Intravenous Fluids

In this edition of Anesthesiology, Maheshwari et al. report on the Saline or Lactated Ringer’s (SOLAR) trial, conducted at the Cleveland Clinic in the United States. The researchers tested the hypothesis that among more than 8,000 patients undergoing elective colorectal and orthopedic surgery, a composite outcome of in-hospital mortality and major postoperative complications would be less common among those given lactated Ringer’s solution than patients given normal saline during surgery. However, contrary to the hypothesis, the researchers found that there was no important difference in postoperative complications between the two fluid groups. Further, there was no important difference in acute kidney injury. The authors concluded: “Clinicians can reasonably use either fluid for routine vascular volume replacement in patients having noncardiac surgery.” So, can we now safely give saline to all noncardiac surgical patients, particularly higher-risk patients? As often happens with clinical questions, the devil is in the detail. When it comes to the question of which crystalloid, there has been debate over several decades as to whether it is clinically wise to use 0.9% saline, the simplest and cheapest fluid, and the favorite of many. An alternative view is that 0.9% saline, often derided as (ab)normal saline, is clinically inferior to more physiologic fluids, also known as balanced or buffered fluids. The most widely used of these more physiologic fluids are the lactate-containing fluids, such as lactated Ringer’s and the very similar, but not identical, Hartmann’s solution. Other alternatives are fluids with acetate anions including Ringer’s Acetate and Plasmalyte. Many think the most important difference between these fluids and saline is the amount of chloride. Concerns about chloride in saline include hyperchloremic metabolic acidosis and kidney injury. The major strengths of the SOLAR study are that it is prospective and involves thousands of patients undergoing the most common major general surgical and orthopedic operations. SOLAR also has limitations. Although the researchers used an intensive multiple crossover approach for 2-week cluster allocation of fluids for all patients, SOLAR is not a randomized trial. A large, well-conducted, pragmatic, randomized trial provides evidence that is less likely to have confounders (biases) that may distort the results. When interpreting SOLAR, we cannot be fully confident that known knowns, known unknowns, and unknown unknowns (thank you Donald Rumsfeld) are truly randomly allocated to minimize bias. To counter this concern the SOLAR authors conducted sensitivity analyses, suggesting an unrecognized confounding factor would need a substantial relative risk to affect the results. Nonetheless, this is evidence at a lower standard than a randomized trial. Further, the trial was unblinded, which may be a bigger risk for bias than being nonrandomized. In part, this is because postoperative fluids were at the discretion of unblinded clinicians, as were decisions around levels of postoperative care. Another important perioperative question is how much fluid? The SOLAR investigators note that the study was conducted when smaller volumes of perioperative fluids were in favor: SOLAR patients received an average of 1.9 l of fluid during surgery. This practice contrasts with the findings of the Liberal Fluid Therapy in Major Abdominal Surgery (RELIEF) study, a large multicenter, randomized trial, that asked: How much fluid for major abdominal