Cardiac cycle: Control and synchronicity

Published on 07/02/2015 by admin

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Cardiac cycle: Control and synchronicity

Brantley D. Gaitan, MD

The cardiac cycle describes the succession of atrial and ventricular events that make up a period of contraction (systole) followed by a period of relaxation (diastole) (i.e., a single heart beat). These periods are further subdivided into phases.

Systole comprises two phases: isovolumic contraction and ejection. On initiation of myocardial contraction, the ventricular pressure rises abruptly, closing the atrioventricular (AV) valves, and continues to increase for approximately 0.03 sec (isovolumic contraction). Once the ventricular pressure sufficiently exceeds the pressure in either the aorta or the pulmonary artery, the semilunar valves open, and ejection occurs.

At the end of systole, the second period, diastole, or relaxation of the ventricle—takes place during four phases: isovolumic relaxation, rapid inflow, diastasis, and atrial systole (the final three phases constitute ventricular filling) (Figures 33-1 and 33-2). During isovolumic relaxation, the ventricular pressure rapidly drops below that of the arterial pressure, and the semilunar valves snap shut. For approximately 0.06 sec, the ventricular pressure continues to decrease without any change in ventricular volume. Once the ventricular pressure drops below the atrial pressure, the AV valves open and blood rapidly fills the ventricles. This period of rapid blood inflow is augmented by a negative intracavitary pressure (diastolic suction) created by the rapid myocardial relaxation. Once relaxation is complete and elastic distention of the ventricle begins, the slowing of blood return, termed diastasis, immediately precedes atrial systole. Effective atrial systole contributes up to about 20% of ventricular filling and completes the period of diastole.

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Figure 33-1 The simultaneous events of the cardiac cycle for left ventricular function, showing changes in left atrial pressure, left ventricular pressure, aortic pressure, ventricular volume, the electrocardiogram, and the phonocardiogram. (From Guyton AC, Hall JE. Textbook of Medical Physiology. 11th ed. Philadelphia: Elsevier Saunders; 2005.)
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Figure 33-2 Relationship between left ventricular volume and intraventricular pressure during diastole and systole. The diastolic pressure curve is determined by filling the heart with blood and measuring the pressure immediately before ventricular contraction occurs. Until the volume of the left ventricle rises above ~ 150 mL the diastolic pressure changes minimally but above this volume the pressure increases rapidly because the myofibrils are stretched to their maximum. During ventricular contraction the systolic pressure increases even at low ventricular volumes and reaches a maximum pressure at volumes of ~ 150 mL. At volumes >150 mL the systolic pressure may decrease as actin and myosin are so stretched that contraction is less than optimal.
The maximum systolic pressure for the left ventricle is between 250 and 300 mm Hg but varies widely between individuals. The heavy red lines show a volume-pressure curve during a normal cardiac cycle, with EW representing the net external work of the heart. (From Guyton AC, Hall JE. Textbook of Medical Physiology. 11th ed. Philadelphia: Elsevier Saunders; 2005.)

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