How does the thirst mechanism (response) guardagainst hypernatremia across a person's lifetime?

Defenses against hypernatremia include the thirst mechanism, which increases free water uptake, and the release of antidiuretic hormone (ADH), which increases renal-free water reabsorption.1 Studies have shown that when plasma osmolarity of humans is increased by as little as 1%-2%, thirst is heightened.4-5 Thirst is such a potent stimulus that it is extraordinarily rare to see hypernatremia in patients with an intact thirst mechanism and adequate access to free water, even if the patient has a near total absence of ADH. It has been demonstrated that thirst drive decreases over a lifetime as evidenced by a 24-hour water deprivation study comparing older to younger men.6 The study results showed that older men had greater increases in plasma osmolality, sodium concentration, and vasopressin levels compared with their younger counterparts.6 A study in which volunteers were administered 5% saline showed that younger individuals drank nearly twice as much as older individuals in response to a similar intravenous infu-sion.7 In addition, decreasing renal function and total body water with advancing age compound the susceptibility for healthy older individuals to develop hypernatremia compared to their younger counterparts.8 Thus, these studies suggest that physicians should be proactive about water balance problems in hospitalized elderly patients by paying close attention to fluid intake and losses. It is also important for physicians to educate the caregivers of the elderly about the importance of monitoring fluid intake in their loved one, especially during periods of illness. The patient in the vignette is not only elderly, but has a history of dementia and so is at high risk of developing a dysnatremia.

Bottom Line: Thirst drive in humans is exquisitely sensitive and is activated with an increase in plasma osmolarity of as little at 1%-2%. However, thirst drive decreases with increasing age, suggesting that physicians should be especially vigilant when monitoring water balance in elderly patients.

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