Thermal energy balance and body temperature: comparison between isolated ultrafiltration and haemodialysis at different dialysate temperatures

Jeroen M. G. Keijman, Frank M. van der Sande, Jeroen P. Kooman and Karel M. L. Leunissen

Department of Internal Medicine and Nephrology, University Hospital Maastricht, Maastricht, The Netherlands

Correspondence and offprint requests to: F. M. van der Sande, MD, Department of Internal Medicine and Nephrology, University Hospital Maastricht, P. Debeyelaan 25, PO Box 5800, 6202 AZ Maastricht, The Netherlands.

Abstract

Background. Haemodynamic stability is better maintained during isolated ultrafiltration (i-UF) than during combined ultrafiltration/haemodialysis (UF+HD). This difference might be explained by differences in thermal energy balances. In this study we compared the thermal energy balance of i-UF with UF+HD at different dialysate temperatures (Td) and determined the Td at which the thermal energy balance during UF+HD is similar to the thermal energy balance during i-UF.

Methods. In the first part of the study, 10 chronic haemodialysis patients were compared during three different treatment sessions, i-UF, UF+HD at Td of 35.5°C and UF+HD at Td of 37.5°C. The second part of the study consisted of one session of 1 h of UF+HD (UF+HD ET-set) with a pre-set energy transfer (ET) at the same level of ET found for that particular patient during i-UF in the first part of the study.

Results. First part of the study: body temperature (BT) decreased significantly during i-UF (-0.25±0.25°C, P<0.05) and UF+HD 35.5°C (-0.24±0.18°C, P<0.05) and increased significantly during UF+HD 37.5°C (+0.18±0.19°C, P<0.05). The differences between the change in BT during UF+HD 37.5°C compared with the other treatments were significant (P<0.05). ET gave a significantly more negative value during i-UF (-30.8±3.1 W, P<0.05) than during UF+HD 35.5°C (-23.6±4.1 W, P<0.05). A slightly positive ET was found during UF+HD 37.5°C (+0.4±4.7 W, P=not significant). Second part of the study: there was a slight, but not significant, decrease in BT during UF+HD ET-set (-0.17±0.26°C). The changes in BT did not differ significantly between i-UF and UF+HD ET-set. After 1 h of UF+HD ET-set, the mean Td was 34.75°C (34.0–36.0°C). The correlation between pre-dialysis BT and Td during UF+HD ET-set was significant (r=0.764, P<0.05).

Conclusion. ET gives a more negative value during i-UF than during UF+HD 35.5°C and than during UF+HD 37.5°C. To obtain the same thermal ET during UF+HD as that achieved during i-UF, a mean Td of 34.75°C is needed, depending on the pre-dialytic BT of the patient. The results of this study may be of relevance in relation to future clinical investigations which can elucidate whether differences in vascular response between i-UF and UF+HD are only related to differences in thermal balance.

Keywords: dialysate temperature; energy transfer; haemodialysis; ultrafiltration

Introduction

Haemodynamic instability is a frequently occurring problem during a dialysis procedure. A decline in blood volume is the initial aetiologic factor which is related to an imbalance between ultrafiltration and blood volume preservation. However, impaired reactivity of resistance and capacitance vessels in reaction to the blood volume decline also plays a major role [1,2].

During standard temperature dialysis (37–38°C) body temperature (BT) increases despite a net energy loss to the extracorporeal system [3,4]. This suggests an effect of the dialysis treatment on the regulation of body temperature [57]. An increase in body temperature causes vasodilatation and might thus interfere with the normal vascular response to a decrease in blood volume [8].

Since the early 1980s the effects of different dialysate temperatures on haemodynamic stability have been investigated and cool dialysis (35–36°C) has been shown to improve haemodynamic stability [912]. We, and others, found that cool dialysis has a beneficial effect not only on peripheral vascular resistance but also on venous reactivity [2]. Moreover, cardiac contractility appears to be increased during cool dialysis [13]. Isolated ultrafiltration (i-UF) has been shown to improve vascular reactivity even more as compared to cool haemodialysis (35.5°C) [2]. This might, among others, be due to differences in energy transfer (ET) between the two treatment modalities.

New devices enable us to study the extracorporeal energy transfer (ET) rather than looking at dialysate temperature (Td), and give us the opportunity to compare thermal energy effects of i-UF with combined ultrafiltration/haemodialysis (UF+HD) using different Td. Although ET has been studied in haemodialysis with different Td, no studies have been done measuring ET during i-UF [3,14].

Therefore in this study we compared the thermal effects of i-UF, UF+HD at a Td of 35.5°C (UF+HD 35.5°C) and UF+HD at a Td of 37.5°C (UF+HD 37.5°C). Secondly, we determined which Td is needed to obtain the same extracorporeal energy transfer as compared to i-UF.

Patients and methods

Patients
Ten patients (five women, five men) were selected from the chronic haemodialysis population in our university hospital. They had an average age of 66.4 years (range 49–79) and an average time on renal replacement therapy of 25.9 months (range 7–65). The diagnosis of renal disease was nephrosclerosis (five patients), diabetes mellitus (two patients), hypertension (one patient), nephrolithiasis (one patient) and polycystic kidney disease (one patient).

During each haemodialysis (HD) session blood flow (Qb) was 300 ml/min, dialysate flow (Qd) was 500 ml/min. Dialysate composition was individualized: bicarbonate (28–36 mmol/l), sodium (136–140 mmol/l), calcium (1.50–1.75 mmol/l), acetate 3 mmol/l, potassium 2 mmol/l, magnesium 0.5 mmol/l, chloride 108 mmol/l. In this study hemophane membranes (GFS 16; Gambro, Lund, Sweden) were used in nine patients and polyamide membrane (Polyflux 14S; Gambro, Lund, Sweden) in one patient. Optimal dry-weight was estimated by echography of the inferior caval vein (15). Room temperature was kept constant between 22–23°C by climate control.

Methods
The Fresenius® 4008H Dialysis Monitor was used during all measurements. The Fresenius® Blood Temperature Monitor (BTM) was used to measure venous (Tv) and arterial (Ta) blood temperatures. By measuring Tv and Ta ET can be calculated according to the following formula:

With c being the specific heat capacity of blood (3.64 kJ/(kg/°C)), {rho} the blood density (1052 kg/m3), Qb the blood flow rate, Tv the temperature of the venous blood and Ta the temperature of the arterial blood. This shows that ET can be controlled by controlling Tv, which in turn can be controlled by changing the Td. The BTM has control over the Td and its software offers a feedback control mechanism over the ET [17]. A negative ET means that energy is withdrawn from the patient (cooling), a positive ET means that energy is transferred to the patient (heating).

The body temperature (BT) of the patient was measured with an ear thermometer (Genius First Temp Model 3000A, Sherwood Medical, Northern Ireland) which correlates well with intra-arterial measured temperature (r2=0.999, P=0.0001) [18]. The first part of study consisted of three measurement sessions (i-UF, UF+HD 35.5°C and UF+HD 37.5°C) per patient in 10 patients and were done during three different dialysis sessions in a randomized order. Each patient served as his or her own control and measurements were done on the same day of the week for each patient, thus eliminating as much bias as possible.

In the first part of the study we determined the ET, Tv and BT during i-UF, UF+HD 35.5°C and UF+HD 37.5°C. All other parameters were kept constant. Pilot studies showed that the ET reached a plateau within 15 min, so all measurements were done during the first hour, after which the Td was reset to the patients regular treatment temperature.

The second part of the study consisted of one session of 1 h with a pre-set ET (UF+HD ET-set) at the same level of ET found for that particular patient during i-UF in the first part of the study. During these measurements we determined the ET and Tv by using the BTM and BT by using the ear thermometer. The Td was assessed using the display of the Fresenius monitor (accuracy±0.5°C). All other parameters were the same as in the first part of the study. The treatment session would be terminated when BT dropped more than 1.0°C, Td went below 34.0°C or the patient experienced severe cold or shivers.

BT and Td were recorded manually every 30 min. All other parameters were registered and recorded continuously via the data-acquisition possibilities of the Fresenius monitor, using a lap-top PC and recorded manually every 30 min.

Statistical analysis
Parameters assessed during the different treatments were analysed with a paired Student's t-test. Pearson's was used for assessing the correlation between two variables. Data are given in mean±SD. A P value <0.05 was considered statistical significant. Calculations were done using SPSS for Windows, release 7.5.

Results

Baseline values
In Table 1Go the baseline values of the four treatment sessions are given. There were no significant differences in body and venous temperatures. The mean ultrafiltration rate during i-UF, UF+HD 35.5°C, UF+HD 37.5°C and UF+HD ET-set were respectively, 726±75 ml/h, 685±128 ml/h, 786±108 ml/h and 684±260 ml/h (not significant).


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Table 1. Baseline values
 
First part of the study
Changes in body temperature ({Delta}BT).
The BT decreased significantly during i-UF (-0.25±0.26°C, P<0.05) and UF+HD 35.5°C (-0.24±0.18°C, P<0.05) and increased significantly during UF+HD 37.5°C (+0.18±0.19°C, P<0.05) (Table 2Go). The differences between {Delta}BT during UF+HD 37.5°C compared with the other treatments are significant (P<0.05). Differences between other combinations of treatment sessions are not significant.


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Table 2. Changes in temperature parameters and energy transfer
 
Changes in venous temperature ({Delta}Tv).
The Tv decreased significantly during i-UF (-2.02±0.22°C, P<0.05) and UF+HD 35.5°C (-1.53±0.19°C, P<0.05). The increase in Tv found during UF+HD 37.5°C was not significant (Table 2Go). The differences in {Delta}Tv between all possible combinations of treatment sessions are all significant (P<0.05).

Changes in energy transfer ({Delta}ET).
A significant negative ET was measured during i-UF (-30.8±3.1 W; P<0.05) and UF+HD 35.5°C (-23.6±4.1 W; P<0.05). A slightly positive ET was found during UF+HD 37.5°C (+0.4±4.7 W) which is not significant (Table 2Go). The differences in ET between all combinations of treatments are again significant (P<0.05).

Second part of the study
Thermal effects.
The negative ET during 1 h of UF+HD ET-set (-32.7±4.9 W) was indeed comparable with the negative ET of i-UF (-30.8±3.1 W). In order to realize this negative ET during UF+HD ET-set, the Td had to be decreased to a mean Td of 34.75°C (34.0–36.0°C). This resulted in a significant decrease of the Tv (-2.20±0.37°C, P<0.05), and a decrease of BT (-0.17±0.26°C) which was not significant. Changes in BT, Tv and ET do not differ significantly between i-UF and UF+HD ET-set. We found a significant correlation between pre-dialysis BT and Td during UF+HD ET-set (r=0.764, P<0.05). None of the treatment sessions had to be terminated.

Discussion

In earlier studies we found a difference in vascular reactivity between i-UF and UF+HD at cool dialysate temperatures [2]. We wondered whether these differences could be related to quantitative differences in thermal balance, we compared in this study the thermal effects of i-UF and cool dialysis (35.5°C) using standard dialysis (37.5°C) as control.

We found that the ET from the extracorporeal system to the patient gave a significantly more negative value during i-UF than during UF+HD 35.5°C. Comparing the ET found during cool dialysis in our study to those described in other studies there were some differences. In the study by Provenzano [14] the ET found with a Td of 34°C is not as low as would be expected when extrapolating our results to a Td of 34°C. This difference during low temperature dialysis sessions between the two centres could be explained by the fact that the Qb is only 190 ml/min in their study compared to 300 ml/min in our study, thus giving a less negative ET according to formula (1). Another explanation might be the use of acetate in their study, which might induce larger heat loss from the body to the environment because of excessive peripheral vasodilatation, provoking more energy influx from the extracorporeal circuit to the patient [14]. However, our results are in agreement with the study of Schneditz [4] who also used a mean pre-set ET of -30.2±3.7 W, which led to a mean Td of 35.3°C. The differences are small and might be caused by the slightly lower Qb (250 ml/min) or a difference in BT at the start of treatment. The conditions in the study of van der Sande from our group are comparable to the present study resulting in the same changes of ET [3].

In our study BT decreased significantly during i-UF and cool dialysis without significant differences between the two treatments. These data are in accordance with previous studies from our group [2,3].

When we compared cool dialysis with the standard dialysis treatment (UF+HD 37.5°C) we found that the ET from the extracorporeal system to the patient gave a significant more negative value during cool dialysis, while the ET during the standard treatment was only 0.4 W. These data are almost in agreement with the data from Schneditz who found a net energy loss of -13.4 W, although in their study the dialysate temperature was set at 37.3°C [4]. The higher value of ET at the higher dialysate temperature in the study of Provenzano could only be explained by, as previously mentioned, a difference in pre-dialysis body temperature [14].

However, despite the fact that ET was only slightly positive during UF+HD 37.5°C there was a significant increase in BT. This significant increase in BT despite a very minor ET from the extracorporeal system to the patient suggests an effect of dialysis on BT, which is in accordance with other studies [24]. It has been suggested that the increase in BT might be caused by a BT increasing factor derived from contact with a bioincompatible membrane or `unpure' dialysate. It has also been hypothesized that a BT decreasing factor might be removed during dialysis [57]. Another contributing factor might be a reduced loss of thermal energy through the skin, caused by increased vasoconstriction as suggested by Gotch, although in earlier studies from our group only minor changes in arterial and venous reactivity during dialysis at a temperature of 37.5°C were observed [2,1921].

In the second part of this study we assessed which Td is needed to obtain the same ET during UF+HD as we found during isolated ultrafiltration. The mean Td at which the ET is comparable is 34.75°C and varied from 34 to 36°C. During this session none of the patients complaint of side-effects. The Td needed to obtain the same ET compared with i-UF was strongly depending on the pre-dialytic body temperature. From this study we conclude that energy transfer from the extracorporeal system to the patient gives a significant more negative value during i-UF than during cool dialysis at a dialysis temperature of 35.5°C. To achieve the same thermal energy balance a dialysate temperature between 34°C and 36°C is needed depending on the pre-dialytic body temperature. The results of this study may be of relevance for further clinical investigations answering the questions whether differences in vascular response between i-UF and UF+HD are only related to differences in thermal energy balances or whether they imply more complex regulatory mechanisms.

Acknowledgments

The authors would like to thank J. Burema, G. Cleeren, P. Hameleers and A. Kerkhofs for their support in operating the equipment and P. Claessens for his technical support.

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Received for publication: 18. 1.99
Accepted in revised form: 20. 5.99