Cardiac Conduction and Cardiac Cycle - Reading PDF

Clinical Note: Broken-Heart Syndrome

• Broken-heart syndrome, also known as stress-induced cardiomyopathy, has been observed in patients experiencing acute emotional stress such as grief.

• Initially noted in Japan in the early 2000s.

• Predominantly affects elderly women presenting with symptoms resembling heart attacks.

• ECGs reveal abnormal electrical activity and dilated left ventricles, but coronary arteries show no lesions commonly associated with heart attacks.

• All symptoms revert to normal with supportive care within one month.

Cardiac Anatomy and Blood Flow

• Blood from the myocardium is drained by cardiac veins into the coronary sinus, which empties into the right atrium.

• Major cardiac veins include:

  • Great Cardiac Vein: Drains blood from regions supplied by the anterior interventricular artery.

  • Middle Cardiac Vein: Drains the area supplied by the posterior interventricular artery.

  • Small Cardiac Vein: Drains the posterior surfaces of the right atrium and ventricle.

  • Anterior Cardiac Veins: Drain the anterior surface of the right ventricle directly into the right atrium.

Cardiac Physiology: Overview

• The heart functions through coordinated electrical impulses leading to contractions.

• Cardiac muscle consists of:

  • Autorrhythmic Cells: Control and coordinate the heartbeat (pacemakers).

  • Contractile Cells: Execute the heart's pumping action.

• The conducting system initiates contraction via action potentials that travel through the myocardium.

The Conducting System: Pacemaker and Contractile Cells

• The heart's automatic contractions (autorhythmicity) are dictated by specialized cells within the conducting system.

• SA Node: Primary pacemaker of the heart, located in the right atrium; generates 60-100 action potentials per minute.

• AV Node: Backup pacemaker, slower than SA Node, generating 40-60 action potentials per minute.

• Electrical impulses stimulate atrial contraction, followed by ventricular contraction through a timed process:

  1. SA Node activity begins.

  2. Stimulus spreads to the ventricles via the AV Node after a delay.

  3. Ventricular contraction is coordinated to prevent backflow into the atria.

Action Potential in Cardiac Muscle

• The contraction process is triggered by calcium influx, essential for the contractile process:

  1. Pacemaker Cells have an unstable resting membrane potential that gradually depolarizes toward threshold.

  2. Action potentials result in contraction lagging behind the depolarization.

• The total duration of a heartbeat is approximately 370 milliseconds.

Phases of Heart Contraction

• Atrial Systole: Atria contract, pushing blood into the ventricles.

• Ventricular Systole: Ventricles contract, pushing blood out into arteries.

• Diastole: Heart refills with blood; all chambers relax for efficient filling.

Electrical Behavior of Cardiac Cells

• Cardiac action potentials display different phases:

  • Rapid Depolarization: Sudden influx of Na+ ions.

  • Plateau Phase: Prolonged phase due to Ca2+ influx, crucial for contraction.

  • Repolarization: K+ exits, returning the membrane potential to resting state.

• The absolute refractory period is critical to avoid tetanic contractions, ensuring effective pumping.

Electrocardiogram (ECG)

• The ECG records electrical activity of the heart.

• Major components include:

  • P Wave: Atrial depolarization.

  • QRS Complex: Ventricular depolarization, associated with ventricular contractile action.

  • T Wave: Ventricular repolarization.

• ECG helps in diagnosing arrhythmias and other heart conditions by evaluating wave patterns and timing.

Cardiac Cycle Events

• The Cardiac Cycle is made up of:

  • Systole: Contraction of heart chambers.

  • Diastole: Relaxation and filling of heart chambers.

• The contraction-relaxation phases must be well-timed:

  • Blood flows from higher pressure areas to lower.

  • Atrial systole occurs slightly before ventricular systole to optimize blood flow.

Heart Sounds

• S₁  Lubb: Closing of AV valves (beginning of ventricular contraction).

• S₂  Dupp: Closing of semilunar valves (beginning of ventricular filling).

• S₃ and S₄ are softer sounds related to blood flow and rarely heard in healthy individuals.

• Murmurs may indicate valve dysfunction or heart problems.