Approval for this prospective, single-blind, randomized, controlled study was issued by the Ethics Committee of the First Affiliated Hospital of Zhengzhou University (2020-KY-176), registered at http://www.chictr.org.cn/index.aspx (ChiCTR2000039162) on 20 October 2020. All patients signed informed consent. The protocol for this work strictly conformed with the international guidelines for randomized clinical studies and the CONSORT Guidelines.
We scheduled 75 patients for elective laparoscopic hysterectomy. Inclusion criteria included:1) Patients aged 40–65 years; 2) patients classified in the American Society of Anesthesiologists’ physical status class of I to II; 3) patients without severe heart, liver, kidney disease; 4) patients with no history of severe respiratory or cerebrovascular disease. Exclusion criteria included: 1) preoperative anemia; 2) intraoperative blood transfusion; 3) transition to open surgery. Using a random number table, we randomly divided the 75 patients into 3 groups: The Control group (A), infusion heating group (B), and incubator group (C).
Premedication was not given to any patient. The operating room was maintained at 22–24 °C. All three groups were provided with warming blankets, which they turned on an hour in advance to achieve the preset temperature (38 °C). Upon entry to the operating room, we first administered the patients in the three groups with an injection of 500 ml Ringer’s lactate solution and followed by 500 ml succinylated gelatin. The infusion speed was maintained at 10 ml/min. Notably, in cases where the above liquids were both infused and replaced with 100 ml Ringer’s lactate solution, we ensured that the infusion path was clear and the infusion speed was nearly stopped. In group A, the infusion fluid was not treated and maintained at room temperature; in group B, we heated the infusion fluid by the infusion thermometer, and set the target temperature of the infusion thermometer 37 °C; in group C, the infusion fluid was incubated with the set target temperature at 37 °C.
ECG, BP, HR, SpO2, and BIS were monitored in all patients after they got into the operating room. Sufentanil 0.5 μg/kg, cisatracurium 0.25 mg/kg and etomidate 0.2 mg/kg were intravenously administered as anesthetic induction. Following the insertion of the laryngeal mask, the ventilator was set at VT 6 ~ 8 ml /kg, FiO250%, I: E 1: 2, RR12 ~ 20 times/min. PETCO2 was maintained at 35 ~ 45 mmHg. Anesthesia was maintained with sevoflurane (gas flow at 2 L/min), remifentanil, and cisatracurium. Sevoflurane was titrated to maintain BIS of 40 to 60 during surgery.
The nasopharyngeal temperature was assessed following the induction of anesthesia in the three groups. Briefly, the nasopharyngeal probe was inserted to of about 1 cm depth beyond the first scale (10 cm). It was then secured with tape to maintain the depth. Vital signs were stabilized during the operation. At lower intraoperative nasopharyngeal temperature below 35 °C, the temperature of the warming blanket was raised to maintain the patient’s nasopharyngeal temperature above 35 °C.
In the three groups, the primary outcome was the nasopharyngeal temperature at 5 min post anesthesia induction (T1), 30 min (T2), 60 min (T3), and 90 min (T4) at the beginning of surgery, whereas the secondary outcome was wake-up time.
SPSS (version 22.0, SPSS Inc., Chicago, IL, USA) was applied to analyze all statistical data. Measurement data were expressed as mean ± standard deviation. We adopted a one-way analysis of variance for comparison between groups; the nasopharyngeal temperature was compared at different time points via repeated measurement ANOVA. For counting data, the Chi-square test was used for comparison. P < 0.05 denoted statistical significance.
The sample size was established using GPower (version 220.127.116.11, Franz Faul, Universitat Kiel, Germany). Reports have demonstrated that a core temperature difference of 0.5 °C is clinically significant; it is the smallest difference associated with hypothermic complications . With the significance level(α) set at 0.05, and power(1-β) at 0.9, each group should include 22 patients, assuming that the withdrawal rate is 10%. Eventually, each group comprises 25 patients.