Lactated Ringer's solution (LRS) has been used as the first choice of perioperative solution for extracellular fluid replacement in surgical operations. However, since the metabolism of lactate to bicarbonate mainly depends on the liver, substantial correction of metabolic acidosis by LRS cannot be expected in patients with hepatic problems or with shock . Sodium acetate is metabolized not only in liver but also in other organs  (particularly in skeletal muscle), and is metabolized faster than lactate [3,4]. However, since both lactate and acetate need metabolic processes to exert their alkalinizing effect, there must be a time-lag required for this effect to appear.
Sodium bicarbonate, a physiological alkalinizing agent contained in bicarbonated Ringer's solution (BRS), needs no metabolic process. However, when sodium bicarbonate is added to Ringer's solution (RS) at room temperature, bicarbonate is easily converted into carbonate, carbon dioxide and water. Calcium carbonate precipitate is formed as a result. Following extensive pharmaceutical investigations, BRS containing sodium bicarbonate as an alkalinizing agent has been finally developed. By adding citrate 5 mEq L−1, which is sufficient to chelate all of the calcium (3 mEq L−1) and magnesium (1 mEq L−1) contained in the solution, production of insoluble calcium and magnesium carbonates is prevented. Furthermore, the pH of BRS can be adjusted to a physiological value (approximately 7.0) using carbon dioxide.
BRS also contains Mg2+ (1 mEq L−1), a concentration approximately equal to that in plasma. Although plasma Mg2+ is known to be increased under conditions of surgical stress , plasma magnesium concentration decreases because of dilution in the perioperative period. The extracellular Mg2+ concentration is much lower than that within the cells, thereby resulting in a poor understanding of the importance of plasma Mg2+ concentrations. The roles of Mg2+ have become gradually clearer as studies of cardiac dysrhythmia and vessel spasm accompanying hypomagnesaemia have accumulated [6-9]. Therefore, administration of BRS containing Mg2+ is supposed to improve the perioperative hypomagnesaemia associated with morbidity in clinical situations. In our other study using the haemorrhagic shock dog model, we showed that the alkalinizing effect of BRS on metabolic acidosis was superior to those of acetated Ringer's solutions (ARSs) and LRSs, and this result agrees with previous report data . However, the alkalinizing effect of BRS has not been examined under conditions of hepatic dysfunction, where BRS would demonstrate its direct alkalinizing effect. The purpose of this study is to confirm the effectiveness of BRS on metabolic acidosis in a partially hepatectomized rabbit model providing moderate to severe surgical invasive state.
Materials and methods
The chemical compositions of the four RSs used in this study are shown in Table 1. BRS (BICARBON® Injection) was prepared at Shimizu Research Center, Shimizu Pharmaceutical Co., Ltd. (Shizuoka, Japan). LRS (SOLITA®), ARS (Veen-F®) and RS (Ringer's solution®) were all of pharmaceutical grade, and obtained from Shimizu Pharmaceutical Co., Ltd. (Shizuoka, Japan), Nikken Chemical Co., Ltd. (Tokyo, Japan) and Otsuka Pharmaceutical Co., Ltd. (Tokushima, Japan), respectively.
Male JW/CSK rabbits weighing 2.6-3.3 kg (15 weeks old) were purchased from Japan SLC Co. Ltd. They were given water and laboratory chow (RC-4; Oriental Yeast Co. Ltd., Kanagawa, Japan) ad libitum. The animals were maintained under controlled conditions at a temperature of 23 ± 3°C with a relative humidity of 55 ± 15% and a 12: 12 h light-dark cycle. They were used for experiments following acclimatization for at least 7 days to these conditions and fasted for 24 h before the experiment but with free access to water. All of the procedures were carried out in accordance with the guidelines of the Laboratories Animal Research Committee in Laboratories, established by Shimizu Pharmaceutical Co., Ltd.
Thirty-six rabbits were randomly divided into six groups (six rabbits each), namely, a sham-operated group without partial hepatectomy and infusion (Sham), a partial hepatectomy group without infusion (PHx), and partial hepatectomy groups given infusions (BRS, ARS, LRS and RS groups). The animals were anaesthetized with an intravenous injection of sodium pentobarbital (20 mg kg−1). The trachea was intubated with a polyethylene catheter to control ventilation and PaCO2 kept at approximately 30-40 mmHg. Anaesthesia was maintained with 0.5% isoflurane in a mixture of 30% O2 and 70% N2O with pancuronium as a muscle relaxant. The femoral artery was catheterized to monitor the mean arterial pressure and to collect arterial blood samples.
The surgery took 1 h; 40 mL kg−1 h−1 of each RS (BRS, ARS, LRS or RS) was infused for 1.5 h via a catheter inserted into the auricular vein, from the start of surgery the Sham and PHx groups did not receive any infusion. Immediately after starting the infusion, the surgical operation was commenced, and then at 25 min, a partial hepatectomy was performed. Briefly, the portal vein was ligated with a nylon suture, followed by excision of the medial segment of the left lobe, the lateral segment of the left lobe and the medial segment of the right lobe. The residual liver tissue was excised at the end of the experiment after collecting the final blood sample and weighed to determine the proportion of the liver represented by the volume removed in the partial hepatectomy. The Sham group was subjected to only a laparotomy.
The blood samples in the infusion groups were obtained before, during (at 30, 60 and 90 min), and after infusion (15 and 30 min) to measure arterial blood gases. The blood samplings in the Sham and PHx groups were performed using the same time schedule as the infusion groups. Blood pressure (BP) was measured just before blood sampling by using a BP transducer (TP-400T; Nihonkohden, Tokyo, Japan) connected to a BP amplifier (AP-641G; Nihonkohden, Tokyo, Japan). Arterial blood gases were analysed by an acid-base analyser (ABL-30; Radiometer, Copenhagen, Denmark). Plasma Mg2+ concentrations were measured by the xylidil-blue method (Magnesium B-test; Wako Pure Chemical Co., Ltd., Tokyo, Japan) using a Cobas Fara analyser (Roche Diagnostics, Mannheim, Germany).
All data were expressed as the mean ± SEM. Two-way analysis of variance (ANOVA) was employed for the comparison of values within each group followed by Dunnett's test. The significance of the differences among the four infusion groups at each point was assessed by one-way ANOVA, followed by Bonferroni's tests correction. Differences were considered to be statistically significant when P < 0.05.
Rates of partial hepatectomy in five groups were 74.5 ± 0.5% (PHx), 76.2 ± 1.1% (BRS), 75.0 ± 0.8% (ARS), 75.8 ± 0.7% (LRS) and 77.2 ± 2.0% (RS); and body weights were 2.80 ± 0.04 kg (Sham), 2.83 ± 0.08 kg (PHx), 2.90 ± 0.06 kg (BRS), 2.87 ± 0.05 kg (ARS), 2.97 ± 0.04 kg (LRS) and 2.97 ± 0.11 kg (RS), showing no significant differences in any variables among groups.
Influences of partial hepatectomy
Effects of partial hepatectomy on mean blood pressure (MBP), blood gases and plasma magnesium concentration are presented in Table 2 and Figure 1a-c, respectively.
In the Sham group, continuous decreases in MBP were observed during the experiment. In the PHx group with no infusion, marked metabolic acidosis, as evidenced by substantial decreases in base excess, bicarbonate concentration and pH, was observed with continuous decreases in MBP. These decreases showed statistically significant differences as compared with their baseline values (P < 0.01). Moreover, significant increases in plasma Mg2+ levels (P < 0.01) were also induced by the hepatectomy.
Changes in BRS, ARS, LRS and RS groups
As shown in Table 2, a sustained and significant (P < 0.05 and/or 0.01) decrease in MBP was observed during the infusion in all infusion groups although MBP tended to recover by the end of the infusion. There were no significant differences in MBP in the four groups throughout the experimental period.
Changes in arterial blood pH, bicarbonate and base excess are shown in Figure 1a-c, respectively. During the infusion, the pH values of all groups were decreased by hepatectomy (Fig. 1a), especially in the PHx group. The decrease continued until the end of the experiment, while in the infusion groups pH reached a nadir just before the cessation of the infusion and this level was sustained. The fall in pH was found to be smallest in the BRS group with almost the same pH changes as that observed in Sham group.
As observed in the PHx group, bicarbonate and base excess in all infusion groups continuously decreased during the experimental period but to a lesser degree (Fig. 1b). BRS most efficiently protected the bicarbonate and base excess decreases observed in PHx and the order of potency was ARS > LRS > RS. With respect to base excess (Fig. 1c), BRS exhibited almost the same changes as did the Sham group. BRS showed significantly higher bicarbonate and base excess values than LRS (P < 0.05) and RS (P < 0.01) during or after the infusion or both. No statistically significant difference was seen in both values between BRS and ARS throughout the experimental period.
Plasma Mg concentrations in ARS, LRS and RS groups significantly decreased (P < 0.05 or P < 0.01) as compared with their 0 min values (Table 2). These changes present an exquisite contrast to that of PHx, which caused a sustained elevation of plasma Mg2+ level. On the contrary, BRS which showed no significant differences in plasma Mg2+ concentration as compared to its 0 min value, significantly inhibited the decrease in the concentration with a statistical difference from those of LRS (P < 0.05 and P < 0.01) and RS (P < 0.05). That is to say, BRS inhibited the increases of plasma magnesium by surgical stress and the decrease of plasma magnesium by dilution.
The metabolic rate of sodium acetate is higher than that of sodium lactate because sodium acetate is metabolized not only in the liver but also in other organs  indicating a proper alkalization effect of ARS in patients with hepatic failure. But acetate is reportedly known to sometimes cause cardiac depression and peripheral vasodilatation, which lead to hypotension [10-14]. Kirkendol and colleagues  reported that sodium acetate depressed the myocardial contractile force more markedly than did sodium bicarbonate, the difference being statistically significant. These facts imply that alkalinizing reagents, such as sodium lactate and sodium acetate are insufficient to satisfy the needs for correcting metabolic acidosis in a clinical situation . On the other hand, sodium bicarbonate is the most biologically suitable alkalinizing agent for its prompt alkalization effect  because bicarbonate can be directly produced from sodium bicarbonate without a metabolic process. In the present study, we have evaluated the effects of several RSs, namely BRS, ARS, LRS and RS, on metabolic acidosis and plasma magnesium level using partially hepatectomized rabbits.
Arterial blood pH, bicarbonate and base excess were regarded as major determinants of metabolic acidosis and were supposed to be improved by infusing RSs. The decreased pH, bicarbonate and base excess by partial hepatectomy were also significantly improved by BRS with an efficacy more potent than those of ARS, LRS and RS. On the contrary, LRS showed little alkalinizing effect in this model because of insufficient metabolism of lactate. These results indicate that BRS causes rapid and potent correcting effects on metabolic acidosis as compared with other Ringer's solutions in hepatic dysfunction and are supported by the previous studies providing comparative evaluations of bicarbonate, acetate and lactate in animals or human beings [15,16]. Sandee and John  reported that bicarbonate showed the most rapid and potent alkalinizing effect among these agents and lactate was least effective because of its slow and incomplete metabolism in dogs with hepatic dysfunction. Thus, the results obtained in the present study agree with these reports and BRS is suggested to be the best Ringer's solution for correcting metabolic acidosis in patients with hepatic failure clinically.
Plasma Mg2+ values in the BRS group were not significantly decreased and were maintained throughout the experimental period, while in the ARS, LRS and RS groups, Mg2+ values were decreased during the infusion to a statistically significant extent as compared with their pre-infusion values. Several investigators have indicated that extracellular Mg2+ deficiency led to derangement of the integrity in the coronary, cerebral and systemic vasculatures [17-24] and it is reportedly known that supplementation of Mg2+ decreased perioperative pain, postoperative cardiac dysrhythmias and atrial fibrillation [25-28]. Thus extracellular Mg2+ plays an important role in regulating vascular tension in the cardiovascular system, indicating the necessity for maintenance of the plasma Mg2+ level by administration of Ringer's solution containing Mg2+ in several medical situations to prevent adverse events associated with hypomagnesaemia.
In this study, BRS exhibited the most prominent alleviating effect on metabolic acidosis. Moreover, BRS is the only solution that could offer satisfactory maintenance of the plasma Mg2+ level. These results indicate that BRS is a highly useful solution for perioperative correction of metabolic acidosis and plasma magnesium balance in hepatic dysfunction.
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