By the end of this chapter the student will be able to:
List and describe the sequence of events (eg, electrical and mechanical) of the cardiac cycle through one complete cycle of contraction and relaxation in the heart.
Describe the pressure and volume relationship of the atria, ventricles, and aortae.
Explain the origin of heart sounds and the events of the electrocardiogram.
Describe the normal tracing of the venous pressure and its abnormality in valvular dysfunction.
Explain the basic principles of clinical imaging techniques used to assess cardiac performance including contraction, blood volume, blood flow, and cardiac output.
The cardiac cycle consists of a series of events that take place from one heartbeat to the next. The sequence and timing of these events are critical as they build on each other in a cyclical pattern with each heartbeat. The initiation of these events is the formation of an electrical impulse at the sinoatrial (SA) node, which is marked in the Electrocardiogram (ECG) tracing.
Electrical impulses cause depolarization and repolarization of the myocardium that precedes the contraction and relaxation of the heart. Consequently, an increase or decrease in atrial, ventricular, and aortic pressure develops. Changes in pressure help drive the blood through the heart and blood vessels. The pressure gradients cause the valves to open and close, which allows the blood to flow in one direction, while the blood volume in each chamber changes accordingly.
The opening and closing of cardiac valves generate turbulent blood flow that causes acoustic vibrations and results in the heart sounds. (Note: It is not the actual movement of the leaflets that produces the heart sounds.) One of the first steps in recognizing the signs and symptoms of heart disease is to have a solid understanding of the cardiac cycle. For example, the ability to recognize abnormal heart sounds and murmurs will aid in the diagnosis of associated pathology in most valvular heart disorders. This chapter begins with a brief overview of the functional aspects of blood flow and the relevant anatomy of the cardiac chambers and valves followed by the events of the cardiac cycle. A discussion of heart sounds and murmurs is also included along with a brief overview of imaging techniques for measuring various cardiac parameters.
Blood Flow through the Heart
For clarification purposes, the heart can be considered to be 2 pumps connected in series to each other as well as to the pulmonary and systemic circulations (Fig. 6.1). Therefore, the blood flow is equal at all points and each pump receives and ejects the same amount of blood flow (ie, venous return = cardiac output). Each pump has a weaker and a stronger chamber (ie, atrium and ventricle, respectively). The right atrium receives deoxygenated blood from the inferior and superior venae cavae and ...