Table 1 – Study groups
Group Age Induction technique
(years)
INH/100 18 to 55 Inhalation: 8% sevoflurane in 100% oxygen
A
INH/50 18 to 55 Inhalation: 8% sevoflurane in 50% oxygen and 50% nitrous oxide*
B
PROP 18 to 55 Intravenous: 2.2 mg Propofol
C
Phyl/PROP 18 to 55 Intravenous: 2.2 mg propofol preceded by 160 mcg phenylephrine
D
INH/100>55 >55 Inhalation: 8% sevoflurane in 100% oxygen**
AA
INH/50>55 >55 Inhalation: 8% sevoflurane in 50% oxygen and 50% nitrous oxide
BB
* Nitrous oxide may have vasoconstricting effects. Thus for a given depth of anesthesia, there may be less vasodilation in a N2O /sevoflurane anesthetic due to a lesser amount of sevoflurane than in a sevoflurane induction without N2O. Ozaki et al found N2O impairs thermoregulation less than sevoflurane or isoflurane.14 With less vasodilation, it seemed plausible that there would be less redistribution hypothermia if N2O was used; hence the reason for studying inhalation inductions using both 100% O2 and 50% O2 / 50% N2O.
** Intravenous inductions were not studied in patients over 55 years. In such patients, the dose of propofol would need to be reduced by varying amounts (difficult to determine objectively). The possible inaccuracies in dose would make such an analysis unreliable.
Table 2 – Inclusion and exclusion criteria
INCLUSION CRITERIA
Age 18 years or greater
Scheduled for general anesthesia where 50% nitrous oxide in oxygen could be used
Endotracheal intubation or laryngeal mask airway insertion would be used
Afebrile (preoperative oral or temporal scan temperature between 36.2 and 37.4°C inclusive)
Positioned supine or lithotomy
Forced air warming would be used
Expected duration of anesthetic to be at least 60 minutes
EXCLUSION CRITERIA
Emergency surgery or any other aspiration risk
Age <18 years
Pregnant
Incarceration
Febrile illness
Anticipated difficult airway
Contraindication to nitrous oxide use
Contraindication to nasal instrumentation
Nasal surgery
Current or recent epistaxis
Requirement for foreign language interpreter
Allergy to propofol
Malignant hyperthermia risk
Inability to oxygenate on less than 50% oxygen
Cardiac surgery
Neuro-surgery
Receiving vasoactive infusions
Significant valvular heart disease
Unstable cardiac disease
Requiring prone or lateral positioning
Inability to provide informed consent
Inability to use forced air warming
Untreated hypo- or hyper-thyroidism
ASA class 4, 5 or 6*
Anticipated inability to tolerate any of the 4 (2 if over 55 years of age) different anesthetic induction options in this study
*Patients with end stage renal disease on dialysis were classified as ASA 3.
Table 3
Demographics and forced air warming data
Group INH/100 INH/50 PROP Phnl/PROP INH/100>55 INH/50>55
Age (years)
Mean (SD) 42.8 (10.1) 43.0 (8.6) 39.0 (11.2) 40.6(9.1) 68.2 (8.4) 66.2 (7.4)
Range 22 to 55 26 to 55 18 to 55 20 to 55 56 to 88 56 to 85
Sex
Male n (%) 20 (40) 31 (62) 20 (40) 21 (42) 32 (64) 26 (52)
ASA classification
1 n (%) 1 (2) 3 (6) 10 (20) 2 (4) 0 (0) 0 (0)
2 n (%) 23 (46) 23 (46) 29 (58) 33 (66) 10 (20) 10 (20)
3 n (%) 16 (52) 24 (48) 11 (22) 15 (30) 40 (80) 40 (80)
BMI (kg/m2)
Mean (SD) 31.9 (7.5) 31.2 (6.7) 26.8 (5.6) 29.9 (6.4) 28.9 (6.5) 29.3 (6.3)
Range 21.7 to 48.9 18.9 to 44.2 17.2 to 43.0 15.1 to 44.4 20.2 to 57.5 18.2 to 43.0
Preoperative screening temperature (oC)
Mean (SD) 36.8 (0.3) 36.8 (0.3) 36.8 (0.3) 36.70 (0.3) 36.8 (0.3) 36.8 (0.3)
Use of upper body forced air warming (FAW) blanket
(remaining patients used lower body FAW)
n 32 39 29 30 25 29
(%) (64) (78) (58) (60) (50) (58)
Time from T0 until FAW turned on, (minutes)
Mean (SD) 16.4 (7.0) 14.7 (7.0) 15.9 (7.9) 17.1 (7.7) 17.4 (7.9) 16.6 (8.1)
Range 5 to 45 4 to 45 4 to 44 6 to 40 5 to 42 4 to 43
Table 4 – List of surgeries
Procedure Groups* having at least one
patient having procedure
UROLOGIC
Cystoscopic surgery A B C D AA BB
Penile procedures A B C AA BB
Suprapubic tube placement A AA
Scrotal procedures A C D BB
Urethroplasty B C D BB
ORTHOPEDIC
Lower extremity orthopedics A B C D AA BB
Upper extremity orthopedics A B C D AA
Anterior cervical discectomy and fusion D BB
GYNECOLOGIC
Vulvoplasty or excision of lesion A B
D&C, hysteroscopy A B C D
Loop endocervical excision procedure (LEEP) A B
Endocervical curettage (ECC) A B
Hysterectomy A B D
Myomectomy B C D
VASCULAR
Dialysis access related procedures A B C D AA BB
Lower extremity vascular – open procedures B AA BB
RFA and/or Lower extremity phlebectomies A B C
Endovascular AAA AA BB
Lower extremity amputations A B AA BB
DENTAL/ENT A C D AA BB
THORACIC
Endobronchial ultrasound (EBUS) A C D AA BB
GENERAL SURGERY
VAC change A AA BB
Non-cavitary procedures A B D AA BB
Inguinal hernia A D AA
Incisional hernia BB
Breast A B C D AA BB
*For this table, the groups have the following designations:
A = INH/100
B = INH/50
C = PROP
D = Phnl/PROP
AA = INH/100>55
BB = INH50>55
Table 5 Comparison of mean core temperature (oC) of the three alternative induction groups to the standard propofol alone group at each time point in patients aged 18 to 55*
T15 T30 T45 T60
Comparison groups
INH/100 minus PROP
Difference (oC) 0.46 0.46 0.47 0.49
95% CI 0.28 to 0.64 0.28 to 0.64 0.25 to 0.69 0.20 to 0.77
INH/50 minus PROP
Difference (oC) 0.47** 0.52** 0.50 0.54
95% CI 0.31 to 0.64 0.36 to 0.69 0.31 to 0.69 0.28 to 0.79
Phnl/PROP minus PROP
Difference (oC) 0.39 0.41 0.45 0.47**
95% CI 0.24 to 0.54 0.25 to 0.57 0.27 to 0.63 0.25 to 0.70
*P≤0.001 for all above comparisons
**These differences are correct to 2 decimal places. Because of rounding to two decimal places in Figure 2, they differ by 0.01 from those that would be calculated using Figure 2.
Table 6 Comparison the three alternative induction groups to the standard propofol alone
group having temperatures <36oC and ≤35.5oC in the first hour in patients aged 18 to 55
One or more temperatures <36oC in first hour
Group INH/100 INH/50 PROP PhnlPROP
n (%) 8 (16%) 8 (16%) 30 (60%) 8 (16%) .
Comparison groups Difference 95% CI p value
(percentage points)
PROP minus INH/100 44% 27% to 61% <0.0001
PROP minus INH/50 44% 27% to 61% <0.0001
PROP minus Phnl/PROP 44% 27% to 61% <0.0001
One or more temperatures ≤35.5oC in first hour
Group INH/100 INH/50 PROP PhnlPROP
n (%) 4 (8%) 2 (4%) 11 (22%) 1 (2%) .
Comparison groups Difference 95% CI p value
(percentage points)
PROP minus INH/100 14% 0.3% to 28% 0.09***
PROP minus INH/50 18% 5% to 31% 0.015
PROP minus Phnl/PROP 20% 8% to 32% 0.004
***The 95% CIs are asymptotic (approximate); the p values are exact. In this one case when the p value and 95% CI yield different conclusions, the exact p value should be used.
Table 7 Comparisons of mean core temperature (oC) at each time point
between patients aged >55 and patients aged 18 to 55
Comparison groups T15 T30 T45 T60
INH/100>55 minus PROP*
Difference (oC) 0.37 0.36 0.29 0.30
95% CI 0.20 to 0.54 0.18 to 0.53 0.09 to 0.50 0.06 to 0.55
p value <0.0001 0.0001 <0.005 0.02
INH/50>55 minus PROP*
Difference (oC) 0.44 0.43 0.41 0.41
95% CI 0.26 to 0.63 0.24 to 0.62 0.19 to 0.63 0.14 to 0.68
p value <0.0001 0.001 0.0004 0.003
INH/100* minus INH/100>55
Difference (oC) 0.09 0.11 0.18 0.18
95% CI -0.10 to 0.27 -0.09 to 0.30 -0.05 to 0.40 -0.08 to 0.45
p value 0.37 0.28 0.12 0.17
INH/50* minus INH/50>55
Difference (oC) 0.03 0.09 0.09 0.13
95% CI -0.16 to 0.22 -0.10 to 0.29 -0.13 to 0.31 -0.13 to 0.38
p value 0.76 0.35 0.41 0.32
*These groups were comprised of patients age 18 to 55 years.
Table 8 Comparisons between the percentages of patients aged >55 and patients aged
18 to 55 having temperatures <36oC and ≤35.5oC in the first hour
One or more temperatures <36oC in first hour
Group INH/100 INH/100>55 INH/50 INH/50>55 PROP
n (%) 8 (16%) 14 (28%) 8 (16%) 14 (28%) 30 (60%)
Comparison groups Difference 95% CI p value
(percentage points)
PROP* minus INH/100>55 32% 14% to 50% 0.002
PROP* minus INH/50>55 32% 14% to 50% 0.002
INH/100>55 minus INH/100* 12% -4% to 28% 0.23
INH/50>55 minus INH/50* 12% -4% to 28% 0.23
One or more temperatures ≤35.5oC in first hour
Group INH/100 INH/100>55 INH/50 INH/50>55 PROP
n (%) 4 (8%) 5 (10%) 2 (4%) 3 (6%) 11 (22%)
Comparison groups Difference 95% CI p value
(percentage points)
PROP* minus INH/100>55 12% -2% to 26% 0.17
PROP* minus INH/50>55 16% 3% to 29% 0.04
INH/100>55 minus INH/100* 2% -9% to 13% 1.0
INH/50>55 minus INH/50* 2% -7% to 11% 1.0
*These groups were comprised of patients age 18 to 55 years.