The Role of Blood Volume Analysis in Assessing and Diagnosing Patients with Possible Hyponatremia

According to a news release, physician researchers at Christiana Medical Center, the largest hospital in Delaware, recently presented results from a study on the diagnosis of hyponatremia in critically ill neurologic patients.

The authors of the retrospective study, “Utilization of Blood Volume Analysis for Distinguishing Cerebral Salt Wasting from SIADH,” noted that “neurocritical care patients frequently develop hyponatremia secondary to cerebral salt wasting or the syndrome of inappropriate antidiuretic hormone release (SIADH). The main distinction between these disorders is the patient’s volume status. Cerebral Salt Wasting presents with a decrease in circulating blood volume, whereas SIADH occurs in patients with a normal circulating blood volume.”

Although, “physical examination and central venous pressures have proven to be unreliable for assessing circulating blood volume,” newer tools and methods for isotopic blood volume analysis have made this procedure less complex and faster administer, facilitating its clinical use in the diagnosis and management of hyponatremia following neurocritical illness.

The researchers performed blood volume analysis (measuring red cell volume, plasma volume, and total blood volume) on 293 patients, 72 of whom “had a neurologic disorder fitting the criteria for the study.” The most common neurologic illnesses among this cohort were subdural hematoma (33%), intraparenchymal hematoma (25%), and subarachnoid hemorrhage (20.8%). They also collected data on: Na+, K+, BUN, Crt, Hct, Urine SG, Urine Na+, Urine Osmolality, Days with a normal serum Na+ level, Days with an abnormal serum Na+ level, fluid and sodium balance, BVA results, and neurological outcome.”

In the study, 51% of the patients were hyponatremic, 36% had normal blood sodium levels, and 12.5% were hypernatremic.

Based on the results of blood volume analysis, the researchers reported that plasma volume was increased in more than half (54%) of hyponatremic patients, red cell volume was decreased in more than 90%, and total blood volume was increased in 21% of patients, decreased in 41%, and normal in 38%.

The authors reported that “Serum Na+, K+, BUN, Crt, Hct, Urine SG, Urine Na+, Urine Osmolality were not predictive of plasma or blood volume status. Sodium administration was not predictive of plasma or blood volume.”

In their discussion of these results, the authors wrote “Using plasma volume criteria, SIADH appears to be more common than CSW in hyponatremic patients. Using total BV criteria, CSW may be present in up to 40.55% of these patients. The traditional markers of volume status are not useful in this neurocritical care population.”

Because patients with neurocritical illness “are at risk for fatal vasospasm, especially when dehydrated,” and “SIADH is treated by judicious fluid restriction,” which could precipitate vasospasm in patients with CSW, the use of blood volume analysis data can improve the assessment and management of hyponatremia and help clinicians avoid contraindicated interventions.

The news release claims the study “shows that the usual physiologic parameters in evaluating these patients were not accurate in identifying correctly which treatment could be used.” In the study, the death rate was 28.6% in patients whose hyponatremia was not corrected, compared to 17.9% in patients in whom serum sodium was corrected. The authors noted that “measurement of the plasma volume was most helpful in correctly categorizing the hyponatremic patients.”

Joseph Feldschuh, MD, president of Daxor Corporation, a company that manufactures blood volume analyzers and diagnostic kits, said the study provide “further evidence that patients with hyponatremia, a relatively common condition seen not only in severely injured neurology patients but in multiple other conditions such as congestive heart failure, should have a blood volume measurement performed so they are not incorrectly treated.”