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Cardiac Cycle

  • The Cardiac Cycle is a basic description of the mechanical, hemodynamic, and electrophysiological events which occur during a single heart beat. We begin the Cardiac Cycle at the point of atrial contraction and divide the remaining events into distinct stages. A few basic principles and notes are worth mentioning before delving fully into the cardiac cycle. Firstly, opening and closure of the heart valves occurs purely passively and is actuated by the differential pressures of the chambers which the valve straddles. Secondly, systole is considered to occur when the ventricles are producing active tension which includes the Isometric Ventricular Contraction and Ventricular Ejection phases. Conversely, diastole is considered to occur when the ventricles are relaxed and thus includes Isometric Ventricular Relaxation, Ventricular Filling, and Atrial Ejection. Finally, because no valves exist between the right atrium and the vena cava, changes in right atrial pressure can be indirectly observed as changes in the Jugular Venous Pressure (JVP).
Atrial Ejection
  • Overview:
    • Atrial Eeection is the phase in which the left ventricle is filled with blood from the atria
    • The majority of filling occurs during the initial portion of this phase while the atria are completely at rest
    • In the final portion of the phase, termed "Atrial Systole", the atria contract and push roughly 25% of final Ventricular End Diastolic Volume (VEDV) into the ventricles
  • Events
    • Valves: Atrioventricular Valves (i.e. tricuspid valve and mitral valve) are open, allowing blood to enter ventricle from atria. This phase ends with Atrioventricular Valve closure
    • ECG: This phase immediately follows the P Wave which represents the spread of depolarization across the atria. Consequently, this phase occurs during the PR interval
    • Ventricular Volume: Increases from the Ventricular End Systolic Volume (VESV) to the Ventricular End Diastolic Volume (VEDV). Again, atrial contraction accounts for roughly 25% of the final VEDV
    • Ventricular Pressure: Slightly increases due to injection of blood from atria but then decreases due to closing of AV Valves
    • Aortic Pressure: Continues decline as blood runs out into circulation
    • Atrial Pressure: During the latter part of this phase, atrial systole causes a transient increase in atrial pressure which is termed the "A Wave"
    • Heart Sound: In individuals with left ventricular hypertrophy injection of atrial blood into the thickened ventricles will manifest as the S4 heart sound which is not audible in normal individuals
Isovolumetric Ventricular Contraction
  • Overview:
    • Isovolumetric Ventricular Contraction is characterized by contraction of ventricles prior to opening of the semilunar valves (i.e. aortic valve or pulmonic valves)
    • Although ventricular tension and pressure are generated during this phase, they are not sufficient to overcome the pressure within the aorta and pulmonary artery
    • Consequently, the pressures within the ventricles increase whilst their volume remains constant (i.e. isovolumetric contraction)
  • Events:
    • Valves: This phase begins with the snapping shut of the atrioventricular valves. Consequently, during this phase all valves are closed
    • ECG: This phase immediately follows the QRS complex which represents the rapid spread of depolarization throughout the ventricles
    • Ventricular Volume: Remains constant at the VEDV as no ejection of blood occurs during this phase
    • Ventricular Pressure: Rapidly rises as ventricles contract
    • Aortic Pressure: Reaches its lowest point at the end of this phase as blood continues to flow out into the circulation
    • Atrial Pressure: The increase in atrial pressure observed in this phase is termed the "C Wave" and represents the bulging of the atrioventricular valves into the atria as the ventricles contract
    • Heart Sounds: The snapping shut of the semilunar Valve results in the S1 heart sound
Ventricular Ejection
  • Overview
    • Ventricular Ejection occurs when the pressure within the ventricles overcomes that in the aorta and the pulmonary artery
    • With the opening of the aortic valve and pulmonic valve, blood effluxes from the ventricles and into the vasculature
    • Nearly 70% of the blood ejects in the first third of ventricular ejection and represents a period of "Rapid Ventricular Ejection" whereas the final 30% of blood ejects in the remainder of the time, representing a period of "Slow Ventricular Ejection"
  • Events
    • Valves: This phase begins with the opening of the aortic valve and pulmonic valve and ends with these same valves closing
    • ECG: The T wave occurs during the latter half of this phase and represents the repolarization of ventricular muscle
    • Ventricular Volume: Most of the ventricular volume is emptied during the first third of this phase (i.e. Raid Ventricular Ejection) while the remainder is ejected during the final two-thirds of the phase (i.e. Slow Ventricular Ejection), ultimately reaching the VESV
    • Ventricular Pressure: Ventricular pressure rises fairly rapidly, peaks roughly half way through this phase, and then begins to fall as ventricular ejection ends
    • Aortic Pressure: Because the Aortic Valve is open during this phase, the aortic pressure matches the ventricular pressure
    • Atrial Pressure: Falls initially during this phase as inward bulging of the atrioventricular valves relaxes but then slowly builds as the atria begin filling with the venous return
    • Heart Sounds: There are no heart sounds during this phase
Isovolumetric Ventricular Relaxation
  • Overview:
    • Isovolumetric Ventricular Relaxation is characterized by the end of active tension in the ventricular myocardium and thus relaxation of the ventricular muscle
    • However, because the ventricular pressure remains above that of the atria, no filling of the ventricles can occur and thus their volume remains constant during this phase
  • Events:
    • Valves: This phase begins with snapping shut of aortic valve and pulmonic valves and ends with opening of the AV valves. All valves are shut during this phase and no blood enters or exits the ventricles
    • ECG: No wave forms occur during this phase and the heart is fully repolarized
    • Ventricular Volume: Ventricular volume does not change as no blood enters or exits ventricles
    • Ventricular Pressure: Rapidly falls as ventricle relaxes
    • Aortic Pressure: The beginning of this phase is characterized by a small blip in the Aortic Pressure curve, termed the "Dicrotic Notch" which occurs due to snapping shut of the aortic valve. Following this event the aortic pressure slowly declines as blood moves into the circulation
    • Atrial Pressure: The atrial pressure continues to rise as venous return fills these chambers
    • Heart Sounds: Snapping shut of aortic valve and pulmonic valves at beginning of this phase results in the S2 heart sound
Ventricular Filling
  • Overview
    • Ventricular Filling occurs when the intraventricular pressure falls below that in the atria, allowing the AV valves to open and atrial blood to enter
    • Entry of blood into the ventricles is not linear and the majority occurs during the first part of this phase
    • The length of the ventricular filling phase is the most variable in all the cardiac cycle
    • When the heart rate is high, this phase is shortened whereas at low heart rates, this phase is lengthened
  • Events
    • Valves: Opening of the tricuspid valve and mitral valves allows entry of atrial blood into the ventricles
    • ECG: The heart is electrophysiologically quiet during this phase; however, the P wave occurs at the end of this phase and represents a wave of atrial depolarization which initiates the subsequent Atrial Systole
    • Ventricular Volume: Ventricular Volume increases rapidly during the initial parts of this phase as blood rushes in from the atria. However, as the ventricles approach their VEDV, the rate of blood entry slowly tables off
    • Ventricular Pressure: Ventricular pressure is slightly lower than the atrial pressure during this phase allowing entry of blood
    • Aortic Pressure: Continues to decline as blood runs out into circulation
    • Atrial Pressure: Atrial pressure is slightly higher than ventricular pressure during this phase allowing movement of blood from atria to ventricles. Because the building atrial pressure in the previous phase is suddenly released as the AV valves open, this is observed as the "V Wave"
    • Heart Sounds: No heart sounds are normally heard in adults but in children the rapid movement of blood between atria and ventricles can cause an S3