Hypovolemia depicts a state of intravascular volume depletion, while dehydration describes a state of reduced total body water volume, primarily affecting the intracellular fluid compartment. These terms are often interchanged in clinical practice as they are frequently encountered simultaneously. 

Body fluid loss (dehydration and/or hypovolemia) emerges when fluid excretion exceeds fluid intake, e.g., due to inadequate fluid intake, vomiting, and/or diarrhea. Young children and the elderly are prone to clinical dehydration due to the differences in body water distribution, the potential inability to express needs to caregivers, and increased use of diuretic in elderly patients. The clinical presentation includes increased thirst, lethargy, prolonged capillary refill, abnormal vital signs, and decreased skin turgor. Further, hypovolemic shock can occur if the severity of hypovolemia is such that the body is unable to compensate, resulting in end-organ damage due to hypoperfusion. 

Hypovolemia, as well as dehydration, are clinical diagnoses and laboratory tests are to be carried out only in patients with suspected associated metabolic disturbances or severe enough fluid loss to cause end-organ damage. The primary goals of treatment are to first address the hypovolemia, if present, to quickly restore the circulatory volume, followed by the management of dehydration through the incremental correction of any remaining fluid deficit (including free water deficit), associated electrolyte abnormalities, persistent fluid losses, and maintenance fluid requirements.

AMBOSS[Internet]. Dehydration and hypovolemia. December 27, 2021. Available from: https://www.amboss.com/us/knowledge/Dehydration_and_hypovolemia/