Patients undergoing a surgical intervention have the possibility to develop perioperative hypothermia and shivering. The normal human core temperature varies between 36.5°C (Celsius) and 37.5°C (Kiekkas, Poulopoulou, Papahatzi, & Souleles, 2005). Several studies define hypothermia as temperature below 36°C (Smith, Sidhu, Lucas, Mehta, & Pinchak, 2007, Sissler, 1997 and Bitner, Hilde, & Duvendack, 2007). Shivering as defined by (Holtzclaw, 2006, pp. 553-555) is the involuntary shaking of the body as a protection against cold. Intraoperative heat loss can occur to conduction by patients coming into contact with cold surface such as the operating table and convection with the use of cold inhalation gases. Further heat is lost through evaporation due to exposure of large surgical sites and radiation caused by exposure to cold operating room temperatures (DeLaune & Ladner, 2002).
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Thermoregulation in humans is accomplished by a physiological control system made up of central and peripheral thermoreceptors, an integrating control centre and afferent response systems, which take compensatory action. The central control mechanism which is located in the hypothalamus, establishes and regulates the body temperature (Buggy & Crossley, 2000). Anaesthetics including opioids and volatile agents affect the thermoregulation and increase the risk of hypothermia (Sessler D. I., 1993, Sessler D. I., 1997, Kurz, Sessler, & Lenhardt, 1996 and Kiekkas et al , 2005).
Perioperative normothermia can be achieved by using warming devices. These include by either warming and humidifying inhaled gases or warming the body by using heated blankets, forced air blankets and fluid warmers. Increasing room temperature is also important, especially when the patient is exposed during skin preparation and positioning (DeLaune & Ladner, 2002). Forced air warming units draw ambient room air through a filter. Filtered air is then passed through a heating element which warms the air to be delivered through a hose into a dedicated blanket (Smith et al, 2007)
Complications which may arise from hypothermia include shivering, which increases oxygen consumption by 400% – 500%, increase mortality in patients less than 55 years old who are exposed to prolonged hypothermia and decreased production of interleukin 2 (Good, Verble, Secrest, & Norwood, 2006). Other consequences include increased risk for, cardiac events, surgical wound infection, need for postoperative mechanical ventilation and need for blood products.
Focused research question
The research question is formulated on the Population, Intervention, Control maneuver and Outcome (PICO) (Straus, Richardson, Glasziou, & Haynes, 2005). As regarding population the author is aiming for surgical patients irrelevant of their age. The intervention and control maneuver the author is comparing active warming system such as forced air warmers and conventional warming such as cotton blankets. The outcome is to evaluate hypothermia and shivering postoperatively.
Hence the PICO question would be “Is a forced air warmer more effective at decreasing hypothermia and shivering postoperatively as compared to conventional warming for patients undergoing surgery?”
A search was conducted using Ebsco host available through the University of Malta, as well as using internet search engines like ‘Google scholar’. Also a library search and a search through references of references was conducted. CINAHL plus with full text was employed in the search using the word ‘warming’ yielded 1055 articles. Additional filtering resulted in 305 full articles available online. Articles with the subject heading ‘warming techniques’ were selected since the PICO question emphasises the comparison between artificial heating and conventional heating. This further reduced the search to 68 articles. Hypothermia was added and further reduced the search to 41 articles, which were further decreased to 3 articles by applying ‘Intraoperatively’ as a filter from the subject column ‘Major Heading’ criteria. Removing the full text option resulted into 15 relevant articles. The author performed a thorough analysis of the references of the final articles from which other articles were chosen. Articles relevant to the PICO question were saved in ‘my folder’ on the Ebsco website.
Another search was performed on CINAHL using the keyword ‘hypothermia’ which resulted in 615 articles. The search was further reduced to 48 full text articles by selecting ‘warming techniques’. Different subjects from the ‘Major Heading’ were selected including ‘surgical patients’, ‘intraoperative care’ and ‘postoperative care’. The final number of articles from each search was quite low, so the author could review titles of the relevant articles and saved any new articles, which were not in the Ebsco folder. A final search was made for full text articles with the word ‘Shivering’, and from the ‘Major heading’, ‘hypothermia’ was chosen thus resulting into 8 applicable articles.
The Ebsco folder was subdivided into two sub folders, one named totally relevant and the other named relevant to the question. The saved articles were further filtered one by one and from the 38 saved articles, the author discovered 4 clinical trials, 2 reviews, 13 research articles and 1 qualitative study.
On the PubMed, several combinations of Mesh keywords were explored. Keywords used were hypothermia, shivering and perioperative. Unfortunately a Mesh word for hot air blower or something similar was not found. From this search 28 articles were obtained but only 5 were full texts of which 3 were randomized trials. The best search result achieved was by using the PubMed advanced search and combining ‘forced air warming’ and ‘perioperative hypothermia’ which resulted in 51 articles. The results were further filtered for free full text articles and 19 articles were obtained. Filters on PubMed can be saved as well as searches. The 19 full text articles were again filtered for ‘meta-analysis’, qualitative research’, randomized control trial ‘and ‘systemic review”. The result of the final filter produced 1 qualitative research and 14 randomized control trials.
Google Scholar was also used to find articles related to the Pico question. The term words for the search included, ‘forced air warming’, ‘perioperative hypothermia’ and ‘shivering’. This resulted in 288 articles of which some were fully accessible. Articles must be opened one by one to identify their reliability and type.
An online search in the University of Malta Library was made for books with the word ‘perioperative’ in the title. This resulted into 17 books, of which 6 were relevant to perioperative nursing. The Google book search engine was an aid in exploring some of the contents, including the table of contents and extract pages. This lead to selecting 3 books which were available locally from the library, and included topics about hypothermia and warming patients. The author reviewed the named books at the library.
Criteria Used for included studies
For the PICO question chosen the articles needed to include warming devices, hypothermia and shivering. Relevant articles were saved in the Ebsco search folder and the abstract of each article was read for relevance. The articles relevant to the PICO question had to incorporate at least one comparison between an active and passive warming system. Shivering as described by Alfonsi, (2001) and Kiekkas et al, (2005) is mainly attributed to hypothermia, hence keywords, which included hypothermia were adequate for the included studies. Studies with relevant keywords included; Scott & Buckland, (2006), Bennet, Ramachandra, Webster, & Carli, (1994), Stevens, Johnson, & Langdon, (2000), Ng, et al. (2003), Lindwall, Svensson, Soderstrom, & Blomqvist, (1998) and Smith et al. (2007). The articles included one systematic review and five randomised control trials.
Critique of a research article should be a balanced evaluation of both the strengths and weaknesses of a study (Burns & Grove, 2003). For the critical appraisal, a systematic review was chosen due to its advantages like clear methods to limit bias by identifying and rejecting studies and hence conclusions are more reliable and precise (Greenhalgh, 2001). Another positive of a systematic review is that it incorporates a large amount of information from different relevant researches and hence results of different studies can be formally compared for consistency and generalisability.
Another article was chosen from the randomised control trials (RCT), for which the author opted for an eligible RCT which was not included into the references of the selected systematic review. The article chosen included the most amounts of subjects to be studied.
Critical Appraisal of a Systemic Review
The systematic review chosen by Scott & Buckland (2006) is a review of intraoperative warming in order to prevent postoperative complications.
Study focus issue
The paper explores previous studies on patients of various ages undergoing a surgical procedure under general or regional anaesthesia. The intervention was designed to prevent hypothermia or treat hypothermia intraoperatively. The outcome included any adverse consequences in the postoperative period in the post anaesthesia care unit (PACU) such as pain, thermal comfort and cost of treatment. Postoperative complications included, shivering, wound infections, pressure ulcers, cardiac events and the need for blood transfusion.
Systematic review cover
This article is a systematic review of randomized trials. The review tries to identify any evidence regarding hypothermia during surgery in previous researches, and if it has any adverse consequences afterwards. Hence this study focused on the use of warming devices during surgery, and the effect on patient outcomes postoperatively either in the PACU or after transfer from the PACU.
Methods used for the search
The authors of the article searched the Cochrane Wounds Group Specialized Trials Register and the Cochrane Central Register of Controlled Trials. The search included MEDLINE, CINAHL, EMBASE, and the National Research Register. The included articles ranged between January 1948 and May 2003. The systematic search included the keywords anaesthesia, perioperative care, hypothermia, normothermia, warming, thermoregulation and postoperative complications. The authors did not identify any search for grey literature.
Assessment of studies
The authors of the article Scott and Buckland included solely randomized trials. Only studies including the postoperative phase were assessed. Since the review was treatment focused the authors did not give great importance to the grade of complications or to identify any specific outcome. On the other hand, eligible studies were identified through clearly defined inclusion criteria. There was no restriction on language of the articles. Articles were reviewed twice independently and only included studies with human participants of any age undergoing a surgical procedure under general or regional anaesthesia. The studies had to include evaluated interventions aiming to prevent hypothermia during surgery. The duration of temperature monitoring had to extend the intraoperatively time and include the postoperative stage. Articles were excluded if hypothermia was induced, patients were undergoing cardiac surgery or if the was an efficacy study. Meta analysis was performed to identify clinically important similarities including type of surgery, anaesthesia used, blinding process and the effectiveness of the warming therapy used.
Any disagreement between the researchers, whether the articles were eligible to be included or not, were dealt through discussion. The authors of this study never verified uncertainty directly with the research authors and hence this might bias the selection criteria. One article was in German, and although it had an English abstract the authors had some of the article’s findings translated.
Consistency of the studies
All studies selected, which included 2,070 patients, compared standard treatment to at least one method of preventing hypothermia. Out of the twenty six studies, seventeen included a comparison with a forced air warming system. The method of anaesthesia was standardised in most of the studies. Some studies included a small number of participants, hence the reliability of these results is debatable.
Change in outcomes
Improvement was shown on the outcome from the twenty-six RCT chosen. The RCTs included 2,070 patients. When a comparison was made between patients being warmed or not the results were as follows:
Shivering in the PACU (RR 0.26, 95%, CI 0.20 – 0.35, ARR 21%)
Morbid cardiac event (RR 0.34, CI 0.20 – 0.57, ARR 21%)
Blood transfusion need (RR 0.39, CI 0.22 – 0.68, ARR 18%)
Wound infection (RR 0.26, CI 0.12 – 0.58, ARR 13%)
Pressure ulcer (RR 0.54, CI 0.25 -1.17, ARR 4%)
Complications in major surgery (RR 0.37, CI 0.27 – 0.51, ARR 13%)
No statistically significant results were obtained regarding pain because the studied groups were very small. Insignificant results were also obtained regarding costs.
The clinical bottom line
This systematic review points out that by warming the patient and thus preventing hypothermia intraoperatively can prevent serious postoperative complications. The results showed that by avoiding hypothermia, shivering can be prevented. Shivering can increase oxygen demands and can thus cause strain on the cardiovascular system which ultimately may lead to other complications, especially in older adults. Warming the patient reduces the need for blood transfusion, less wound infections and fewer pressure ulcer formations. The authors also included another meta analysis done by Mahoney & Odom (1999) that identified improved outcomes when normothermia was achieved, and hence the cost of warming a patient is much less than treating any adverse consequences.
Critical Appraisal of an RCT
The RCT chosen for the critical appraisal by Ng et al. (2003) compares three different warming interventions for effectiveness regarding normothermia.
Study focus issue
Ng et al. (2003) wanted to explore the most effective therapy between three different interventions in warming patients. The interventions included warming with two cotton blankets, warming with a reflective blanket and one cotton blanket, and warming with a forced air warming blanket. The standard practice before the study was the use of two cotton blankets. The authors sought to evaluate if the other techniques would yield better results in warming patients.
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This study (Ng et al. 2003) was a randomised control trial involving three hundred patients scheduled for a unilateral total knee replacement. The participants were divided into three groups, and they were randomised using the sealed envelope method. The control group was those patients warmed with two cotton blankets whilst the other two groups were the intervention participants. Similarity in the group included the selection of patients using the American Society of Anesthesiologists (ASA) physical status of I or II, although any specific medical conditions were not mentioned. The ambient temperatures of both the operating theatre and the recovery room were kept between 19° C and 22° C by the relevant personnel.
Four nurse researchers were trained on data collection and treatment application as to avoid inaccuracies in the results. On the day of the intervention, one of the researchers picked a sealed envelope which included the type of warming technique to be used and a data collection form. One single type of thermometer was used for accuracy and all patients were brought into the operating room 45 minutes before the commencement of surgery. Patients were all donned in a patient gown and covered with one cotton blanket folded into two.
Ng et al. (2003) gave a very detailed account on the application of the three different interventions. According to the sealed envelope opened the nurse researcher applied or another cotton blanket, or added a reflective blanket or applied the forced air warming blanket. The common practice of the hospital included a warm water circulating blanket underneath. The cleaning solutions and type of drapes were standardised for all patients.
Temperature difference was the most prominent in all the results obtained. The researchers measured the difference in patient temperature from the induction room to the recovery room. Temperature dropped significantly for both the two cotton and the reflective blanket group. The forced air warming group had significantly higher temperatures when measured in the recovery room compared to the reflective blanket group (temperature 0.577° C, 95%, CI 0.427 – 0.726, P<0.001) and the two blanket group (temperature 0.510° C, 95% CI 0.349 - 0.672, P<0.001). In the recovery room, the percentage of patients who reached 36.5° C in one hour was 75% in the forced air warming vice 45% in the two blanket group and 40% in the reflective blanket group.
Shivering in this study, was observed in four patients with two cotton blankets, three patients in the reflective blanket group and one in the forced air warming group. However it was not considered statistically significant. Due to this result, the researchers found that there was no connection between shivering and the warming intervention used.
Discussion on citations chosen
Both articles included, identify the importance of warming patients intraoperatively. Postoperative complications can be prevented if patients are actively warmed. In the systematic review by Scott & Buckland (2006) the authors focused on the outcomes rather than the costs. Although all researches included were RCT, the randomization quality was not described in each of the final twenty six researches eligible for the systematic review. Forced air warming was used in most of the RCT (17 out of 26) which ameliorated the outcome postoperatively. This favoured the choice of the systematic review. Ouellette (1993) in a study comparing four different types of patient warming concluded that forced air warming systems were the most effective compared the other three. Blinding was not described in all chosen RCT’s. The article included citations of all studies selected and includes a table with detailed review of important content relevant to the systematic review.
The RCT written by Ng et al. (2003) compare three different warming techniques. The authors took extreme precautions to eliminate variables, and included a very detailed account of the warming techniques used and selection criteria of patients. The chosen clients all underwent the same operation in the same hospital. This eliminated any inconsistency in the results obtained. The authors used a Bair Huggerâ„¢ warming device which is one of the most used warming devices at Mater Dei Hospital besides the Warmtouchâ„¢. In a study by Perl et al. (2003) which compared four forced air warming devices, highlighted that the Warmtouchâ„¢ device was superior to the Bair Huggerâ„¢ warming device. This does not limit the findings if applied locally since both devices are used, hence whichever system is used the incidence of hypothermia can be avoided.
An aid to maintain normothermia, which is not discussed in these studies is, to pre warm the patient prior surgery. Kiekkas & Karga (2005) and Cooper (2006) in their studies on warming patient preoperatively, concluded that when pre, intra and postoperative warming are offered to the patient the possibility of completely avoiding the risk of developing hypothermia and its associated complications can be achieved. Stevens et al. (2000), Lindwall et al. (1998), Smith et al. (2007) and Bennett et al. (1994) had congruent results with the forced air warming system as to maintain normothermia. This emphasizes the reliability of results obtained from the studies chosen for critique, if applied locally.
To conclude both articles gave evidence that warming a patient intraoperatively will decrease hypothermia postoperatively. Ng et al. (2003) did not find any difference in warming techniques used as regarding shivering. Contrarily, the systematic review performed by Scott & Buckland (2006) demonstrated positive results if patients were actively warmed. Patients who are warmed conventionally are more prone to hypothermia postoperatively. This can lead to undesirable complications and surely can be avoided locally due to the number of forced air warming devices available in each theatre and in the recovery room. Hence it is of utmost importance to introduce locally a protocol which highlights the importance of patient warming.
The clinical bottom line to the PICO question asked would be, “patients undergoing surgery, with the use of a forced air warmer rather than conventional warming, will definitely benefit due to a decrease in hypothermia and shivering postoperatively”.
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