The patient is presenting with symptoms and findings consistent with atrial fibrillation. This is a predisposing factor for thromboembolic events that can occlude arteries in a number of organs. The dual blood supply (hepatic artery/portal vein) and collateral circulation of the liver decreases the possibility of ischemic infarction.
Atrial fibrillation is characterized by a chaotic rhythm with very high atrial/ventricular rates and no discernible P waves. It is a dangerous arrhythmia because of its ability to decrease cardiac output and increased risk for thromboembolism due to blood stasis in the atrium. Organs most at risk in descending order are the brain, myocardium, kidney, spleen, and liver. High metabolic demand coupled with lack of collateral circulation are two important factors that predispose an organ to ischemic damage secondary to an embolus.
The first step in answering this question is understanding that the ECG is consistent with atrial fibrillation. Remember that atrial fibrillation puts that patient at risk for systemic thrombo-embolization secondary to atrial blood stasis. Therefore, the question is asking which of the following organs are less likely to experience thrombo-embolization, and thus subsequent ischemia/infarction. An important fact to have in mind when answering this question is understanding that parenchymal necrosis of the liver is an uncommon complication due to the dual blood supply the liver receives from the hepatic artery and portal vein. Therefore, if there were to be an occlusion of one hepatic blood supply, the other vascular system can compensate.
Figure A is an EKG strip displaying irregularly irregular rhythm with no discernable p wave, indicative of atrial fibrillation.
Answer 1: The heart's metabolic demands are relatively high for its oxygen supply and myocytes are especially vulnerable to ischemic damage.
Answer 2: Stroke is a common complication of atrial fibrillation due to the sensitivity of many neurons to ischemic damage. Irreversible damage may occur in less than ten minutes.
Answer 3: The kidney is one of the most over-perfused organs in terms of oxygen demand and is therefore able to withstand a moderate degree of ischemia but not as much as the liver.
Answer 5: Splenic hemorrhage is a complication of infarction of the artery supplying this organ. Splenic infarcts are known to occur despite the spleen having collateral blood flow in the setting of atrial thromboembolism, sickle cell anemia, and endocarditis.
In general rate control (<80 bpm) is preferable to rhythm control except in patients in whom rate control is not achievable or in those who continue to have symptoms despite rate control. Rhythm control is an option for patients in whom rate control cannot be achieved or who have persistent symptoms despite rate control. The decision to anti-coagulate should be based on scoring systems such as the CHADS2 score which takes into account comorbidities, age, and prior incidence of stroke. (1)
Gutierrez C, Blanchard DG. Atrial fibrillation: diagnosis and treatment. Am Fam Physician. 2011 Jan 1;83(1):61-8. Review.
PMID:21888129 (Link to Abstract)
USMLE World Step 1 QBank Question #1875. Copyright © USMLEWorld, LLC 2012