
and the size.
Pulse oximetry was developed in 1972, by
Aoyagi at Nihon Kohden using the ratio of red to infrared light
absorption of pulsating components at the measuring site. It was
commercialized by BIOX/Ohmeda in 1981 and Nellcor in 1983. Nellcor
incorporated in 1982, and introduced it into the US operating room
market in 1983. Prior to its introduction, a patient’s oxygenation was
determined by a painful arterial blood gas, a single point of measure
which typically took a minimum of 20-30 minutes of processing by the
lab. (Remember that in the absence of oxygenation, damage to the brain
starts in five minutes, with brain death in another 10-15 minutes.)
In the U.S. alone, approximately $2 billion
was spent annually on this measurement. With the introduction of pulse
oximetry, a non-invasive, continuous measure of patients' oxygenation
was possible, revolutionizing the practice of anesthesia and greatly
improving patient safety. Prior to its introduction, studies in
anesthesia journals estimated U.S. patient mortality as a consequence
of undetected hypoxemia at 2,000 to 10,000 deaths per year, with no
known estimate of patient morbidity.
By 1987, the standard of care for the
administration of a general anesthetic in the U.S. included pulse
oximetry. From the operating room, the use of the pulse oximetry
rapidly spread throughout the hospital, first in the recovery room,
and then into the various intensive care units.
How Does a Pulse Oximeter Work
A pulse oximeter consists of a peripheral
probe together with a microprocessor unit, displaying a waveform — the
oxygen saturation and the pulse rate.
Most oximeters have an audible pulse tone, the
pitch of which is proportional to the oxygen saturation — which can be
useful when you cannot see the display. The probe is placed on a
peripheral part of the body such as a finger, toe, earlobe or the
nose.
Within the probe are two light-emitting diodes
(LEDs). One is visible red spectrum and the other is infrared
spectrum. The beams of light pass through the tissues; some light is
absorbed by the blood and soft tissue depending on the concentration
of hemoglobin.
Saturation values are averaged out over five
to 20 seconds. The pulse rate is also calculated from the number of
LED cycles between successive pulsatile signals and average out over a
similar variable period of time, depending on the particular monitor.
Calibration and Performance
Oximeters are calibrated during manufacture
and automatically check their internal circuits when they are turned
on. They are accurate in the range of oxygen saturations of 70
percent-100 percent (+/- 2 percent), but less accurate under 70
percent. The pitch of the audible pulse signal falls with reducing
values of saturation.
The size of the pulse wave is displayed
graphically. Some models automatically increase the gain of the
display when the flow decreases; in these, the display may prove
misleading. The alarms usually respond to a slow or fast pulse rate or
an oxygen saturation below 90 percent. At this level, there is a
marked fall in PaO2, representing serious hypoxia.
Limitations/Pitfalls
Pulse oximetry is a measure solely of
oxygenation, not of ventilation, and is not a substitute for blood
gases checked in the laboratory. Pulse oximetry does not give an
indication of carbon dioxide levels, blood pH, or sodium bicarbonate
levels.
False low readings may be caused by
hypoperfusion of the extremity being used for monitoring (often
because the part is cold or because of the vasoconstriction secondary
to the use of vasopressor agents); incorrect sensor application;
highly calloused skin; and movement (such as shivering), especially
during hypoperfusion. Ambient light, abnormal hemoglobin; pulse rate
and rhythm and cardiac function can also cause abnormal pulse oximeter
reading. To ensure accuracy, the sensor should return a steady pulse
and/or waveform. Falsely high or falsely low readings will occur when
the hemoglobin is bound to something other than oxygen. In cases of
carbon monoxide poisoning, the falsely high reading may delay the
recognition of hypoxemia. Cyanide poisoning can also give a false high
reading.
What factors cause errors in the pulse
oximeter?
Abnormal hemoglobin -- Blood may contain
abnormal hemoglobin such as carboxyhemogolobins and methemoglobin,
which do not contribute to oxygen delivery.
Medical dyes -- If dyes such as cardio green,
intravascular dyes and indocyanine green have been injected into the
blood, they may influence the level of transmission of the red and
infrared light.
Manicure and pedicure -- If the users wear
nail polish, it may absorb the light emitted from the LED and change
the light transmitted through the body, influencing the values
calculated.
Major body motion -- Body motion may cause
noise that affects the values calculated. When noise -- including that
caused by body motion -- is reduced, the reliability of the values
calculated falls, and the pulse oximeter displays a warning.
Blood flow blocked by pressure on the arms or
fingers -- The pulse oximeter measures oxygen saturation based on
changes in the blood flow. Therefore, if the blood is blocked, correct
measurement becomes impossible. In addition, if the fingers are flexed
at a uniform place, the pulse oximeter may interpret the pressure
changes in pulse rate, causing errors.
Peripheral circulatory failures -- The pulse
oximeter utilizes blood flow to monitor changes in the amount of light
transmitted to calculated values. If peripheral blood flow is reduced,
adequate data may not be obtained, and the result is inaccurate
measurement. In this case, it is necessary to promote blood flow by
massaging or warming the fingers.
Excessive ambient light -- The pulse oximeter
can usually cancel out the effects of ambient light. However, if the
ambient light is too strong, the device will not be able to cancel out
the effects and this may cause errors.
Ambient electromagnetic waves -- If electric
appliances such as television, mobile telephones, or medical devices
that produce high levels of electromagnetic waves are used near the
pulse oximeter, the electromagnetic waves from these devices may
interfere with accurate measurement.
Probe attached incorrectly -- If the probe is
not attached properly, it may detect a variety of noise, resulting in
inaccurate measurement.
What exactly does the pulse oximeter measure?
Blood carries oxygen in two forms. The
majority is bound to hemoglobin (oxyhemoglobin), and the rest is
dissolved in the aqueous phase of blood (the plasma). The pulse
oximeter measures the saturation of hemoglobin with oxygen. This is
expressed as a percent saturation. Each gram of normal hemoglobin can
hold 1.34 milliliters of oxygen. The dissolved fraction is dependent
upon the partial pressure of oxygen. As the partial pressure
increases, the dissolved fraction of oxygen increases. For each 1
mm/Hg pressure of oxygen partial pressure, 0.003 milliliters dissolves
in the plasma. So under normal conditions, each 100 mL of blood
contains about 20 mL of oxygen bound to hemoglobin and about 0.3 mL
dissolved in plasma. The dissolved fraction is available to tissues
first, and then the fraction bound to hemoglobin. So as tissues
metabolize oxygen, or if oxygen becomes difficult to pink up through
the lungs, the dissolved oxygen and the hemoglobin-bound oxygen will
eventually become depleted.
The dissolved oxygen can be measured by
arterial blood gas analysis, but this is not yet practical for field
application. This fraction is not measured by pulse oximetry. The
pulse oximeter waits to sense the pulse of capillary blood from the
side of the capillaries, then, using two different wavelengths of
light, calculates the percent of oxyhemoglobin from the total
hemoglobin present. If oxygen transfer across the lungs or lung
function is compromised and tissues continue to metabolize oxygen, the
percentage of oxyhemoglobin will decrease. This becomes our
quantitative indicator of hypoxia.
When should supplemental oxygen be
administered despite a normal pulse ox? Oxygen should be delivered
whenever the underlying problem is suspected to be organ or tissue
ischemia. This would include things like stroke, intracerebral bleed,
head injury, altered mental status from any cause, chest pain of
suspected cardiac origin, cardiac dysrhythmias, shortness of breath
from any cause, vascular emergencies like aortic dissection or
aneurysm or vascular occlusions, shock from any cause, sickle cell
disease and multisystem trauma.
What respiratory function will pulse ox not
measure? The pulse oximeter measures oxygenation. It does not measure
ventilation. Ventilation is the process of removing carbon dioxide
from the blood. Hypoventilation from any cause will result in the
accumulation of carbon dioxide in the blood. This leads to respiratory
acidosis. If hypoventilation goes on long enough, blood oxygen will
begin to deplete and the pulse oximeter oxygen saturation will begin
to decrease. However, short of apnea, the rate of carbon dioxide
accumulation and the development of respiratory acidosis may be
greater then the rate of onset of hypoxia by pulse oximetry. The
assessment of adequate ventilation is based on respiratory rate and
depth.
Should I hold on administering oxygen in order
to check a baseline pulse ox? The decision to administer supplemental
oxygen is based on available history and initial examination. If your
first impression is that the patient is really sick, then they
probably are. So don’t feel compelled to document a baseline pulse ox
in a patient you think is in trouble. If the flow of the call results
in an off-oxygen pulse ox reading being available, it certainly can be
useful to correlate to symptom severity, but don’t hold off on oxygen
waiting to get this value.
Where this might be useful is if you have a
patient who is symptomatic and looks OK for the moment, but you would
like to get a better assessment of the severity of the symptoms.
Practical Tips for Successful Use of Pulse Oximetry
Oximetry is not a complete measure of
respiratory sufficiency. A patient suffering from hypoventilation
(poor gas exchange in the lungs) given 100 percent oxygen can have
excellent blood oxygen levels while still suffering from respiratory
acidosis due to excess carbon dioxide.
It is also not a complete measure of
circulatory sufficiency. If there is insufficient blood flow or
insufficient hemoglobin (anemia), tissues can suffer hypoxia despite
high oxygen saturation in the blood that does arrive.
It is important to remember that pulse
oximetry is only one way of monitoring breathing. It is also
necessary, as a minimum, to record respiratory rate, and if pulse
oximetry is used, the amount of oxygen the patient is receiving must
be recorded. As with all clinical assessments, you must look at the
"whole picture."
Sharon Lesser, RN, is a pulmonary clinical
nurse II in the department of pulmonary and critical care medicine at
the University of Maryland Hospital in Baltimore, Md.
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