Straight to the Heart

As EMS providers, we are inundated with information, guidelines and protocols regarding cardiac emergencies and conditions. Even in the absence of a cardiac emergency, patients commonly give a history of a "heart murmur," "a hole in my heart," "mitral valve prolapse" and other cardiac-related conditions. This article provides a review of cardiac anatomy as it relates to conditions you may encounter in the field.

THE BASICS
     In its normal position, the heart occupies the inferior part of the mediastinum (chest cavity) with two-thirds of its bulk to the left of midline. The terms "right" and "left" as they refer to the heart are slightly misleading. The heart lies in such a position within the chest cavity that the right atrium and ventricle are, in fact, mostly anterior structures, while the left atrium and ventricle are mostly posterior structures.

     The heart is a four-chambered pump that beats approximately 100,800 times per day. The top two chambers are the right atrium and left atrium; the bottom chambers are the right and left ventricles. Blood returns to the heart's right atrium chiefly from three vessels: the superior vena cava, the inferior vena cava and the coronary sinus. The superior vena cava is a large vein that drains blood from the head, arms and chest. The inferior vena cava is a large vein that returns blood from the legs, pelvis and abdomen. The coronary sinus is the vessel into which the cardiac veins drain. These vessels all drain into the right atrium.

     Flow of blood through the cardiac circulation starts in the right atrium, with low-oxygenated blood returning from the body, and ends in the left ventricle, with highly oxygenated blood leaving the heart to nourish the body's tissues. Blood flows through the heart's chambers and valves, which act as gates to prevent blood from going backwards.

     Blood passes from the right atrium through the tricuspid valve and into the right ventricle. The right ventricle contracts, and the pressure inside the ventricle increases. The pressure surpasses the pressure in the right atrium, closing the tricuspid valve and preventing regurgitation of blood from the right ventricle back into the right atrium. The pressure in the right ventricle rises until it is greater than the pressure in the next stop, a large vessel called the pulmonary trunk. This causes the pulmonic valve to open, allowing blood to eject into the pulmonary trunk. The pulmonary trunk divides into a pair of left pulmonary arteries and a pair of right pulmonary arteries. You may think of arteries as bearing bright red oxygen-rich blood. The pulmonary arteries are unique, as they carry blood that is not highly oxygenated because it has not yet reached the lungs.

     So pulmonary arteries bring blood into the lungs, where it is oxygenated and returned to the left atrium by pulmonary veins. The valve between the left atrium and left ventricle is the bicuspid or mitral valve, so named because, to early anatomists, it resembled a bishop's two-pointed hat--the miter. Pressure in the left ventricle soon eclipses that in the left atrium and closes the bicuspid (mitral) valve, preventing regurgitation of blood from the left ventricle back into the left atrium. Soon pressure in the left ventricle rises until it is greater than the pressure in the aorta. This causes the aortic valve to open, allowing oxygenated blood to eject into the aorta and go on to perfuse the body organs.

     From the left ventricle, oxygenated blood is pushed up past the aortic valve into the ascending aorta. Note that blood is pumped "up" from the left ventricle into the aorta, not down. The left ventricle is thicker than the right ventricle, as it needs to generate more force to pump blood up into the aorta than the right ventricle does to pump blood "next door" to the lungs.