Cutaneous heat loss in children during anaesthesia

Prewarming operating rooms for prevention of intraoperative hypothermia during total knee and hip arthroplasties

Prewarming operating rooms has been shown to limit hypothermia in pediatric surgical patients but may be associated with extreme discomfort for surgeons. We examined the effect of prewarming operating rooms on core temperatures during knee and hip arthroplasties. Sixty-six patients were randomized to the prewarmed group at 24 °C or control group at 17 °C. The prewarmed group core temperature (mean, 36.14 °C) before active warming was significantly higher (P = .018) than that of the control group (mean, 35.83 °C). By the start of surgery, the difference was 36.01 °C prewarmed vs 35.83 °C control, P = .038. There was no significant difference in the last recorded mean temperatures between groups: 36.35°C (prewarmed) vs 36.16 °C (control). A prewarmed operating room for adults undergoing knee or hip arthroplasty had minimal effect on preventing intraoperative hypothermia.

Esophageal, tympanic, rectal, and skin temperatures in children undergoing surgery with general anesthesia

The purpose of this study was to determine the degrees of agreement between various sites of temperature measurement and examine the trend of body temperature in children during surgery under general anaesthesia. Thirty-six consecutive children who underwent surgery with general anaesthesia, had temperatures measured at the oesophagus, skin, ear canal and rectum at baseline, every 15 minutes for the first hour and every 30 minutes thereafter. Spearman correlation and Bland-Altman analyses were used to compare data and trends of mean differences assessed by line graphs. The median age of the sample was 48 months. There were 575 temperature measurements taken. The inter-method correlation coefficients was highest for the oesophageal vs rectal (r = 0.96) temperature and lowest for rectal vs skin (r = -0.11) temperature. The lowest mean difference (95% CI) in temperature at commencement of surgery was between the oesophageal and rectal sites, -0.03°C (-0.08, -0.01) while the highest mean difference (95% CI) temperature was between oesophageal and skin sites, 3.24°C (2.65, 3.85). The trend in differential temperatures between sites remained throughout the duration of surgery. Bland-Altman plots showed that the least difference (bias) at baseline (0.3°C) was between the oesophageal and tympanic temperatures while at 1 hour (0.13°C ) was between the oesophageal and rectal temperatures. The oesophageal site was the closest to rectal for monitoring core temperature while the skin was the least reliable site in the study population. In the situation where oesophageal probe is not routine or functioning, rectal or tympanic temperatures may be used.

Is there thermal benefit from preoperative warming in children?

AIM

We aimed to quantify the impact of a raised preoperative ambient temperature (T(ambient)) on core temperature (T(core)) after induction of anesthesia in children.

BACKGROUND

It has been suggested that prewarming of patients before anesthesia induction reduces postinduction drop in T(core). Neither the prewarming temperature nor its duration is established for adults or children. Nevertheless, it remains common practice to either warm the operating theatre and induction room or employ radiant heaters prior to induction of anesthesia, particularly for infants and neonates. We aimed to quantify the benefit, if any, of this warming practice.

METHODS

We conducted a prospective clinical study to assess T(core) behavior in children randomized to either raised or standard ambient temperature as a prewarming technique prior to induction and until the operation commenced. We have called this 'preoperative' warming. Well, children scheduled for elective surgery where presurgical anesthetic duration exceeded 20 min were randomized to a T(ambient) of either 26 or 21 degrees C. Esophageal temperature was monitored continuously until the operative procedure commenced.

RESULTS

There were 30 children in each group. Those in the warmed group (26 degrees C) had a statistically significant higher initial T(core) (0.4 degrees C warmer) and less drop in their T(core) (0.18 degrees C benefit at 20 min). Although younger/lighter/shorter individuals were more likely to drop their T(core), a warmer T(ambient) had only 0.1 degrees C thermal benefit irrespective of age.

CONCLUSIONS

There are statistically significant thermal advantages to preoperative environmental warming. This study provides data to assist the anesthetist in deciding when these are likely to be clinically relevant.

Wet forced-air warming blankets are ineffective at maintaining normothermia

BACKGROUND

Forced-air warming systems have proven effective in preventing perioperative hypothermia. To date, reported adverse events relate primarily to overheating and thermal injuries. This study uses a simple model to show that forced-air warming blankets become ineffective if they get wet.

METHODS

Temperature sensor probes were inserted into three 1-liter fluid bags. Group C bags served as the control. Groups D (dry) and W (wet) bags were placed on Bair Hugger(R) Model 555 (Arizant Healthcare, Inc., Eden Prairie, MN, USA) pediatric underbody blankets. The warming blanket for Group W bags was subsequently wet with irrigation fluid. Temperature was documented every 5 min. This model was repeated two times for a total of three cycles. Statistical analysis was performed using anova for repeated measures.

RESULTS

Starting temperatures for each model were within a 0.3 degrees C range. Group C demonstrated a steady decline in temperature. Group D maintained and slightly increased in temperature during the observation period, while Group W exhibited a decrease in temperature at a rate similar to Group C. These results were significant at P < 0.005.

CONCLUSIONS

A wet forced-air warming blanket is ineffective at maintaining normothermia. Once wet, the warming blanket resulted in cooling similar to the control group.

Prevention of hypothermia in children under combined epidural and general anesthesia: a comparison between upper- and lower-body warming

BACKGROUND

Children receiving combined epidural and general anesthesia may be at greater risk of hypothermia. Active warming should be undertaken to combat heat loss. With combined epidural and general anesthesia heat loss from the lower body may be greater than from the upper body because of shift of blood towards the vasodilated lower body. We assumed that application of the warming blanket to the lower body might provide better protection against hypothermia. To test this hypothesis, lower-body warming (LBW) was compared with upper-body warming (UBW) in a randomized comparative study.

METHODS

Children subjected to open urologic surgery under combined epidural and general anesthesia were randomly allocated to either UBW n = 38 or LBW n = 35 using a forced-air warming blanket. Core and peripheral skin temperatures were monitored. Temperature gradients between forearm and fingertip during LBW and between leg and toe during UBW were calculated. The warmer was set at 32 degrees C, room temperature was around 22 degrees C and fluids were infused at ambient room temperature.

RESULTS

The changes in core temperature were comparable and parallel in both groups. Core temperature decreased significantly in each group at 1 h after induction compared with basal values. Temperature gradients at forearm-fingertip and at leg-toe were also comparable in both groups. Recovery was uneventful and no patient shivered in the recovery room.

CONCLUSIONS

Lower body warming is as effective as UBW in prevention of hypothermia in children subjected to combined epidural and general anesthesia.

Convection heating in pediatric general surgery - a comparison of warming alternatives in a mannequin study

BACKGROUND

Numerous methods of patient warming are used to prevent intraoperative hypothermia in children. Commercially available forced air warming blankets are effective, but are single-use items. We tested a custom-designed heat dissipation unit (HDU) against one such commercially available blanket.

METHODS

Air temperatures at various points around a mannequin under simulated operating conditions were recorded using thermistors and thermal imaging. The only variable changed was the heating method: a forced air blanket or a customized HDU with two draping techniques - cotton drapes with and without a plastic 'undersheet'.

RESULTS

The three methods produced similar temperature increases and plateaux across the 11 thermistor points measured. There were no significant differences between temperatures at 1 h. A plastic sheet did not appear to enhance the effectiveness of the HDU in this study. Thermal imaging photography suggested more uniform heating of the mannequin with the HDU arrangements.

CONCLUSIONS

The custom-built HDU compares favorably in our mannequin study with a Bair Hugger forced air warming blanket. As it is reusable, it offers considerable potential savings.

What are the most important risk factors for a patient's developing intraoperative hypothermia?

Anesthesiologists attempt to maintain perioperative normothermia for surgical patients. We surveyed clinical anesthesiologists and physician researchers and asked them to prioritize risk factors for a patient to develop intraoperative hypothermia. The questionnaire included 41 factors associated with changes in patient temperature identified during a computerized literature search. We asked respondents to estimate the relative importance of each risk factor on a 10-point scale. The survey was mailed to two groups: 1) 180 anesthesiologists (n = 84 respondents) randomly selected from the 1999 American Society of Anesthesiologists Members Directory and to 2) 24 physician researchers (n = 12 respondents) in thermoregulation. Researchers rated the following to be the most important risk factors for hypothermia (in sequence): neonates, a low ambient operating room temperature, burn injuries, general anesthesia with neuraxial anesthesia, geriatric patients, low temperature of the patient before induction, a thin body type, and large blood loss. The results for the clinician group were similar, because the median differences between the groups' results were two or fewer units for all items. The risk factors identified to be most important can now be further evaluated in clinical trials to develop a multivariate predictive tool for calculating a patient's a priori risk for developing hypothermia.

IMPLICATIONS

Surveys of clinicians and physician researchers identified what they consider to be the most important risk factors for perioperative hypothermia (e.g., neonates, a low ambient operating room temperature, burn patients, and general anesthesia with neuraxial anesthesia).

Surgical materials and wound closure techniques

Recent innovations in surgical materials have helped to enhance wound healing and protect surgical wounds from infection. Materials such as polyglyconate have been evaluated in equine tissues and found efficacious. Other materials, such as poliglecaprone 25, appear to have qualities useful to equine surgery but are untested in equine tissues. Care must be taken to fully evaluate a new surgical material because its usefulness in equine tissues may not match that in human or laboratory animal tissues. Furthermore, use of disposable materials in surgery, although considered ideal for maintenance of sterility, may come under more scrutiny in the future as environmental issues gain prominence. We tend to reserve disposable materials for prolonged procedures and for procedures that carry high risk of failure if infection develops.