Cardiac Cycle Phases of Systole (Lecture Notes)

Introduction

  • Lecture presented by Jack O'Lee
  • Second part of a series on the cardiac cycle focusing on systole
  • Goals: Understand phases four to seven of the cardiac systole, including:
    • Pressure changes across chambers
    • Volume movement between chambers
    • Blood flow directions
    • Heart sounds
    • EKG changes and their correlation with cardiac mechanics

Overview of the Cardiac Cycle

  • Vickers Diagram: An essential tool for understanding the phases of the cardiac cycle
  • Systole includes four phases: numbered from four to seven
    • Phase Four: Isovolumic Contraction Phase
    • Phase Five: Rapid Ejection Phase
    • Phase Six: Reduced Ejection Phase
    • Phase Seven: Isovolumic Relaxation Phase

Phase Four: Isovolumic Contraction Phase

  • Duration: From the end of atrial systole to the start of ventricular ejection

  • Key Events:

    • Depolarization of ventricles indicated by the QRS complex on the EKG precedes contraction.
    • Ventricular pressure increases, starting at 5 mmHg
    • Mitral Valve Closure: At this point, left ventricular pressure exceeds left atrial pressure, preventing blood backflow into the atrium.
    • First Heart Sound: S1 is generated by the turbulence from the closure of the mitral valve.
    • Aortic valve remains closed throughout this phase.

  • Physiological Implications:

    • Ventricular chamber becomes a closed space, meaning volumes remain constant:
    • No blood enters from the atrium (mitral valve closed)
    • No blood flows into the aorta (aortic valve closed)

  • Pressure Changes: Rapid increase in left ventricular pressure due to contraction, approaching diastolic pressure in the aorta (up to 80 mmHg).

  • Venous Pressure: Also increases slightly as the atria continue to receive blood, indirectly reflecting ventricular pressure changes.

Phase Five: Rapid Ejection Phase

  • Initiation: Begins with the opening of the aortic valve when left ventricular pressure exceeds aortic pressure.
  • Blood Flow: Blood rapidly flows from the left ventricle into the aorta.
  • Aortic Pressure Increase: Blood inflow from the ventricle raises aortic pressure to around 120 mmHg (systolic pressure).
  • Volume Changes:
    • Left ventricular volume decreases from about 125 mL to approximately 75 mL.
  • Heart Sounds: No sound associated with valve opening; only closure produces sound.
  • Electrical Activity: At this point, electrical activity stabilizes (isoelectric on EKG).

Phase Six: Reduced Ejection Phase

  • Timing: Follows the peak systolic pressure.
  • Continuation of Blood Flow: Blood continues to flow into the aorta but at a decreasing rate.
  • Pressure Trends:
    • Ventricular and aortic pressure both start to decline after reaching their peak.
  • Volume Decrease: Further drop in ventricular volume from around 75 mL to about 50 mL.
  • Atrial Pressure Increase: Blood is still returning to the atria, causing an increase in atrial and venous pressure.
  • EKG Changes: The T wave begins during this phase, indicating ventricular repolarization.
  • Aortic Valve Closure: Marks the end of this phase when ventricular pressure falls below aortic pressure.

Phase Seven: Isovolumic Relaxation Phase

  • Start: Initiated by the closure of the aortic valve; marks the transition from contraction to relaxation.
  • Pressure Changes: Ventricular pressure declines significantly due to relaxation; a dicrotic notch is noted in the aortic pressure tracing due to backflow against the closed aortic valve.
  • Volume Stability: Ventricular volume remains constant as both valves are closed (no blood entering or leaving).
  • Second Heart Sound (S2): Produced by the closure of the aortic valve, heard uniformly in healthy subjects.
  • End Systolic Volume: Typically around 50 mL remains in the ventricle; important for defining cardiac output parameters:
    • End Diastolic Volume (about 120-125 mL) vs. End Systolic Volume (about 50 mL) creates the Stroke Volume of approximately 70 mL.
  • Physiological contributions: Ventricular muscle is completely relaxed, characterizing no further contraction.
  • Conclusion of Phases:
    • Review of the key aspects of systolic phases.
    • The systematic drop in ventricular and aortic pressure throughout the phases; the heart sounds correlated with valve closures (S1 & S2).

Summary of Cardiac Cycle Phases

  • Phases Four to Seven:
    • Phase Four: Isovolumic contraction (pressure rises rapidly, volume stable)
    • Phase Five: Rapid ejection (pressure peaks, ventricular volume decreases)
    • Phase Six: Reduced ejection (pressure declines, ventricular volume continues to decrease)
    • Phase Seven: Isovolumic relaxation (pressure declines steeply, volume remains constant)
  • Importance of EKG: Understanding the correlation between electrical activity (QRS and T wave) and mechanical ventricular events.