Fluid and Electrolytes (3)

Fluid balance has two separate questions: Is total body water adequate, and is enough blood volume circulating to perfuse organs? A patient can have oedema but still have inadequate effective arterial volume when fluid has moved into the interstitial space or a third space. A fluid deficit is a hypovolaemic state in which loss of total body water greater than approximately 5% of body weight reduces intravascular volume. The resulting fall in renal perfusion activates the renin–angiotensin–aldosterone system and antidiuretic hormone. The kidneys retain sodium and water, urine output falls, and BUN and creatinine rise in a prerenal pattern. Common triggers include poor intake, gastrointestinal losses, fever, excessive diuresis, and renal losses. Fluid excess occurs when intake or retention exceeds the ability to excrete fluid. Acute kidney injury, chronic kidney disease, heart failure, and excessive IV replacement can all contribute. The earliest dangerous consequence may be pulmonary interstitial oedema: increasing work of breathing, crackles, hypoxaemia, and a rising oxygen requirement. Peripheral oedema is less immediately threatening than impaired oxygenation, but rapid weight gain and increasing oedema often signal a worsening...