Short interruptions between pre‐warming and intraoperative warming are associated with low intraoperative hypothermia rates

Prevention of inadvertent hypothermia is recommended for procedures >30 minutes because hypothermia increases the risk of myocardial ischemia, intraoperative blood loss, transfusion and wound complications. Therefore, short warming interruptions between pre‐warming and intraoperative warming might result in lower hypothermia rates. The aim of this retrospective investigation was to determine whether the incidence of inadvertent intraoperative hypothermia was affected by the warming interruption.


| Data analysis from the anaesthesia records
The total number of patients, the number of patients with intraoperative hypothermia and the warming interruption times were calculated from the anaesthesia records.

| Standard thermal management
The thermal management measures followed a standard operating procedure. After positioning on the operating table, the patients were covered with a forced-air warming blanket and pre-warming was started immediately after arrival in the anaesthesia induction area. Warming was continued during the induction of anaesthesia and stopped with the beginning of preparatory surgical measures.

| Measurement and documentation of core temperature
Body core temperature was continuously measured during anaesthesia either with an oesophageal probe (Assmuth D-0S4, Philips 21075A-compatible, Philips Healthcare, Germany), whose tip was positioned under laryngoscopic view with a Magill forceps 30 cm distal from the lower incisor teeth, or a bladder catheter thermometer (UROSID ® sensor 400; Asid Bonz, Herrenberg, Germany), or an intraarterial temperature probe (PiCCO-Catheter; Pulsion, München, Germany). Temperature values were continuously recorded by the anaesthesia monitor. The lowest recorded intraoperative temperature value was entered into a special checkbox for digital data acquisition in the anaesthesia record by the attending anaesthesiologist.

Editorial Comment
Unplanned intraoperative or post-operative hypothermia can increase risk for complications for patients. This investigation shows that short (rather than long) warming interruptions between pre-warming and intraoperative warming can reduce the rate of intraoperative hypothermia. no normal distribution. The Mann-Whitney U Test was used to analyse continuous parameters regarding significant differences between normo-and hypothermic patients; the chi-squared test was used to analyse significant differences between normo-and hypothermic patients in categorical parameters. A P-value of <.05 was considered statistically significant. Receiver operating characteristic (ROC) analysis was used to determine a cut-off value for the forced-air warming interruption time.

| RE SULTS
During the analysed time period, 6471 patients fulfilled the inclusion criteria. In 1387 (21.4%) of the analysed anaesthesia records, documentation of temperature values or of the used temperature probe was insufficient, so that 5084 records could be used for retrospective analysis. Patient data and process times are shown in Table 1. Receiver operating characteristic analysis revealed a cut-off value for forced-air warming interruption time of >20 minutes (AUC 0.652 with a specificity of 67.9% and a sensitivity of 54.4%; P < .001). The incidence of intraoperative hypothermia was significantly lower in patients with a forced-air warming interruption time of ≤20 minutes than those with an interruption of >20 minutes (9.6% vs 21.2%; P < .0001).

| D ISCUSS I ON
This large retrospective analysis of more than 5000 patients undergoing general anaesthesia revealed that longer forced-air warming interruption times were associated with significantly higher intraoperative hypothermia rates. Patients with forced-air warming interruptions >20 minutes showed significantly higher intraoperative hypothermia rates than those with interruptions of ≤20 minutes (21.2% vs 9.6%; P > .0001).
The effect that long interruption times between pre-warming and intraoperative warming can increase the risk of hypothermia has recently been shown for the first time in a prospective randomized-controlled trial with 200 patients. 9 In contrast to the present analysis, where a standard warming blanket was used for pre-warming in the operating room, in the study of Lau et al 9 a patient controlled pre-warming gown was used in the preoperative unit. A pre-warming gown is specially designed for the purpose of pre-warming awake patients in holding areas or general wards.
Patient-controlled pre-warming with a pre-warming gown has already been shown to be effective in preventing perioperative hypothermia, but when conducted outside the operating room, In previous studies it has been shown that the intraoperative hypothermia rates varied between 30.4%-69% with no pre-warming at all, 24%-42% with pre-warming in general wards or holding areas and 15.3% with pre-warming in the operating room directly before and during induction of anaesthesia. [5][6][7][8]10 The different hypothermia rates of pre-warmed patients might now be explained by higher amounts of heat loss during longer forced-air warming interruption times. In addition, the presented analysis showed, that forced-air warming interruptions of more than 40 minutes were associated with hypothermia rates of ≥30% that have also been described in patients without pre-warming. 5 warming interruptions, the effort to prevent intraoperative hypothermia through conducting pre-warming can obviously be reduced dramatically.
The physiology of heat exchange and the clear association of short warming interruption times with low intraoperative hypothermia rates suggest that short warming interruption times are important to prevent perioperative hypothermia. The present analysis shows that a warming interruption time shorter than 20 minutes is desirable.

| Limitations of the study
As this investigation was based on a retrospective analysis of anaesthesia records, it is methodologically inferior to prospective randomized studies. However, a prospective investigation with  shown effectiveness were not analysed. [12][13][14] The concept of focusing on the dichotomous outcome value hypothermia (body core temperature <36.0°C) is recommended in the NICE and German and Austrian guidelines. 2,3 Alternatively, it could be discussed to use the combination of temporal duration and low core temperature (area under the curve for body core temperature <36°C). 4 However, as the extent to which the degree of hypothermia experienced by the patients determines the occurrence and extent of complications, has not been evaluated yet, such an analysis was not intended nor possible with the present data. Furthermore, in clinical practice, the same measures will be necessary to avoid intraoperative hypothermia no matter whether hypothermia AUC curves or a dichotomous value (core body temperature <36°C) are the objectives.

| CON CLUS ION
Intraoperative hypothermia rates increased significantly with longer forced-air warming interruptions between pre-warming and intraoperative warming. Short warming interruptions can preserve the effect of pre-warming and are associated with low intraoperative hypothermia rates.