A 72-year-old man is brought to the emergency department after passing out while getting out of bed in the morning. The episode lasted a few minutes, and the patient rapidly recovered consciousness. Currently he reports generalized weakness but no chest pain, shortness of breath, or abdominal pain. He had decreased appetite and watery diarrhea for 2-3 days before admission. The patient takes atorvastatin for hyperlipidemia and lansoprazole for frequent heartburn. He has a strong family history of coronary artery disease. Blood pressure is 120/70 mm Hg when supine and 98/50 mm Hg when standing. The jugular veins are flat when the patient is in a sitting position. Auscultation of the chest reveals clear lung fields and normal cardiac sounds. The abdomen is soft and nontender, without organomegaly. ECG is normal. Which of the following laboratory values is the most sensitive indicator of the patient's current condition?

Question

A 72-year-old man is brought to the emergency department after passing out while getting out of bed in the morning.  The episode lasted a few minutes, and the patient rapidly recovered consciousness.  Currently he reports generalized weakness but no chest pain, shortness of breath, or abdominal pain.  He had decreased appetite and watery diarrhea for 2-3 days before admission.  The patient takes atorvastatin for hyperlipidemia and lansoprazole for frequent heartburn.  He has a strong family history of coronary artery disease.  Blood pressure is 120/70 mm Hg when supine and 98/50 mm Hg when standing.  The jugular veins are flat when the patient is in a sitting position.  Auscultation of the chest reveals clear lung fields and normal cardiac sounds.  The abdomen is soft and nontender, without organomegaly.  ECG is normal.  Which of the following laboratory values is the most sensitive indicator of the patient's current condition?

Quizplus · Accepted Answer

This patient's recent history of diarrhea and poor appetite as well as flat neck veins on physical examination strongly suggests hypovolemia (depletion of water and salt). Elderly patients often have impaired baroreceptor sensitivity (autonomic failure), making them particularly susceptible to orthostatic hypotension and orthostatic syncope in the setting of hypovolemia. orthostatic hypotension Hypovolemia causes decreased renal perfusion, leading to activation of the renin-angiotensin-aldosterone system. Aldosterone stimulates aggressive sodium reabsorption in the collecting tubules of the kidney in an effort to sustain blood volume. Consequently, most patients with hypovolemia (unless taking diuretics or experiencing significant renal impairment) have decreased urine sodium. The fractional excretion of sodium (ratio of renal sodium clearance to renal creatinine clearance) is typically <1%. (Choice A) Water and salt depletion typically leads to an increase in hematocrit (hemoconcentration) rather than a decrease. (Choice B) Plasma renin activity is increased, rather than decreased, in the setting of hypovolemia. (Choice C) Hypovolemia triggers the nonosmotic release of antidiuretic hormone, which can sometimes lead to decreased serum sodium concentration (hypovolemic hyponatremia). However, many patients with volume depletion have a normal serum sodium concentration, and some demonstrate hypernatremia (water deficiency > salt deficiency). (Choice D) The urine osmolality in patients with hypovolemia is typically increased (rather than decreased) as urine is concentrated in an effort to conserve water. (Choice F) Secretion of brain natriuretic peptide is triggered by increased left or right ventricular blood volume (eg, heart failure). Ventricular blood volume is decreased in hypovolemia, leading to decreased (rather than increased) levels of this peptide. (Choice G) Aldosterone stimulates sodium exchange for potassium in the collecting tubules, resulting in increased potassium excretion and a tendency toward decreased (rather than increased) serum potassium. (Choice H) Urea is reabsorbed by the kidneys in the setting of hypovolemia, leading to decreased (rather than increased) urine urea nitrogen levels. In patients taking diuretics, low fractional excretion of urea (eg, <35%) is often used as an indicator of decreased renal perfusion. Educational objective: Hypovolemia is a common cause of orthostatic hypotension and orthostatic syncope, especially in elderly patients. Due to decreased renal perfusion and activation of the renin-angiotensin-aldosterone system, decreased urine sodium is usually present in patients with hypovolemia. References: A review of the etiology, associated comorbidities, and treatment of orthostatic hypotension. (http://www.ncbi.nlm.nih.gov/pubmed/23656967) Urinary diagnostic indices in acute renal failure: a prospective study. (http://www.ncbi.nlm.nih.gov/pubmed/666184)

A 72-Year-Old Man Is Brought to the Emergency Department After