European Journal of Cardiovascular Medicine

Peri-operative hypothermia as defined by the American Society of Peri- Anaesthesia Nurses (ASPAN) is a core temperature below 36 °C and is known to increase the risk of surgical wound infections, poor wound healing, blood loss, prolonged and altered drug effects, increased duration of hospital stay, cardiac events, morbidity, and mortality.[1]

Hypothermia is associated with many detrimental physiological alterations and increased morbidity. These derangements include decreased cardiac output, decreased metabolic rate, metabolic acidosis, and prolonged recovery from anaesthesia, deranged clotting, and an increased incidence of post-operative infections. Postoperative shivering may also lead to increased oxygen consumption, norepinephrine release, and myocardial ischemia.[2]

Pre-operative forced-air warming (i.e., pre-warming) reduces the potential for heat loss that occurs during post-induction redistribution by cutaneous transferring heat to peripheral tissues, thereby decreasing the core-to periphery temperature gradient and in turn reducing the overall incidence of hypothermia. [3,4,5]

Perioperative hypothermia refers to a condition where the core body temperature drops below 36.0oC. This occurs because the control of body temperature is disrupted by the widening of blood vessels caused by anesthetic medicines, as well as exposure to a cold environment and cleansing chemicals. [6] Unintentional hypothermia is frequently observed during the perioperative phase when central neuraxial anesthesia is administered, and its severity can be comparable to that of general anesthesia. [7] Perioperative hypothermia has numerous clinical repercussions, including the activation of the sympathetic nervous system, resulting in shivering, patient discomfort, platelet dysfunction, blood coagulation dysfunction, increased vasoconstriction, heightened risk of surgical site infection, increased urine output, bradycardia, and other effects. [6]

Although preventive hypothermia treatments are widely implemented, perioperative hypothermia continues to be a major contributor to morbidity and unfavorable outcomes in elderly trauma patients. The occurrence of intraoperative hypothermia ranges from 60% to 90%, while the occurrence of perioperative hypothermia ranges from 50% to 90%. [6] The prevalence of femur fractures in elderly adults has increased primarily due to minor trauma on weak bones affected by osteoporosis.

Elderly people experience increased heat loss by radiation and convection due to variations in circulation and tissue mass in the skin, particularly in the amount of adipose tissue that serves as insulation against heat loss. Perfusion, however, can contribute to alterations in both radiative and convective heat dissipation through the skin. Consequently, older individuals are more susceptible to hypothermia during surgical procedures. [8].

There is a lack of research on the prevention of hypothermia during the perioperative period, notably in older patients undergoing broken femur procedures with central neuraxial anesthesia. Further studies are needed to address this gap.

This randomised comparative prospective study was conducted in the Department of Anaesthesiology and Pain Management at Max Super speciality Hospital, Patparganj, New Delhi after approval by the institutional ethics committee.

Study Population:

170 adult persons of either sex, ASA physical status 1-III, admitted in our hospital and scheduled for different surgeries under general anaesthesia who met eligibility criteria were enrolled for the study after their voluntary consent. The patients were randomly assigned using computer generated table of random numbers, either to group A (who were pre-warmed) or to group B (who were not pre-warmed).

Study Design:

This is a randomized comparative prospective study. The randomization was done using the table of random numbers with the aid of website randomization.com.

Inclusion Criteria

1. ASA grade 1-3
2. Age 18-75 years
3. Undergoing general anaesthesia for surgeries of duration of 1-3 hours

Exclusion Criteria

1. ASA grade 4-5
2. Patients with h/o current infection
3. Intake of antipyretics within 24 hours of intake of anaesthesia
4. Hypo or hyperthyroidism
5. Preoperative temperature of more than 37.2 degree Celsius
6. BMI > 35 kg/m2
7. Refusal to give consent for pre-warming

Secondary outcome measures of the study:

1. Amount of blood loss
2. Recovery time from anaesthesia
3. Incidence of adverse cardiac event
4. Incidence of post- operative shivering

Measurements

Pre-operative period:

• Axillary temperature
• BP
• SpO2
• Heart rate

Intraoperative period:

• Nasopharyngeal temperature
• Axillary temperature
• BP
• SpO2
• Heart rate
• ECG

Post-operative period:

• Axillary temperature
• Blood pressure
• SpO2
• Heart rate
• ECG

Our study comprised of two groups:

Group A: In this group forced air warming for 30 minutes was done pre- operatively.

Group B: In this group forced air warming was not done pre-operatively.

FORCED-AIR WARMING

Sample Size

The primary outcome of this study is the incidence of peri-operative hypothermia in pre-warmed group and in non-pre-warmed group.

In a study by Horn et al these have been reported as 13% and 69% respectively. With these values and to be able to detect a difference of minimum of 20% in this incidence, the sample size comes to 85 in each group using the following formula.

η = [Ζ-Ζ)]/8

Z=1.96 corresponding to 5% level of significance

Z= 0.84 corresponding to 80% power

π. = 0.69

π₂ = 0.13

π= π1 + π2/2

8 = difference to be detected (0.20)

SAMPLE TECHNIQUE: By computer generated random numbers with the aid of website www.randomization.com

Study Intervention:

Forced air pre-warming for 30 mins at temperature setting of 45 degree Celsius in patients undergoing general anaesthesia.

Duration Of Study:

The study was carried out over a span of 10 months starting from August 2018 to May 2019.

Methodology:

Primary outcome measure of the study:

1. Incidence of peri-operative hypothermia

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Table: 1. Demographic parameters comparison between groups

Table: 2. Association of the primary and secondary study outcomes between groups

Table 3: Association between Variables and Groups

Table 4: Comparison between Variables and Groups

Table 5: Comparison between Recovery time from anaesthesia and Groups

Table 6: Comparison between Postoperative shivering grade and Groups

Table 1

In Group A (n=85), n (%), the mean Age in years (mean± s.d.) of patients was 45.44±14.38.

In Group B (n=85), n (%), the mean Age in years (mean± s.d.) of patients was 43.72±15.17.

Distribution of mean Age in years with Group was not statistically significant (p=0.42).

In Group A (n=85), n (%), the mean BMI in kg/m² (mean± s.d.) of patients was 24.71±3.09.

In Group B (n=85), n (%), the mean BMI in kg/m² (mean± s.d.) of patients was 24.97±4.21.

Distribution of mean BMI in kg/m² with Group was not statistically significant (p=0.65).

In Group A (n=85), n (%), 39 (45.88) patients were Male and 46 (54.11) patients were Female.

In Group B (n=85), n (%), 36 (42.35) patients were Male and 49 (57.64) patients were Female.

Association of Gender with Group was not statistically significant (p=0.643).

In Group A (n=85), n (%), 6 (7.1) patients had CAD Comorbidities, 19 (22.3) patients had DM Comorbidities, 35 (41.1) patients had Hypertension Comorbidities, 5 (5.9) patients had Obesity Comorbidities and 1 (1.7) patient was Post-chemotherapy Comorbidities.

In Group B (n=85), n (%), 3 (3.5) patients had Asthma Comorbidities, 5 (5.9) patients had CAD Comorbidities, 22 (25.9) patients had DM Comorbidities, 26 (30.6) patients had Hypertension Comorbidities, 5 (5.9) patients had Obesity Comorbidities, 1 (1.7) patient was Post-chemotherapy Comorbidities and 1 (1.7) patient was Post-CABG Comorbidities.

Association of Comorbidities with Group was not statistically significant (p=0.4728).

Table 2

In Group A, the mean Just after induction (mean± s.d.) of patients was 36.03±0.16.

In Group B, the mean Just after induction (mean± s.d.) of patients was 35.95±0.13.

Distribution of mean Just after induction with Group was not statistically significant (p=0.5).

In Group A, the mean At 10 minutes (mean± s.d.) of patients was 36.02±0.14.

In Group B, the mean At 10 minutes (mean± s.d.) of patients was 35.85±0.18.

Distribution of mean At 10 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 20 minutes (mean± s.d.) of patients was 36±0.17.

In Group B, the mean At 20 minutes (mean± s.d.) of patients was 35.83±0.19.

Distribution of mean At 20 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 30 minutes (mean± s.d.) of patients was 36.01±0.17.

In Group B, the mean At 30 minutes (mean± s.d.) of patients was 35.82±0.2.

Distribution of mean At 30 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 40 minutes (mean± s.d.) of patients was 36.02±0.19.

In Group B, the mean At 40 minutes (mean± s.d.) of patients was 35.82±0.23.

Distribution of mean At 40 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 50 minutes (mean± s.d.) of patients was 36.03±0.2.

In Group B, the mean At 50 minutes (mean± s.d.) of patients was 35.81±0.23.

Distribution of mean At 50 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 60 minutes (mean± s.d.) of patients was 36.04±0.21.

In Group B, the mean At 60 minutes (mean± s.d.) of patients was 35.82±0.23.

Distribution of mean At 60 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 70 minutes (mean± s.d.) of patients was 36.03±0.22.

In Group B, the mean At 70 minutes (mean± s.d.) of patients was 35.78±0.24.

Distribution of mean At 70 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 80 minutes (mean± s.d.) of patients was 36.03±0.23.

In Group B, the mean At 80 minutes (mean± s.d.) of patients was 35.77±0.25.

Distribution of mean At 80 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 90 minutes (mean± s.d.) of patients was 36.03±0.24.

In Group B, the mean At 90 minutes (mean± s.d.) of patients was 35.78±0.25.

Distribution of mean At 90 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 100 minutes (mean± s.d.) of patients was 35.98±0.26.

In Group B, the mean At 100 minutes (mean± s.d.) of patients was 35.76±0.27.

Distribution of mean At 100 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 110 minutes (mean± s.d.) of patients was 35.98±0.26.

In Group B, the mean At 110 minutes (mean± s.d.) of patients was 35.75±0.28.

Distribution of mean At 110 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 120 minutes (mean± s.d.) of patients was 35.97±0.26.

In Group B, the mean At 120 minutes (mean± s.d.) of patients was 35.76±0.26.

Distribution of mean At 120 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 130 minutes (mean± s.d.) of patients was 36.06±0.2.

In Group B, the mean At 130 minutes (mean± s.d.) of patients was 35.55±0.09.

Distribution of mean At 130 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 140 minutes (mean± s.d.) of patients was 36.06±0.2.

In Group B, the mean At 140 minutes (mean± s.d.) of patients was 35.55±0.07.

Distribution of mean At 140 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 150 minutes (mean± s.d.) of patients was 36.03±0.23.

In Group B, the mean At 150 minutes (mean± s.d.) of patients was 35.55±0.07.

Distribution of mean At 150 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 160 minutes (mean± s.d.) of patients was 36.02±0.23.

In Group B, the mean At 160 minutes (mean± s.d.) of patients was 35.54±0.05.

Distribution of mean At 160 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 170 minutes (mean± s.d.) of patients was 36.02±0.23.

In Group B, the mean At 170 minutes (mean± s.d.) of patients was 35.54±0.05.

Distribution of mean At 170 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At 180 minutes (mean± s.d.) of patients was 36.05±0.23.

In Group B, the mean At 180 minutes (mean± s.d.) of patients was 35.54±0.05.

Distribution of mean At 180 minutes with Group was statistically significant (p<0.001).

In Group A, the mean At end of surgery (mean± s.d.) of patients was 36.06±0.21.

In Group B, the mean At end of surgery (mean± s.d.) of patients was 35.84±0.23.

Distribution of mean At end of surgery with Group was statistically significant (p<0.001).

Table 3

In Group A (n=85), n (%), the mean Preoperative area (mean± s.d.) of patients was 19.28±0.71.

In Group B (n=85), n (%), the mean Preoperative area (mean± s.d.) of patients was 19.32±0.60.

Distribution of mean Preoperative area with Group was not statistically significant (p=0.73).

In Group A (n=85), n (%), the mean Operation theatre (mean± s.d.) of patients was 19.20±0.74.

In Group B (n=85), n (%), the mean Operation theatre (mean± s.d.) of patients was 19.24±0.78.

Distribution of mean Operation theatre with Group was not statistically significant (p=0.76).

In Group A (n=85), n (%), the mean Postoperative area (mean± s.d.) of patients was 21.47±0.52.

In Group B (n=85), n (%), the mean Postoperative area (mean± s.d.) of patients was 21.47±0.50.

Distribution of mean Postoperative area with Group was not statistically significant (p=0.99).

Table 4

In Group A (n=85), n (%),29(34.1) patients had Hypothermia .

In Group B (n=85), n (%),54(63.5) patients had Hypothermia .

Association of Hypothermia with Group was not statistically significant (p<0.0001).

Table 5

In Group A (n=85), n (%), the mean Recovery time from anaesthesia (mean± s.d.) of patients was 12.64±4.49.

In Group B (n=85), n (%), the mean Recovery time from anaesthesia (mean± s.d.) of patients was 15.55±5.87.

Distribution of mean Recovery time from anaesthesia with Group was statistically significant (p<0.0001).

Table 6

In Group A (n=85), n (%), 64(75.13) patients had shivering grade 0 Postoperative.

In Group B (n=85), n (%), 36(42.4) patients had shivering grade 0 Postoperative.

Association of Postoperative shivering grade with Group was statistically significant (p<0.0001).

In Group A (n=85), n (%), 17(20) patients had shivering grade 1 Postoperative.

In Group B (n=85), n (%), 28(32.9) patients had shivering grade 1 Postoperative.

Association of Postoperative shivering grade with Group was not statistically significant (p=0.05).

In Group A (n=85), n (%),4(4.7) patients had shivering grade 2 Postoperative.

In Group B (n=85), n (%),20(23.5) patients had shivering grade 2 Postoperative.

Association of Postoperative shivering grade with Group was statistically significant (p<0.0001).

In Group B (n=85), n (%),1(1.2) patient was shivering grade 3 Postoperative.

Association of Postoperative shivering grade with Group was not statistically significant (p=0.99).

Intraoperative hypothermia is characterized by a three-phase pattern. The initial rapid decrease in core temperature observed within the first hour of general anesthesia is caused by the impairment of centrally mediated thermoregulatory control induced by the anesthesia. This impairment inhibits thermoregulatory vasoconstriction and primarily causes the redistribution of heat from the core to the periphery. This redistribution is a result of peripheral vasodilation caused by the administration of anesthetic drugs. There is a gradual and consistent drop that occurs over the next 2 or 3 hours. It occurs when there is an imbalance in thermal energy, with heat loss exceeding the body's metabolic heat production. Ultimately, the core temperature enters a phase of stability where it remains constant, even after an extended period of surgery. [9]

Forbes et al conducted A study has suggested the use of forced-air warming systems and raising the room temperature to 22 °C for surgical procedures that are anticipated to last longer than 30 minutes. [10] Multiple studies have indicated that a period ranging from 10 to 60 minutes effectively decreased shivering and significantly prevented hypothermia in healthy adult patients after general anesthesia. [11]

NICE has established that a temperature difference of 0.2 degrees between intervention and control groups is considered clinically significant in hypothermic patients. The recommendation is to utilize the highest setting of the forced air warming unit starting from the commencement of the surgical procedure. [12] For the duration of the surgery, the forced-air warming unit was maintained at a constant temperature of 45 °C. In our investigation, the average difference in core temperature at 40, 60, 70, 80, 100, and 120 minutes was greater than 0.2 °C.

In a similar study by Shin et al,

The patients in the pre-warmed group were heated 30 minutes before to the start of the procedure using a forced-air warming device. The core temperatures of the pre-warmed group were consistently higher than those of the control group starting 20 minutes after the induction and throughout the entire procedure. [13]

Research has indicated that pre-warming patients for a duration of 30 minutes before surgery can effectively decrease the occurrence of low body temperature during the operation. There were notable disparities in the fluctuations of core temperature between the group that did not undergo pre-warming and all the groups that did undergo pre-warming.[14]

At the conclusion of the procedure, the core body temperature in group A was significantly higher (p<0.001) according to our study. Upon admission at the Post-Anesthesia Care Unit (PACU), the axillary temperature was significantly higher in group A compared to other groups (p<0.001). A substantial difference was seen in the prevalence of hypothermia between group A (34.1%) and group B (65.9%) (p<0.001).

During procedures lasting one hour, 57.5% of patients in group B and 16.6% of patients in group A experienced hypothermia, which was statistically significant (p < 0.001). During procedures lasting between 60 to 120 minutes, 65.9% of patients in group B and 27.4% in group A experienced hypothermia, with a statistically significant difference (p<0.001). During procedures lasting 120 to 180 minutes, 83.3% of patients in group B and 14.2% of patients in group A experienced hypothermia, which was statistically significant (p<0.001). Treating intraoperative hypothermia is challenging due to the significant time it takes for heat delivered to the skin surface to reach the core thermal compartment.

Our study found that 24.7% of patients in the pre-warmed group experienced postoperative shivering, while 57.6% of patients in the non-pre-warmed group experienced postoperative shivering. This difference was statistically significant (p < 0.001). 23.5% of patients in the non-pre-warmed group experienced grade 2 shivering, which was statistically significant (p<0.001). Only one patient who was not pre-warmed experienced grade 3 shivering.

Two separate investigations conducted by Park B et al and Lopez MB demonstrated that the occurrence of shivering was notably lower in the groups that underwent pre-warming, as opposed to the group that did not get pre-warming.[15]

Postoperative cutaneous warming reduces thermal discomfort, intensity of shivering, and maximum oxygen use during shivering. However, it does not prevent or alter the duration of shivering. Nevertheless, our study found that preoperative forced-air warming decreased the occurrence of postoperative shivering.

Hypothermia is not simply a source of discomfort for sufferers. Additionally, it induces physiological stress, leading to increased blood pressure, heart rate, and amounts of catecholamines in the plasma. These factors result in a threefold rise in severe heart-related events. The exact cause of the heightened risk of cardiac complications after surgery due to moderate hypothermia remains uncertain. However, the study demonstrated that maintaining normal body temperature during surgery is linked to a lower occurrence of severe heart-related events and rapid heart rate in the ventricles. In a study conducted by Frank et al on patients with a high risk of coronary artery disease, it was found that those who experienced hypothermia had a higher occurrence of cardiac events after surgery. Although the levels of norepinephrine, epinephrine, and cortisol increased in all patients after surgery, the hypothermic group had considerably greater levels of norepinephrine compared to the normothermic group.[16]

The systemic evaluation conducted by Sajid et al found that the existing trials did not provide enough data to evaluate the impact of hypothermia on cardiac dysfunction.[17] In the current investigation, no cases of adverse cardiac events were seen in either the pre-warmed or non-pre-warmed groups.

In our study, the mean intraoperative blood loss in group A was 83.36 ml and group B was 99.36 ml which was not significant. In a study by Tedesco NS et al, The study did not find any impact on perioperative blood loss due to hypothermia. They deduced that the impact of temperature on blood loss can be elucidated by a robust correlation with the variables of operating time and surgical type. [18]

In our study, mean blood loss between pre-warmed and non-pre warmed groups when compared did not give significant results and this could be attributed to the confounding factors of operating time and surgery type.

One additional problem caused by mild hypothermia during surgery is that it leads to a longer recovery time after anesthesia. The average duration of recovery after anesthesia was 12.64 minutes in group A and 15.55 minutes in group B, according to our research. The p-value is less than 0.001, indicating statistical significance. Research has established that mild hypothermia prolongs the effects of and the time needed for natural recovery from vecuronium-induced neuromuscular inhibition. [19]

Our investigation confirms the prior research indicating that peri-operative hypothermia extends the duration of post-anesthesia recovery. Applying heat to patients for 30 minutes at a temperature of 45 °C before to administering general anesthesia decreased the occurrence of hypothermia compared to individuals who did not receive pre-warming. Pre-warming additionally decreased the occurrence of postoperative shivering and shortened the duration of recovery from anesthesia.

According to the observations and findings of this study, the occurrence of peri-operative hypothermia was decreased in patients who underwent a 30-minute pre-warming period before receiving general anesthesia, compared to patients who did not get pre-warming. Pre-warming additionally decreased the occurrence of post-operative shivering and shortened the duration of recovery from anesthesia. Nevertheless, there was no significant difference in the occurrence of negative cardiac events and the amount of blood loss between the two groups. The study was conducted within a limited time frame, and a bigger sample size would provide more definitive results.

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