USMLE Question Breakdown: Structural Changes to the Heart
- Sep 05, 2017
The following is a sample USMLE Step 1 question, with explanation:
A 62-year-old man presented to the emergency room with a 45-minute history of severe chest pain. EKG shows ST elevations in leads II, III, and aVF. Shortly after being admitted to the hospital, the patient developed ventricular fibrillation and passed away despite resuscitation attempts. On autopsy, the heart was found to have an enlarged left atrium, an increase in left ventricular mass, and an increase in left ventricular wall thickness.
Which of the following conditions is the most likely the cause of structural changes to the patient’s heart?
- Atrial septal defect
- Mitral insufficiency
- Long-standing hypertension
- Mitral Stenosis
- Recent myocardial infarction
- Ventricular fibrillation
Approach to the Step 1 Question:
This question begins with a typical presentation of a myocardial infarction, but then pivots its focus to autopsy findings and structural changes to the heart. While most questions require attention to demographic information and past medical history, the autopsy findings provide the key information here that allows us to select the correct answer. So, let’s break down the structural findings first!
We’ll begin with the enlarged left atrium. Left atrial enlargement is a chronic process, which is important to keep in mind while selecting an answer choice. It is unlikely that a myocardial infarction or subsequent ventricular fibrillation, both acute processes, would cause major structural remodeling of the heart in such a short period of time. Left atrial enlargement also tells us that there has been a long-term increase in left atrial volume. This can be caused by poor atrial pumping (i.e. atrial fibrillation), obstruction of flow from the atrium to the ventricle (mitral stenosis), backflow to the atrium through a weak valve (mitral insufficiency) or a backflow of blood from a dilated or hypertrophic left ventricle (systolic/diastolic heart failure, aortic stenosis). Now lets look at the ventricular findings to see how they relate to our left atrial enlargement.
The ventricle is described as having an increase in muscle mass and in wall thickness, which we should be able to recognize as ventricular hypertrophy. There are two types of ventricular hypertrophy that we need to know for Step 1: Eccentric hypertrophy and concentric hypertrophy. Understanding the difference between these two presentations, and their pathogenesis, is very important for the exam.
Eccentric LVH is associated with a dilation of the left ventricular cavity and generally reflects chronic volume overload. Increases in left ventricular volume are often due to aortic or mitral regurgitation. It is also a common sequela of myocardial infarctions, as a dysfunctional ventricular myocardium leads to pooling of blood. This will often lead to systolic heart failure caused by diminished ventricular contraction strength, which can be identified on auscultation by the presence of S3.
Concentric LVH is associated with a thickened ventricular wall and decreased ventricular cavity volume. These changes reflect a chronic pressure overload and are most commonly seen due to aortic stenosis or hypertension. Concentric LVH will lead to diastolic heart failure, characterized by poor ventricular filling, which can be identified on auscultation by the presence of S4.
Applying our Medical Knowledge:
The structural changes in the left ventricle (increased mass and wall thickness) are most consistent with those of concentric hypertrophy. As discussed above, concentric hypertrophy is caused by a chronic increase in pressure on the ventricle. The left atrial enlargement is due to an increase in ventricular pressure being transmitted to the atria, ultimately causing dilation of the chamber.
Once identifying a chronic pressure overload as the cause of the structural heart changes, we can easily identify “C. Long-standing hypertension” as the correct answer choice for this question.
For exam-prep and USMLE good practice, let us effectively eliminate the other answer choices:
- An atrial septal defect will be characterized by a left-to-right shunt due to higher left atrial pressures. This would cause a right-sided volume overload, leading to dilation of both the right atrium and right ventricle (eccentric hypertrophy).
- Mitral insufficiency would lead to a volume overload of the left atrium causing left atrial enlargement. The left ventricular end diastolic volume would increase as well, ultimately causing left ventricular volume overload and eccentric hypertrophy, not concentric.
- Mitral stenosis would increase the pressure in the left atrium causing a hypertrophy and dilation of the left atrium.
- A recent myocardial infarction could cause an acute volume overload in both the left ventricle and left atrium, but would not cause immediate structural changes to either chamber.
- Ventricular fibrillation, like a myocardial infarction, would acutely compromise ventricular systolic function leading to volume overload of both the left ventricle and left atrium without causing permanent structural change.