Dehydration

Dehydration is a reduction in total body water. It develops when fluid losses exceed intake, including losses through vomiting or diarrhea, excessive urine production, sweating and fever, or third-spacing into tissues and body spaces. A patient may also become dehydrated simply because illness, weakness, dysphagia, confusion, or lack of access to fluids prevents adequate drinking. The immediate clinical danger is falling circulating plasma volume. Less venous return means less ventricular filling and a lower stroke volume, so the heart often compensates with tachycardia. As volume loss progresses, blood pressure and tissue perfusion fall. The kidneys detect reduced renal blood flow and conserve water through the renin–angiotensin–aldosterone system and antidiuretic hormone; urine output decreases as the body tries to preserve circulation. Dehydration and hypovolemia overlap but are not identical terms. Dehydration describes a body-water deficit, whereas hypovolemia describes inadequate intravascular volume. A patient with third-spacing can have substantial fluid somewhere in the body while remaining intravascularly depleted. That is why visible edema does not automatically rule out clinically important volume loss. The bedside priority is perfusion, not thirst alone. A dry...