EFFECTIVENESS OF HEAT MOISTURE EXCHANGERS (HMES) IN PREVENTING PERIOPERATIVE HYPOTHERMIA AMONG ADULT PATIENTS UNDERGOING ABDOMINAL SURGERY UNDER GENERAL ENDOTRACHEAL ANAESTHESIA

Abstract

Background: Heat Moisture Exchangers (HMEs) conserve heat and moisture during expiration and make this available to inspired gases during subsequent inspiration.

We sought to evaluate the effectiveness of HMEs in the prevention of perioperative hypothermia in patients scheduled for abdominal surgery under general anaesthesia relaxant technique with endotrachael intubation (GART.)

Setting: Lagos University Teaching Hospital, in Modular theatre,

Anaesthesia unit.

Study Design: The study was a randomized, controlled, longitudinal, interventional study

Methods: 100 ASA I, II and III patients aged 18 to 65 years scheduled for abdominal surgery under GART were randomly assigned to 2 groups, groups H and C. Group H had HMEs, while group C served as controls. Core temperature measured using tympanic probe was every 10 minutes till end of anaesthesia

Data from total 99 patients, 49 in group H and 50 in group C were eventually analysed. Although patients in both groups developed hypothermia in the course of anaesthesia, core temperature was significantly lower p< 0.05 after one hour in the control group than the intervention group.

Conclusion: The use of HMEs during general anaesthesia with endotrachael intubation did not prevent hypothermia but resulted in higher core temperature and should be part of a multimodal approach in the prevention of perioperative hypothermia.

Keywords: Heat Moisture Exchangers, General endotracheal anaesthesia, Hypothermia, abdominal surgery

Introduction

During spontaneous breathing, inspired gases are heated and humidified in the nose and pharynx. On reaching the bifurcation of the trachea, the temperature gets up to 37° Centigrade and absolute humidity reaches 44mg/L1. However when natural airways are bypassed during endotrachael intubation, cold and dry gases are delivered to the patients, which need to be heated and humidified2.

Hypothermia is defined as core temperature of less than 36º centigrad3.Hypothermia has many complications namely, increased perioperative blood loss due to impaired blood coagulation pathways, altered drug metabolism, which leads to prolonged drug actions, delayed recovery and morbid cardiac events such as arrhythmias. Others are post operative shivering, prolonged hospital stay and patients’ less satisfaction with surgical and anaesthesia experience4. Lastly, hypothermia results in impaired wound healing and susceptibility to wound infection5. Better postoperative outcomes are achieved when normothermia is maintained perioperatively6.

Commonly used methods of humidifying inspired gases include heated humidifiers (HH), heat and moisture exchangers (HME) and hygroscopic condenser humidifiers. Heated humidifiers provide warm, saturated gas at the artificial airway; but they are bulky, require frequent care giver attention to function well and may be a source of infection secondary to aspiration of water that runs out of the ventilator circuit7.

On the other hand, HMEs function by conserving heat and moisture during expiration and make these available to the incoming dry medical gases during the subsequent inspiration8. Advantages of HMEs include, portability, light weight, simple, passive generation of heat and low cost compared to heated humidifiers. They serve three main purposes, warm and humidify air in the respiratory tract, protect patients from contamination from anaesthetic equipment and from contaminating the anaesthetic breathing circuit9.

Surgery associated with exposed intra- abdominal cavity is more likely to be associated with heat loss than peripheral surgeries.

Presently there is a dearth of literature on the use of HMEs in the prevention of perioperative hypothermia in the West African sub-region. This study therefore sought to evaluate the effectiveness of HMEs in preventing perioperative hypothermia among abdominal surgeries.

Reports

PATIENTS AND METHOD

Study Population: Inclusion criteria was all the patients with American Society of Anesthesiologist (ASA) risk stratification class I to III within the age of 18 and 65 scheduled for elective abdominal surgery under general anaesthesia relaxant technique. The patients with pulmonary disease, hypothyroidism and febrile illnesses were excluded.

Randomization schedule: The patients were randomly assigned using opaque sealed envelopes by a blinded observer into two groups, groups H and C. Group H had Heat Moisture Exchanger, while Group C was the control group .

Ethical Approval: Approval was obtained from the ethical committee of the Lagos University Teaching Hospital, and informed consent was obtained from the patients who were eligible.

Hypothesis: The Null hypothesis was that there is no difference in perioperative temperature in patients who use Heat and moisture exchanger compared to patients who do not use HME, during abdominal surgery. The alternative hypothesis was that patients who use HME during abdominal surgery will have better perioperative temperature profile.

Sample size: A prehoc power analysis indicated that a sample size of 100 patients with 10% attrition was needed to determine a difference between the two groups.

Intervention:

Fasting guidelines were observed for all patients and were placed on oral diazepam 5mg at 2200hours the night before surgery. The trend of temperature was monitored on the ward using the nursing temperature chart and febrile patients were excluded.

On arrival at the theatre, a multi-parameter monitor Cardio cap 5 GE Healthcare Finland Oy, Helsinki REF 60510000164-01 was attached for baseline vital signs namely blood pressure, pulse rate, oxygen saturation and electrocardiography.

Preoxygenation with 100% oxygen with a tight fitting face mask was carried out for three minutes. This was followed by induction with a 2mg/kg propofol and suxamethonium 2mg/kg to facilitate tracheal intubation. Correct placement of tube was confirmed before the tube was secured. Group H patients had HME (Hygrovent Medisize 0000165877, Vertrieb: B+P Beatmungs-Produkte GmbH) immediately attached between the endotracheal tube and the Y connection tubing. Mechanical ventilation was done with Datex Ohmeda Aespire machine.

Ventilation was volume controlled and time cycled. The respiratory rate, tidal volume, FiO2 and PEEP was adjusted to maintain SpO2 at 98-100%. Anaesthesia was maintained with Isoflurane (1-2%) and oxygen at 6l/min . Muscle relaxation was achieved with Pancuronium 0.1mg/kg , and one third of initial dose was given for supplementation as required. Fluids were warmed to 38 ^o C in AMSCO warming cabinet model M70WCE with thermostatically controlled thermometer. Blood was also warmed in warm water to body temperature before transfusion. The theatre environment was maintained at 22 to 24°C by controlling the room cooling unit. Patient’s temperature was measured with Braun thermoscan infrared ear thermometer, model 6220, Kronberg, Germany. Temperature was taken within 1minute of arrival at the operating room, and every 10 minutes thereafter up till end of anaesthesia.

At the end of surgery residual muscle paralysis was reversed with intravenous Atropine 0.02mg/kg and Neostigmine 0.04mg/kg. When the patient was able to obey commands and had adequate tidal volume, the trachea was extubated. Primary outcome variables included the temperature profile of both groups and incidence of hypothermia.

Data Analysis: Continuous variables were reported as mean value with variability expressed as +/- standard deviation. The descriptive variables were reported as proportions. The core temperatures were compared by student t test. Data was analysed with SPSS 17.0. Demographic data was compared using Chi squared test. T- test was done for parametric variables and comparison of means. The Chi square test was done where applicable. P value less than 0.05 was considered statistically significant.

Results

One hundred patients were recruited for this study but one patient was withdrawn from the Heat and moisture (HME) group, for unplanned admission to the intensive care unit (ICU) post operation for ventilation due to delayed recovery. There were 49 patients in the HME group (Group H) and 50 patients in the control group (Group C).

Table 1 illustrates the socio-demographic characteristics of the patients namely Age, Weight, Height, Body mass index (BMI) and American Association of Anesthesiologists Risk stratification of the patients per group. The patients’ mean age was 40.04±8.9 in the HME group (Group H), and 40.28±9.6 in the control group (p=0.900). The Body mass index of the patients in Group H was 28.49±5.80 Kg/m², and 30.61±5.7 kg/m² in Group H.

Table 1: Patients’ Demographics

	HME (n=49)Mean ±SD	Control (n=50)Mean ±SD	p-value
Age(Years)	40.04±8.94	40.28±9.63	0.900
Weight (Kg)	72.69±10.4	78.76±10.0	0.004*
Height (m)	1.61±0.10	1.61±0.09	0.807
BMI (Kg/m)	28.49±5.80	30.61±5.7	0.070
ASA (I/II/III)	22/22/5	22/23/5
*statistically significant p <0.05

Indication for surgery is as illustrated in fig I. , The commonest indication for surgery was symptomatic uterine fibroid (42 patients) the least common was cholecystitis and renal stone.

Table II shows the mean systolic blood pressure was 148.98±4.22 mmHg in Group H and was 144.78±2.88 mmHg in the control group.

Table 2: Baseline Parameters

Haemodynamic parameters	HME (n=49)Mean ±SD	Control (n=50)Mean ±SD	p-value
SBP (mmHg)	148.96±4.22	144.78±2.88	0.001*
DBP(mmHg)	98.96±4.22	94.78±2.88	0.001*
PR	84.07±3.08	83.64±2.79	0.469
*statistically significant.

Intra-operative values viz. duration of anaesthesia, duration of surgery, volume of fluid given are represented in table III. The duration of anaesthesia and volume of infused fluid was similar in both groups.

Table 3: Intraoperative parameters

	HME (n=49)Mean ±SD	Control (n=50)Mean ±SD	p-value
Duration of anaesthesia (hours)	2.59±0.788	2.85±0.69	0.298
Duration of surgery (hours)	2.56±0.50	2.53±0.64	0.815
Volume of fluid infused (ml)	2997.96±658.44	2720±605.41	0.031*
Volume of blood transfused (ml)	802±247.39	1145.92±858.28	0.008*
*statistically significant

Table IV shows the mean temperature difference between the 2 groups at regular intervals. At one hour, the mean temperature dropped to 36.09±0.95ºC in group H, while it fell to 35.87±0.82ºC in the control group, which was statistically significant p value 0.004. The highest temperature drop was within the first hour.

Table IV: Mean difference in temperature between the two groups at regular intervals

Time (Minutes)	Temperature HME (n=49)Mean±SD	Temperature Control (n=50)Mean±SD	P-value
0	36.59±0.48	36.58±0.52	0.107
10	36.44±0.72	36.43±0.54	0.004*
20	36.20±0.84	36.13±0.69	0.103
30	36.19±0.89	36.01±0.72	0.001*
40	36.20±0.95	35.99±0.82	0.001*
50	36.17±0.95	35.89±0.82	0.001*
60	36.09±0.95	35.87±0.82	0.001*
70	36.04±0.95	35.79±0.82	0.001*
80	36.01±0.95	35.69±0.82	0.001*
90	35.66±0.86	36.59±0.95	0.001*
100	35.59±0.95	35.46 ±0.86	0.001*
110	35.55±0.99	35.49±1.07	0.053
120	35.51±1.88	35.44±0.98	0.824

Figure 1 .

Indication for surgery and distribution of male and female in each group.

Discussion

This prospective randomised controlled study, evaluated the effectiveness of heat and moisture exchanger (HMEs) in the prevention of perioperative hypothermia among adults undergoing abdominal surgery under general endotrachael anaesthesia. The study demonstrated that, although HME has minimal effect in preventing perioperative hypothermia, the temperature in patients who had HME was significantly higher than those without HME. At present, the methods used to prevent perioperative hypothermia in our centre are, warming of intravenous fluids, fluids used for lavage and controlling room temperature. Considering the dire consequences of perioperative hypothermia, it is imperative to employ a multimodal approach which will include the use of HME.

One of the earliest researchers that studied the effect of HME was Weeks10 who studied 28 patients requiring endotracheal anaesthesia for at least one hour. Mean airway temperature of inspired gases increased by 3.0 +/- 0.5ºC above ambient temperatures present with the use of HME. Libo 11 and workers observed significant difference in temperature fall between patients who had HME and those who did not, his study conclude that the use of HMEs increased the temperature of inspired gases. Our findings are in agreement with these past studies.

Johanson12 et al evaluated the use of HMEs in the prevention of hypothermia at different fresh gas flows. They observed that at low flow, there was no much difference in temperature between patients with HMEs and those without. It would therefore appear that the effectiveness of HMEs in the prevention of temperature drop during anaesthesia and surgery depends also on the flow of fresh gas. In our study, we utilized a uniform high flow for all our patients, thus the impact of FGF on observed temperature difference is negligible.

Furthermore, the nature of surgery plays a significant role in the development of hypothermia. This index study was carried out on patients undergoing abdominal surgery. Similarly, Goldberg13 et al studied 51 patients undergoing lower abdominal surgery to evaluate the effect of heated humidifiers and HMEs on perioperative hypothermia. They found that the use of HH and HMEs provided little benefit against perioperative hypothermia.

In the index study, there was a significant difference in the baseline systolic and diastolic blood pressures. However, this may unlikely to influence the difference in temperature observed in both groups. Kasai13 and colleagues while evaluating the influence of preoperative blood pressure and intraoperative hypothermia in patients undergoing lower abdominal surgery found that core temperature was higher in patients with high systolic blood pressure (>140 mmHg) from 15 minutes after induction of anaesthesia to 2 hours after the induction of anaesthesia. The authors opined that, suppression of distribution of heat from the core to the peripheral compartment was prevented by a higher level of vasoconstriction among patients with a high systolic blood pressure.

The results from this study showed there was a fall in core temperature from baseline in the first hour but was more in control group. The first hour of anaesthesia and surgery is usually when the temperature gradient between patient and room temperature is greatest. Before surgical draping, patients are exposed maximally, while the operation site is prepared by unwarmed solutions. Libo11 and colleagues noticed in their study that the core temperature fell to the lowest observed point at 1 hour after the induction of anaesthesia. Although their patients had craniotomy, while the patients in the incumbent study had exploratory laparotomy. The effect of induction agents on the incidence of hypothermia was evaluated by Ikeda14 et al. They demonstrated that propofol caused significantly lower core temperature than inhalational agents when used for induction of anaesthesia. The use of propofol for induction of anaesthesia in the index study may be a contributory factor to the hypothermia observed.

The use of HME alone for the prevention of hypothermia is not sufficient since less than ten percent of metabolic heat production is lost through respiration. Approximately 65 to 85% of respiratory heat loss is insensible and results from high latent heat of vaporization15. During surgery heat is mainly lost through evaporation of water from wet tissue exposed to air16. If the area of wound is increased, the increase of energy loss exceeds what can be compensated for by any increase in inspiratory humidity17. Therefore, Heat Moisture Exchanger may not prevent hypothermia during abdominal surgeries, but further studies on non-abdominal surgeries may prove its efficacy in preventing hypothermia. A multimodal approach with the incorporation of HME to prevention of hypothermia is therefore advised under general anaesthesia with endotrachael intubation, although its advantage is minimal during abdominal surgeries; however a little gain is better than none at all.

Conclusions

The results and findings of this study indicate that heat and moisture exchangers (HMEs) have little benefits in preventing perioperative hypothermia during abdominal. Although both groups of patients in this study had hypothermia, patients who had heat and moisture exchangers tended to have a significantly higher temperature than those without heat and moisture exchanger, a multimodal approach is hereby proposed for the prevention of hypothermia in this group of patients.