Metabolic acidaemia is a common complication in intensive care patients and can have a negative influence on outcome . Low tidal volume ventilation has been shown to improve mortality in patients who suffer from acute respiratory distress syndrome (ARDS). This led to the concept of low tidal volume ventilation with permissive hypercapnia [2,3], but respiratory acidaemia can exert a negative influence on haemodynamic function . Thus, early correction of respiratory acidaemia despite the low tidal volume ventilation may be beneficial . The use of sodium bicarbonate as a buffer is adversely discussed in lung protective ventilation, as CO2 is eliminated via the lungs. The buffer tris-hydroxymethyl aminomethane (THAM) is excreted via the kidneys. THAM can be used as a buffer in case of metabolic, respiratory or mixed acidaemia, but known adverse effects of THAM are hypoglycaemia and a decrease in mean arterial pressure (MAP) [4–7]. Furthermore, critically ill patients often require volume support, but possible interactions of volume support with the infusion of THAM, especially the control of blood glucose, have not been investigated yet. In a former study, we had observed that pigs had higher blood glucose values during prolonged anaesthesia if volume support was done with 6% hydroxyethyl starch 130 kDa/0.4 (HES130) compared with 4% polysuccinated gelatin (GEL). After 8 h of anaesthesia, HES130 pigs had a median blood glucose of 5.6 mmol l−1 (100 mg dl−1) vs. 4.3 mmol l−1 (77 mg dl−1) in GEL pigs (n = 6 in each group) . Thus, in the present study, we investigated whether a volume support with a low molecular weight starch-based colloid (HES130) may be beneficial when the risk of hypoglycaemia is increased by prolonged anaesthesia, mixed acidaemia and THAM infusion.
The study was conducted in male pigs (German Landrace × Large White, median body weight of 41 kg), which were anaesthetized and randomized to HES130 or GEL. Mixed acidaemia was induced in each group (n = 6 each, see Fig. 1).
The ‘University Animal Care Committee’ and the federal authorities for animal research in Berlin, Germany, approved the experimental protocol. The principles of laboratory animal care were followed according to the guidelines of the European and German society of laboratory animal sciences. The experiments were performed at the Department of Experimental Medicine (certified 9001: 2000 by International Organization for Standardization).
Survey of the experimental protocol
After the induction of anaesthesia and instrumentation, the measurements for ‘baseline’ were performed and a bolus infusion of the colloid was done and a mixed acidaemia was induced (Fig. 1).
Afterwards, measurements ‘after bolus’ were taken. Thereafter, the targeted pH was adjusted and acidaemia was maintained for 3 h, which was followed by measurements ‘before therapy’. For the last 2 h, THAM was infused aiming at a normalization of the pH values, which was followed by measurements ‘after therapy’. The respective volume was infused continuously in all groups. At the end of the experiment, animals were killed with thiopentone (1 g) followed by potassium chloride (60 mmol).
The animals were fasted overnight but had free access to water. Premedication was performed by intramuscular (i.m.) injection of azaperon 3 mg kg−1 (Stressnil; Janssen-Cilag, Neuss, Germany), atropine sulfate 0.03 mg kg−1 (Atropin sulfat; B. Braun Melsungen AG, Melsungen, Germany), ketamine 25 mg kg−1 (Ursotamin; Serumwerk, Bernberg, Germany) and xylazinhydrochlorid 3.5 mg kg−1 (Rompun; Bayer Vital GmbH, Leverkusen, Germany).
Afterwards, anaesthesia was induced with intravenous (i.v.) propofol (5–10 mg kg−1) and maintained with continuous infusion of thiopentone (21 mg kg−1 h−1) and fentanyl (5.5 μg kg−1 h−1), as described in a former work . Initial normoventilation targeted an expiratory CO2 [end-tidal CO2 (etCO2)] within the range of 35–40 mmHg and oxygen saturation (SpO2) above 95%.
A cut-down procedure was used to expose blood vessels and the Seldinger technique to place an 8.5 F sheath (for the introduction of a pulmonary artery catheter), a 4 F central venous catheter (for acid infusion), an 11 F central venous catheter (for the infusion of THAM separate from the acid), an arterial catheter and a Foley catheter as described in a former work .
The volume support was done with either HES130 [Voluven; Fresenius Kabi Deutschland GmbH, Bad Homburg, Germany; 1000 ml contain 60 g poly(O-2-hydroxyethyl)starch with a molecular weight of 130 kDa] or GEL (Gelafundin 4%; B. Braun Melsungen AG, Melsungen, Germany; 1000 ml contain 40 g polysuccinated gelatine with a molecular weight of 30 kDa). The colloid infusion was started as a bolus (15 ml kg−1 infused in 60–90 min), which was paralleled to the acid bolus (see below). Thereafter, the respective colloid was infused continuously (2.5 ml kg−1 h−1). In total, the pigs received approximately 1200 ml of HES130 or GEL. The colloid to crystalloid ratio was 1: 4.
Induction, maintenance and therapy of acidaemia
After ‘baseline’ (pH∼7.45, paCO2∼35 mmHg), a mixed acidaemia was induced with infusion of an acid bolus (5 ml kg−1 of 0.9% isotonic saline containing 0.2 mol l−1 lactic acid and 0.2 mol l−1 hydrochloric acid) and low tidal volume ventilation (6–8 ml kg−1). The acid bolus was followed by continuous infusion (5.5 ml kg−1 h−1) until the start of treatment. Low doses of 3 mol l−1 THAM (Tris 36.34% Braun; B. Braun Melsungen AG, Melsungen, Germany) were used to titrate a target pH of 7.19–7.24 when the pH was below 7.19 (1.3 mmol kg−1 h−1 of THAM in the period of acidaemia). After 3 h of mixed acidaemia, the acid infusion was stopped and THAM was infused following examples from case reports and experimental works in swine [5–7,9], resulting in 2.1 mmol kg−1 h−1 of THAM during treatment.
The arterial blood glucose concentration was measured every 30 min using a radiometer analyser (Radiometer ABL700; Radiometer, Copenhagen, Denmark). When glucose levels dropped below 4 mmol l−1 (72 mg dl−1), glucose was substituted with a bolus of a 5% glucose solution (5% glucose in sterile water, G5W; Glucosteril 5%; Fresenius Kabi Deutschland GmbH, Bad Homburg, Germany).
Haemodynamic, pulmonary and renal function and laboratory analyses
The following parameters were monitored as described in a former work : MAP, heart rate, cardiac output (CO), central venous pressure (CVP), oxygen delivery (DO2), oxygen consumption (VO2), etCO2, SpO2 and diuresis. The blood samples were drawn from arterial, pulmonary and central venous catheters and analysed for pH, paO2, paCO2 and lactate, haematocrit and protein content.
All data were analysed using SigmaStat 3.1s for Windows (Systat Software GmbH, Erkrath, Germany). Results are presented as median and percentiles. Statistical comparison between groups was done with the Mann–Whitney rank sum test. Friedman repeated measures analysis of variance on ranks was used to identify significant changes within one group, followed by an all-pairwise multiple comparison procedure (Tukey test). Statistical significance was assumed at a P value of less than 0.05. Spearman rank order correlation was used to identify a correlation between the infused volume of G5W and THAM.
In both groups, values for pH were significantly reduced during the period of acidaemia compared with ‘baseline’ values (difference in median pH of 0.24), which was paralleled by a significant increase in paCO2 (difference in median paCO2 of 37 mmHg). In both groups, final values demonstrated a significant increase in the pH of 0.1 and a parallel reduction of the paCO2 of 6 mmHg (difference in median values during acidaemia and values ‘after therapy’) Table 2.
Blood glucose and glucose substitution
The blood glucose values decreased during the course of the experiment (Fig. 2 and Table 1). The infusion of the HES130 bolus resulted in a temporary increase in blood glucose in four HES130 pigs. An increase of blood glucose by 2.2 mmol l−1 (40 mg dl−1; from 4.8 to 7 mmol l−1/86 to 126 mg dl−1) was the highest increase after the HES130 bolus. Such peak values did not occur during continuous HES130 infusion. In the GEL group, the infusion of GW5 was necessary 2.5 h after the ‘after bolus’ measurements (during acidaemia). In the HES130 group, an infusion of G5W was not required until the last 2 h of the experiment (during treatment). The cumulative infused volume of THAM (ml) and G5W (ml) correlated positively in the GEL group (0.691 ml, P < 0.01) but not in the HES130 group (0.15 ml, P < 0.19).
MAP and CVP decreased significantly during acidaemia in both groups and did not reach ‘native baseline’ values after treatment with THAM. CO values were significantly lower after the 3 h of acidaemia compared with values after the colloid bolus, but even the lowest value for CO was 2.7 l min−1. The DO2 values were significantly lower after 3 h of acidaemia compared with the respective ‘native baseline’ values. However, even the lowest DO2 value was always at least 100 ml min−1 greater than the respective VO2 value, and lactate values remained within or below the ranges found for ‘native baseline’ values (Table 2).
In a swine model of mixed acidaemia, we studied whether volume management with a starch-based colloid (HES130) may impact differently on blood glucose than a gelatine-based colloid.
Colloid infusion and blood glucose
HES130 alone could not completely counteract hypoglycaemia, but it delayed the need for G5W infusion for 2 h and reduced the required GW5 amount compared with GEL. This effect was also observed in a former study  (see Introduction), in which the glucose-specific glucokinase reaction was used to measure blood glucose. Thus, analytical artefacts are not likely. The degradation of the starch into smaller molecules is probably caused by α-amylase. It is also known that starch molecules are distributed into the body's tissues, especially in the reticuloendothelial system. But in how far other tissues play a role in the degradation of the starch is not completely investigated yet [10–12]. The results of our study suggest that the infusion of HES130 impacts differently on blood glucose than the infusion of GEL. The risk for sudden hypoglycaemia (during prolonged surgery, in the first hours after admission to the ICU when parenteral feeding is not started and so on) may be reduced by a continuous infusion of HES130, but peaks in blood glucose may be a concern after a bolus infusion of HES130. Further investigations are necessary to determine the clinical relevance of our findings because patients receive additional therapies, which impact on blood glucose (e.g. enteral or parenteral feeding, catecholamines, steroids and insulin therapy). Until this question is satisfyingly answered, a control of blood glucose seems to be recommendable whenever a major change in the volume support has occurred (start or stop of infusion of HES130 and major change of the dosage).
Recent clinical trials [13–16] did not report whether the incidence of hypoglycaemia was influenced by the choice of volume support (e.g. 10% HES 200 kDa/0.5 vs. Ringers lactate ). The results of our work suggest the investigation of a possible interaction of volume support with the control of blood glucose in these studies.
Tris-hydroxymethyl aminomethane infusion and blood glucose
Hypoglycaemia is described for continuous THAM infusion , which is in concordance with the observations in this study. The positive correlation between the infused volume of THAM and G5W, which was found in the GEL group, could not be found in the HES130 group, likely because HES130 pigs required less G5W. Both groups had received roughly 6000 ml of fluid in 9 h. In a former study  without acidaemia or infusion of THAM, a similar volume management was used. In this former study, the blood glucose values also decreased during anaesthesia but did not decrease below 4 mmol l−1, and the swine did not receive G5W. Thus, the infusion of THAM likely contributed to the decrease of blood glucose in the current study.
Tris-hydroxymethyl aminomethane infusion and acidaemia
The pH -values increased significantly in both groups during treatment with THAM, but ‘native baseline’ values were not reached. Martini et al.  corrected metabolic acidaemia (pH of 7.1) in pigs of a comparable body weight with a THAM infusion of 7 mmol kg−1 (infused in 50 min) and Rehm and Finsterer  corrected intraoperative hyperchloraemic acidaemia (mean pH 7.28) with 2 mmol kg−1 of THAM (infused in 20 min). Thus, in future experimental set-ups, the complete correction of pH values with higher dosages of THAM seems desirable to stress the adverse effects of THAM.
Despite the infusion of 1200 ml of a colloid and roughly 4800 ml of a crystalloid, the infusion of volume did not completely compensate for the decrease in MAP and CVP during the induction of acidaemia. Thus, a fluid overload of the swine is not likely, especially as the high fluid intake of swine, the overnight fasting (which can disturb the watering habits of swine) and a relative volume deficit due to the supine position (which can promote an aortocaval compression syndrome in swine) have to be taken into account.
The model does not allow a direct transfer of quantitative results to investigations in patients because of creating a swine model of mixed acidaemia without organ failure or sepsis. However, excluding organ failure and sepsis was necessary to restrict the factors that may influence blood glucose such as differences in hepatic function, microcirculatory or cellular disturbances of oxidative metabolism or the different phases of sepsis (e.g. hypometabolic vs. hypermetabolic phase) . Thus, we challenged the control of blood glucose with 9 h of anaesthesia, an instrumentation period of 90 min, a mixed acidaemia that was maintained for 3 h and an infusion of THAM for treatment.
HES130 infusion (6% HES 130 kDa/0.4) impacted differently on blood glucose than GEL infusion (4% GEL) in this porcine model. The swine of the HES130 group required less glucose compared with GEL to prevent a decrease of blood glucose. The bolus infusion of HES130 may lead to peak values in blood glucose. Thus, these findings suggest a control of blood glucose to avoid hypo or hyperglycaemia whenever a major change in the volume support has occurred.
This study was supported by a grant from the Else Kröner-Fresenius Foundation, by Gambro Dialysatoren, Hechingen, Germany and by Fresenius Kabi Deutschland GmbH.
We would like to thank our veterinary and doctoral student Birgit Grossmann for her substantial participation. Some data will be part of a doctoral thesis.
This study was conducted at the Department of Experimental Medicine (FEM), Charité, University Medical Centre, Humboldt University, Campus Virchow, Berlin, Germany.
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