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Abstract
Triage measurements of oxygen saturation
by pulse oximetry (O2Sat) is usually obtained
with a finger probe. It is generally thought
that changes in hand temperatures can
influence digit bloodjlow and pulse oximetry
accuracy. The purpose of this study was
to determine the effect of average ambient
outside temperatures on initial triage
measurements of °2Sat in patients coming
in from the external environment. Pulse
oximetry measurements of °2Sat and oral
body temperatures were obtained from ambulatory
patients presenting to the emergency department
for 12 different days. The averages of
these measurements for each day were correlated
by linear regression analysis with the
average ambient outside temperature for
the 24-hour period as recorded by the
National Weather Service. In the 397 patients
studied, the ambient outside temperature
was found to be negatively correlated
with O2Sat measurements (r = 0.51). However,
there was a significant correlation of
the patients' average oral body temperature
with the ambient outside temperatures
(r = 0.84). There was poor correlation
of the patient's oral temperature and
the triage 0 )at (r = 0.26). There also
appears to be no significant differences
in the values obtained after cold water
immersion in the 15 volunteer subjects.
Despite the intuitive impression that
external temperatures may effect pulse
oximetry measurements, we found no correlation
between the average ambient outside temperatures
and the O2Sat recordings at triage.
 Introduction:
Vital signs have traditionally included heart rate, blood pressure, respiratory rate and temperature. Recently, pulse oximetry has been used in triage as a screening for potential cardiopulmonary complications. In fact, the measurement of oxygen saturation
(O2Sat) is often regarded as the
"fifth vital sign" (1). Because of its current inexpensive costs and low margin of error over the range of 70 to 100 percent saturation ( + or -2%, + or -1 SD), pulse oximetry now has the potential for widespread use (2). It has also been proven to be a sensitive and reliable tool for monitoring
O2Sat and may help direct triage classification (3,4). Triage systems categorize patients according to the level of severity of illness in an attempt to prioritize service. Therefore, it is important to understand the limitations of the technology used in this decision making process. While it is known that factors such as skin pigmentation, low flow states, carbon monoxide and certain diagnostic dyes can influence
02Sat readings, the effect of the ambient external temperature on routine triage pulse oximetry measurements has not been studied (5). Triage measurements
of O2Sat by pulse oximetry is usually obtained with a
finger probe. It is often thought that changes in hand temperatures can influence digit blood flow and pulse oximetry accuracy (6,7). Finger-tip pulse oximetry readings may change with limb hypothermia due to temperature dependent arteriovenous shunts in the periphery (7). The observed change in
O2Sat probably reflects altered transmission of arterial pulsations to venous blood in the
finger. The purpose of this study was to determine the effect of average ambient outside temperatures on initial triage measurements of
O2Sat in patients coming in from the external environment.
Methods:
Pulse oximetry measurements of o zSat
and oral body temperatures were obtained
from ambulatory patients (both black and
white) presenting to the emergency department
triage of an urban university hospital
for 12 different days (24 hour periods)
during the month of January. This included
all adult patients (age> 16) with a triage
classification of nonurgent or minor emergency.
Patients excluded from the study were
those found to have markedly abnormal
vital signs, febrile illness, pathologic
hypothermia or significant trauma. Also
excluded were all ambulance transports
and patients with respiratory problems
such as congestive heart failure, asthma
or chronic obstructive pulmonary disease.
The stringent and broad exclusion criteria
were designed to minimize the effects
of other factors on the O2Sat measures.
All O2Sat measurements were obtained using
a finger probe portable pulse oximeter
(Onyx model 9500 by Nonin Medical, Inc.)
at the same time that routine triage vital
NONIN Medical, Inc (8). An established
protocol was used to help to insure consistency
of measurement despite the fact that the
triage was performed by several different
nurses throughout the study period. The
averages of these measurements for each
day were correlated by linear regression
analysis (significance at p < 0.05)
with the average ambient outside temperature
for the corresponding 24-hour period as
recorded by the National Weather Service.
Results:
A total of 397 patients that met criteria
were included in the ambulatory portion
of the study. The average outside temperatures
varied widely from a low of 2700 F to
a high of 8000 F during the 12 days. Surprisingly
the ambient outside temperature was found
to be negatively correlated with O2Sat
measurements (r = 0.51) (Figure 1). However,
there was a significant correlation of
the patients' average oral body temperature
with the ambient temperatures (r = 0.84)
(Figure 2). Also noted was a lack of correlation
of the 'patient's oral temperature and
the measured triage O2Sat (r = 0.26) (Figure 3).
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Fig.1
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Fig.2
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Fig.3
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Conclusion:
There is the general impression among
emergency personnel that external temperatures
may effect pulse oximetry measurements.
It seems intuitive that cold environments
would reduce digit blood flow and therefore
result in inaccurate pulse oximetry readings.
In fact, cold exposure is often cited
as a potential source of error in O2Sat
measurements. This has also been of some
concern in the surgical literature in
patients made hypothermic during cardiopulmonary
bypass. A number of studies looking at
patients' core temperatures during surgery
have not found a significant effect of
hypothermia on pulseoximetry accuracy
until the temperatures fall below 2700C
(9). The known effect of hypothermia on
the hemoglobin-oxygen saturation curve
prohibits the extrapolation of these surgery
findings to the triage setting in which
mainly skin temperature is affected (6).
Generalized core hypothermia may also
have! variable effects on general circulatory
flow as opposed to the regional limb flow
variations that may be seen in the patient
coming in from the external environment
(9). In fact some literature suggests
that localized hand hypothermia (1500)
results in paradoxical changes in the
O2Satmeasurements (7). Warmer temperatures
appear to shunt blood to the venous side
of the circulation, creating venous pulsations
and decreasing O2Sat readings (7). Likewise
the hypothermia reduced the natural shunting
and increased the O2Sat levels measured.
This phenomenon could explain the negative
temperature-O2Sat correlation seen in
our study.
Prior to this study there is very little
hard information available in the
literature concerning the effects that
typical: ambient temperature changes may
have on pulse oximetry recordings. Since
the changes seen due to ambient temperature
fluctuations do not appear to be clinically
very significant, these findings may dispel
the "common wisdom" that environmental
temperatures interfere with pulse oximetry
measurements (6). This clinical based
finding is supported by observations in
artificially induced hypothermia in hands
of normal volunteer subjects (7). Likewise,
the lack of effect of minor body temperature
fluctuations to influence °2Sat measurements
is also consistent with the core temperature
studies previously cited (6,9). The strong
correlation of the triage oral body temperature
with the ambient external temperature
may have some clinical importance when
the ambient temperatures are extreme and
when a subtle difference in the patient's
temperature is meaningful.
References:
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