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Three Part Question

In [patients presenting with mild hypothermia in the pre-hospital environment] are [Mediwrap blankets superior to other passive warming blankets] in the [correction of low core body temperature and improving the patients perception of warmth and comfort?]

Clinical Scenario

A 35 year old female known to suffer with severe depression and suicidal tendencies goes missing from her home address. Search teams are deployed and following a 3 day search the missing person is found alive but very cold and in a remote area of woodland inaccessible to other emergency services. Further medical support is requested to aid with rewarming the MISPER (missing person) and evacuate her to hospital.

Search Strategy

OVID MEDLINE 1946 – January Week 3 2014
Exp hypothermia/ OR hypothermia.mp
AND
Exp Rewarming/ OR exp "Bedding and Linens"/ OR passive warming blanket.mp.
AND
Exp emergency medical services/ OR prehospital.mp
Limit to human and English language
EBSCOHOST CINAHL
(MH "Hypothermia") OR "hypothermia"
AND
(MH "Bedding and Linens") OR "passive warming blanket" OR (MH "Warming Techniques")
AND
(MH “Prehospital Care”) OR “prehospital”
OVID Nursing Database
exp Hypothermia/ OR hypothermia.mp.
AND
exp "Bedding and Linens"/ OR passive warming blanket.mp. OR exp Warming Techniques/
AND
prehospital.mp OR exp Emergency Medical Services/ OR exp Prehospital Care/
Cochrane Database
“Passive warming blanket”
GOOGLE SCHOLAR
“Mediwrap” “passive warming blanket”

Search Outcome

Ovid Medline = 49 of which 1 were relevant
EBSCOHOST CINAHL = 33 of which 0 were relevant
OVID Nursing Database = 24 of which 2 were relevant
Cochrane Database – no additional papers
Google Scholar = 2 extra relevant paper

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Henriksson et al 2009 Sweden	Thermal manikin inside a climatic chamber using various ensembles of blanket	Level 2 Review of individual passive warming ensembles in controlled environment	Evaluate thermal insulation properties in different wind conditions of 12 different blankets and rescue bags used in prehospital care	Thermal insulation properties vary between blankets at different wind speeds.	Study done on a manikin and not humans. Does not take into account the human aspects of hypothermia, nor does it assess comfort and ease of use.
			Evaluate thermal insulation properties in different wind conditions of 12 different blankets and rescue bags used in prehospital care	Mediwrap blankets performed best in low insulation group at low wind speed but were surpassed by the RC20 rescue blanket and bubble wrap at higher wind speeds
Rathinam et al 2009 UK	30 patients undergoing major thoracic surgery between November 2005 and September 2006 at Birmingham Heartlands Hospital randomly allocated to either Mediwrap (16) or forced air warming (14)	Level 2 Individual RCT in a single centre	Compare the effectiveness of Mediwrap and the current standard of forced air warming in the maintenance of normothermia in thoracic surgery	Mediwrap group attained core baseline temperature sooner than forced air warming group post-operatively	1) Does not cover pre-hospital care setting although does compare methodologies 2) compared 2 different warming strategies and not just 2 different devices 3) small numbers in study although power calculations performed and deemed significant
				Core temperatures comparable at baseline and end of surgery
Cohen et al 2002 USA	313 patients arriving to the Trauma Resuscitation Unit at Broward General Medical Centre. Patients were required to have a documented history of trauma and a minimum arrival body temperature of 35 degrees Celsius	Level 2 Quasi experimental design with prospective randomisation of patients to 1 of 3 groups. Warmed cotton blankets, reflective blanket or forced air warming blanket	Compare the effectiveness of different warming blankets and the nurses attitudes and perceptions of each method	1) Reflective blankets were most effective atmaintaining normothermia and/or increasing body temperature mean temperature change +0.66, Warmed cotton blanket +0.46, forced warm air blanket +0.49. Nurses perceived the cotton blankets to be the most user friendly with reflective blankets coming second	1) Study was based in Emergency Department and not prehospital. 2) The study did not appear to consider the patients perception of each method. 3) Despite large study population unsure of the cohorts’ power. 4)Does not specify type of reflective blanket used nor does it identify if any specific wrapping methodology was used or indeed if it was the same between groups
Alex et al 2013 Sweden	48 people in the intervention of which 9 were excluded. 24 agreed to the study with a further 4 being withdrawn due to cognitive dysfunction	Level 3 Qualitative study using interviews about patients’ perception of treatment and comfort.	Understand patients’ perception of hypothermia and the discomfort encountered.	Patients’ perception of warming was that it took too long with blankets alone. Patients described the heat pad as warming and comforting which provided a positive feeling. Patients reported that blankets helped protect against the cold but more heat was needed.	1)Long time period between incident and conducting interviews. 2)Type of blankets not identified. 3)Fails to compare types of passive warming blanket instead by referring to them all as blankets could suggest they are all the same.
			Patients preference towards passive or active warming	Receiving warmth seems to induce greater comfort
Thomassen, O et al 2011 Norway	8 fit and healthy male volunteer participants Power calculations showed minimum of 6 required. 1 participant withdrew part way through	Level 3 – non blinded, randomised cohort comparative study	Outcome measures 1)mean skin temperature 2)core temperature 3)metabolic heat production 4) participants perception of thermal comfort, thermal sensation and degree of shivering Done by comparing methods	Hibler Method (vapour tight layer and insulating layer) is significantly better at managing hypothermia than bubble wrap (least effective), authors suggest may be due to the insulating layer. Hibler method is also the preferred wrapping method according to the participants, with bubble wrap and ambulance blankets leaving them feeling colder and experiencing more shivering.	1)Participants were all fit and healthy – not necessarily representative of hypothermia sufferers 2)No blinding may have affected the qualitative data although authors suggest this is unlikely 3)Small cohort although power studies suggest this number to be significant 4) Not actually done in pre-hospital environment although authors attempted to reproduce similar temperatures and other weather factors.

Comment(s)

Warming casualties and/or preventing worsening of hypothermia in the pre-hospital environment can be very challenging especially during periods of adverse weather. There are a number of factors to be considered when choosing a method for warming a casualty or attempting to prevent worsening of hypothermia symptoms. • Weight – this is particularly important in Search & Rescue where you may be a long way from somewhere with vehicular access and may be carrying great amounts of equipment for extrication of a casualty through adverse terrain • Size – In addition to extra weight it can be very challenging to transport bulky items • Environmental influences – terrain, weather and environment can often mean that different choices may need to be made in the most appropriate methods. For the authors of the above work these factors did not seem to be of consideration to them. However, in the implementation phase all of these things need to be considered along with cost. The papers I have appraised do vary in design and outcome measurement. I feel this is good as it enables the review of not just the quantitative data of how good the methods are but starts to assess the patients’ perspective along with the clinician’s perspective of the effectiveness of a given method. A number of the studies have tried to compare passive warming methods with active warming methods which have in the pre-hospital setting shown that active warming is better. This is mainly due to the compressive nature of wind reducing the planned level of efficacy of the blanket. Where active warming is not possible the studies above suggest that a passive warming blanket that is insulating, vapour tight and reflective would afford the best outcomes for the patient as defined in the Hibler Method. The Mediwrap blanket is constructed around the Hibler method principle. All the studies have relatively low sample sizes and whilst in some of the studies power calculations suggest the small sample is all that is required larger studies would be required to further solidify the findings. One of studies did not use people in the study instead favouring a thermal manikin. This may have produced anomalous results, although this is unlikely. At present there is very little research in the pre-hospital setting and most of the current guidelines have been derived from in-hospital settings and adapted to try to fit pre-hospital care. This is a trend that is changing but as yet seems not to have encompassed hypothermia and the rewarming of cold casualties.

Clinical Bottom Line

There is as yet not enough evidence to firmly state that one passive warming blanket is significantly better than another. However research does clearly show that a vapour tight, reflective and insulating blanket will afford the best results. Further research into the warming of mildly hypothermic casualties in the pre-hospital environment needs to be done. At this time Mediwrap blankets would be a cost effective and practical solution in the UK. However consideration should be given to the adversity of the environment these patients are in.

Level of Evidence

Level 2 - Studies considered were neither 1 or 3.

References

1. Henriksson, O et al 'Protection against cold in pre-hospital care - thermal insulation properties of blankets and rescue bags in different wind conditions.' 2009
2. Rathinam, S et al 'A randomised controlled trial comparing Mediwrap® heat retention and forced air warming for maintaining normothermia in thoracic surgery.'
3. Cohen, S et al 'Thermal efficiency of prewarmed cotton, reflective, and forced-warm-air inflatable blankets in trauma patients.' 2002
4. Alex, J et al 'Being cold when injured in a cold environment - patients' experiences.'
5. Thomassen, O et al 'Comparison of three different pre-hospital wrapping methods for preventing hypothermia - a crossover study in humans.' 2011