Anaesthesia, 1991, Volume 46, pages 44-48 APPARATUS

A comparison of patient rewarming devices after cardiac surgery

M. M. IMRIE, M. WARD

AND

G. M. HALL

Summary

Three regimens for rewarming patients after cardiac surgery involving hypothermic cardiopulmonary bypass were studied in 30 patients. The control group (n = 10) received no active rewarming, the oesophageal group (n = 10) was warmed centrally using an oesophageal heat exchanger and the radiant group (n = 10) was warmedperipherally with an overhead radiant heater. There were no statistically SignEficant diFerences between the groups apart from the higher mean skin temperatures in the peripheral group. Key words

Temperature; body. Surgery; cardiovascular. Equipment. A device for noninvasive central rewarming of hypothermic patients by recirculation of warm water through an oesophageal tube has recently become available (The Exacon thermal therapy system) and was effective in experimental hypothermia in dogs,' accidental hypothermia in humans2 and in prevention of peroperative hypothermia.' There have been no studies using this technique after cardiopulmonary bypass procedures; however, peripheral warming using a radiant overhead heater was demonstrated to be effective in this situation."h This study was designed to compare the efficacy of central oesophageal rewarming with peripheral rewarming using a radiant overhead heater (the Aragona mobile thermal ceiling) after cardiac surgery. Methods Thirty patients after routine coronary artery bypass grafting were randomly allocated to one of three groups: a control group, no active rewarming in the postoperative period; an oesophageal group, rewarming using the Exacon TT8200 thermal therapy system. This consists of a disposable double lumen oesophageal tube and a base unit comprising water heater, circulating pump and monitor/ alarm module. Sterile distilled water is circulated through the oesophageal tube at a rate of 3 litres/minute. The temperature of the circulating water is variable and in this study the maximum temperature of 42°C was used. The oesophageal tube was inserted in theatre after heparinisation had been reversed; a radiant group, rewarming using

the Aragona mobile thermal ceiling. The heating surface was mounted approximately 1 m above the patients and the output adjusted initially to maximum but then decreased to maintain skin temperature at 37°C. The patients were treated identically in all other respects. Anaesthesia consisted of premedication with papaveretum, hyoscine and droperidol given 1-2 hours pre-operatively and induction with a sleep dose of thiopentone and fentanyl (50 pg/kg). Neuromuscular blockade was achieved using pancuronium (0.15 mg/kg) and anaesthesia was maintained with 50% nitrous oxide in oxygen; muscle relaxation was not reversed at the end of surgery. Bypass temperatures of 27-28°C were used for all patients. Temperature monitoring probes were attached on arrival in the intensive care unit (ICU) and allowed to stabilise for 15 minutes before the readings started. Intermittent positive pressure ventilation was continued after operation with oxygen-enriched air. Humidification of inspired gases was achieved using the Engstrom Edith heat and moisture exchanger. Postoperative sedation and pain relief were provided using intermittent intravenous boluses of midazolam and papaveretum. Vasodilators (sodium nitroprusside or glyceryl trinitrate) and inotropes (dopamine) were administered by intravenous infusion as required to control systolic blood pressure. All patients were nursed covered to the waist with an aluminised reflecting blanket. The following demographic and clinical data were recorded for all patients: weight, sex, age, current medication, bypass duration, and time from termination of bypass

M.M. Imrie, MB, BS, BSc, FFARCS, Senior Registrar, G.M. Hall, MB, BS, PhD, FI Biol, FFARCS, Professor of Clinical Anaesthesia, The Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London W12 OHS, M. Ward, MB, BS, FCAnaes, Registrar, Edgeware General Hospital, Edgeware, Middlesex HA8 OAD. Accepted 22nd June 1990 0003-2409/9 1 /010044 + 05 %03.00/0

@ 1991 The Association of Anaesthetists of Gt Britain and Ireland 44

45

Comparison of patient rewarming devices to arrival in ICU. Ambient, tympanic membrane and skin temperatures were measured every 15 minutes during the study period. Skin temperature was measured at the following sites: anterior surface of chest above nipple, lateral surface of midpoint of upper arm, anterior surface of midpoint of thigh, lateral surface of midpoint of calf. Ellab (Electrolabatoriet, Copenhagen) thermocouples and recording equipment were used for all temperature measurements and the equipment was calibrated against a National Physical Laboratory mercury in glass thermometer before the study started. Skin probes were attached using adhesive orthopaedic felt to provide insulation from external heat sources. Cardiovascular variables, systolic arterial pressure, diastolic arterial pressure, mean arterial pressure, mean central venous pressure and heart rate were also measured every 15 minutes. Shivering. Shivering and muscle tone were assessed using the method of Joachims~on.~ This method grades both shivering and muscle tone as 0, none; 1, moderate; 2, severe. The two scores were combined to give a shivering score with five levels, W. Drugs. The doses of all sedative or analgesic drugs and the infusion rates of vasodilators and inotropes were recorded. Rewarming in groups 2 and 3 started once the first set of readings had been made on the intensive care unit and continued until the tympanic membrane temperature reached 37°C. Readings continued for 2 hours after rewarming was stopped. Calculations. Mean skin temperature (Tskin)was calculated according to the method of Ramanathan' and mean body temperature (Tbody) derived using; T,,,, = (0.66 x T,,,) (0.34 x TEkin). Total body heat (TBH) was calculated as from this: TBH = Tbody x body weight(kg) x 3.475 kJ where 3.475 kJ/"C. (0.83 Cal/"C.) is the specific heat of the human b ~ d y . ~ , ~ Data analysis. Data were analysed using Statview 512 on the Apple Macintosh. One- and two-way analysis of variance (ANOVA) were used for within-group and betweengroup differences. Sheffes test of significance was used where ANOVA detected a difference.

+

Results

All data are given as mean values (standard deviation). The patient characteristics for the three groups are shown in Table 1. There were no significant differences between Table 1. Patient characteristics. Values are expressed as mean (SW. Group 1 (Control) Number of patients Gender; rnaIe/female Age; years Weight; kg Duration of bypass; minutes Time after bypass; minutes*

Group 2 Group 3 (Oesophageal) (Radiant)

10

10

8/2 57.4 (7.5) 78.8 (13.6)

911 54.6 (7.9) 75.9 (10.4)

10 1010 57 (9.5) 77.1 (13.3)

94.5 (20.2)

98.5 (18.4)

100.5 (22.6)

84.3 (21.9)

78.2 (12.8)

82.1 (12.4)

*Time after bypass is the time from the end of bypass to arrival in the intensive care unit. There were no significant differences between groups.

Table 2. Rewarming data. Group 1 Group 2 Group 3 (Control) (Oesophageal) (Radiant) Number of patients Ambient temperature; "C Rate of rise of Tcore; "Clhour Rate of rise of Tskin; "C/hour Rate of rise of TBH; kJ/hour Total shivering score (arbitrary units)

10

10

25.3 (1.2)

10 25.2 (1.2)

25.2 (0.8)

1.0 (0.3)

1.4 (0.6)

1.1 (0.3)

1.2 (0.2)

1.3 (0.5)

1.9 (0.4)

303 (78)

361 (148)

355 (94)

8.5 (5.7)

10.2 (11.7)

4.1 (4.1)

Data given as; Mean value (standard deviation). No significant differences between groups.

the two groups with regard to any of the patient variables measured before the start of the study. Drugs. The three groups were comparable regarding preoperative and peroperative medication and postoperative requirements for sedative and vasodilator agents. Cardiovascular data. There were no significant differences in arterial pressures, central venous pressure and heart rate between the groups at any time throughout the study period. Rewarming data. The ambient temperatures during the study period are shown in Table 2. The rate of change ot' tympanic membrane temperature during the rewarming period was calculated as; First temperature > 37°C Temperature on arrival on ICU "C/hour time taken for temperature increase The rates of change of mean skin temperature and total body heat were similarly calculated over the same time interval (Table 2). There were no differences in the tympanic membrane temperatures between the groups at any time during the study period (Fig. 1). The slightly more rapid increase in tympanic membrane temperature in the oesophageal group failed to reach statistical significance (Table 2). The mean skin temperature in the radiant group was significantly greater than the oesophageal group at 45 minutes (radiant group 32.1(0.7)"C, oesophageal group 30.9( 1.2)"C. p < 0.05) and the control group at 1 hour 15 minutes (radiant group 33.3(0.7)"C, control 32.1(0.8)"C. p < 0.05). These differences were maintained for the remainder of the study period. There were no differences in total body heat or change of total body heat between the groups at any time during the period of the study (Fig. 1). To examine the postwarming period the temperature data were aligned around the first temperature greater than 37°C and data points for the next 2 hours compared. The tympanic membrane temperatures of all groups continued to increase in the postwarming period and there were no significant differences between the groups (Fig. 2 ) . Similarly, there were no statistical differences between the groups with regard to total body heat and change in total body heat in this period (Fig. 2 ) . The mean skin temperatures in the radiant group were significantly higher than those in the other two groups throughout the postwarming period; p < 0.01 throughout (Fig. 2). Shivering. The radiant group showed less severe shivering than the other two groups; however, there were no significant differences in the shivering scores for the three groups

M.M. Imrie,M . Ward and G.M. Hall

46

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Fig. 1. Tympanic membrane temperature ("C), mean skin temperature ("C), total body heat (kJ) and A total body heat (change of total body heat in the preceding 15 minutes) (kJ) for the whole of the study period. All data are presented as mean values. Control group, closed circles; oesophageal group, open circles; radiant group, open squares. Time 0 is the time of arrival the patient on the intensive care unit. The first recordings were 15 minutes later.

Fig. 2. Tympanic membrane temperature ("C), mean skin temperature ("C), total body heat (kJ) and A total body heat (change of total body heat in the preceding 15 minutes) (kJ) during the post warming' period. The post warming period began with the first tympanic membrane temperature greater than 37°C and continued for 2 hours thereafter. Ail data are presented as mean values. Control group, closed circles; oesophageal group, open circles; radiant group, open squares.

at any time interval (Fig. 3) or in the total shivering scores for the whole study period (Table 2).

heated humidification of the inspired ga~es.'~.'' None of these techniques is totally successful, but if untreated, hypothermia in the postoperative period results in thermoregulatory efforts on the part of the patient that can produce potentially harmful increases in metabolic rate with increased oxygen consumption, carbon dioxide production and peripheral vasoconstriction.5JSHeavy sedation with opioids,lg neuromuscular blockadez0 or active rewarming with radiant heater^^.^.*' have previously been demonstrated to be effective methods of preventing shivering and thus the resulting cardiovascular and meta-

Discussion

Many studies have demonstrated that hypothermia is very difficult to avoid after cardiopulmonary bypass procedures.'*14 Techniques aimed at preventing postoperative hypothermia include prolonged rewarming6 high bypass flows and vasodilator therapy during r e ~ a r m i n g , regula'~ tion of bypass flows to ensure muscle ~xygenation'~ and

Comparison of patient rewarming devices

47

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180 195 210 225 240 255 270

Time (minutes)

Fig. 3. Mean shivering scores during the whole of the study period (see methods for details). Control group, solid bars; oesophageal group,

open bars; radiant group, hatched bars. bolic changes. Sedation and myoneural blockade will both obtund the shivering response to hypothermia, slowing the rate of rewarming but allowing rewarming to occur at a normal metabolic rate. This will inevitably mean a longer period of postoperative ventilation. External heat supply in the form of radiant overhead heaters will suppress thermoregulatory shivering by stimulation of the cutaneous thermoreceptors; therefore, intrinsic heat production by the patient is reduced while heat gain from the environment is increased. The rate of rewarming will depend on the balance of these two effects. The vasodilatation produced by cutaneous heating may also be of advantage in the postoperative patient after cardiopulminary bypass. Radiant heaters, however, are not widely used in the UK, probably because they create an uncomfortable working environment for the nursing and medical staff and cannot easily be used in theatre during surgery. Oesophageal rewarmers were first developed for use in the treatment of accidental hypothermia,2,2zwhere they are claimed to be effective, although no controlled trials have been undertaken. In vitro tests show the calculated rate of heat transfer over the temperature range 19-37°C to be 339 kJ/hour.I This applied to a 70-kg man would result in a rate of increase of mean body temperature of 1.39"C/hour (assuming no losses to the environment). Thus, oesophageal rewarming devices promised to provide rapid postoperative warming without the environmental drawbacks of the radiant heaters. An additional benefit would be the use of the rewarming device in theatre after cessation of bypass to help minimise the afterdrop in body heat. The effect of these devices on shivering has never been studied, but potential problems resulting from their use include thermal injury to the oesophagus, local trauma to the oropharynx and oesophagus and leakage of fluid into the oesophagus (the Exacon device stops circulating water if more than 500ml is lost from the system). The Exacon rewarmer proved to be simple to use and no complications associated with its use occurred during the course of the study. This study set out to investigate the efficacy of the two methods of rewarming and to compare them with normal practice in this hospital, which is for no active rewarming to be employed. The only statistically significant findings in the study were the higher mean skin temperatures produced by radiant heat. It must be noted that the formula used, in

this and several other s t u d i e ~ ~ , ~to. ~derive J' mean skin temperatures has not been validated during application of external heat sources and may overestimate values under these conditions. Shivering was reduced in the radiant group, tympanic membrane temperature increased slightly more rapidly in the oesophageal group and both treatment groups gained total body heat more rapidly than the control group. However, all differences were small and failed to reach statistical significance, but there are several possible reasons why the rewarming devices did not produce a clearer benefit. The ambient temperature in our ICU was relatively high and the patients were not profoundly hypothermic before the start of the trial. Both these factors will tend to minimise the temperature gradients over which the rewarming devices work and thus the heat transfer to the patients. The degree of shivering recorded in our study was low, indeed the control group in this study showed roughly the same degree of shivering as the treatment group in a recent study of radiant heating after cardiopulmonary bypass5 The reason for this is not clear, but may be because of different usage of sedative and vasodilator agents peri-operatively and the high ambient temperature in our ICU. Also, for reasons of modesty on our open plan ICU, the patients in this study were all nursed covered to the waist. This reduces the effective skin area available for radiant heating. We have been unable to find any clear improvement in the rate of rewarming from the use of either method of heating. A large study may demonstrate small differences between the groups, but it is unlikely that this will be clinically significant. The mobile radiant heatcr was cumbersome to use and disliked by the medical and nursing staff, but may be useful if the ambient temperature in the ICU is low or shivering is a clinical problem. The Exacon thermal tube is expensive, invasive and does not suppress shivering and therefore cannot be recommended for routine use after cardiopulmonary bypass procedures.

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References 1.

KRISTENSEN G, GRAVENSEN H, BENVENISTE D, J0KI)ENING H. An oesophageal thermal tube for rewarming in hypothermia

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M.M. Imrie, M . Ward and G.M. Hall

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