Most dangerous dehydration is-

Hypernatraemia Hypernatraemia is defined as existing when the serum Na is >145 mmol/L. Pathophysiology Hypernatraemia occurs due to inadequate concentration of the urine in the face of restricted water intake. This can arise because of failure to generate an adequate medullary concentration gradient in the kidney due to low GFR or loop diuretic therapy, but more commonly is caused by failure of the vasopressin system. This can occur because of pituitary damage (cranial diabetes insipidus ) or because the collecting duct cells are unable to respond to circulating vasopressin concentrations in the face of restricted water intake (nephrogenic diabetes insipidus). Whatever the underlying cause, sustained or severe hypernatraemia generally reflects an impaired thirst mechanism or responsiveness to thirst. Clinical features Patients with hypernatraemia generally have reduced cerebral function, either as a primary problem or as a consequence of the hypernatraemia itself, which results in dehydration of neurons and brain shrinkage. In the presence of an intact thirst mechanism and preserved capacity to obtain and ingest water, hypernatraemia may not progress very far. If adequate wateris not obtained, dizziness, delirium, weakness and, ultimately, coma and death can result. Ref Davidson 23rd edition pg 360