Table 1 – Study groups
Group Induction technique
INH/100 Inhalation: 8% sevoflurane in 100% oxygen
INH/50 Inhalation: 8% sevoflurane in 50% oxygen and 50% nitrous oxide*
PROP Intravenous: 2.2 mg Propofol
Phyl/PROP Intravenous: 2.2 mg propofol preceded by 160 mcg phenylephrine
* 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.31 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.
Table 2 – Inclusion and exclusion criteria
INCLUSION CRITERIA
Age 18 to 55 years inclusive
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 or >55 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 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 of the 200 patients analyzed
Group INH/100 INH/50 PROP Phnl/PROP
Age (years)
Mean (SD) 42.8 (10.1) 43.0 (8.6) 39.0 (11.2) 40.6(9.1)
Range 22 to 55 26 to 55 18 to 55 20 to 55
Sex
Male n (%) 20 (40) 31 (62) 20 (40) 21 (42)
ASA classification
1 n (%) 1 (2) 3 (6) 10 (20) 2 (4)
2 n (%) 23 (46) 23 (46) 29 (58) 33 (66)
3 n (%) 16 (52) 24 (48) 11 (22) 15 (30)
BMI (kg/m2)
Mean (SD) 31.9 (7.5) 31.2 (6.7) 26.8 (5.6) 29.9 (6.4)
Range 21.7 to 48.9 18.9 to 44.2 17.2 to 43.0 15.1 to 44.4
Preoperative screening temperature (oC)
Mean (SD) 36.8 (0.3) 36.8 (0.3) 36.8 (0.3) 36.7 (0.3)
Use of upper body forced air warming (FAW) blanket
(remaining patients used lower body FAW)
n 32 39 29 30
(%) (64) (78) (58) (60)
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)
Range 5 to 45 4 to 45 4 to 44 6 to 40
Table 4 – List of surgeries
Procedure Groups having at least one patient having the listed
procedure are denoted by an x
INH/100 INH/50 PROP Phnl/PROP
UROLOGIC
Cystoscopic surgery x x x x
Penile procedures x x x
Suprapubic tube placement x
Scrotal procedures x
Urethroplasty x x x
ORTHOPEDIC
Lower extremity orthopedics x x x x
Upper extremity orthopedics x x x x
Anterior cervical discectomy and fusion x
GYNECOLOGIC
Vulvoplasty or excision of lesion x x
Dilation and curettage, hysteroscopy x x x x
Loop endocervical excision procedure x x
Endocervical curettage x x
Hysterectomy x x x
Myomectomy x x x
VASCULAR
Dialysis access related procedures x x x x
Lower extremity vascular – open procedures x
Radiofrequency ablation and/or Lower extremity phlebectomies
x x x
Lower extremity amputations x x
DENTAL/ENT x x x
THORACIC
Endobronchial ultrasound x x x
GENERAL SURGERY
Vacuum assisted closure change x
Non-cavitary procedures x x x
Inguinal hernia x x
Breast x x x x
Table 5 Differences between the mean core temperature (oC) of each of three alternative induction groups and the standard propofol alone group at each time point*
Comparison groups T15 T30 T45 T60
INH/100 minus PROP
Difference (oC) 0.46 0.46 0.47 0.49
95% CI of difference 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 of difference 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 of difference 0.24 to 0.54 0.25 to 0.57 0.27 to 0.63 0.25 to 0.70
*p≤0.001 for each of the above comparisons separately. p< 0.006 when applying Bonferroni’s correction for the 12 multiple comparisons (3 groups compared to propofol only group at 4 time points).
**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.