Cardiac Cycle
The electrocardiogram (EKG) is a key to the mechanical events of the cycle because electrical events precede mechanical events and the phases of contraction and relaxation are described based on the activity of the ventricle.
The cardiac cycle consists of the following events:
Diastasis: The cycle begins at the end of diastole. The pressure in the aorta is falling and the venous pressure exceeds the pressure within the chambers. The major portion of ventricular filling occurs during the period known as diastasis. This extends from the end of the rapid filling phase in one cycle to the atrial contraction in the next cycle. This phase is the P wave on the EKG. Duration = 0.19 sec.
Atrial Contraction: During the diastolic period (after P wave) the atria contract. This increases the intra-atrial pressure by about 5 mm HG. The ventricular volume and pressure increase slightly due to the atrial ejection of blood. The atria eject blood into the ventricles, this produces a rise in ventricular pressure and the A-V valves close as the blood starts to flow back into the atria. Atria contribute about 20% to ventricular filling. The ventricles begin to depolarize as shown by the beginning of the QRS complex. Duration = 0.11sec.
Ventricular Isometric (Isovolumetric) Contraction: Begins shortly after the onset of QRS. The aortic and pulmonary valves are closed and A-V valves are either closed or close almost immediately after the ventricular contraction begins. At this point the volume of the blood remains constant while the rpessure rises rapidly. Aortic pressure reaches about 80 mm Hg at the beginning of the ventricular contraction. The ventricular muscle fibers contract, however, since the A-V are closed no blood flows retrograde into the atria. Duration = 0.05sec.
Rapid Ejection: As soon as the pressure in the left ventricle exceeds the pressure in the aorta, the aortic valves open and blood flows rapidly from the ventricle into the aorta. This corresponds with a sharp decrease in ventricular volume. Atrial pressure falls below venous pressure, and the atria begin to fill at this time. At the end of maximal ejection the onset of the T wave occurs signalizing the beginning of ventricular repolarization. Duration = 0.09 sec.
Decreased Ejection: Following rapid ejection the rate of outflow from the ventricle decreases and the ventricular and aortic pressures start to decrease. At this point muscle fibers have reached a shorter length and can no longer contract forcefully. The venous pressure is still greater than atrial pressure, and atria are still filling. This is characterized by the end of the T wave on the EKG. Duration = 0.13sec.
Isometric (Isovolumetric) Relaxation: When the ventricular ejection falls to zero, the LV pressure falls below the pressures in the aorta and pulmonary artery which causes the aortic and pulmonary valves to close. Ventricles relax and A-V valves remain closed while the ventricular pressure exceeds atrial pressure. The amount of blood cannot change because the valves at both ends of the ventricles are closed. Duration = 0.08sec.
Rapid Ventricular Filling: As soon as ventricular pressure falls below atrial pressure the A-V valves open and ventricular filling begins. During this period the flow of blood from the aorta to the peripheral arteries continues and the aortic pressure falls slowly. The phase of rapid inflow is followed by a variable phase of slow filling during which filling is much less rapid, and the ventricle is almost at its maximum diastolic size. Duration = 0.11 sec.