Core Temperature—The Intraoperative
Difference Between Esophageal Versus
Nasopharyngeal Temperatures and the Impact
of Prewarming, Age, and Weight: A Randomized
Clinical Trial
Anne Erdling, CRNA, MSc
Anders Johansson, CRNA, PhD
Unplanned perioperative hypothermia is a well-known
complication to anesthesia. This study compares
esophageal and nasopharyngeal temperature measured in the same patient for a period of 210 minutes
of anesthesia. Forty-three patients undergoing colorectal surgery were randomly assigned in 2 groups, with
or without a prewarming period (group A = prewarming [n = 21] or group B = no prewarming [n = 22]).
Demographics were similar in both groups. Mean
temperatures at 210 minutes were statistically different between the groups at both sites of measurement.
Esophageal temperature in group A was 36.5 ± 0.6 vs
35.8 ± 0.7 in group B (P = .001), and nasopharyngeal
D
espite recommendations and several clinical
guidelines, prevention of unplanned perioperative hypothermia (UPH) is still missing
in many anesthetized patients and the incidence of inadvertent hypothermia remains
high.1-4 Considering the high incidence of UPH and the
accompanied complication rates in hemostatic function,
cardiac morbidity, healing problems, enhanced incidence levels of pressure ulcers, surgical site infection,
and patient discomfort, core body temperature should be
recorded continually.5-9 Implementation of guidelines for
the prevention of UPH imposes technical concerns, since
thermometry carries inadequacies, and warming strategies diverge.10,11 Unplanned perioperative hypothermia is
defined as body core temperature <36°C, at any moment
in the perioperative period.12 UPH is related to the environmental temperature in the operating room and altered
thermoregulation in the body. General anesthesia (volatile
gas, hypnotics, and analgesics) and neuroaxial anesthesia
(spinal, epidural) act both in the central nervous system
and peripheral tissues.13-15 Vasodilatation properties of
general anesthetics cause a core-to-peripheral redistribution of body heat during the first hour of anesthesia
(phase I) followed by several hours of heat loss exceeding
heat production, leading to a linear reduction in core tem-
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temperature was 36.7 ± 0.6 and 36.0 ± 0.6 in group A
and group B, respectively (P = .002). A negative correlation was found between esophageal temperature
and age (r 2 = –.381, P < .012). Esophageal temperature
was different with respect to BMI below or above 25.
The temperatures were 35.81 ± 0.66 in the lower BMI
group vs 36.46 ± 0.59 (P < .001). These results demonstrate a difference between the 2 measurement
techniques and that prewarming, age and BMI have an
impact on measured temperatures.
Keywords: Anesthesia, core temperature, esophageal,
nasopharyngeal, prewarming.
perature (phase II). In the final plateau phase (phase III),
hypothermia stabilizes due to vasoconstriction induced by
the low temperature. Internal heat redistribution depends
on heat storage in peripheral tissues and changes, such as
heat lost over the skin (radiation and convection), alter
the core temperature.14 Neuroaxial anesthesia decreases
the thresholds for shivering and vasoconstriction and
impairs central control of thermoregulatory responses
resulting in a similar redistribution of body heat.14
Temperature Measurements
Efficiency in preventing UPH depends on valid temperature measurement and effective warming routines.1,2
Body temperature is regulated in the hypothalamus in
a narrow thermodynamic range and maintained to optimize synaptic transmission of biochemical reactions.
During anesthesia, the brain temperature is the most
clinically relevant site for normothermia. Brain and core
temperatures (in internal organ tissues) correlate to each
other during normothermia but may decouple during
anesthesia.10 Since the temperature of blood perfusing the hypothalamus is not readily measured, invasive
temperature measurement in the pulmonary artery is
the gold standard. When noninvasive core temperature
measurements are wanted during surgery, the tympanic,
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esophageal, and nasopharyngeal temperatures are regarded as reliable alternatives.10,16,17 These methods enable
a continuously monitored temperature measurement
during surgery. The esophageal temperature permits a
placement fairly close to the aorta and the surgical site,
particularly for abdominal surgery. This method is considered to be low risk and provides the most accurate
measurement of changes in core temperature, especially
during rapid temperature alterations, and may be preferred whenever possible.18-21 Positioning of the esophageal probe may involve considerations at insertion, such
as placement verification and esophageal anomalies.21-24
The nasopharyngeal temperature, known as an easy and
reliable method, well accepted by the awake patient,
permits a method for continuous measurement useful
pre- and postanesthesia. Positioned above the palate,
the probe will be placed close to the brain. However,
bleeding and tissue injury may complicate insertion of
the nasopharyngeal probe, and alterations in nasopharyngeal perfusion may have an impact on measurement
reliability.20 Several studies evaluating esophageal and
nasopharyngeal temperatures are conducted,17-20 but to
our knowledge there have been no reports directly comparing these 2 measuring techniques in the same patient.
Concept of Prewarming and Intraoperative
Warming
Prewarming, defined as both actively warming the skin
before anesthesia starts12 and active intraoperative
warming, is an effective way to prevent UPH during
surgery.18,25-27 Warming prior to anesthesia induction
reduces the core-to-peripheral temperature gradient,
increases the total body heat content and is thus essential to the prevention of intraoperative hypothermia.28
One reason why UPH prevention still fails may be the
lack of consensus in prewarming time, warming devices,
and warming methods. Intraoperative warming prevents
intraoperative heat loss from exceeding the heat production.29 Circulating water, resistive heating, and forced-air
are effective warming techniques for the prevention of
UPH. Recommendations in prewarming times differ from
10–120 minutes.11,16,25,30,31 However, without prewarming, these techniques often fail to prevent the temperature drop during the first hour of anesthesia.32
Prewarming may be perceived as disturbing to the
surgical preparation process and may raise concerns as a
source of contamination, enhancing the risk of surgical
site infections (SSIs).23,30 However, since most warming
equipment has bacterial filters, this risk has been considered negligible.33 In a recent study, it was concluded that
there is a need for improved intake filtration in order to
reduce contamination and emission risks.34 A later study
found no evidence of increased risk when using forcedair warming intraoperative.35
Patients undergoing abdominal surgery are at specific
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risk of developing intraoperative hypothermia due to
massive heat loss from tissues. Age and BMI are factors
known to affect core body temperature, partly explained
by a more severe thermoregulatory impairment in the
elderly and by increased insulation by excessive fat in the
obese.36,37 Awareness of the risks of perioperative hypothermia is the key to prevention and an important component in the prevention of UPH-related adverse effects
is to maintain a normothermic condition constantly
during surgery. This requires the use of a temperature
monitoring method that provides accurate and consistent
measurements.
This study was conducted to determine the intraoperative temperatures with 2 different measurement
techniques (esophagus vs nasopharynx). This issue was
evaluated in 2 groups with and without an extended
warming period.
Materials and Methods
•Participants. All patients were recruited from a waiting
list for colorectal surgery at a general hospital in southern Sweden. Patients included were adult and of both
genders with ASA physical status 1 and 2, who were to
undergo elective open colorectal surgery under general
anesthesia, combined with regional analgesia for an anticipated anesthesia time of at least 210 minutes. Patients
who did not give their informed consent or understand
the information were not eligible for the study, as were
patients with known nasal or esophageal anomalies.
Patients with thyroid dysfunction and known ischemic
peripheral vessel disease were also excluded.
•Study Design. In this explorative parallel-group
study, patients were randomly assigned to 1 of 2 groups.
The study was approved by the Ethics Committee of the
Medical Faculty, Lund University (VEN 15-11) and was
performed in accordance with the Helsinki declaration.38
The study was further accepted by the heads of the anesthesia and surgical departments. In order to reduce
performing bias, all the measurements were carried out
by the author of this study. Consent to participate in
the study was obtained from each patient. The patients
signed a consent form a week prior to surgery and a
second session with information was given the day before
surgery. The study was designed to perform repeated core
temperature measurements in two groups: A (pre- and
intraoperative warmed) and B (intraoperative warmed),
from probes in the esophagus and nasopharynx. In the
OR, patients were randomly assigned by a sealed envelope technique to group A (n = 26) or group B (n = 26).
To minimize diurnal variation in body temperature as a
confounding factor, all studies started at 7:30 AM.
•Temperature Measures. In all patients, both esophageal and nasopharyngeal thermometers were used to
collect core temperatures. The Level 1 disposable general
purpose temperature probes (Smiths Medical ASD Inc.)
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AANA Journal
X X X XX XXXX
X
X
Warming Outflow
Table 1. Time Schedule for Documentation of Temperatures
X
X
X X X XX XXXX

Air (OR)
X
Esophagus
April 2015
X
X
X X X XX XXXX
X
X
Nasopharynx
X
X X X XX XXXX
X
X
X
XX XXXX

Skin (thorax)
X X
X X X XX XXXX
X
X
X
X
X
Skin (calf)
450 510
390
330
270
210
150
120
90
Post
30 min
Surgery
start
Anesthesia
start
After EA
test dose
Before
epidural
Reading
sites
were used to measure core temperature via placement
in the lower esophagus and nasopharynx. Prior to insertion of the epidural catheter, a nasopharyngeal probe
was placed 6–8 cm beyond one of the nostrils using the
individual nose-to-ear distance, and confirming that
the probe was not visible in the mouth. Thus a baseline temperature was achieved. Following intubation, a
temperature probe was immediately placed in the distal
esophagus at an individually adjusted distance of 40 ± 4
cm from the nostrils using the Mekjavić–Rempel formula
(calculated from patient height in cm x 0.228, resulting in placement at level T8/T9 corresponding to the
aorta).39 Documentation of temperatures was according
to schedule (Table 1) with the intention to reflect the
3 phases (peripheral redistribution, heat loss exceeding
heat production, and the plateau phase) of hypothermia.17 Skin temperatures were obtained from probes
positioned on the right calf and upper thorax to detect
any overheating from the warming device (Siemens
Temperature Probe 5204644E530U Adult). Ambient
room temperature was read at a site not affected by the
warming device and recorded at the same time as body
temperatures. A temperature probe was also placed in the
warming equipment. All probes were connected to the
KION vital signs monitor (Maquet Critical Care, Solna,
Sweden) and values were displayed continuously on the
monitor screen, but recorded according to the study protocol. The probes were all calibrated prior to use and the
monitors were calibrated yearly by the engineering department according to manufacturer’s recommendations.
•Warming Procedure. The warming procedure started
in the operating room, where all the anesthesia and surgical preparations took place. Pre- and intraoperative
warming in this study was defined as warming the patients’
skin using a forced-air device (Warm Touch, Nellcor, or
Gaymar, Smiths Medical), set to 43°C, covering both arms,
head, and thorax. Prewarming (extended warming) in
group A started after epidural catheter insertion but before
epidural anesthesia test dose was given and this warming
was continued during 210 minutes of surgery. Warming
intraoperatively in group B started after surgical preparation was completed using the same warming equipment
and continued similarly. All fluids given intravenously
were warmed to 39°C in both groups and passive insulation was applied to a layer of quilted cotton on the legs
only and covered with the surgical drape.
•Anesthesia Procedure. The epidural anesthesia was
initiated with a 3–6 mL test dose of mepivacaine (20 mg/
mL) and epinephrine (5 µg/mL) just before anesthesia
start and the dermatome level of sensory blockade was
verified to T9–10 by loss of cutaneous cold sensation.
General anesthesia was induced with propofol (1.5–3 mg/
kg) and remifentanil (0.3–0.5 µg/kg) and orotracheal intubation was facilitated with suxamethonuim (1 mg/kg).
Anesthesia was maintained with remifentanil (0.15–0.3
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101
Characteristics
All, N=52
Group A, Group B,
n=26
n=26
Assessed for eligibility (N=52)
Excluded (n=0)
Gender
Female, n (%)
29 (55.8%)
14 (48.3%)
15 (51.7%)
Male, n (%)
23 (44.2%)
12 (52.2%)
11 (47.8%)
24 (46%)
12 (23%)
12 (23%)
70±13 (32–92)
70 ±15
(32–92)
72 ±11
(41–90)
Beta blocking
medication
Age (y); mean SD
(range)
Body mass index (kg/m²); mean
(SD: range) 2626
(5: 16–34)
(4: 19–37)
Table 2. Demographic Data of the Patients Presented
in Absolute and Relative Numbers
µg/kg/min) and sevoflurane at an end-tidal concentration of 0.7%–1.0% due to age. The lungs were mechanically ventilated with a KION anesthesia machine (Maquet
Critical Care, Solna, Sweden) using a low-flow recirculation system, with Fio2 0.4 and rate and volume sufficient
to maintain normocapnia. Neuromuscular blockade was
maintained with rocuronium 10 mg/mL guided by appropriate monitoring. According to the surgical care plan, the
patients were given Ringer’s solution 3 mL/kg/h. In case
of hypotension, the patient was given vasopressive drugs
(norepinephrine or ephedrine) in order to keep MAP
between 70–100 mm Hg. A continuous epidural infusion
of fentanyl (2 μg/mL) and ropivacaine hydrochloride (1
mg/mL) 10 mL/h was started just before surgical incision. Vital signs (arterial blood pressure, HR, etco2, Spo2,
airway pressure) were recorded every 5 minutes and the
patient data sheet was completed with demographic characteristics as well as vital signs, temperature readings, and
length of surgery, and the use of vasopressive drugs was
recorded (Tables 1-2).
•Data Analysis. Data were analyzed using SPSS for
Windows version 20 Chicago, IL, USA. A power calculation revealed that a clinically relevant temperature
difference of 0.2°C40 would reach a power of 0.80 at P
< 0.05 with 16 patients per group. The distribution of
data collected in the different groups was tested for normality using the one-sample Kolmogorov–Smirnov test.
A paired two tailed t-test was used to compare groups
A and B with respect to temperatures at 210 minutes.
Spearman’s rank correlation was used for testing correlation between age and esophageal temperature. A postpower calculation was done to ensure the power included
a population of 21 patients in each group. Our results
demonstrate an intergroup difference of approximately
0.6°C with an average standard deviation of at least 0.6°C
and the result reached a power of 0.90.
Results
Fifty-two adults aged 32–92 years were enrolled in the
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Allocated to Group A (n=26)
Analyzed (n=21)
• Excluded due to loss of
final analysis (n=5)
Allocated to Group B (n=26)
Analyzed (n=22)
• Excluded due to loss of
final analysis (n=4)
Figure 1. Flow Diagram for Enrolled Patients
(Group A Pre- and Intraoperative Warmed, Group B
Intraoperative Warmed)
study. Forty-three patients (21 in group A and 22 in group
B) were analyzed at 210 minutes since 9 patients were
excluded due to shorter surgery (Figure 1). Three patients
(1 in group A and 2 in group B) did not receive epidural
anesthesia due to anticoagulant treatment or spine anomalies. There were no problems or complications due to
temperature probe insertion. Groups A and B were not different with respect to demographic variables (Table 2) and
the warming period in group A was 42 ± 10 minutes longer
than in group B. Overall, preoperative use of beta blockers was 46% and was similar in both groups (Table 2).
All patients were given norepinephrine 0.02–0.20 μg/kg/
min to keep MAP between 70 and 100 mm Hg. BMI was
≥25 in 2/3 (67.3%) of the patients and they were evenly
distributed between the groups. Ambient air temperature
at anesthesia induction was similar in both groups (21.9 ±
0.8°C) and differed slightly at 210 minutes, group A 22.1
± 0.6°C and group B 22.4 ± 0.7°C. Just after test dose the
skin temperature had increased in both groups, and was
0.2°C higher in group B. The baseline of the nasopharyngeal temperature was also higher in group B (Figure 2).
In all patients (group A and B), the temperatures in
the esophagus and the nasopharynx showed a significant
mean difference of 0.2 °C throughout the procedure
(Figure 3). The esophageal temperature in group B
declined at 30 minutes post-surgical start. A similar
decrease was observed in the esophageal temperature in
group A at 60 minutes. This was not seen in the nasopharyngeal temperatures (Figure 2).
At 210 minutes, there were statistically significant
differences between the groups A and B in both measuring techniques, ES 36.5 ± 0.6 vs 35.8 ± 0.7, (P = 0.001),
and NF 36.7 ± 0.6 versus 36.0 ± 0.6, (P = 0.002) (Table
3). From anesthesia start to the 210-minute mark, the
esophageal temperature in A had increased a mean 0.65 ±
0.63°C (P = 0.001). In B, this difference was smaller (0.27
± 0.62°C) and not statistically significant (P = 0.052).
A negative correlation was found between the esophageal temperature and age (r2 = –0.381, P < 0.012).
Statistical differences could also be seen between 2 BMI
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Figure 2. Nasopharyngeal and Esophageal Temperatures During Surgery in Group A and B
Figure 3. Mean Values for Nasopharyngeal and Esophageal Temperatures in All Patients
Group
Temperature site
nMean SD
P value
A prewarmed
Esophageal
21
36.46
0.59
B not prewarned
Esophageal
22
35.81
0.66
A prewarmed
Nasopharyngeal
21
36.65
0.63
B not prewarned
Nasopharyngeal
22
36.02
0.60
0.001
0.002
Table 3. Esophageal and Nasopharyngeal Temperatures and Differences Between the Groups of Prewarmed (A)
and Not Prewarmed (B) Patients at 210 Minutes of Surgery
Note: The values were significantly lower in the B group (Paired t-test; P < 0.05).
groups (BMI <25 and ≥25, respectively) measured with
the esophageal temperature at 210 minutes (35.81 ± 0.66
vs 36.46 ± 0.59, P < 0.001).
Discussion
This study compared 2 different core temperature measurements in the same patient; one from a probe placed
in the lower esophagus and the other placed in the
nasopharynx. A statistical significant difference was
found between the esophageal and nasopharyngeal temperatures during the scheduled measurement (Figure 3).
The higher temperature values in the nasopharyngeal
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readings were continued during surgery and exceeded
the temperatures measured in the esophagus on average
by 0.2°C. Consistent to other studies we found that the
trend for both techniques was similar, although the
esophageal temperatures seemed to react stronger than
the nasopharyngeal to temperature alterations.18-21 This
may be of importance in how to maintain a normothermic
condition constantly during surgery, and requires the use
of a temperature monitoring method giving accurate and
consistent measurements. As the nasopharyngeal probe
was placed before induction of general and epidural
anesthesia, it provided measurement both before and
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during anesthesia. Owing to probable patient discomfort,
the esophageal probe was placed post induction and no
esophageal baseline temperature was therefore obtained.
All patients were hypothermic before start of anesthesia
(Figure 2) but no further decrease in core temperature was
noted during the first hour of anesthesia. Several studies
have shown the positive effect of prewarming,11 while in
a recent study it was demonstrated to have no benefit.41
The duration of prewarming has been explored and the
effects on daily routines and prewarming–related delays in
anesthesia start have been discussed.27,32 Horn et al25 concluded that only 10 or 20 minutes of prewarming might
reduce the incidence of UPH. In this study, we evaluated
the temperature difference in a shortened prewarming
routine that did not affect the anesthesia start. Group A
was prewarmed for 42 minutes during anesthesia and surgical preparations while group B was warmed according
to daily routines, when all these preparations were done.
The positive influence of prewarming seen in group A was
probably an effect of the early cutaneous warming, but
may also be referred to impacts of the preoperative beta
blockade and vasopressors used during anesthesia.16,42
Patients in group A were more likely to be normothermic
at end of surgery than patients in group B (Table 3). When
compared to core temperature at the start of anesthesia,
esophageal temperature at 210 minutes had increased
twice as much in group A as in group B. In this study, prewarming started immediately after epidural catheter insertion and patients had an intraoperative benefit of a longer
warming time as demonstrated by Horn et al.25 Since there
was only a small temperature drop before surgical start (in
esophagus, group B, warmed only 15 minutes during the
first hour of anesthesia), we conclude that warming for a
short period seems to be valuable.
In this study, an upper body forced-air warming
device was used and in agreement with other studies
we find it a sufficient method to prevent UPH.11,18,25,27
Similarly to other findings, the outflow temperature in
the warming device varied considerably during warming,
by –1°C to +5°C, and differed from the preset of 43°C.42
The skin temperature on the upper thorax was measured
continuously and no skin reaction from warming device
was observed. A smaller increase in ambient air temperature of 0.2–0.5°C was seen during surgery and the
warming device may have contributed to this. There were
no complaints from OR personnel or the surgeon.
Results of the current study confirm that age and
BMI is of importance to core temperature. The elderly
are more likely to get hypothermic and the risk of hypothermia is reduced in the obese. This is consistent with
findings during both general and regional anesthesia, and
may partly be explained by a more severe thermoregulatory impairment in elderly and by increased insulation by
excessive fat in obese patients.36,37
Because anesthesia most often is multimodal, it is
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difficult to evaluate the effect on thermoregulation of
each drug during surgery. Twelve patients in each group
were using beta blockers preoperatively. Beta blockers
have been shown to reduce hypothermia by decreasing
cardiac output.43 The impact this may have had on the
core temperature was not evaluated; neither was possibly augmented precapillary vasoconstriction induced by
vasopressors given during the procedures. The effect of
these drugs on tissue at the measurement sites is unclear
and further studies on this subject are warranted.
•Limitations. Blinding was not possible in this study.
Because of limitations in population size, types of surgery,
and anesthesia techniques, results from this study may
not be generalized to all populations and institutions. It
is possible that different placement of the temperature
probes occurred and affected our results; however, we
used the Mekjavić–Rempel formula for the length of
esophageal insertion in order to minimize differences
in placement. Another limitation could be referred to
patient conditions, such as perfusion and tissue disorders; however, the preoperative use of beta blockade was
similar in both groups and all patients were preoperatively given a low dose of vasopressive drugs.
Conclusion
We conclude small but statistical significant difference
between the esophageal and the nasopharyngeal temperatures. Though the temperature differences were statistically significant, the mean difference of 0.2°C may be of
clinical significance only when patients are at extreme
risk of hypothermia. In this study, core temperature was
recorded continuously and this enabled the detection of
hypothermic periods during surgery. Within the first hour
of surgery, there were decreasing temperatures registered
at the esophagus in both groups and this was not noted at
the nasopharyngeal site. This may indicate a higher accuracy in the esophageal measurement. Results in this study
also confirm that prewarming has a positive effect on core
temperature even in patients only warmed intraoperatively. We suggest that even a shorter prewarming time may
be of benefit for the prevention of unplanned hypothermia.
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AUTHORS
Anne Erdling, CRNA, MSc, is a nurse anesthetist and student manager at
Kristianstad General Hospital, Sweden. At the time this paper was written, she was a student in the master’s program in Medical Science at Lund
University, Sweden. Email: anne.erdling@skane.se.
Anders Johansson, CRNA, PhD, is an associate professor in the faculty
of medicine at Lund University and Lund University Hospital, Sweden.
Email: anders.johansson@med.lu.se.
DISCLOSURES
The authors have declared they have no financial relationships with any
commercial interest related to the content of this activity. The authors did
not discuss off-label use within the article.
AANA Journal
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April 2015
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Vol. 83, No. 2
105