a review of cooling patients with severe cerebral insult in icu (part 1)

LITERATURE REVIEW

Nursing in Critical Care 2003 • Vol 8 No 130

A review of cooling patients with severe cerebral insult in ICU (Part 1)Theresa Price and Sarah McGloin

SUMMARY• A review of the literature was undertaken to determine the existence of any evidence to support cooling strategies used in the ICU• The focus of the review was to examine previous findings on the specific problems of cooling patients with severe cerebral insult• Cooling methods that use external physical cooling strategies as well as the effects of antipyretics and vasoactive drugs were examined• Overall, it would appear that conclusions for practice remain unclear• The findings led to the implementation of a pilot study, the findings of which will be discussed in Part 2

Key words: Cooling strategies • Head injury • Severe cerebral insult • Thermoregulation

INTRODUCTIONAn elevated body core temperature (BCT) in the critic-ally ill patient is potentially dangerous. It increases meta-bolic rate, oxygen consumption and carbon dioxideproduction, which if untreated can compromise majororgan functions and tissue repair processes (Manthouset al., 1995). Medical researchers have also indicatedthat increases in the oxygen demand and consumptionthat arise as a consequence of an elevated BCT can bedetrimental to the outcome in those patients who havereceived a severe cerebral insult (Childers et al., 1994;Marion, 1997; Bruder et al., 1998). It is also suggestedthat the outcome, from severe cerebral insult, can beimproved through the use of therapeutic body cooling(Metz et al., 1996; Marion, 1997; Signorini and Alderson,2001; Nolan, 2002). As a consequence, staff in the inten-sive care unit (ICU) must respond quickly to increasesin a patient’s BCT.

Personal experience of practice revealed that ICUnursing staff regularly use a wide variety of cooling strat-egies to reduce BCT in all groups of patients in theircare. It would seem that a ritualistic, traditional approachis perpetuated, which may be owing to a lack of explor-ation and valuable evidence (Walsh and Ford, 1989).

This review will present some of the findings pre-sented in the literature. It is not the intention of the

review to consider the benefits and outcomes of cooling.It is, however, recognized that cooling is a major com-ponent of the head injury protocol in use in manyhead injury units throughout the country. Similarly, itis acknowledged that antipyretics and vasoactivedrugs are important in the management of temperature,but for the purpose of this review, their effects will notbe compared. The aim is to determine an effectivestrategy that can be used to cool patients who havesustained a severe cerebral insult, to baseline normalor to the hypothermic range.

BACKGROUNDPyrexia or fever occurs in response to the presence ofa pyrogen, which stimulates the hypothalamus toincrease ‘set point’. The ‘set point’ refers to a temperaturewithin the range of 36–37 °C that the body functionsat normally (Henker, 1997; Mackowiak, 1998; Marik,2000). The stimulation of the hypothalamus by pyrogenswill initiate physiological mechanisms that willreduce body heat loss through vasoconstriction orincrease heat production by the initiation of shivering.The result is an increase in BCT and set point toa range between 38 and 42°C.

The process of increasing the set point is com-monly associated with infection. It is widely believedthat the ensuing elevated BCT aids in the destructionof bacteria and hence assists in the protection of thebody from the invasion of bacteria. Elevations intemperature may also occur in response to tissue dam-age owing to the release of inflammatory substancesthat stimulate and increase the set point (Whyte et al.,1993; Mackowiak, 1998; Lenhardt et al., 1999; Marik,2000).

Authors: T Price, RN, MSc, BSc (Hons), Cert Ed.F.E. RNT, Senior Lecturer, Critical Care, University of the West of England, Bristol; S McGloin, RN, BSc (Hons), ENB 100, Practice Development Nurse, ITU/HDU Oldchurch Hospital, RomfordAddress for correspondence: T Price, School of Nursing and Midwifery, University of Paisley, Royal Alexandra Hospital, Corsebar Road, Paisley PA2 9PN, UKE-mail: [email protected]

A review of cooling patients

Nursing in Critical Care 2003 • Vol 8 No 1 31

An elevated BCT is not always accompanied byan increase in the set point. Patients, who have hada severe head injury, cerebrovascular haemorrhage orneurosurgery, may develop an elevated BCT. Thismay be because of the disruption to the autonomicnervous system or injury to the hypothalamus, both ofwhich are the key elements in the control of BCT(Holtzclaw, 1992; Whyte et al., 1993). Disruption ordamage to these regions of the brain can result in thebody inappropriately generating heat or inactivatingheat loss mechanisms. BCT will therefore increase, butthe set point usually remains within the normal range(Holtzclaw, 1992; Whyte et al., 1993).

Knowing the cause of the elevation in BCT mayhave important consequences for the method chosento reduce the temperature, as different causal mechan-isms may require different management strategies(Segatore, 1992; Rowsey, 1997; Wong, 2000). Forexample, fever owing to infection may respond toantipyretics, whereas fever, as a consequence of thermo-regulatory failure owing to neural pathway disruption,may not.

The diagnosis of an elevation owing to thermoregu-latory failure may be established in several ways.Firstly, an elevated BCT of patients who have suffereda severe cerebral insult is often noted within the first48h of admission (Young et al., 1988). This is thoughtto be because of the presence of inflammatory mediatorsreleased in response to damage to cerebral tissue(Segatore, 1992). In addition, the diagnosis of disrup-tion to tissue in and around the hypothalamic nervetracts in the brain may be made from computerizedtomography scan (CT scan) or magnetic resonanceimaging (MRI scan). Conversely, patients who developraised BCT in response to a hospital-acquired infectiondo not normally present with a change in temperatureuntil 48h after admission.

ICU patients with elevated BCT, whatever be thecause, will require cooling to reduce the BCT to thenormal range (36–37 °C). However, cooling strat-egies may also be required to induce the therapeuticuse of hypothermia (BCT below the normal range).The evidence of Signorini and Alderson (2001),Bruder et al. (1998) and Marion (1997), suggests thatmoderate hypothermia (33–34°C) can reduce intra-cranial hypertension and subsequently improveoutcome from severe traumatic brain injury. Theevidence for improved outcome, as a result ofinducing hypothermia is considered to be so signifi-cant that it is integrated into a head injury protocol.The protocol, devised in the USA (American BrainTrauma Foundation, 1995) also forms the guidelinefor management in many head injury units in the UK(Kirkpatrick, 1997).

EXTERNAL COOLING METHODS WITHOUT ADDITIONAL DRUG THERAPYAn approach to cooling includes the use of externalstrategies that do not involve the administration ofantipyretics or other pharmacological preparationsthat may influence BCT.

The most important heat exchanger in the humanbody is the skin. Heat loss results from the transfer ofheat from core blood vessels and tissues to peripheralvessels located in the cutaneous tissue. Processes ofconductive and convective heat flow will promoteheat transfer (Plattner et al., 1996). Peripheral vasodila-tion will facilitate the process of heat transfer throughconvection and conduction, therefore promoting theloss of heat from the body to the surrounding environ-ment (Kurz et al., 1996). These processes therefore aidthe body to maintain its BCT at ‘set point’ or 36–37°C.Methods include the use of sponging the skin withcool water, the application of ice packs, fanning theface and/or body using a portable rotary fan and theuse of cooling or hypothermia blankets/mattresses.Personal observation of practice suggests that most ofthese methods are commonly used in the community,the hospital setting, as well as in the ICU environment.

Ice coolingOne early study was identified that explored the use ofice packs, ice water immersion and cold water/air sprays.

A controlled laboratory setting was used by Keil-block et al. (1986) to examine three cooling strategiesthat used a combination of approaches. Methods evalu-ated included whole body immersion in ice water, icepacks applied to large vessels near the surface of skinat neck, axillae and groin and ice packs applied allover the body. Evaporative cooling using water and airsprays and a combination of evaporative cooling withice packs were also evaluated.

Findings suggested that peripheral vasoconstric-tion was counterproductive in the method that usedtotal body immersion in cold water. Ice packs placedon strategic points resulted in little significant changein body temperature. Temperature did reduce whenthe body was covered with ice packs which was a sur-prising result, as vasoconstriction was demonstratedin those areas of the skin in close contact with thepack. However, the researchers suggest that as con-tact with the ice pack is incomplete in that not all skinis in contact with ice, the counterproductive effectsof vasoconstriction are overcome. Nevertheless, themost effective methods, in relation to time taken fortemperature to fall to baseline, were those that usedevaporative cooling, with or without the application



of ice packs. The results of Keilblock et al.’s (1986)study are interesting, but there are a number of limita-tions in applying this information to the hospital setting.

The thermoregulatory responses of fit, young menin a laboratory setting will be very different to thecritically ill patient. The ill patient may have an over-whelming infection or receiving drugs that impairvasomotor tone or have cerebral tissue damage andswelling, causing impaired thermoregulatory responses(Campbell, 1997; Henker, 1997). As a result, thepatients’ ability to control heat transfer will be limited.In addition, if cooling is required over a longer timeperiod than that needed for Keilblock et al.’s studypopulation, shivering will be induced, further increas-ing the patients’ oxygen consumption.

However, despite the suggestion that ice packs maybe effective, coupled with their widespread uselocally, it is interesting to note a dearth of literaturethat evaluates their use in reducing BCT.

FansRotary, mobile, fans blow air, using a variety ofspeeds, over the surface of the body. They are fre-quently requested by conscious patients and appear tobe liberally employed by nurses to make the patientfeel more comfortable. Their efficacy in the general orcritical care population does not appear to have beenevaluated. In addition, their place in the ICU is beingquestioned for other reasons. Fans have been impli-cated in the spread of airborne pathogens, creatingenvironmental contamination and transmitting patho-gens to adjacent patients (Rutala et al., 1983 (cited inMorley, 1997)). Fans can also induce shivering andperipheral skin vasoconstriction (Campbell, 1997).

One study that considered the effects of forced airand water on skin cooling was found.

A randomized and controlled study by Lenhardtet al. (1999) involved nine healthy male volunteerswho had received human recombinant interleukin-2 toinduce fever (38·4 ± 0·5°C). Each volunteer was stud-ied over a 3-day period after having been randomlyallocated one of three strategies. These strategiesincluded: a cotton blanket (the control group), forcedair and circulating water set at 15°C and forced air andwater set at a temperature adjusted by the volunteerfor confront.

It is not clear but assumed that the forced air andwater circulation is contained within a blanket ormattress. Pharmacological preparations that are knownto have an effect on BCT such as antipyretics, anaes-thetics and sedatives were not administered to thevolunteers.

The findings of the study suggested that activeexternal cooling did not reduce the BCT. Vasocon-

striction and shivering were noted, as were increasesin the mean arterial blood pressure and oxygenconsumption. Lenhardt et al. (1999) therefore suggestthat the metabolic rate had increased. Volunteerssubjected to the 15°C forced air and water blanketalso felt very uncomfortable and distressed, leadingto concern about the methods used on patients whomay be unable to express their discomfort. Lenhardtet al. (1999) argue that active cooling without pharma-cological support to prevent vasoconstriction andshivering is inappropriate. They also acknowledgethat their study of volunteers does not replicate theresponses found in a sicker population who may havevery different thermoregulatory responses owing totheir illness and/or the use of pharmacological inter-vention.

Overall, there appears to be a limited exploration orevidence to support external cooling strategies with-out pharmacological support.

PHARMACOLOGICAL METHODS OF COOLINGAntipyreticsAntipyretics such as aspirin, paracetamol and ibu-profen have been widely used to control BCT (Holtzclaw,1992; Arons et al., 1999). The drugs work by blockingthe production of cyclooxygenase, an enzyme that isrequired for the synthesis of prostaglandins. Inflamma-tion and infection will increase the level of pyrogens,which stimulate the production of prostaglandins,which will then elevate the set point. Blocking the pro-duction will induce vasodilation and hence promoteheat transfer from the skin to air (radiant heat loss)and increased diaphoresis (evaporative heat loss)(Rosenthal and Silverstein, 1988; Holtzclaw, 1992).

The search did not reveal any studies that werespecifically related to antipyretic use only, to treatelevated BCT within an ICU setting. However, litera-ture was found that appeared to focus on the use ofantipyretics in children within the community or hos-pital setting (Agbolosu et al., 1997; Aksoyalar et al.,1997; Vauzelle-Kervroedan et al., 1997; Bernarth et al.,2002). These studies were not considered in thereview as the pharmacological management of theadult critical care population may differ to that ofchildren. Relevance and conclusions may be difficultto achieve.

An extensive search did not reveal any studies thatexplored the use of antipyretics alone in the popu-lation who had received a severe cerebral insult.Segatore (1992) had implied that antipyretics mightnot be appropriate for those patients who had devel-oped ‘central fever’ because of the damage to thethermoregulatory structures.



Anaesthetic and sedative drugsIt is well known that other vasoactive drugs widelyused peri-operatively and within the ICU setting havean effect on BCT (Illievich et al., 1996; Kurz et al., 1996).Thermoregulatory control of vasomotor tone can beimpaired by the administration of general anaesthet-ics. The resulting vasodilation will result in rapid heattransfer and a lowered BCT (by 0·5–1·5 °C) in the firsthour of anaesthetic induction. However, anaestheticdrugs will also reduce metabolic heat production byabout 20%, which slows down the rate of heat loss.This process of passive cooling can result in a BCT ofnear 34 °C (Illievich et al., 1996). In practice, anaestheticdrugs such as propofol are used to provide continuoussedation. It would therefore seem reasonable toassume that these drugs have control over thermoregu-lation within the ICU. Similarly BCT can be loweredthrough the pharmacological effects of opioids, suchas morphine and fentanyl, which through their seda-tive effects, reduce or prevent shivering and reducemetabolic heat production (Ault, 2000; Negishi et al.,2000).

External cooling methods combined with a pharmacological approachCombining external cooling methods with the use ofdrugs would appear to be the most frequently studied,and possibly the most used strategy in ICU. Currentcustom and practice on the project site involved theadministration of paracetamol and one or more exter-nal cooling methods.

Drugs and sponging with cool waterPoblete et al. (1997) studied, in the ICU setting, theeffects of evaporative cooling with antipyretics onenergy expenditure. Causes of the elevated tempera-ture of the 20 patients recruited into the study werenot provided, although diagnosis was indicated.Energy expenditure was used as an indicator of meta-bolic activity and was measured using indirect calor-imetry. Twenty patients with an elevated BCT wereallocated one of the three cooling methods. Two drugswere evaluated, propacetamol (paracetamol) andmetamizol. Evaporative cooling using cloths soaked inice-cooled water was applied to the body, with clothsbeing changed every 15–30min.

The findings suggested that energy expenditurewas less in those patients who received external cool-ing. This was a surprising result, as it was expectedthat these patients would shiver, in response to thecold stress. The lack of shivering is attributed to theuse of continuous administration of sedation and opi-oids. The authors conclude that critically ill patientswho are receiving sedation and/or analgesia can be

more effectively cooled using external methods thanthrough the use of antipyretics. Little change to BCTwas observed following administration of antipyretics.

The authors acknowledge within the limitationsthat the methods used were not randomized andhence selection bias may have been introduced.A further limitation includes the pathological prob-lems presented by patients within the study sample.Poblete et al.’s (1997) study group included patientswith neurologic trauma; however, it is unclearwhether the elevated BCT is because of damage tobrain tissue, or as a result of infection or because ofinflammation.

Therefore, while this study may be relevant to theICU population, it is difficult to apply these findingsto those patients who have an elevated temperatureowing to severe cerebral insult and damage to thethermoregulatory structures. A further confoundingvariable includes the duration of the study period,which appears to be a 2-h period. It is not clearwhether shivering may be induced on exposure toexternal cooling for a longer period of time.

Grossman et al. (1995) examined the strategies usedby staff to cool acutely ill patients on medical and sur-gical wards. The intent was not to determine the beststrategy but to examine the decision-making processesused by staff when managing patients’ BCT. Informa-tion was compiled retrospectively from 150 patients’records. It appeared that some patients had receivedtreatment for their fever, which included the adminis-tration of acetaminophen (paracetamol). A smallergroup received the drug and either the removal orthe addition of linens, ice packs, tepid sponging ora hypothermia blanket.

One of the additional findings of this surveyrevealed that the use of the drug in combination withan external cooling method was more effective inreducing BCT than the use of drugs alone. Effective-ness was defined as the quickest time to reduce BCT.An acknowledged limitation of this finding is thatvery few of the sample appeared to receive both para-cetamol and an external cooling method. This wasthought to be owing to a staffing concern that shiver-ing would be induced should external cooling beused. Grossman et al. (1995) also suggest that it wasquicker and easier to administer an antipyretic, butthat there was perhaps a lack of available time for thenurse to administer an external cooling strategy.

However, care needs to be taken when interpretingthe results of this study. Although the questionnairewas subjected to content validity, retrospective analy-sis of data can be misleading. The patients in thissample could have been able to control and reducetheir own BCT by adding or removing bed linen or



drinking cool fluids. An inspection of patient observa-tion charts would not reveal this important confound-ing variable, nor would it accurately depict responsessuch as shivering and vasoconstriction.

An added consideration when considering theapplicability of these findings is that the samplereviewed is not representative of the ICU populationor of those who have sustained a severe cerebralinsult.

The use of combining evaporative cooling and anti-pyretics appears to have limited evidence to endorseits use in practice. The two studies that have beenconsidered have recruited small sample groups,which make it difficult to suggest recommendationsfor practice. In addition, it is not clear what effectsvasoconstriction and shivering are having on BCTwhen the skin is cooled externally. This strategy there-fore warrants wider research examination.

Drugs and cooling mattresses/blanketsA number of studies were identified that evaluatedthe use of cooling blankets and antipyretics; threewere considered relevant to this review.

Cutaneous (skin) cooling can also be achievedthrough the use of cooling blankets and mattresses, orhypothermia blankets. Cooling through conductiveprocesses is achieved through the use of water-filledblankets that are placed either under or over thepatient. Convective cooling employs the use of airflowblankets.

One study focused on cooling the neurologic patient.Morgan (1990) compared the effects of three strategies on21 patients who developed a fever (defined as >101°F).The sample group appeared to include patients whohad neuromedical and surgical problems, but the sever-ity of their illness is not clear. Strategies evaluatedwere the use of acetaminophen (paracetamol) alone,acetaminophen and tepid-water sponging and thehypothermia blanket with acetaminophen. The water-cooled blanket used in the study involves the circula-tion of water through a machine (where it is cooled)and the blanket. The desired BCT is pre-set, and themachine adjusts the temperature of the water accord-ingly. Each strategy was tested on 7 patients. BCT wastaken every 15 min until temperature returned to base-line normal (100 °F).

One of the determinants of effectiveness was therate of temperature fall. Morgan (1990) explains thatsupport will be given to the method that results in thequickest return to normal, as cerebral metabolism willbe increased during periods of elevated BCT. This willhave serious implications for intracranial pressure andcerebral perfusion. Morgan also acknowledges thatantipyretics will be of little value to the patient who

has thermoregulatory disruption, and that externalcooling methods may be of more value.

The most clinically significant finding of this studywas that the hypothermia blanket reduced BCT morerapidly. However, it is acknowledged that thesepatients were unconscious, and it is not clear whetherthey were receiving other pharmacological interven-tions that reduced the incidence of vasoconstrictionand shivering.

Patients who received tepid sponging required alonger time period to reduce BCT, when comparedwith both the antipyretic group alone and the hypo-thermic blanket group. However, none of the spong-ing group appeared to shiver. This is an interestingfinding as the lack of shivering during cool-watersponging in Poblete et al.’s (1997) study was said to beprevented through the use of sedation or anaestheticdrug use. It is not clear whether Morgan’s (1990)group similarly had other forms of thermoregulatoryresponse control.

Again in contrast to Poblete et al. (1997), Morganpoints out that antipyretics did reduce fever and weretherefore a useful adjunct to temperature control.However, their rate of effect in the group who hadthermoregulatory disruption was apparently slow.Morgan further comments that support for any onephysical method in this group of patients was notestablished.

The consequences for practice would suggest thathypothermic blankets are effective, but this may beowing to their combined use with other pharmaco-logical preparations. This finding would support theneed for a further study.

O’Donnell et al. (1997) investigated the role ofwater-cooled hypothermia blankets in a number ofICUs within one regional area. O’Donnell et al. (1997)aimed to examine the decision-making processesincorporated into determining which patient receivedan hypothermic blanket as well as how effective theblankets were in reducing BCT. About 83 patientswith fever (defined as >102·5°C) who had a variety ofpathological conditions including those involving thecentral nervous system were followed for 48h. Hypo-thermia blankets appeared to be used for only thosepatients with very high BCT (104°C). Antipyretics andexternal cooling methods (not involving the hypother-mia blanket) were administered to the remainingpatients over several febrile episodes.

O’Donnell et al. (1997) implies that the hypothermiablanket has no more effective control over elevatedBCT than any other external physical cooling meth-ods. However, there are a number of acknowledgedlimitations with this statement. Firstly, a comparisonof effectiveness between study groups was difficult to



achieve because of the presence of many confoundingvariables. These include the lack of randomization,the use of antipyretics and, thirdly, the nature andelevation of the BCT. In addition, it is not clear whichphysical cooling methods are used or whether patientswere receiving other pharmacological preparationsthat could influence vasomotor tone.

Theard et al. (1997) and Plattner et al. (1996) exploredthe effectiveness of convective cooling blankets toinduce hypothermia in the theatre setting. Both groupsimply that they are safer, induce less complications(e.g. shivering) and were faster in reducing BCT thanwater-cooled mattresses. Creechan et al. (2001) com-pared the effectiveness of airflow and water-cooledblankets on a convenience sample of 41 critically illpatients in three different ICUs. Fever was defined as>38·5°C, and was thought to be caused by thepresence of an infection. Both blankets were set to atemperature of 10°C. In addition to the measurementof effectiveness, nursing staff were asked to completea satisfaction survey to explore their views on the useof each blanket. Antipyretics were not administered,nor were neuromuscular blockers; however, the patientswere sedated.

The findings reveal that shivering was not a signifi-cant problem in either group and that the airflowblanket brought BCT down, more rapidly. However,interestingly, this research group implies that the lackof shivering was owing to wrapping the extremitieswith towels rather than the effects of antipyretics(Morgan, 1990) or the presence of sedation (Pobleteet al., 1997). It was also found (but not understood)that temperature stayed down longer in the airflowgroup, following removal of the blanket.

The satisfaction survey showed that nurses pre-ferred the airflow blankets, perceiving them to reducetemperature more rapidly and easier to use. While thisstudy tentatively suggests that the way forward forcooling critically ill patients in the ICU will eventuallybe through the use of the airflow blanket, there aresome concerns.

A limitation of this research is that the sample size issmall and also not representative of the patient popu-lation who have sustained severe cerebral insult.In addition, the extent of effect from sedation is notclear.

CONCLUSIONOverall, while it would appear that external andpharmacological approaches are the most studiedmethods of cooling, conclusions for practice remainunclear. There appears to be a lack of agreement as towhich method is the most effective, which causes theleast shivering and a lack of exploration on their use

for the critically ill patient or for those who havesustained a severe cerebral insult.

Exploration of the literature also seems to suggestthat little distinction is made between the differentcauses of elevated BCT or that they may require a dif-ferent cooling approach. Furthermore, the commentsthat suggest that antipyretics are of little benefit to thepatient with damage to the thermoregulatory structuresdo not appear to be supported by existing evidence.

Overall the studies reviewed were small andincluded a variety of methodologies. These factors limitthe generalizability of the findings to the patient witha severe cerebral insult.

However, in the light of current support for the useof evidence-based practice, it would seem inappropri-ate to continue to manage care, using methods that arebased on ritual or tradition. Furthermore, the demandfor new equipment and resources to assist in themanagement of care requires an evidenced-basedrationale.

The findings of this review led to the implementationof a pilot study undertaken on one ICU. The studyaimed to determine the most effective cooling strategyfor critically ill, ventilated patients who had sustaineda severe cerebral insult. The findings will be presentedin Part 2.

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