Effects of a Forced-Air Warming System and Warmed Intravenous Fluids on Hemodynamic Parameters, Shivering, and Time to Awakening in Elderly Patients Undergoing Open Cardiac Surgery

ABSTRACT

Background:

As one of the most common postoperative complications, hypothermia is the main cause of numerous problems during treatment, especially in elderly patients with decreased cardiac reserves.

Objectives:

The aim of the present study was to compare the effects of forced-air warming system (FAWS), warmed intravenous fluids (WIVFs) and routine method on hemodynamic parameters, arterial blood gases (ABGs), shivering, and time to awakening in elderly patients undergoing open cardiac surgery (OCS).

Methods:

This clinical randomized controlled trial was conducted on 94 elderly patients who underwent OCS at Ayatollah Rouhani Hospital, Babol, Iran. They were divided into three groups, namely FAWS (n=31), WIVFs (n=31) and routine rewarming method (RRWM, with a blanket) (n=32). The data were then recorded in a checklist. Descriptive and inferential statistics were performed using SPSS 26 at a significance level of less than 0.05.

Results:

The findings demonstrated that the degree of hypothermia had a significant decreasing trend in the groups receiving FAWS and WIVFs (P=0.002). Additionally, increased respiratory rate (P=0.013), higher bicarbonate (HCO3-) levels on arrival up to 4 hours after surgery (P=0.045), reduced lactate level (P=0.005), normal base excess (BE) and accelerated time to awakening (P=0.004) were observed in patients receiving FAWS. There was no significant difference in the study groups regarding shivering.

Conclusion:

The results revealed that FAWS could mitigate the degree of hypothermia, increase body temperature and decrease the postoperative serum lactate levels. Moreover, BE as one of the most important parameters for ABGs, could remain at a normal level. Besides, the use of FAWS could lead to early awakening, and thus facilitate weaning and extubation of these patients.

INTRODUCTION

Cardiovascular disease (CVD) is the leading cause of death worldwide,[1] and more than three-quarters of deaths caused by CVDs occur in middle- and low-income countries. In Iran, the highest death rate is caused by CVD,[2,3] which is currently treated by pharmacotherapy and invasive techniques, for example, percutaneous coronary intervention (PCI) and coronary artery bypass grafting (CABG).[4]

In this line, cardiovascular surgery is the most common therapeutic intervention for patients with coronary artery disease and valvular heart disease.[5,6] Among the age groups, the number of individuals over 75 undergoing such surgery is increasing.[7] Currently, CABG is the most commonly performed procedure with a prevalence rate of 60%, followed by heart valve repair or replacement (15%) and other open cardiac surgery (OCS).[8,9] After OCS, patients are transferred directly to the intensive care unit (ICU), and monitored until they waken from anesthesia and their hemodynamic status stabilizes.[10] In this context, hypothermia is one of the critical postoperative complications.[11] Postoperative hypothermia is related to some sequelae, such as bleeding and disturbed hemodynamic parameters. Hypothermia leads to decreased O2 consumption, cardiac arrhythmias, and huge treatment costs.[6] Cold infusions, inhalation of cold gases, long duration of surgery, large exposure, and excessive bleeding are also among the causes of hypothermia.[12] Other complications include shivering[13] and a physiological response to peripheral vascular contraction.[14] Shivering can increase the risk of complications by increasing the body’s demand for oxygen.[15] Of note, older patients are at higher risk of hypothermia and its complications than other age groups because they lose subcutaneous fat and have a lower metabolism.[14]

Another common problem that occurs after cardiac surgery is the fluctuation of hemodynamic parameters.[16] In this regard, monitoring such parameters by direct or invasive techniques in critically ill patients is an integral part of intensive care nursing.[16] In addition, analysis of arterial blood gases (ABGs) provides more information about oxygenation and acid–base balance.[17] The changes in ABGs can be influenced by body temperature and environment,[18] which requires close monitoring of ABG parameters by nurses.

In this regard, the use of some common sedatives after OCS in ICUs may help to improve mechanical ventilation and stabilize hemodynamic status while lowering metabolic demand,[19] which may influence patients’ time to awakening.[20] Various medical and mechanical methods are now used to maintain normal body temperature and prevent postoperative shivering in operating theatres.[21] Techniques to treat and manage hypothermia include the use of active external heat, moist warm gases, radiant vapors, warmed intravenous fluids (WIVFs), cardiopulmonary bypass, rewarming, and forced-air warming systems (FAWS).[22] In one study, it was found that the most effective method of maintaining normal body temperature was the use of FAWS, warmed fluids, and raising the ambient temperature.[10]

In this light, cardiopulmonary monitoring and rewarming of patients after cardiac surgery are among the most important tasks of nurses working in the OCS department. Although the effects of WIVFs as a standard nursing intervention on hypothermia after some surgeries, such as renal surgery,[13] have been studied, a few studies have simultaneously addressed the effects of the interventions in terms of hypothermia, hemodynamic parameters, ABGs, shivering, and time o awakening.[21-23] Given that the complications resulting from hypothermia have irreparable consequences in elderly patients undergoing OCS, the aim of the present study was to compare the effects of FAWS and WIVFs on hemodynamic parameters, ABGs, shivering, and time to awakening in these patients.

MATERIALS AND METHODS

Study design

This clinical randomized controlled trial (RCT), registered in the Iranian Clinical Trials Registry with code number IRCT20220310054240, was conducted on elderly patients who underwent OCS at Ayatollah Rouhani Hospital, Babol, Iran, over a 6-month period in 2021-22.

Study population

Based on the G*power software package and considering the effect size of 0.9,[6] type-I error of 5% and type-II error of 10%, the sample size was estimated to be 94. These patients were accordingly selected using purposive sampling by block randomization and then divided into one of three groups, namely, FAWS (n = 31), WIVFs (n = 31), and routine rewarming method (RRWM, with a blanket) (n = 32). It should be noted that this study was single-blinded because patients admitted from the operating theatre were not awake on arrival in the ICU and their awakening occurred at least 3 hours after their arrival in the ward, which coincided with the completion of the intervention, so it was not possible to know that they were part of the study groups. Inclusion criteria included: Patients were at least 60 years old, had an esophageal temperature of less than 36°C (based on Saadat Monitoring temperature sensor), and had a cardiac output fraction of more than 40%. Moreover, the use of high-dose inotropic drugs (epinephrine, norepinephrine), antiarrhythmic drugs (lidocaine), drugs affecting hemodynamic status such as esmolol, labetalol, verapamil, procainamide, severe anemia (Hb ≤8), the need for resuscitation and cardiopulmonary resuscitation after surgery, and administration of more than 10 mg of midazolam and 10 mg of morphine sulfate to sedate the patient in the ICU-OH department were considered as exclusion criteria.

Intervention

Interventions were performed on patients immediately after they entered the ICU. In all groups, a typical double-layer blanket of the same brand, measuring 114 × 175 cm, produced by the same brand was used to cover the patients from the armpits to the toes. In the first group (group A), FAWS with a temperature of 38-42°C was used via Covidien Nellcor Warm Touch System (2012) (SOMA TECH INTL-166 HIGHLAND PARK DRIVE-BLOOMFIELD, CT 06002- USA) along with a blanket for patients undergoing OCS for 4-5 hours. In the second group (group B), one liter of WIVFs was infused over 6 hours at a temperature of 37-40°C, together with a blanket if necessary. In the patients receiving RRWM (group C), only a typical blanket was used. Of note, all patients received 4 cc/kg/6 h serum after surgery according to the treatment protocol. In addition, a serum warming device (this device is used to warm sera and other equipment needed in the operating theatres). This device, which uses an intelligent control system to create a constant and even flow in the chamber, was only used with the group with WIVFs, whereas the other groups received fluids without heating. The temperature in the ICU was still maintained between 26 and 30°C. Patients were also given inotropic support and when the infusion rate was increased or decreased during these time points, as well as during significant events such as extubation or other interventions such as drainage, chest drainage, and bedding changes.

In addition, a radial arterial pathway was implanted in all patients by a cardiac anesthetist for monitoring arterial blood pressure (BP) and ABG measurement. Note that direct BP monitoring is usually performed by inserting a catheter into a peripheral artery and then connecting it to a BP transducer and monitor system. Thereafter, ABG parameters (i.e., bicarbonate [HCO₃^-], BE, PaO₂, oxygen saturation [O₂SAT]), electrolytes (namely, sodium–potassium pump [Na, K-ATPase] and lactate levels), hemodynamic parameters (i.e., arterial BP, heart rate, and respiratory rate), shivering criteria based on Crossley and Mahajan grading, and time to awakening were measured and compared across the study groups using the Ramsay Sedation Scale (RSS).

Instruments

The following questionnaires were used for data collection. Demographic data including age, gender, weight, preoperative ejection fraction (EF), and the history of smoking and chronic diseases were obtained from the patients’ medical records. Data on hemodynamic status and shivering, RSS, ICU temperature, and fluids and blood products received were then recorded in a checklist. Arterial BP, heart rate, respiratory rate, temperature using a monitoring device (Saadat model, Iran), and arterial blood O₂SAT were also measured using pulse oximetry connected to the monitor. Temperature was monitored every 15 minutes after entry into the ICU and for up to 6 hours until awakening. Moreover, core body temperature was monitored with a probe in the esophageal region. ABG parameters (i.e., HCO₃-, BE, PaO₂, O₂SAT, and lactate levels) and Na, K-ATPase were then checked at the time of the patient’s arrival in the ICU and hourly for up to 6 hours using an ABG meter (GEM Premier 3000, United States).[24]

The RSS was also used to measure time to awakening and anesthesia. The sedation score based on the RSS was further classified as agitated = 1, calm and alert = 2, responsive to verbal commands = 3, sleepy = 4, confused but responsive to commands = 5, no verbal response, no response to verbal commands, and painful stimuli = 6.[25,26] This instrument was first developed by Rasheed (2019), and its reliability was confirmed with a Cronbach’s alpha of 0.82.[27]

Statistical analysis

Descriptive and inferential statistics were used for data analysis using SPSS 26. The Kolmogorov–Smirnov test and the Shapiro–Wilk test were also applied to check the normality of the variables before the study. The Chi-squared test and analysis of variance (ANOVA) were then used to determine the homogeneity of the three groups in the pre-intervention phase. Repeated-measures ANOVA and analysis of covariance (ANCOVA) were then used to investigate the effects of the above interventions on hemodynamic parameters, shivering, and time to awakening in the patients.

Ethical considerations

The Ethics Committee of Babol University of Medical Sciences, Babol, Iran, approved this research (code no. IR.MUBABOL.HRI.REC.1400.256). All participants also signed the written informed consent form, and their rights were respected (i.e., all data were kept anonymous and confidential).

RESULTS

The mean and standard deviation (SD) of the age of the patients in the present study in the groups receiving FAWS, WIVFs, and RRWM (with a blanket) were 67.81 ± 7.49, 67.16 ± 5.49, and 68.69 ± 7.89, respectively. No statistically significant difference was found in all groups with respect to their demographic characteristics (P > 0.05) [Table 1].

Table 1.

Demographic characteristics of the elderly patients undergoing OCS (n=94)

Demographic characteristics/Groups	FAWS (n=31)		WIVFs (n=31)		RRWM (n=32)		Test results
	Ferquency	Percentage	Ferquency	Percentage	Ferquency	Percentage
Gender
Male	13	41.9	13	41.9	14	43.8	X2 (2) = 0.028
Female	18	58.1	18	58.1	18	56.2	P=0.986
Number of underlying diseases
1	4	12.9	6	19.4	4	12.5	X2 (6) = 4.314
2	6	19.4	10	32.3	8	25	P=0.634
3	10	32.3	10	32.3	9	28.1
More than 3	11	35.5	5	16.1	11	34.4
Type of surgery
CABG	25	80.6	25	80.6	30	93.7	X2 (4) = 5.213 P=0.266
Valve repair	2	6.5	2	6.5	1	3.1
CABG-Valve repair	4	12.9	4	12.9	1	3.1
Age (years old) Mean ± SD	67.81±7.49		67.16±5.49		68.69±7.89		F (2.91) = 0.373 P=0.690
Weight (kg) Mean ± SD	74.13±13.77		7.29±11.69		78.31±9.61		F (2.91) = 2.164 P=0.121
EF Mean±SD	45.97±5.44		47.06±6.07		48.28±6.92		F (2.91) = 1.103 P=0.336
Pumps Mean ± SD	74.13±13.77		67.48±19.46		65.75±25.14		F (2.91) = 2.934 P=0.058

X²=Chi-square, F=ANOVA

The results of the Shapiro–Wilk test confirmed the normality assumption for the data. The results of Mauchly’s sphericity test, determining the sphericity condition of the variance–covariance matrix, also showed no significance at the 0.05 level (P < 0.001). Therefore, the assumption of equality of variances was not met. The range of temperature among the patients was 34.9-36.

The results of the repeated-measures ANOVA results also revealed a significant difference in heart rate measurements in patients receiving FAWS (P = 0.044) and WIVFs (P = 0.021). There was a significant difference in mean respiratory rate between patients, such that it increased more in patients receiving FAWS than in the other groups.

Repeated-measures ANOVA also showed no significant difference in mean systolic BP (SBP) between all groups (P > 0.05), but a significant difference was observed in diastolic BP (DBP) in patients receiving FAWS (P = 0.048). Furthermore, the results indicated a significant difference in mean DBP between the groups at the fourth (P = 0.010) and fifth (P = 0.016) hour [Figure 1].

Figure 1.

Comparing the mean values of DBP in the elderly patients undergoing OHS during seven measurement intervals

The findings of the current study represented no significant difference between the mean pH values of all groups at different time points and even between groups (P = 0.29). Furthermore, the results of the repeated-measures ANOVA revealed a significant difference between the eight time points of HCO_3- measurement in the groups with FAWS, WIVFs, and RRWM (P < 0.001). There was also a significant difference between the mean HCO₃^- values in the intervention groups at the measurement intervals. For example, the mean HCO3 value in the group receiving FAWS was higher on arrival and for the duration of the first to third hour than the other two methods compared to RRWM [Figure 2].

Figure 2.

Comparing of mean values of HCO3- in the elderly patients undergoing OHS during eight measurement intervals

In addition, the results showed a significant difference between the mean values of SPO₂ in the three intervention groups only during the third hour of measurement (P = 0.003), such that the mean SPO₂ in the group receiving WIVFs increased during the third hour compared to RRWM.

Besides, there was a significant difference between the mean values of O₂SAT in the three intervention groups. Thus, the mean O₂SAT in the group receiving RRWM was lower in the second and third hours than in the group receiving FAWS and WIVFs. Furthermore, BE status was normal in the group receiving FAWS during the first, third, and fourth hours, whereas it was considered acidotic in the groups receiving WIVFs and RRWM, during the first, third, and fourth hours. Repeated-measures ANOVA also showed a significant difference during the eight measurement time points of lactate levels in all groups (P < 0.001). The results further demonstrated a significant difference between the mean lactate values in the three groups. This value was also lower in the group receiving FAWS than RRWM throughout the second to fifth hour.

The results of the present study showed that the mean Na level of the patients before the intervention was significantly different in the study groups (P = 0.001). Therefore, repeated-measures ANOVA was used as a covariate related to time before intervention. The results also revealed no significant difference during the seven time points of Na measurement in the groups receiving FAWS (P = 0.57), WIVFs (P = 0.72), and RRWM (P = 0.28). There was no significant difference in mean K levels among three study groups (P > 0.05).

Based on the results of the current study, there was a significant difference between the mean time to awakening in the three groups. This value was also lower in the group receiving FAWS than in the groups receiving WIVFs and RRWM.

Moreover, the results of the ongoing study suggested no significant difference in bleeding rates between the three groups (p = 0.92). However, the variations in bleeding rate during the measurement intervals within each of the three interventions were significant, such that the bleeding rate decreased from the fourth to the sixth hour. The repeated-measures ANOVA results accordingly indicated a significant difference during the seven time points of serum and blood product measurements in the groups with FAWS (P < 0.001), WIVFs (P < 0.001), and RRWM (P < 0.001).

DISCUSSION

Postoperative hypothermia is related to some sequelae, such as bleeding and disturbed hemodynamic parameters, cardiac arrhythmias, and shivering.[13] Considering the hemodynamic parameters, the mean respiratory rate in the group with FAWS was higher than RRWM in the first hour. There was also a significant difference in heart rate measurements between the groups with FAWS and WIVFs. These observations were consistent with the study findings of Baradaranfard et al. (2019) who had reported that the use of active rewarming techniques during laparoscopic surgery could have positive effects on respiratory and heart rates,[27] in contrast to the recent results of Hassani et al. (2018), where the changes in respiratory and heart rates were not significantly different in the two study groups.[28] Hasankhani et al. (2007) pointed out that heart rate was increased during anesthesia in the group receiving normothermia.[29]

The anesthesia group had a higher heart rate than the normothermia group. In addition, the mean temperature was higher in the group with FAWS and WIVFs than in the group with RRWM during the second to fifth hours, but it was higher in the group receiving FAWS than RRWM until the sixth hour, which is consistent with the results of other studies. In this context, Esfandiari et al. (2021) had shown that the mean normal body temperature was higher in the group receiving FAWS than RRWM. It is noteworthy that the study was conducted in the age group of 18-82 years. Considering the impact of age on complications caused by hypothermia,[5] reduced metabolism, and vascular contraction, these small differences in the findings of the present study and those in Esfandiari et al. (2021) were justified. Dixon et al. (2002) had additionally shown that both methods (namely, fully covering the trauma patient with a cap, blanket, and WIVFs) were effective in preventing hypothermia, but that tissue perfusion was better in the group receiving WIVFs than in the group using blankets. The results of the study by Dixon et al. (2002) have also shown that FAWS effectively minimizes neurothermia recovery time in patients undergoing CABG.[30] In contrast to these results, Baradaranfard et al. (2019) have reported that the use of active rewarming methods such as WIVFs and FAWS in laparoscopic surgery has no effect on physiological indicators.[27]

The study results showed no significant difference in mean SBP in all groups, but a significant difference was found in DBP between the three groups. In addition, Dixon et al. (2002) reported a significant relationship between hypothermia during surgery and SBP/DBP.[30] Accordingly, the results reported by Wang et al. (2022) indicated that the use of FAWS for the groups with different levels of warmth (i.e., below 38, between 38 and 42 and above 42°C) had no effect on the occurrence of arrhythmias and hypotension.[32] Thus, the inconsistency in the findings of the recent study may be related to warmed serum temperature and FAWS affecting vasodilation.

As for the results of the ABGs, no significant difference was found in the mean pH values in all groups. Baradaranfard et al. (2019) pointed out that the use of active rewarming methods (i.e., WIVFs and FAWS) had no effect on SPO₂ before, during, and after surgery.[27] In contrast to these results, Mohammadi et al. (2020) found that rewarming could lead to an equilibrium in ABG-related and some physiological parameters.[33] One reason for the difference was related to the type of rewarming method used, the duration and the type of surgery. Among the parameters concerned, patients showed an increasing trend in lactate levels after the interventions were carried out. However, this was in contrast to recent findings by Hasanshiri et al. (2017), in which hyperlactatemia was associated with factors such as increased PH and bicarbonate levels, longer bypass duration and aortic clamping time, and increased use of vasopressors during surgery.[34] Kachoueian et al. (2020) also concluded in their study that profound hypothermia was associated with higher blood lactate levels.[35] Regarding the experimental results, Na levels in the elderly patients undergoing OCS were lower in the groups receiving FAWS and WIVFs compared to RRWM during the first to sixth hour, which is contrary to the results reported by Hassani et al. (2018).[28] Moreover, mean K levels increased in all groups. These results are in contrast to those reported by Hassani et al. (2018), in which no significant difference was found in K changes among the study groups.[28]

The results of the present study suggested no significant difference in the mean levels of shivering between all intervention groups, which is consistent with the findings of Baradaranfard et al. (2019) and contradicted the results of some previous studies. Baradaranfard et al. (2019) also reported that the use of active rewarming methods (including WIVFs and FAWS) during laparoscopic surgery had no effect on shivering.[27] However, Wang et al. (2022) found that rewarming before joint replacement surgery could reduce patient shivering.[32] The reason for this discrepancy was related to the characteristics of the statistical population and the research methods used.

Based on the present study results, the differences in the patients’ time to awakening after the interventions were significant as it decreased during the intervention phase. In this line, Esfandiari et al. (2021) observed that the mean score of the FOUR score test was significantly higher in the group with FAWS than in the control group. Moreover, Lewis et al. (2002) declared that the group receiving FAWS had a faster recovery time after anesthesia than the group with RRWM.[31]

Finally, the study results showed no significant difference in bleeding rate between patients. However, the bleeding rate in patients increased from the time of arrival to the third hour and decreased from the fourth to the sixth hour. These results contradicted those results reported by Bezerra et al. (2021), who confirmed that the bleeding rate was lower in the intervention group than in the control group.[36] In Haddadzadeh et al.’s (2012) study of patients undergoing cardiac surgery, the mean bleeding rate was higher in the hypothermic group than in the normothermic group.[37]

CONCLUSION

The results of the current study suggested that the use of FAWS could have a positive effect on some hemodynamic parameters (including respiratory rate, heart rate, DBP, and HCO₃-) by reducing hypothermia, maintaining lactate, and BE at normal levels and speeding up time to awakening in elderly patients undergoing OCS. The effect of FAWS on time to awakening was also stronger than that of WIVF. Therefore, it is suggested to use FAWS to improve respiratory rate, heart rate, DBP, and HCO3^-, to keep lactate and BE at normal levels and to speed up time to awakening in elderly patients undergoing OCS.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.