A 34-year-old woman comes to the hospital with a 4-day history of abdominal cramps, nausea, and watery diarrhea. Today she developed dizziness on standing. Her child has had similar symptoms recently. The patient has no prior medical conditions and takes no medications on a regular basis. Blood pressure is 124/82 mm Hg while supine and 100/70 on standing; pulse is 98/min. Examination shows dry mucous membranes. The abdomen is soft and nontender. Laboratory results are as follows: Which of the following changes are most likely to be seen in this patient?

Question

A 34-year-old woman comes to the hospital with a 4-day history of abdominal cramps, nausea, and watery diarrhea. Today she developed dizziness on standing. Her child has had similar symptoms recently. The patient has no prior medical conditions and takes no medications on a regular basis. Blood pressure is 124/82 mm Hg while supine and 100/70 on standing; pulse is 98/min. Examination shows dry mucous membranes. The abdomen is soft and nontender. Laboratory results are as follows:

Which of the following changes are most likely to be seen in this patient?

Quizplus · Accepted Answer

This patient with gastroenteritis has evidence of hypovolemia (dry mucous membranes, orthostatic dizziness/hypotension) and acute kidney injury. Decreased extracellular fluid volume stimulates compensatory mechanisms directed at maintaining systemic blood pressure and tissue oxygenation. This is largely driven by the sympathetic nervous system and the kidneys: Activation of the renin-angiotensin-aldosterone system (RAAS) leads to elevated levels of angiotensin II (ATII), a potent vasoconstrictor that stimulates the release of aldosterone and endothelin 1. These increase sodium and water reabsorption and systemic vascular resistance to help maintain blood pressure. Vasopressin (antidiuretic hormone) is released by the posterior pituitary in response to increased serum osmolarity and decreased systemic pressure; it increases urea and free water reabsorption by the renal collecting duct. Increased sympathetic activity results in release of circulating norepinephrine and other catecholamines, which increases systemic vasoconstriction, renal sodium and water reabsorption, and heart rate. These neurohumoral mechanisms promote volume expansion and increase blood pressure, helping to maintain tissue perfusion. Laboratory studies characteristically reflect sodium, water, and urea reabsorption by the kidney, including low urine sodium (<20 mEq/L), low fractional excretion of sodium (<1%), and elevated blood urea nitrogen to creatinine ratio (>20:1). Urinary sediment is typically bland or may show hyaline casts (which suggest increased urine concentration). Educational objective: Hypovolemia triggers a variety of compensatory mechanisms to improve tissue perfusion. These include activation of the renin-angiotensin-aldosterone system (resulting in increased aldosterone and endothelin release), increased vasopressin release, and increased sympathetic tone.

A 34-Year-Old Woman Comes to the Hospital with a 4-Day