USMLE Step 3 - Question 5

An 18 year old male with no significant past medical history passes out while running. He states that he was feeling a little dizzy prior to the event, but no chest pains or palpitations. His blood pressure is 115/85, heart rate 80, respirations 12, and he is afebrile. His physical examination reveals normal lung sounds, a II/VI mid-systolic crescendo-decrescendo murmur is heard at the right upper sternal border which increases in intensity with Valsalva, an S4 heart sound is also present. Laboratory studies are normal. What is the most appropriate treatment at this time?

A. Start a diuretic

B. Implantation of a pacemaker

C. Implantation of a defibrillator

D. Observation

E. Atrial septal defect

Hypertrophic obstructive cardiomyopathy or HOCM (previously called IHSS or idiopathic hypertrophic subaortic stenosis) is the most common cause of sudden cardiac death in young athletes. HOCM is associated with mostly exertional symptoms. During exercise (when the heart contracts harder), the asymmetrically hypertrophied interventricular septum obstructs blood from flowing out of the aortic valve resulting in a markedly reduced cardiac output. This leads to dizziness and syncope. Life threatening ventricular arrhythmia such as ventricular tachycardia also occur which can cause syncope. The murmur of HOCM can mimic that of aortic valve stenosis: a mid-systolic crescendo-decrescendo murmur heard at the right upper sternal border. The main differences that the murmur of HOCM increases in intensity with Valsalva (due to decreased left ventricular volume/chamber size allowing more obstruction to occur).


A defibrillator is indicated in any HOCM patient with syncope, documented ventricular arrhythmias, a family history of sudden cardiac death, interventricular septal thickness of > 30 mm or systolic dysfunction. Medical management includes beta-blockers or verapamil to prolong diastole and improve left ventricular filling resulting in less obstruction. Dehydration leads to reduced preload and less left ventricular filling which can worsen obstruction, this diuretics are contraindicated. Exercise should be completely restricted.