From the Guidelines
High osmolality is not a reliable indicator of intravascular volume depletion. While dehydration can cause increased serum osmolality (normally 275-295 mOsm/kg), many other factors can affect osmolality independently of volume status, as noted in the ESPEN guideline on clinical nutrition and hydration in geriatrics 1. Conditions like hyperglycemia, alcohol intoxication, and certain medications can elevate osmolality without significant volume depletion. Conversely, patients can be volume depleted while maintaining normal osmolality, particularly in isotonic fluid losses such as bleeding or gastrointestinal losses, as discussed in the commentary to recommendations 74 e 79 1.
Key Points to Consider
- Better indicators of intravascular volume depletion include clinical signs like:
- Orthostatic hypotension
- Tachycardia
- Decreased skin turgor
- Dry mucous membranes
- Laboratory findings such as:
- Elevated BUN-to-creatinine ratio
- Increased hematocrit
- Decreased central venous pressure
- When assessing volume status, clinicians should rely on multiple parameters rather than osmolality alone, including physical examination findings, vital sign trends, urine output, and other laboratory values to guide appropriate fluid resuscitation decisions, as suggested by the systematic reviews and guidelines 1.
Clinical Implications
The ESPEN guideline recommends using an action threshold of directly measured serum osmolality >300 mOsm/kg to identify low-intake dehydration in older adults, with a grade of recommendation B and strong consensus (94% agreement) 1. However, this does not necessarily apply to intravascular volume depletion, where other signs and symptoms are more relevant. Clinicians should be cautious when interpreting osmolality values and consider the broader clinical context, including the presence of other conditions that may affect osmolality, as noted in the commentary to recommendations 74 e 79 1.
From the Research
Reliability of High Osmolality as an Indicator of Hypovolemia
- High osmolality can occur due to various factors, including defects in thirst and arginine vasopressin (AVP) release, as well as the presence of impermeable solutes in excessive quantities in extracellular fluid 2.
- Hyperosmolality can lead to cellular dehydration, and serum sodium levels may be reduced by water drawn out of cells along an osmotic gradient 2.
- Hypovolemia, or intravascular volume depletion, can be associated with hyponatremia, hypernatremia, or isotonic conditions, and careful clinical assessment and laboratory tests are necessary for diagnosis and effective management 3.
- The use of hypertonic saline for fluid resuscitation in critically ill patients can result in volume expansion and less total infusion volume, which may be beneficial in patients with intravascular volume depletion 4.
Clinical Assessment and Diagnosis
- Clinical signs such as delay of capillary refill, tachycardia, and orthostatic hypotension can indicate early volume depletion 3.
- Laboratory tests, including serum sodium concentration and urine sodium concentration, can help differentiate between various categories of fluid deficits 2, 3.
- The Evans blue and mannitol test can be used to distinguish between hypernatremic volume depletion and dehydration 3.
- In patients with anorexia nervosa, plasma volume depletion and hypovolemic hyponatremia can be common, and clinicians should be aware of the potential for hemoconcentration to mask underlying anemia 5.
Treatment and Management
- Treatment of fluid deficits depends on the type, severity, and associated symptoms, and may involve oral or parenteral fluid replacement, as well as treatment of the underlying cause 3.
- In hemodynamically compromised individuals, replacement with isotonic saline until hemodynamic stabilization is crucial 3.
- The use of hypertonic saline for fluid resuscitation in critically ill patients may have beneficial effects, but further clinical studies are needed to assess its efficacy 4.