Cardiac Cycle
Aortic pressure is measured by inserting a pressure catheter into the aorta from a peripheral artery, and the left ventricular pressure is obtained by placing a pressure catheter inside the left ventricle and measuring changes in intraventricular pressure as the heart beats. Left atrial pressure is not usually measured directly, except in investigational procedures. Ventricular volume changes can be assessed in real time using echocardiography or radionuclide imaging, or by using a special volume conductance catheter placed within the ventricle.
A single cycle of cardiac activity can be divided into two basic stages. The first stage is diastole, which represents ventricular filling and a brief period just prior to filling at which time the ventricles are relaxing. The second stage is systole, which represents the time of contraction and ejection of blood from the ventricles.
To analyze these two stages in more detail, the cardiac cycle is usually divided into seven phases. The first phase begins with the P wave of the electrocardiogram, which represents atrial depolarization. The last phase of the cardiac cycle ends with the appearance of the next P wave. In order to understand the events of the cardiac cycle, the reader should first review basic cardiac anatomy.
The entire cardiac cycle diagram, which contains information on aortic, left ventricular and left atrial pressures, along with ventricular volume, heart sounds and the electrocardiogram, is shown above. Detailed descriptions of each phase can be obtained by clicking on each of the seven phases listed below.
- Phase 1 - Atrial Contraction
- Phase 2 - Isovolumetric Contraction
- Phase 3 - Rapid Ejection
- Phase 4 - Reduced Ejection
- Phase 5 - Isovolumetric Relaxation
- Phase 6 - Rapid Filling
- Phase 7 - Reduced Filling
Mini-Lecture: Cardiac Cycle (Time = 16.5 minutes)
RK Revised 12/02/09