By Hayder Aledan, MD, FASN
Introduction and background
This Communities discussion compared the results of two randomized clinical trials (SMART and SALT-ed) reported on in the New England Journal of Medicine that compared the effects of normal saline and a “balanced solution” in preventing AKI. Isotonic saline (0.9% NS) is the main crystalloid solution used for resuscitation of hypovolemic critically or non-critically ill patients. The SMART trial in which 7,942 critical care patients were randomized to either 0.9% NS or a more balanced electrolyte fluid preparation (Lactated Ringers or Plasma-Lyte A) showed a higher rate of composite of death from any cause, new renal replacement therapy or persistent renal dysfunction in those patients resuscitated by the isotonic saline compared to balanced crystalloids. The absolute risk reduction was 1.1% for major adverse kidney event (15.4% v 14.3%), 0.8% for mortality (11.1% v 10.3%). Based on these results, shall we change our treatment strategy for the use of isotonic saline in the initial resuscitation of hypovolemic patients?
Communities participants highlighted many different points in deciding whether to change patient care and debated whether or not these findings were “game changers”.
Is 0.9% NS at Na 154 meq/l isotonic or hypertonic to plasma?
One liter of 0.9% saline has a [Na] (and Cl) of 154 mEq/L so the final osmolality is 308 mOsm. But this is the same osmolality as the water content of the blood. The measured osmolality of blood is lower because there is a 7% solid phase of blood that contains no NaCl. So a liter of blood is actually 70 ml of solids and 930 ml of water. In the water phase a [Na] of 154meq/L contains 154X0.93=143 meq/l, which is the final concentration when mixed to a liter with the 70ml of solid. 0.9 saline is thus considered “isotonic”.
What is thought to be the cause of the adverse events seen with isotonic saline?
The main concern with isotonic saline is felt to be related to the high Cl concentration relative to the plasma Cl concentration the former being markedly different from the patient’s blood. The excessive Cl concentration may decrease renal perfusion by causing renal vasoconstriction and thus leading to acute kidney injury. It also causes a dilutional non-anion-gap metabolic acidosis and may also cause inflammation, hypertension, all of which have the potential to increase mortality.
What is the relative precautions of balanced crystalloids?
The relative hypotonicity of the balanced crystalloids (276 mOsm) may increase intracranial pressure so use cautiously in patients with traumatic brain injury and in patients who are hyperkalemic.
The use of balanced solutions was associated with a lower rate of major adverse renal events and death in hypovolemic patients as compared with isotonic saline. This difference, while meager, may be warranted since the cost difference between the two solutions is minimal. The effects on morbidity and mortality may be more important in septic patients in which the use of large volume resuscitation is often required.
Read the whole discussion here