BY 312 Lecture (3/9)

Overview of the Cardiac Cycle

  • The cardiac cycle consists of alternating contractions and relaxations of the atria and ventricles.
    • During atrial systole, the ventricles are relaxed.
    • During ventricular systole, the atria are relaxed.
    • These phases are always opposite each other.

Principal Events in Cardiac Cycle

  • Two key events aid in understanding the cardiac cycle:
    1. Ventricular Filling
    • Occurs during ventricular diastole when ventricles are relaxed.
    1. Ventricular Ejection
    • Occurs during ventricular systole when ventricles contract.
  • Blood pressure measured reflects the pressure developed by the left ventricle before and after systole.
  • Measuring pulmonary blood pressure from right ventricular function is more challenging.

Sequence of the Cardiac Cycle

  • Refer to Figure 20.14 in the textbook for a visual representation of the cardiac cycle.

Atrial Systole

  • Atrial Systole: Atria contract, causing depolarization of the atria.
    • Blood is pumped through open valves into the ventricles.
  • The end of atrial systole coincides with the end of ventricular diastole.
  • End Diastolic Volume (EDV):
    • The volume of blood in the ventricle at the end of relaxation.
    • Typically around 130 mL of blood.
    • This volume signifies the end of atrial systole and the end of ventricular diastole.

Ventricular Systole

  • Begins after atrial systole.
  • While the ventricles contract, the atria relax.
  • Isovolumetric Contraction:
    • Occurs at the beginning of ventricular systole.
    • Characterized by a rise in pressure inside the ventricle without blood flow (lasts about 0.05 seconds).
    • Valves remain closed preventing blood from flowing back into atria.
    • It is a quick phase where pressure builds up within a closed chamber.
  • After isovolumetric contraction, ventricular pressure rises, leading to Ventricular Ejection:
    • Occurs when ventricular pressure exceeds the pressure of semilunar valves, forcing them open.
    • Blood is ejected into the aorta and pulmonary artery.

End Systolic Volume (ESV)

  • End Systolic Volume (ESV):
    • Volume of blood left in the ventricles after contraction (about 60 mL).
  • Stroke Volume (SV):
    • The amount of blood ejected per beat from each ventricle.
    • Calculated as:
      SV = EDV - ESV
    • Typical stroke volume is around 70 mL.

Relaxation Period

  • Lasts 0.04 seconds where both atria and ventricles are relaxed.
  • This phase also involves:
    • Isovolumetric Relaxation:
    • All four heart valves are closed.
    • Blood filling begins when atrial pressure exceeds ventricular pressure, and AV valves open.

Heart Valves and Auscultation

  • During the cardiac cycle:
    • AV valves are open during ventricular diastole and closed during ventricular systole.
    • Semilunar valves open during ventricular systole and close during diastole.
  • Auscultation:
    • Listening to heart sounds helps diagnose heart conditions.
    • Sounds:
    • S1 (Lubb): Closure of AV valves at the beginning of ventricular systole.
    • S2 (Dubb): Closure of semilunar valves at the end of ventricular systole.
    • Murmurs are abnormal sounds that can result from valve dysfunction.
    • Can indicate issues like stenosis (narrowed valves) or incompetency (leaky valves).

Summary of Cardiac Cycle Duration

  • Average time for one complete cardiac cycle is typically 0.8 seconds or about 75 beats per minute:
    • 0.1 seconds: Atrial contraction.
    • 0.3 seconds: Ventricular contraction.
    • 0.4 seconds: Relaxation of all chambers.

Cardiac Output and Cardiac Reserve

  • Cardiac Output (CO):
    • Defined as the amount of blood pumped per minute, calculated by:
      CO = SV imes HR where HR is heart rate.
    • For a male at rest, average cardiac output is 5.25 liters/min.
  • Cardiac Reserve:
    • Difference between resting cardiac output and the maximum cardiac output achievable during exercise.
    • Normal cardiac reserve is around four to five times resting output.

Factors Affecting Cardiac Output

  1. Preload: The amount of blood in the ventricles before contraction.
  2. Contractility: The strength of ventricular contraction.
  3. Afterload: The resistance opposing the blood ejection from the ventricles.
  • According to Frank Starling's Law: Increased stretch (preload) leads to a stronger contraction.
  • Exercise increases venous return, stretches heart muscles further, elevating cardiac output.

Selected Pathologies Related to Cardiac Function

Coronary Artery Disease (CAD)

  • Leading cause of death among men and women caused by atherosclerotic plaques in coronary arteries.
  • High levels of low-density lipoproteins (LDL) contribute to plaque formation.
  • Risk factors include smoking, high blood pressure, diabetes, high cholesterol, obesity, sedentary lifestyle, and family history.
  • Diagnosis involves stress testing and imaging techniques (like echocardiograms).
  • Treatment options:
    • Pharmacological (medications).
    • Surgical interventions (CABG, PTCA).
    • Physical therapy.

Congestive Heart Failure (CHF)

  • Characterized by the heart's inability to pump efficiently, leading to blood accumulation in ventricles.
  • Symptoms include swelling in extremities and cough with frothy sputum due to backflow of blood.

Patent Ductus Arteriosus

  • Failure of ductus arteriosus to close, often benign and easily surgically remedied.

Arrhythmias

  • Includes:
    1. Supraventricular Tachycardias: Abnormal fast heart rate originating in the atria.
    2. Premature Ventricular Contractions (PVCs): Rapid depolarization of the ventricles.
    3. Ventricular Tachycardia: Rapid heartbeat originating in the ventricles.
    4. Atrioventricular (AV) Blocks:
    • Three degrees of blocks defined by:
    • First-degree: Prolonged PR interval.
    • Second-degree: Dropped QRS complexes.
    • Third-degree: Complete heart block without transmission.