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ü
, MDDepartment of Anesthesiology, Medical Faculty of Marmara University, Istanbul, Turkey
Address correspondence and reprint requests to Dr. Arzu Takil, Atakent Burç cad. Burç apt. N0: 1B/45 81240 Ümraniye, 
stanbul, Türkiye. Address e-mail to takilarzu{at}hotmail.com
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Abstract |
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IMPLICATIONS: The infusion of large-volume lactated Ringer’s solution does not cause hyperchloremic metabolic acidosis as does 0.9% saline during major surgery, but leads to postoperative mild hyponatremia and respiratory acidosis.
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Introduction |
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The aim of our study was to compare the intraoperative effects of large-volume infusion of NS or LR on electrolytes and acid base balance during major spine surgery and to evaluate the duration of these effects and whether they had any other effects postoperatively.
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Methods |
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All patients were premedicated with atropine 0.5 mg and midazolam 0.07 mg/kg IM preoperatively. General anesthesia was induced with thiopental 5 mg/kg and vecuronium 0.1 mg/kg IV. After endotracheal intubation, anesthesia was maintained with oxygen in 70% nitrous oxide and 1.5%–2% sevoflurane.
In Group I, NS solution, and in Group II, LR solution were infused 20 mL · kg · -1h-1 intraoperatively (15 patients in each group). The patients with >20% blood loss received blood transfusions. For the first 500-mL blood loss, 500 mL of colloid solution (Gelofusine) was administered to all patients.
After the induction of anesthesia, radial arterial and central venous catheters were inserted. Mechanical ventilation was performed to maintain PaCO2 as close as possible to 40 mm Hg.
Intraoperative monitoring included continuous mean arterial blood pressure, heart rate, oxygen saturation, end-tidal carbon dioxide pressure, central venous pressure, and esophageal temperature in all patients, and the values were recorded every 15 min. Intraoperative crystalloid infusion volumes, urinary output, blood loss, and blood transfusion volumes were recorded hourly.
Electrolytes (Na+, K+, Cl-) and arterial blood gases were measured preoperatively, every hour intraoperatively and at the 1st, 2nd, 4th, 6th, and 12th hours postoperatively. Also, serum albumin values were measured and anion gaps (Na+ - [Cl- + HCO3]) calculated preoperatively and at the end of the surgery.
Postoperatively, the same solutions were infused at 2.5 mL · kg -1 · h-1 for 12 h and IV patient-controlled analgesia with morphine was administered to all patients.
The length of intensive care unit and hospital stay of all patients were also recorded.
All data were compared with two-way repeated measures of analysis of variance and post hoc testing was performed according to Tukey-Kramer test statistically. Student’s t-test was performed for unpaired data and a P < 0.05 was considered significant.
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Results |
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pHa decreased significantly at the 2nd hour intraoperatively and returned to normal ranges at the 12th hour postoperatively in the NS group. No major pHa difference was observed in the LR group intraoperatively but it was decreased significantly at the 1st hour postoperatively. The difference in pHa between groups was significant at the 2nd hour intraoperatively and persisted until the 6th hour postoperatively (Table 4).
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No significant difference was observed in HCO3, Cl-, or base excess values in the LR group during the study period.
During the postoperative period, although there was no difference in respiratory rate between groups, PaCO2 increased significantly at the 1st hour in the LR group and remained increased for 12 h (Table 6).
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The anion gap and albumin concentration showed similar changes in both groups with an insignificant decrease after crystalloid infusions.
There were no significant differences in intensive care unit (ICU) time (42 ± 18 h for NS and 47 ± 23 h for LR) and discharge time (10 ± 2 days for NS and 11 ± 2 days for LR) between the groups.
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Discussion |
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Hyperchloremic metabolic acidosis is a consequence of large volumes of chloride-rich crystalloid solutions during surgery (3,4). Although the acidifying potential of large volumes of sodium chloride solutions on plasma has been discussed by McFarlane and Lee (4) in 1994 and recognized in another randomized controlled trial (3) and a series of case reports and letters (6–13), the etiology and clinical relevance of this metabolic acidosis remains controversial. Some investigators call this "dilutional acidosis," which implies that plasma expansion and dilutional reduction of plasma bicarbonate concentration are the underlying mechanism (3,5). In contrast, the Stewart model emphasizes the importance of hyperchloremia resulting in a reduction of the strong ion difference (6,8,10–13).
Most of these studies, other than those by Williams et al. (7) and McFarlane and Lee (4), did not evaluate the duration of acidosis. McFarlane and Lee reported that plasma chloride concentrations after large-volume administration of NS returned to baseline values by 24 hours postoperatively because they made plasma biochemistry measurements only before and after and at 24 hours postoperatively. One of the main implications of our study is that the increase in Na+ and Cl- concentrations and the decrease in HCO3, base excess and pH with 20 mL · kg -1 · h-1 NS infusion, returned to baseline values at the 12th hour postoperatively in patients continuing to receive NS infusion at a rate of 2.5 mL · kg -1 · h-1. This can be explained by the fact that a relatively slow infusion rate was coupled with an appropriate cellular buffering effect of increasing extracellular HCO3 and hyperchloremia was corrected by subsequent chloriduria (5,6,11).
Most of the authors suggested that hyperchloremic acidosis does not have major pathophysiologic implications in the clinical setting unless it is confused with hypoperfusion and leads to further large-volume infusions (3–5). However, hyperchloremic metabolic acidosis may impair end organ perfusion and interfere with cellular exchange mechanisms (7,9,14). Williams et al. (7) found a significantly increased time to first urination and an increased frequency of abdominal discomfort in healthy volunteers who developed metabolic acidosis with 50 mL/kg NS infusion. Hyperchloremia alone has a negative effect on renal blood flow and glomerular filtration rate (14). Also Wilkes et al. (9) studied the effects of LR and NS infusions on gastric mucosal pH in elderly surgical patients and concluded that the use of balanced IV solutions can prevent the development of metabolic acidosis and is associated with better indices of gastric mucosal perfusion than saline-based solutions. In a recent outcome study done by Waters et al. (15), it was concluded that an acid-base imbalance occurred in the NS group with the development of a hyperchloremic metabolic acidosis but with this acidosis, no difference was seen in the postoperative complications, in the ventilator time, in ICU time, or hospital stay. However, more blood products were transfused in the group receiving NS, although no statistical difference in blood loss was found. In our study, we observed that the LR group had less estimated blood loss but, similar to Waters et al. (15), we found no statistically significant difference. Also, there were no significant differences in blood transfusion volumes, or in ICU and hospital stay of our patients.
In all the studies evaluating the acidotic effect of NS, the authors advocated the use of balanced IV solutions such as LR for large-volume infusion. During the infusion of LR, the removal of lactate from circulation will increase the strong ion difference and reduce acidosis. This effect may be supplemented by a further increase in the strong ion difference associated with lactate metabolism (11). According to the results of controlled trials and our study, large-volume LR infusions do not cause hyperchloremic metabolic acidosis. However, the possible deleterious postoperative effects of lactate load in spontaneously breathing patients were not evaluated in these controlled trials (3,9,15). Another important implication of our study, not previously mentioned, was that intraoperative 20 mL · kg-1 · h-1 LR infusion leads to early postoperative respiratory acidosis as a result of lactate metabolism to CO2. Although the mean PaCO2 was 45 ± 4 mm Hg at the postoperative 2nd hour, in one patient, PaCO2 increased to 52 mm Hg and pH decreased to 7.25. The postoperative increase in PaCO2 can be misinterpreted as a side effect of opioid analgesics and can lead to inadequate postoperative pain treatment or can be clinically significant in patients with pulmonary disease. The clinical effects of this hypercarbia after LR should be evaluated in patients with severe pulmonary disease. The major limitation of our study is that we evaluated ASA physical status I and II patients with intact compensatory mechanisms.
There was a significant decrease in postoperative Na+ concentrations in our patients treated with LR intraoperatively. Although the mean Na+ concentration at the postoperative 6th hour was 136 ± 4 mEq/L in the LR group and this decrease has no clinical relevance in healthy patients, in one patient it decreased to 126 mEq/L. Williams et al. (7) investigated the effect of large volumes of IV LR or NS on serum osmolality and concluded that although the decrease in osmolality (4 ± 3 mOsm/kg) after LR did not cause any clinical signs of increased intracranial pressure in healthy volunteers in the presence of compensatory mechanisms, these mechanisms may not be intact in patients with intracranial pathology. The decrease in Na+ concentration and serum osmolality will tend to increase brain water and it may be clinically significant in patients at risk for intracranial hypertension (7,10).
We conclude that 20 mL · kg-1 · h-1 LR infusion does not cause hyperchloremic metabolic acidosis as does NS infusion; however, it may lead to early postoperative respiratory acidosis. Although this hypercarbia did not have any significant clinical effect, it can be misinterpreted as opioid overdose, leading to inadequate analgesia, or it can have clinical significance in patients with severe pulmonary disease. The combination of LR and NS infusions (such as switching between the two solutions every other liter) during major surgery may be a better choice, but the effects of this combination await confirmation with further clinical trials.
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