Comparing Patient Temperatures in Adults During Lower Spinal Surgery Using Either a Heated Ventilator Circuit or a Standard Ventilator Circuit With a Heat-Moisture Exchanger
Phase: NA
Conditions: Hypothermia
Sponsor: Mayo Clinic

 
 
 
 
 
Comparing Patient Temperatures in 
Adults during Lower Spinal Surgery using 
either a Heated Ventilator Circuit or a 
Standard Ventilator Circuit with a Heat-
Moisture Exchanger 
 
NCT 03050775 
 
Version date: October 26, 2015 

1 
 
 
 
 
 
 
 
 
 
Comparing Patient Temperatures in Adults during Lower Spinal Surgery using either a Heated 
Ventilator Circuit or a Standard Ventilator Circuit with a Heat-Moisture Exchanger.  
Thomas Jorgensen, BSN, BS 
 Jean Guyer, APRN, CRNA, DNP 
 Tim Curry, MD, Ph.D. 
 Mary Shirk Marienau, APRN, CRNA, Ph.D.  
Darrell R. Schroeder, MS 
October 26, 2015: Revised 
 
 
 
 
 
 
 
 

Evaluating Adult Patient Temperatures 
 
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BACKGROUND: 
 
Hypothermia is a core temperature less than 36◦C in surgical patients,1,2  affecting up to 
sixty-four percent.3 All surgical patients are at increased risk as intrinsic thermoregulation is 
impaired by anesthetics’ effects,4-8 increasing the core temperature threshold from 0.2◦C to 4◦C; 
this delays normal compensatory mechanisms.9,10 This effect is compounded by the cold surgical 
environment,4,5,8,11 IV fluids,4,5,12 and airway gases.4,9,13 Hypothermia is known to have several 
deleterious complications.10,14 
 
Surgical site infections occur more frequently,15-17 as there is increased vasoconstriction 
near surgical wounds, which decreases oxygen and nutrient delivery.9,16 Immune function also is 
impaired9,18 from raised interleukin-6 levels19 and damaged neutrophils.4,9 Coagulopathies are 
more prevalent9,11,20 because of impaired clotting cascade enzymes11,20-22 and platelet function as 
well as increased fibrinolysis.11,20,22 As a result, greater blood loss11,14,23-25 and subsequent 
transfusions are observed,16,17,23,24 up to 16% and 22%, respectively.26 Colder patients experience 
increased cardiac-related events,9,17,27,28 as norepinephrine levels can be 7-fold, 29 precipitating 
hypertension,9,30 angina, and myocardial ischemia.31 In addition, drug metabolism is 
decreased,4,28 prolonging medication effects22,32 and increasing potency.2 Post-anesthesia 
shivering is more observed in hypothermic patients,1,4,12,15-17,23,27,29,33-37 which can increase 
oxygen consumption,12 with estimates 110% or greater.29,35,38 Pressure ulcer formation39 and 
greater postoperative pain15,38 are further described. These events significantly impact surgical 
patients as there is increased time to extubation,19 post-anesthesia recovery,10,12,40 
hospitalization,3,16 and mortality.4,11,28  
 
 
Heat is lost through radiation, convection, conduction, and evaporation,2,5,11,12 with 
radiation and convection being the most significant sources.6,9,33 Patients try to maintain 

Evaluating Adult Patient Temperatures 
 
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normothermia, defined by Hooper et al41 as a core temperature between 36◦C and 38◦C. This 
mechanism, however, is disrupted through three phases during surgery.6 Post-induction is the 
first phase, with anesthetics inducing vasodilation, even in response to cold-stressors; this 
abruptly shifts core heat peripherally,6,36,42,43 as there is a decrease of 1-1.5◦C within the first 
hour.6,7 The second phase occurs over the next 1-2 hours6,7,44 and is associated with a slow, linear 
heat loss as the heat deficit exceeds heat production.6,33,44 Phase III occurs after three hours of 
induction6,33,44 and is associated with a plateau state as peripheral vasoconstriction is triggered in 
an attempt to reestablish thermoregulation.6,9,33,44  
 
The standards of practice for the American Society of Anesthesiologists and the 
American Association of Nurse Anesthetists recommend that patient temperature should be 
monitored and maintained when significant changes are anticipated or suspected.45,46 This is also 
supported through the clinical practice guideline by the American Society of PeriAnesthesia 
Nurses, which states “frequent intraoperative temperature monitoring should be considered in all 
cases”(p.353) as well as proper assessment for hypothermia.41 The Centers for Medicare and 
Medicaid Services (CMS) initiated the Surgical Care Improvement Project-Infection-10 (SCIP-
Inf-10) in 2009 as means for maintaining normothermia in surgical patients undergoing general 
or neuraxial anesthesia for greater than one hour,47 which was required for hospitals participating 
in CMS.48 SCIP-Inf-10 applied reimbursement costs the following two ways47: 1) maintaining a 
patient’s temperature at least 36◦C within 30 minutes before or 15 minutes after anesthesia’s end 
time or 2) attempting to maintain patient normothermia by active warming methods. SCIP-Inf-10 
was retired January 1, 2014 as an active measure for The Joint Commission and CMS.49 
However, CMS in December 2014 as part of their “List of Measures under Consideration Table” 
included the normothermia outcome defined as the following to be addressed50: “This measure 

Evaluating Adult Patient Temperatures 
 
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evaluates whether patients having surgical procedures under general or neuraxial anesthesia of 
60 minutes or more in duration are normothermic within 15 minutes of arrival in PACU.”(np)  
 
Achieving normothermia on arrival to the PACU is only a partial goal as deleterious 
effects can be potentiated intraoperative while seemingly corrected postoperative. Coagulation 
during surgery is dynamic as fluctuations according to patient temperature may or may not be 
noticed if the patient is normothermic in the PACU, even if they were hypothermic 
intraoperative.11,20 Sun et al included over 58,000 patients in their study to evaluate temperature 
patterns perioperative, demonstrating approximately 50% and 20% of patients having 
temperatures less than 36◦C and 35.5◦C for over an hour, respectively.3 Furthermore, 20% of 
these patients had temperatures less than 36◦C for greater than two hours.3  
 
Several interventions are commonly utilized to manage patients’ temperatures 
intraoperative, such as blankets, circulating fluid mattresses,2,36 forced air warming,15,27,30,35,36,43 
IV fluid warmers,1,15,29,34,51 and heated breathing circuits.13,23,37,51-56 The most used heating 
intervention is forced air warming as it is relatively inexpensive3,5 and increasing ambient 
temperature isn’t usually an option as the surgical staff can get too hot from their additional 
layers.4,5,9 Maintaining normothermia can be challenging utilizing only forced air warming in 
certain populations, such as patients undergoing spinal surgery.23 These patients are positioned 
prone and active warming is often delayed more than 30 minutes. Employing multiple active 
warming devices has been suggested in several studies and may be a viable option to prevent 
hypothermia from developing.  
 
Less than 10% of heat loss occurs via the airway; although, this has been suggested as a 
possible location for warming patients.9,13,33,52 Several studies have incorporated heated 
humidification as a warming method, but the results are conflicting regarding its effectiveness. 

Evaluating Adult Patient Temperatures 
 
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Some suggest no effects are achieved,13,51,54,57 while others have demonstrated higher initial 
temperatures following induction23,37,55 as well as maintaining these temperatures during 
surgery.23,37,52,53,55,56 Heated humidifiers use active warming and are shown to be superior 
compared to heat-moisture exchangers (HMEs), which passively warm the airway.37,52,58 Several 
studies have observed temperature changes of 0.5◦C or greater in patients provided active heat 
and humidification compared to patients given none or passive humidification.13,23,37,52,55-57 A 
temperature difference of 0.5◦C has demonstrated clinical relevance, as patients have less 
intraoperative blood loss,23,25,59 fewer transfusions,23 decreased post-anesthesia recovery,12,56 and 
less shivering.12  
 
Most of the current literature regarding heated breathing circuits focuses on surgeries 
three hours or less, with small to medium patient populations, and healthier adults. This study 
will expand on the previous works by concentrating on surgeries greater than three hours and 
including less healthy adults. This will allow for evaluation of active heat and humidification in 
regards to patients experiencing Phase II and Phase III, as it is suggested that active heat and 
humidification would have an increased effect on temperature as surgery length progresses 
compared to patients receiving none or passive humidification.23,37,52,53,55,56 The purpose of this 
study is to compare core temperatures in adult patients having lower spinal surgery for at least 
three hours in duration using either a heated ventilator circuit (ANAPOD™ Heat and 
Humidification System; Westmed) or a standard ventilator circuit with a HME (Thermovent® 
600; Portex). The ANAPOD™ Heat and Humidification System (Westmed) provides active heat 
and humidification during anesthesia by warming inspiratory gases for intubated patients.60 The 
HME connects between the endotracheal tube (ETT) and the standard ventilator circuit and is our 
hospital’s standard practice for all spine surgery patients.  

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SPECIFIC OUTCOMES AND HYPOTHESIS: 
 
The primary outcome of this study is to compare core body temperature four hours after 
induction in patients having lower spinal surgery, of at least three hours in duration and using 
either a heated ventilator circuit (treatment group) or a standard ventilator circuit with a HME 
(control group). For any patient surgery with a core temperature not reaching four hours post-
induction, the final esophageal temperature will be used. We hypothesize that the treatment 
group will have higher statistically significant temperatures four hours after induction or using 
the last recorded esophageal temperature for surgeries not reaching that timeframe compared to 
the control group.   
 
A secondary outcome will compare overall patient temperature trends between the two 
groups, perioperatively. We hypothesize that the treatment group will have higher statistically 
significant temperatures perioperatively 60 minutes post-induction compared to the control 
group. An additional secondary outcome will compare patient shivering between the two groups. 
We hypothesize that the treatment group will have less shivering compared to the control group. 
MATERIALS AND METHODS: 
 
Following Institutional Review Board approval, a randomized, prospective study will be 
conducted on Saint Mary’s Campus at Mayo Clinic Hospital in Rochester, MN. Study 
participants will be patients ≥18 years old (including possible pregnant women), with an ASA 
status of I-III, and the following surgery inclusive criteria: elective spinal surgery greater than 
three hours in duration, posterior approach, an operation site between lumbar one and sacral one, 
involving two or more levels with fusion and/or instrumentation and/or revisions, and 
encompassing any anticipated possibilities for these mentioned criteria prior to surgery. 
Exclusion criteria are the following: patients with a tracheostomy, preoperative body temperature 

Evaluating Adult Patient Temperatures 
 
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>38◦C or <36◦C on the day of surgery, active infection or erythema to the back, or white blood 
cell count greater than 10,500/mcL (Figure 1).  
  
Patients will be randomly assigned to either the treatment group or the control group prior 
to surgery. Patients in the treatment group will be given heated and humidified inspiratory gases 
prior to being positioned prone following induction. Induction will be defined for this study as 
intubation since this will be the first opportunity to place an esophageal stethoscope with a 
temperature sensor as is the usual practice at our hospital. Distal esophageal temperature is a 
measure of patient core temperature61,62 and has been demonstrated to not be affected by tracheal 
temperatures if placed correctly in the retrocardiac portion of the distal esophagus.63-66 The 
retrocardiac position is represented by maximal heart sounds66 or 38-42cm as measured from the 
central incisors in adults.63 Education with anesthesia providers, incorporating visual aids and 
descriptive explanations, for placing the esophageal temperature probe at this location will be 
provided by a team member associated with this study prior to study enrollment. Even with 
proper education, this may be a study limitation if the probe isn’t placed at the retrocardiac 
portion of the esophagus; however, this limitation should be minimized with the planned 
educational approach. All patients will receive a forced air warming blanket to their lower 
extremities and upper back at 43◦C (high) after draping as is the usual practice at our hospital 
(Figure 2). If any patient has a temperature of 36◦C for at least 15 minutes, both forced air 
warming controller devices will be turned off but remain in place under the drapes, as is the 
recommended practice at our hospital. If a patient’s temperature decreases below 36◦C, both 
forced air warming devices would again be turned on high setting (43◦C). The operating room 
temperature will initially be set to 21◦C by a team member associated with this study each 
morning prior to enrolled surgeries. The operating room temperature will be maintained at 20-

Evaluating Adult Patient Temperatures 
 
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22◦C, but may be adjusted beyond this range at the surgeon’s discretion and/or subject’s 
condition. The room temperature will be documented for all subsequent subjects every hour 
using an in-room digital display temperature device. It is important to standardize flow settings 
and tidal volumes as both may impact the amount of delivered heat to the patient, and it is 
recognized that these settings may be difficult to maintain during induction or patient positioning 
as higher volumes are generally given. The ventilator flow setting will be maintained at ≤2L/min 
or adjusted to meet the patient’s needs during anesthetic maintenance, with tidal volumes 6-
8ml/kg following patient positioning and induction. Patient randomization will assist with 
reducing any other potential sources of variation that may occur, such as patient exposure time 
during positioning. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
All surgical 
patients 
Potential study 
patients 
Qualified study 
patients (N=70) 
INCLUSION CRITERIA: ≥18 years old 
(including possible pregnant women), ASA I-
III, elective spinal surgery at L1-S1 for 
greater than three hours, posterior approach, 
two or more levels with fusion and/or 
instrumentation and/or revisions, 
encompassing any anticipated possibilities for 
these criteria prior to surgery. 
EXCLUSION CRITERIA: Patients with a 
tracheostomy, preoperative body temperature 
>38◦C or <36◦C on the day of surgery, active 
infection or erythema to the back, or a white 
blood cell count greater than 10,500/mcL. 
FIGURE 1: Describes patient selection process 

Evaluating Adult Patient Temperatures 
 
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Patient’s temperature will be measured and recorded in the preoperative area at least once 
prior to entering the OR suite. Intraoperative, patient’s temperature will be measured and 
recorded within ten minutes or the first stable temperature reading after induction to establish a 
baseline. To allow for patient repositioning, the next temperature will be recorded within 30 
minutes of the baseline esophageal temperature and every 10 minutes thereafter for the first hour 
post-induction, followed subsequently every 30 minutes until the patient is repositioned supine.  
Following the surgery, the patient will be transferred to the PACU, where the temperature will be 
measured and recorded at least once within 15 minutes of arrival (Figure 2). Oral temperatures 
(or axillary temperatures if an oral temperature cannot be obtained) will be used in the 
preoperative area, PACU, and ICU while esophageal temperatures will be measured in the OR 
suite after induction. 
 
Patients will be monitored in the PACU for shivering as this can be an indicator of 
hypothermia. Any patients transferred directly to ICU from surgery will have a temperature 
recorded within 15 minutes of arrival. Any patient transferred to ICU will not be monitored for 
shivering. All patients’ electronic medical records will be monitored through their hospital 
discharge, up to 30 days post-surgery to record any postoperative complications. 
 
Shivering in the PACU will be assessed at least every 30 minutes for a maximum of two 
hours using the Bedside Shivering Assessment Scale (BSAS) described by Badjatia et al  
(Table 1).67 This is a 4-point scale and rates shivering as the following67: absent, mild, moderate, 
or severe (Table 2). Absent means “no shivering noted on palpation of the masseter, neck, or 
chest wall.”67(p3243) Mild means “shivering localized to the neck and/or thorax only.”67(p3243) 
Moderate means “shivering involves gross movement of the upper extremities (in addition to 

Evaluating Adult Patient Temperatures 
 
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neck and thorax),”(p3243) and severe means “shivering involves gross movements of the trunk and 
upper and lower extremities.”67(p3243) The BSAS demonstrated reliability among multiple 
users67,68 and was tested for validity using indirect calorimetry (IDC) at rest.67 IDC is often 
performed at rest and provides reliable, precise measurements of resting energy expenditure, 
which contribute largely to the total daily energy expenditure.69 Nurses in the PACU will be 
educated to use this scale by a team member associated with this study. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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QUALIFIED STUDY 
PATIENTS 
CONTROL GROUP: 
Standard ventilator 
circuit with HME 
TREATMENT 
GROUP: Heated 
ventilator circuit 
(ANAPOD™ Heat and 
Humidification System; 
Westmed) 
SURGERY 
Randomization 
Inspiratory gas at 
ambient air 
temperature (20-
22◦C) 
Inspiratory gas 
at 40-41◦C 
ALL: Induction 
ALL: 
Esophageal 
temperature 
within ten 
minutes or first 
stable temp. 
ALL: Patient 
positioned prone 
ALL: Esophageal temperature within 30 
minutes of the baseline esophageal 
temperature and every 10 minutes 
thereafter for the first hour post-induction.   
ALL: a blanket and force air 
warming to their lower extremities 
and upper back at 43◦C (high) after 
draping 
PACU 
ALL: Surgery 
end-time  
Primary Outcome 
Secondary 
Outcomes 
FIGURE 2: Describes perioperative timeline and outcome measurement events 
AM Admit areas 
(Alfred-1 or 
EPACU) 
ALL: Esophageal temperature continued every 30 
minutes until the patient is repositioned supine.   
ALL: Four hours 
post-induction 

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The ANAPOD™ Heat and Humidification System (Westmed) controller will be turned 
on prior to connecting the ventilator circuit to the ETT and will be set to 45◦C with an acceptable 
fluctuation between 44-45.6◦C. The temperature setting will be monitored and recorded every 30 
minutes until extubation.  This will be monitored by the anesthesia provider and documented in 
the patient’s electronic medical record. The delivered temperature is expected to be 4-5◦C less 
OUTCOME 
LOCATION 
DESCRIPTION 
Primary: 
 
Post-induction 
temperature 
Intraoperative 
Patient core temperature four hours post-induction  or 
using the last recorded esophageal temperature for 
surgeries not reaching that timeframe compared to 
initial baseline esophageal temperature 
Secondary 
 
Perioperative 
temperature 
Intraoperative, 
postoperative 
Patient temperature trends intraoperative to 
postoperative.  
 
Shivering 
Postoperative 
Presence of shivering among patients in the PACU as 
described by Badjatia et al.67 Assessed every 30 
minutes for a maximum of two hours. 
TABLE1: Describes study outcomes 
TABLE 2: Describes Bedside Shivering Assessment Scale developed by Badjatia et al.67  
Score 
0 
2 
1 
3 
Definition 
None: no shivering noted on palpation of the masseter, neck, or chest wall 
Mild: shivering localized to the neck and/or thorax only 
Moderate: shivering involves gross movement of the upper extremities (in addition 
to the neck and thorax) 
Severe: shivering involves gross movements of the trunk and upper and lower 
extremities 

Evaluating Adult Patient Temperatures 
 
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than the administered temperature, which will provide airway gases at 40-41◦C to patients in the 
treatment group.70 If the patient’s temperature is >37.4◦C for at least 15 minutes using only the 
ANAPOD™ Heat and Humidification System (Westmed), then the controller unit would be 
turned off until the patient’s temperature is <37◦C, at which point the controller’s temperature 
setting would follow the previous outlined parameters. The ventilator circuit associated with the 
ANAPOD™ Heat and Humidification System (Westmed) is comprised of a heated wick70 and 
has a water trap on the expiratory limb to prevent humidity impacting the ventilator. A 0.5 
micron hydrophobic filter (Smiths-Medical) will be placed at the patient-end of the gas sample 
line to prevent humidity impacting the gas analyzer. The ANAPOD™ Heat and Humidification 
System (Westmed) is a FDA approved device for all patient populations and uses sterile water 
for adding humidification to the ventilator circuit. This device has been approved by the 
Anesthesiology Equipment Committee and Anesthesia Biomed Department for this study. A 
team member associated with this study will provide education to anesthesia providers for using 
this equipment.  
 
An investigator associated with this study will meet patients in the Preoperative 
Evaluation Clinic located at Gonda or the AM admit areas (on Alfred-1 or EPACU) located at St. 
Mary’s campus. The investigator will review the consent form with patients meeting the study’s 
eligibility requirements and enroll patients who are willing to provide written, informed consent. 
Patients will be randomized to the study after consent and enrollment as described in the section 
below.  
STATISTICAL CONSIDERATIONS/ANALYSIS 
 
Based on previous studies,15,54 we anticipate the standard deviation of patient temperature 
is 0.6◦C. Several studies have demonstrated that a 0.5◦C temperature change is clinically relevant, 

Evaluating Adult Patient Temperatures 
 
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as patients have less intraoperative blood loss,23,25,59 fewer transfusions,23 decreased post-
anesthesia recovery,12,56 and less shivering.12 Using the assumption that a standard deviation of 
patient temperature is 0.6◦C and assuming that a difference between groups of 0.5◦C is clinically 
relevant, we have determined that a total sample size of N=64 (N=32per group) will provide a 
statistical power (two-tailed, alpha=0.05) of 90% to detect a clinically relevant difference 
between groups. In order to account for potential attrition due to cancelled surgery, inability to 
place an esophageal stethoscope with a temperature sensor, and/or unexpected surgery length <3 
hours, we propose a total sample size of N=70 (N=35 per group). Group randomization will be 
performed using a randomization schedule prepared by the Division of Clinical Statistics with 
subjects randomized in blocks of 4. This will ensure that after every 4th patient is enrolled, there 
are an equal number assigned to each treatment arm. Using this randomization schedule, 
individuals who will not have any subject contact for the present study will prepare 
randomization envelopes, which will be labeled according to subject ID number and contain an 
index card indicating the intervention assignment (control group or treatment group) for the 
given subject.  For enrolled and consented patients on the day of surgery in the AM admit areas 
(Alfred-1 or EPACU), a subject will be assigned the next sequential subject ID number and the 
appropriate sealed envelope will be opened to reveal the subject’s randomized intervention 
assignment. This will occur before the patient is transferred to the OR suite.  
 
Data will be summarized using mean±SD (or median 25th, 75th) for continuous variables 
and frequency percentages for nominal variables. For the primary analysis, the esophageal 
temperature four hours post-induction or using the last recorded esophageal temperature for 
surgeries not reaching that timeframe will be compared between the control group and treatment 
group using analysis of covariance (ANCOVA) with pre-induction temperature included as a 

Evaluating Adult Patient Temperatures 
 
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covariate. This will account for any variation in pre-induction temperature, and the potential 
correlation between pre-induction temperature and the primary outcome temperature. In addition, 
a repeated measures analysis will be performed using all intraoperative esophageal temperatures 
recorded until surgery end-time, which will be defined as the last recorded esophageal 
temperature (for all patients). This analysis will be performed using mixed linear models with 
temperature as the dependent variable, time as a within subject effect, and treatment group as a 
between subject effect. Additional secondary outcomes will include shivering and temperature at 
PACU arrival. The frequency of any shivering during the PACU will each be compared between 
treatment groups using the chi-square test (or Fisher’s exact test). Temperature at PACU arrival 
will be compared between groups using analysis of covariance (ANCOVA) as previously 
described. In all cases p-value ≤ 0.05 will be considered statistically significant. 
FINANCIAL CONSIDERATIONS 
 
Three ANAPOD™ Heat and Humidification System (Westmed) controllers will be 
provided by Westmed at no cost for the study’s duration (David Christensen, Westmed Territory 
Sales Manager, email communication, January 21, 2015). Ventilator circuits for the duration of 
the study are $410.00 per case, and each case contains 20 individually packaged circuits (David 
Christensen, Westmed Territory Sales Manager, email communication, March 25, 2015). We 
plan to enroll 70 patients (35 patients for each arm), which requires two cases of ventilator 
circuits. The 0.5 micron hydrophobic filter is $108.85 per case, and each case contains 50 
individually packaged filters (Pam Rasmussen, Mayo Clinic Surgical Process Analyst, email 
communication, April 30, 2015). One case will be required for the study. No staff education 
costs are expected, and all statistical analyses’ costs will be provided at no charge using the 
Center for Clinical and Translational Science as provided to students. The current cost for a 

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standard ventilator circuit is $6.62 (Jean Guyer, Mayo Clinic Director of Nurse Anesthesia, email 
communication, March 12, 2015). Taking this into consideration, 35 standard ventilator circuits 
cost $231.70 and will be deducted from the cost analysis as these circuits will not be used. The 
total cost associated with this study is $697.15 ($928.85-$231.70) and will be supported by the 
Department of Anesthesiology. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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