Modern perioperative thermal management aims at the reduction of postoperative morbidity. The key element is the implementation of prewarming in addition to intraoperative warming
Anselm Bräuer MD PhD
Martin Bauer MD PhD
Department of Anesthesiology
Center for Anesthesiology,
Emergency and Intensive Care Medicine,
University Hospital Goettingen,
Germany
Perioperative hypothermia is one of the longest known adverse effects of anaesthesia. It was first described by von Bibra and Harnass(1) in Erlangen, Germany, only one year after the first successful general anaesthesia in 1846. In the 1950s, severe intraoperative hypothermia was described in newborns even during short procedures, and severe hypothermia was associated with perioperative death.(2) In the 1960s and 1970s, thermal blankets and infusion warming became popular to prevent perioperative hypothermia. However, in the 1980s, studies still showed an incidence of postoperative hypothermia of 60-80%.(3,4) As a consequence, the first forced-air warming systems were introduced into clinical practice. To date, these warming devices are recommended for all patients having anaesthesia lasting longer than 30 minutes.(5) However, even with the consequent intraoperative use of forced-air warming systems, perioperative hypothermia is still common.(6–9)
Several prospective, randomised trials have shown that perioperative hypothermia is associated with numerous relevant adverse outcomes, such as increased blood loss and higher transfusion requirements, longer duration of postanaesthetic recovery, morbid cardiac events, surgical site wound infections, longer duration of hospitalisation(10) and even increased mortality in special high-risk patient groups.(11,12) To understand the emerging thermal management concepts, it is necessary to take a closer look at the pathophysiology of perioperative hypothermia.
Pathophysiology
Perioperative hypothermia is the result of a number of contributing factors. Many patients already start to lose heat during their way from the ward to the operating room (OR) due to cold environmental temperatures in the hospital, insufficient insulation of the body and the influence of anxiolytic premedication (usually benzodiazepines).(13–15) This heat loss of the peripheral compartments of the body is aggravated in the OR because patients are only minimally dressed and temperatures are reduced to 18–21°C.(16)
During and after induction of anaesthesia, intravenous as well as volatile anaesthetics reduce the body’s heat production and impair thermoregulatory control. This leads to a redistribution of heat from the warm core of the body to the colder periphery. The magnitude of the redistribution of heat depends on the temperature difference between the core and the periphery of the body.(16) The higher this temperature gradient, the larger the redistribution of heat. Skin preparation and the use of unwarmed intravenous or irrigation fluids further contribute to the development of hypothermia.
Modern perioperative thermal management
Modern perioperative thermal management aims at the prevention of hypothermia during the whole perioperative process to protect the patient from the adverse outcomes associated with hypothermia. A number of studies(17–19) clearly show that the intraoperative use of forced-air warming without prewarming of the patient is often ineffective. Without prewarming of the patient, the redistribution of heat after induction of anaesthesia often causes hypothermia.(20) This is especially true in patients with a low core temperature and a cold periphery before induction of anaesthesia. After induction of anaesthesia, these patients are hypothermic during the first hours of surgery,(21,22) which is the decisive period for surgical wound infections.(23) In this time hypothermia also causes relevant coagulation disturbances and increases blood loss.(24)
Therefore it is necessary to combine consequent intraoperative forced-air warming with prewarming. If large amounts of intravenous fluids and blood products are necessary during the operation, the additional use of a blood and fluid warming system is necessary to maintain normothermia.(25)
Organisational aspects of a modern perioperative thermal management
Organisation of intraoperative warming therapy is easy. The OR has to be equipped with a forced-air warming system and sometimes with an infusion warmer. Organisation of prewarming is more challenging. Typical barriers are:
- Many physicians and nurses are not convinced that it is really necessary to change their daily practice that has been the norm for many years. However, as already pointed out, prewarming is an important part of a modern perioperative thermal management and cannot be substituted by additional intraoperative measures.
- Some physicians and nurses are convinced that patients do not want active prewarming before induction of anaesthesia. However, prewarming increases thermal comfort of patients and only few patients refuse prewarming or stop prewarming because of intolerance to heat (<5%).(26)
- For a number of years it was thought that duration of prewarming must be between at least 30 and 60 minutes.(27) It is true that a prewarming period of 60 minutes is difficult to realise even with a highly effective OR management. However, a new study by Horn et al(28) showed that even 10–20 minutes of prewarming is very efficient. This makes it much easier to realise a prewarming programme. Prewarming does not automatically lead to inefficiency in the perioperative processes. In combination with efficient OR management, prewarming can help to reduce waiting times, avoid underused OR utilisation and can contribute to the economic success of a hospital.
Until now, it was not possible to recommend a single strategy that suits all hospitals. However, there are several possibilities for organising a prewarming programme. Patients can be prewarmed on the ward, in the preoperative holding area, in the induction room or in the delivery room. Some of the possibilities including their specific advantages and disadvantages are described below.
Prewarming on the ward
Active prewarming on the ward can be initiated with a warming mattress placed under the back of the patient two hours before surgery.(29) However, this time schedule is difficult to plan.
Another effective method is active prewarming using a warming gown with forced-air warming.(17) With this method patients can regulate the air temperature of the forced-air warmer themselves. This increases thermal comfort and patient satisfaction and allows safe and effective prewarming even if the time of surgery is not absolutely clear.
Prerequisites
The following prerequisites are required to allow prewarming with one of these two methods on the ward:
- A number of forced-air warmers with warming gowns or power units and warming mattresses must be available on every surgical ward
- There must be sufficient storage room for the devices and the warming gowns and warming mattresses
- There must be reliable organisation of the OR schedule
- Information about the time when the patient will be called into the OR is necessary on the ward
- The patient must be positioned on the OR table with his/her warming gown or warming mattress
- The same warming systems must be used on the ward and in the OR
- Physicians and nurses on the ward must be trained in prewarming as an integral part of the preoperative preparation and as part of their job
- The patient must be informed adequately about the benefits of prewarming
- Patients must receive their warming mattress or warming gown together with the premedication
- Responsibility for, and organisation of, cleaning and transportation of the reusable warming mattresses must be clarified. This is not necessary when single use gowns are used.
Advantages
- Patients are prewarmed as soon as possible
- It is an effective form of prewarming(17,29)
- Prewarming is still possible in the holding area or in the induction room if prewarming on the ward was not possible or has failed.
- The use of the warming gown or warming mattress can be continued in the holding area or during induction of anaesthesia and during the operation. This reduces costs and time for intraoperative warming therapy.
Disadvantages
- The anaesthetist has no control of the prewarming process
- The number of forced-air warming power units and warming gowns or power units and warming mattresses that are required is very high
- Training of all physicians and nurses on the wards is time consuming
- Prewarming means additional work for the staff on the ward. The time required for the initiation of prewarming on the ward is approximately 10 minutes.
- If a warming mattress under the back is used for prewarming, an additional forced-air warming system is still necessary for most surgical procedures. Additionally, knowledge of the intraoperative positioning of the patient is necessary. For example, procedures in the lithotomy position require a shorter warming mattress. Single-use warming gowns give more flexibility.
In the holding area
Prewarming started on the ward can be continued in the holding area. It is also possible to start prewarming there.(28) Because patients normally stay for some time in the holding area, this waiting time can be used for prewarming.
Prerequisites
The following prerequisites are necessary for safe and effective prewarming in the holding area:
- The holding area must be large enough to accommodate several patients during prewarming
- A sufficient number of forced-air warmers with warming gowns or blankets must be available in the holding area
- There must be sufficient storing room for the devices and the warming gowns or blankets
- Sufficient time of at least 10 minutes for prewarming in the holding area must be available
- The patients have to be informed adequately about the benefits of prewarming in the holding area
- Nurses in the holding area must be trained for prewarming as an integral part of the preoperative preparation and as part of their job
- Prewarming must be started immediately after arrival of the patient in the holding area, because sometimes the stay in the holding area is unpredictably short.
Advantages
- Prewarming in the holding area is effective(28)
- If prewarming in the holding area was not possible, or has failed, it is still possible in the induction room
- The anaesthetist can have control of the prewarming process
- Training of the nurses in the holding area requires less time because fewer people require training
- The number of forced-air warmers or power units used is lower because not every surgical ward has to be equipped.
Disadvantages
- Sometimes patients bypass the holding area and arrive in the OR without prewarming
- Prewarming means additional work for the staff of the holding area. The time required for the initiation of prewarming is about 10 minutes.
In the induction room
Prewarming started on the ward or in the holding area can be continued in the induction room, or it can be started there.(19) To allow convenient prewarming, it is sensible to position the patient on an adequate underbody warming blanket that will be used during the operation as soon as he/she enters the operating suite. On arrival in the induction room, the underbody blanket is used for prewarming as one of the first measures using the highest flow and temperature of the forced-air warmer; and the surgical safety checklist is completed (patient has confirmed identity, site, procedure and consent; the operation site is marked, the anaesthesia safety check completed, etc). After ECG monitoring, and pulse oximeter and blood pressure measurements, an adequate intravenous access is established. During these procedures, sometimes more than ten minutes of prewarming has been achieved. If a thoracic epidural catheter is placed for large abdominal or thoracic procedures, the prewarming time is further increased.
Prerequisites
The following prerequisites are necessary for safe and effective prewarming in the induction room:
- An induction room in front of the OR is required
- The induction room must be equipped with a forced-air warming power unit in addition to the forced-air warming power unit in the OR itself. The placement of the power units should enable all routine measures without interference of the prewarming process and vice versa
- The underbody blankets should have several inlets for the hose of the forced-air warming nozzle to enable several set ups
- Sufficient time for prewarming in the induction room is necessary
- The patients have to be informed adequately about the benefits of prewarming either during the preoperative preparation or in the induction room
- The anaesthesia nurses must be trained for prewarming as an integral part of the preoperative preparation and as part of their job.
- Prewarming must be started immediately after arrival of the patient in the induction room, because there is not much time for prewarming.
Advantages
- Prewarming in the induction room is effective and is becoming more so in cases of long induction times, because patients are warmed continuously during the whole induction period
- The anaesthetist has control of the prewarming process
- Training of the anaesthesia nurses requires little time, because the number of people that require training is small. Also, the problem of perioperative hypothermia and the warming equipment are familiar to anaesthesia nurses
- Prewarming in that setting needs approximately one minute of additional time.
Disadvantages
- If prewarming in the induction room is the only prewarming measure and it fails, the patient cannot be prewarmed
- A large number of forced-air warming power units is required
- The underbody forced-air warming blankets are more expensive than normal upper body blankets.
- In the delivery room
- Prewarming before elective caesarean delivery with spinal(30) or epidural anaesthesia(31) is effective and practical. It can be implemented according to the principles described for the prewarming on the ward or in the induction room.
Conclusions
Prewarming in combination with intraoperative warming is an effective and well evaluated method for the prevention of perioperative hypothermia.(32) It is important to implement a prewarming programme on a locally adjusted basis, because no single strategy suits all hospitals and all situations. Prewarming can help reduce postoperative morbidity and increase economical effectiveness, even if the introduction of prewarming necessitates some investments in warming equipment and training of staff.
Declaration of interest
Dr Bräuer has received consulting honoraria from 3M Germany, LMA Germany GmbH in the past.
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