CARDIAC CYCLE

The Cardiac Cycle

  • Phases of Contraction and Relaxation: The cardiac cycle alternates between systole (contraction and emptying) and diastole (relaxation and filling).

  • Systolic Pressure: Refers to the peak pressure in blood vessels during systole.

  • Diastolic Pressure: The lowest pressure during ventricular diastole.

  • ECG Correlation:

    • Atrial Systole: Starts after the P wave.

    • Ventricular Systole: Begins near the end of the R wave and ends just after the T wave.

The cycle will be explained by correlating

  • ECG readings,

  • intracardiac pressures,

  • blood volume changes,

  • cardiac valve activity,

  • and heart sounds.

Slide 6-13: Phases of the Cardiac Cycle

The following sequence details each phase of the cardiac cycle:

  1. Mid Ventricular Diastole:

    • Corresponds to the TP interval on the ECG, where both atria and ventricles are relaxed.

    • Blood passively flows from the venous system into the atria and through open AV valves into the ventricles.

    • Ventricular volume gradually increases due to passive filling.

  2. Late Ventricular Diastole:

    • The SA node reaches threshold, causing atrial depolarization (seen as the P wave on the ECG).

    • Atrial contraction follows, increasing atrial pressure and pushing additional blood into the ventricles.

    • The AV valve remains open as atrial pressure exceeds ventricular pressure, allowing complete ventricular filling.

  3. End of Ventricular Diastole:

    • Atrial contraction concludes, and ventricular filling is complete.

    • The volume in the ventricle at this stage is termed end-diastolic volume (EDV), averaging approximately 135 mL.

  4. Ventricular Excitation and Onset of Ventricular Systole:

    • The QRS complex signifies ventricular depolarization, initiating ventricular contraction.

    • Ventricular pressure rises sharply exceeding atrial pressure as the ventricles begin to contract, closing the AV valve and producing the first heart sound (S1).

  5. Isovolumetric Ventricular Contraction:

    • During this brief phase, both the AV and aortic valves are closed, creating a closed chamber.

    • Ventricular pressure rises without a change in blood volume.

  6. Ventricular Ejection:

    • As ventricular pressure surpasses aortic pressure, the aortic valve opens, initiating blood ejection into the aorta.

    • Stroke Volume (SV): The amount of blood ejected per beat, approximately 70 mL.

    • This phase marks the active ejection component of ventricular systole.

  7. End of Ventricular Systole:

    • The ventricles do not completely empty; the remaining blood volume is termed end-systolic volume (ESV), around 65 mL.

  8. Ventricular Repolarization and Onset of Ventricular Diastole:

    • The T wave on the ECG indicates ventricular repolarization.

    • As ventricular pressure falls below aortic pressure, the aortic and pulmonary valves close, producing the second heart sound (S2).

  9. Isovolumetric Ventricular Relaxation:

    • Both the aortic valve and AV valve remain closed as ventricular pressure is still higher than atrial pressure, resulting in no change in blood volume.

  10. Ventricular Filling:

    • Once ventricular pressure drops below atrial pressure, the AV valve opens, allowing blood to flow from the atria into the ventricles.

    • The Ventricular Diastole Phase: Encompasses both isovolumetric relaxation and the filling period, reinitiating the cardiac cycle as the SA node fires again.

Slide 14: Duration of the Cardiac Cycle

  • Resting Cardiac Cycle Duration: One cycle lasts 0.8 seconds when at rest.

    • Ventricular Systole: 0.3 seconds.

    • Ventricular Diastole: 0.5 seconds.