USMLE Guide: Heart

Introduction

The USMLE (United States Medical Licensing Examination) is a three-step examination for medical licensure in the United States. This guide aims to provide an overview of key topics related to the heart that may be encountered in the USMLE.

Anatomy

The heart is a muscular organ located in the chest cavity, slightly to the left of the midline. It consists of four chambers: two atria and two ventricles. The atria receive blood returning to the heart, while the ventricles pump blood out of the heart. The heart is surrounded by a protective sac called the pericardium.

Physiology

The heart functions as a pump to circulate blood throughout the body. It performs two main actions: systole (contraction) and diastole (relaxation). The cardiac cycle consists of various phases, including atrial and ventricular systole, and atrial and ventricular diastole.

Cardiac Conduction System

The heart's electrical conduction system coordinates the contraction of its chambers. The system consists of the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, and Purkinje fibers. The SA node initiates the electrical signal, which then travels through the AV node, bundle of His, and Purkinje fibers, causing the heart muscle to contract in a coordinated manner.

Cardiac Cycle

The cardiac cycle is divided into systole and diastole. Systole consists of atrial and ventricular contraction, while diastole involves relaxation and filling of the chambers. Key events during the cardiac cycle include atrial and ventricular depolarization, opening and closing of heart valves, and ejection of blood into the circulation.

Heart Sounds

Heart sounds are important clinical indicators. The first heart sound (S1) is produced by the closure of the mitral and tricuspid valves at the beginning of systole. The second heart sound (S2) is produced by the closure of the aortic and pulmonary valves at the beginning of diastole. Abnormal heart sounds, such as murmurs, may indicate underlying heart conditions.

Coronary Circulation

The heart receives its own blood supply through the coronary arteries. The left coronary artery branches into the left anterior descending artery (LAD) and the circumflex artery. The right coronary artery supplies the right atrium, right ventricle, and part of the interventricular septum. Coronary artery disease can lead to myocardial infarction (heart attack) if blood flow is compromised.

Cardiac Output

Cardiac output is the amount of blood pumped by the heart per minute. It is calculated by multiplying heart rate (beats per minute) by stroke volume (amount of blood pumped by each beat). Cardiac output is influenced by factors such as heart rate, stroke volume, and systemic vascular resistance.

Common Cardiac Pathologies

• Coronary Artery Disease (CAD): Narrowing or blockage of the coronary arteries, often due to atherosclerosis.
• Myocardial Infarction (MI): Death of heart muscle tissue due to prolonged lack of blood supply.
• Congestive Heart Failure (CHF): Inability of the heart to pump blood effectively, leading to fluid accumulation in the lungs and other tissues.
• Arrhythmias: Abnormal heart rhythms, which can be tachycardia (fast) or bradycardia (slow).
• Valvular Heart Disease: Abnormalities of the heart valves, such as stenosis or regurgitation.

Conclusion

This USMLE guide provides a brief overview of key topics related to the heart. Familiarizing yourself with the anatomy, physiology, conduction system, and pathologies of the heart will help you succeed in the USMLE and in your future medical practice.