Given this patient's history of myocardial infarction and findings on physical exam, his lower extremity edema is likely due to an increase in capillary pressure secondary to congestive heart failure. An increase in capillary pressure occurs in congestive heart failure as a result of increased plasma fluid volume backing up into the pulmonary and systemic circulations.
Starling forces determine fluid movement through capillary membranes. Net filtration pressure is dependent on the balance between the hydrostatic pressure gradient and the oncotic pressure gradient across the capillary. In heart failure, increased plasma fluid volume results in an increase in capillary pressure, thereby pushing fluid out of the capillaries and into the interstitium. There is a compensatory increase in lymphatic drainage in order to prevent the development of interstitial edema; however, as more fluid accumulates, this compensatory mechanism is overcome and overt peripheral edema develops.
King et al. review the diagnostic features and evaluation considerations in patients with heart failure. Patients most commonly present with decreased exercise tolerance with dyspnea, fatigue, generalized weakness, and fluid retention, with peripheral or abdominal swelling and possibly orthopnea. They highlight that the most important consideration when categorizing heart failure is whether left ventricular ejection fraction is preserved or reduced (less than 50 percent).
McMurray provides a review of the guidelines for recommended treatment in ambulatory patients with systolic heart failure. Pharmacologic agents include those that provide relief of symptoms only (i.e. diuretics) and those that also modify the course of the disease (i.e. ACE, ARB, beta-blocker). Furthermore, lifestyle changes, such as restriction of sodium intake are routinely recommended, although this recommendation is based on little evidence.
Illustration A summarizes the four Starling forces that determine fluid movement through capillary membranes.
Answer 2: A decrease in plasma proteins would cause excess fluid outflow into the interstitium and occurs in conditions such as nephrotic syndrome and liver failure.
Answer 3: An increase in capillary permeability would cause excess fluid outflow into the interstitium and occurs in conditions such as infections and burns.
Answer 4: An increase in colloid osmotic pressure would cause excess fluid outflow into the interstitium and occurs in conditions such as lymphatic blockage.
Answer 5: An increase in interstitial fluid pressure would not cause excess fluid outflow into the interstitium.
King M, Kingery J, Casey B. Diagnosis and evaluation of heart failure. Am Fam Physician. 2012 Jun 15;85(12):1161-8.
PMID:22962896 (Link to Abstract)
McMurray JJ. Clinical practice. Systolic heart failure. N Engl J Med. 2010 Jan 21;362(3):228-38.
PMID:20089973 (Link to Abstract)
USMLE World Step 1 QBank Question #1578. Copyright © USMLEWorld, LLC 2012