Vomiting, Hypernatremia, and Orthostatic Change in an Older Adult
An 78-year-old woman is brought to the emergency department after 3 days of vomiting and almost no oral intake. She is drowsy and has dry mucous membranes. She lives alone and has had no fever, diarrhea, or blood in the vomitus. Supine vital signs: HR 118, BP 96/60; on standing: HR 132, BP 82/50. Urine output over the past 6 hours is 15 mL of dark, concentrated urine. Weight is 2.5 kg below her recorded weight from one week ago. Labs: serum sodium 154 mEq/L, potassium 3.1 mEq/L, BUN 38 mg/dL, creatinine 1.4 mg/dL (baseline 0.8). Serum osmolality is elevated. The provider orders IV fluids, and the nurse must anticipate how the fluid will shift between compartments and what signs indicate correction or overload.
1. Which fluid compartment has lost the most volume, and how does the serum sodium explain the patient's drowsiness?
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The extracellular fluid (ECF) has lost the most volume: vomiting removes isotonic fluid, but the patient has replaced almost none of it, so both plasma and interstitial volume fall. The serum sodium of 154 mEq/L makes the remaining ECF hypertonic. Water moves by osmosis from the intracellular fluid (ICF) into the hypertonic ECF, so cells, including brain cells, shrink. Brain-cell shrinkage produces the drowsiness and confusion. The elevated BUN (38 mg/dL) with a creatinine only mildly above baseline also supports volume depletion rather than isolated kidney failure, because a low effective circulating volume increases urea reabsorption. The history of vomiting, dry mucous membranes, weight loss, and scant dark urine all point in the same direction. The key reasoning is that sodium is a measure of concentration, not total body sodium, and a high sodium here reflects water loss that is proportionally greater than sodium loss.
2. Explain why the urine is dark and scant, naming the hormonal pathways involved.
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The effective circulating volume is low, so two systems activate. First, hypothalamic osmoreceptors detect the elevated serum osmolality and trigger antidiuretic hormone (ADH) release from the posterior pituitary. ADH inserts aquaporin-2 channels in the collecting duct, increasing water reabsorption and concentrating the urine. Second, reduced renal perfusion and reduced stretch at the baroreceptors activate the renin-angiotensin-aldosterone system. Renin leads to angiotensin II, which causes vasoconstriction and stimulates aldosterone; aldosterone increases sodium reabsorption in the distal nephron, and water follows. The combination of ADH-driven water retention and aldosterone-driven sodium retention produces a small volume of concentrated urine. The darkness reflects concentrated solutes, not blood, unless a dipstick shows otherwise. The low urine output also reflects a reduced glomerular filtration rate from hypovolemia. These are appropriate compensatory responses, but they cannot restore volume without fluid intake or IV therapy.
3. Explain the orthostatic fall in blood pressure and the rise in heart rate using the baroreceptor reflex.
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When the patient stands, blood pools in the lower extremities and venous return falls. Less blood fills the heart, so stroke volume and cardiac output drop, and blood pressure begins to fall. Baroreceptors in the carotid sinus and aortic arch sense reduced stretch and reduce their firing to the medulla. The medulla increases sympathetic outflow and reduces parasympathetic outflow. Sympathetic effects include a faster heart rate and more forceful contraction, plus arteriolar and venous constriction, which raises peripheral resistance and helps return blood to the heart. In this patient the reflex is insufficient because volume is already depleted, so standing produces a fall from 96/60 to 82/50 and a further rise in heart rate from 118 to 132. The orthostatic drop is therefore evidence of inadequate compensation, not a separate disease. A fall of 20 mmHg systolic or 10 mmHg diastolic on standing is considered abnormal.