534 Week 4 May 20 8 am Cardiac Physiology and Anatomy Notes

Cardiac Anatomy and Electrophysiology

Cardiac Conducting System

  • Key anatomical characteristics of the heart's conducting system:
    • Sinoatrial (SA) node
    • Atrioventricular (AV) node
    • His bundle
    • Bundle branches (right and left)
    • Purkinje system

Cardiac Muscle Cells

  • Two types of cardiac muscle cells exist, differing morphologically and histologically.
    • Muscle cells in the SA/AV nodes (pacemakers are different from the majority of heart muscle cells.
    • Most heart muscle cells are involved in the electrical tracing and conduction system of the heart.

Pacemaker Cells

  • SA and AV nodes contain "pacemaker cells."

    • These specialized cardiac muscle cells can generate electrical potentials/signals.
    • SA node: 60-100 beats per minute (BPM)
    • AV node: 40-70 BPM (slower than SA node)

  • His bundle and Purkinje fibers have different conduction velocities, with the AV node being the slowest.

  • Pacemaker cells trigger/stimulate non-pacemaker cells.

Atrial Depolarization

  • Impulse from SA node rapidly depolarizes both atria.
  • The interval between SA node signal and AV node arrival corresponds to atrial depolarization.
  • Conduction velocity slows from 1 m/s in atrial tissue to 0.05 m/s in nodal tissue.
  • After AV node delay, the signal moves rapidly (1 m/s in His bundle, 4 m/s in Purkinje fibers).

Ventricular Synchronization

  • Rapid conduction in the ventricles:

    • His bundle: 1 m/s
    • Purkinje fibers: 4 m/s (very fast)

  • The arrangement allows synchronized atrial and ventricular contractions.

    • Minimizes electrical feedback between chambers, preventing dysrhythmias.
    • Dysrhythmias occur when abnormal electrical activity arises.

Electrocardiogram (ECG)

  • ECG basics:
    • P wave: atrial depolarization
    • QRS complex: ventricular depolarization

Histology of Cardiac Myocytes

  • Atrial and nodal myocytes are smaller than ventricular and conduction system myocytes.
  • Cardiac myocytes exhibit branching and intercalated discs.
  • Intercalated discs:
    • Borderlines between cells.
    • Contain channels for cell-to-cell connection.

Cardiac Cycle

  • Pressure is measured in millimeters of mercury (mmHg).
  • The right side of the heart is often overlooked but essential.
  • Key events:
    • MC: Mitral valve closing
    • AO: Aortic valve opening
    • AC: Aortic valve closing
    • MO: Mitral valve opening

Ventricular Filling and Contraction

  • The left ventricle fills with blood.
  • Isovolumic contraction occurs with both mitral and aortic valves closed.
    • The volume of blood remains constant while pressure increases.
  • The aortic valve opens when left ventricular pressure exceeds aortic pressure.
  • Blood flows from the left ventricle into the aorta.

Ventricular Relaxation

  • The aortic valve closes as left ventricular pressure decreases.
  • Isovolumic relaxation occurs with both valves closed.
  • The mitral valve opens, and blood flows from the left atrium to the left ventricle again.

Ventricular Volume

  • Additional information about ventricular volume changes during the cardiac cycle.

Heart Sounds

  • Heart sounds during physical examination:
    • S1: Mitral and tricuspid valve closure
    • S2: Aortic and pulmonic valve closure
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Factors Affecting Ventricular Relaxation

  • Rate and extent of ventricular relaxation depend on:
    • Heart rate
    • Wall thickness
    • Chamber volume and shape
    • Aortic pressure
    • Sympathetic tone
    • Myocardial ischemia (scar tissue post-MI)

Ventricular Filling

  • After mitral valve opening, rapid ventricular filling contributes 70-80% of blood flow volume.
  • Pulmonary vascular resistance is lower than systemic resistance.
  • Right-side pressure is lower due to anatomical differences.

Pressure-Volume Loops

  • Pressure-volume loop for the left ventricle.
  • D to A: Ventricular filling with blood from the atrium.
    • Mitral valve opens.
  • A: Mitral valve closes; aortic valve is closed.
    • Isovolumic contraction of the left ventricle.
    • No change in volume.
  • A to B: Isovolumic contraction; pressure increases.
  • B: Maximum pressure generated by the left ventricle.
    • Aortic valve opens.
    • Blood flows from the left ventricle to the aorta.
  • C: Aortic valve closes; volume decreases.

Pressure-Volume Relationship Variability

  • Different scenarios (A, A', A''):
    • Changes in diastolic pressure-volume relationship.
    • Factors: Chamber characteristics, shape, size.
    • Different scenarios lead to different pressures.
    • Relaxation properties of the ventricle.
    • Elastic recoil of the ventricle.

Cardiac Adaptation

  • Organs adapt to disorders and pathologies.
  • The heart's primary function is to pump blood.
  • Stroke volume: The volume of blood pumped with each beat.

Effects of Afterload and Preload

  • Increasing afterload (B to B'): Decreases stroke volume (BC to B'C').
  • Increasing preload (A to A'): Volume increases (e.g., 150).
    • The heart may struggle to adapt.