Treatment of the hypernatremia patient in veterinary medicine can be challenging, and appropriate fluid therapy and careful monitoring is imperative. The speed of correction of hypernatremia will depend on the speed of onset of hypernatremia in the patient. Normal body sodium in both the dog and cat is approximately 140 mEq/L. If you look at your bags of intravenous fluids used in the hospital, the majority are isotonic and have a sodium content similar to that of the patient (e.g., most balanced crystalloid fluids have approximately 132-154 mEq/L of sodium in them). As a quick review, LRS has approximately 132 mEq/L of sodium, while 0.9% NaCL has 154 mEq/L of both sodium and chloride in it. 0.9% NaCl is actually the highest containing sodium crystalloid fluid, which is one of the reasons why VETgirl hardly uses it anymore (after all, most of our patients don't have a sodium of 154 mEq/L).
Secondly, remember the key rule when it comes to treating critically ill hypernatremic veterinary patients: What happens chronically, treat chronically. What happens acutely, treat acutely.
So, what's an example of a chronic hypernatremic case? The classic case is a cat or dog that was accidentally locked in a garage for several weeks without any food or water (Yes, this happens, often by accident when the owner has moved and the pet has run back to the old house and gotten trapped inside). We can also see chronic hypernatremia in patients that have excessive free water loss [e.g., a newly diagnosed cat with diabetic ketoacidosis (DKA), a cat with renal disease that has excessive PU/PD that is accidentally chronically deprived of water, etc.] In these situations, the patient is chronically losing too much free water. Alternatively, it can be from chronic water deprivation (e.g., again, the pet locked in a garage for 2 weeks without food or water). With chronic hypernatremia, very slow treatment of IV fluid replacement should occur. In this situation, you want to lower the body’s sodium very, very, very slowly. Why? Because with chronic hypernatremia, the formation of idiogenic osmoles within the brain develop; these are originally designed to prevent the brain from undergoing rapid fluid shifts. With rapid fluid resuscitation (e.g., boluses of IV crystalloids), that fluid shifts to the brain (which is hyperosmolar from the chronic dehydration and idiogenic osmoles), resulting in severe cerebral edema. Clinical signs of cerebral edema may then develop, such as worsening obtundation, ataxia, tremoring, seizuring, etc.). The general rule is that with CHRONIC cases, the sodium should never be changed more than 0.5 mEq/kg/hour (some sources will say 1 mEq/kg/hour). Since there are 24 hours in a day, one doesn't want to change the sodium more than 12 mEq/L day! (In other words, don't change the sodium from 180 to 160 in one day with chronic hypernatremic patients!)
With acute hypernatremia, we can change the patient's sodium quickly. When do we see acute hypernatremia? This is typically seen certain toxicants (e.g., paint balls, salt ingestion, etc.). With paint ball toxicosis, hypernatremia is due to the presence of sorbitol and polyethylene glycol (e.g., GoLytely); the paint balls don't contain sodium chloride. With salt toxicity (from homemade play dough, salt emetic ingestion, etc.), patients will develop acute hypernatremia within several hours (e.g., 140 to 180 mEq/L). Treatment should be initiated promptly and includes rapid correction of the sodium levels. We can drop the sodium levels rapidly by resuscitating or hydrating the patient with a low-sodium fluid. VETgirl's general rule? Hydrate the patient first with a lower-sodium fluid such as LRS or Plasmalyte-A.
Once the patient is hydrated, you can switch to a much lower sodium fluid (e.g., 0.45% NaCL + 2.5% dextrose, D5W). Now keep in mind that these lower sodium fluids are rarely used in veterinary medicine (as they are not an isosmotic fluid). Keep in mind that when we are administering 0.45% NaCL + 2.5% dextrose, that extra 2.5% dextrose is mixed in to the 500 ml bag to make the fluid isosmolar! It’s not to help treat hypoglycemia or anything. Giving 0.45% NaCL ALONE is contraindicated, as it’s not an isotonic fluid (it’s osmolality is only 154 mEq/L TOTAL, as it’s half of what’s in a normal bag of 0.9% NaCL). Our normal body’s osmolality is approximately 300 mosml/L, so we should always give a fluid that is approximately 300 mosm/L (any balanced, isotonic fluid is usually 300 mosm/L). That extra dextrose increases the osmolality of a bag of 0.45%NaCL to approximately 300 mosm/L, making it a safe fluid to give. That dextrose is also metabolized by cells and provides free water to the patient, helping to lower the sodium in the body. With acute hypernatremia, one doesn't have to worry about dropping the sodium slowly with the traditional “0.5mEq/kg/hr of sodium” formulation.
Remember, what happens acutely, treat acutely. What happens chronically, treat chronically.
Regardless, make sure to monitor the patient and the sodium levels q. 4-6 hours, or as needed, depending on the patient's clinical signs.
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