Ch18 pt4

Cardiac Impulse and Conduction System

  • The cardiac impulse begins at the sinoatrial (SA) node, known as the heart's natural pacemaker, generating impulses at approximately 75 beats per minute.

  • The path of the cardiac impulse is as follows:

    • SA Node

    • Atrioventricular (AV) Node

    • Bundle of His

    • Right and Left Bundle Branches

    • Purkinje Fibers

Sequence of Conduction

  • The electrical impulse spreads through the atria, reaching the AV node via internodal pathways, delaying it by approximately 0.1 seconds to allow for atrial contraction before ventricular contraction occurs.

  • If the SA node fails, the AV node can take over as a pacemaker at a slower rate (40-60 bpm).

  • Other components like the Bundle of His and Purkinje fibers can also function as pacemakers at even slower rates (30 bpm).

Features of Cardiac Conduction

  • Gap junctions support impulse transmission across cardiac cell membranes.

  • The conduction system maintains the rhythm of the heart essential for effective pumping.

Electrocardiogram (ECG or EKG) Basics

  • An electrocardiogram records the heart's electrical activity through electrodes placed on the body.

  • P Wave: Indicates depolarization of the atria during atrial contraction.

  • QRS Complex: Represents ventricular depolarization; signifies ventricular contraction.

  • T Wave: Reflects ventricular repolarization; represents the relaxation phase of the ventricles.

  • PR Interval: Duration from the beginning of atrial depolarization to the start of ventricular depolarization.

  • QT Interval: Extends from the onset of ventricular depolarization to the end of ventricular repolarization.

Arrhythmias

  • Arrhythmia: An abnormal rhythm caused by irregular electrical impulses, leading to ineffective heart contractions. Types include:

    • Atrial Fibrillation: Rapid and irregular contractions in the atria.

    • Ventricular Fibrillation: Disorganized electrical activity in the ventricles, preventing effective pumping and can lead to death if untreated.

  • Treatment for serious arrhythmias includes:

    • Defibrillation: Uses electric shocks to restore normal rhythm.

    • Artificial Pacemaker: Delivers impulses to maintain heart rhythm.

Factors Affecting Heart Rhythm

  • Factors such as caffeine or nicotine can cause premature contractions known as extrasystoles, affecting normal heart rhythm and possibly leading to symptoms like jitteriness.

  • The autonomic nervous system (ANS) modulates the heart's activity:

    • Sympathetic Nervous System: Increases heart rate and contractility during stress.

    • Parasympathetic Nervous System: Decreases heart rate during restful states.

Action Potentials in Cardiac Muscle

  • Action Potential: Involves the movement of sodium (Na+) and calcium (Ca2+) ions, causing depolarization and repolarization phases.

    • Rapid Depolarization: Caused by sodium influx, leading to a quick rise in membrane potential.

    • Plateau Phase: Maintained by calcium influx, preventing tetanus and allowing heart chambers time to fill.

    • Repolarization: Potassium (K+) efflux re-establishes resting membrane potential, returning to the original state.

Difference between Skeletal and Cardiac Muscles

  • Cardiac muscle action potentials: 200+ milliseconds vs. Skeletal muscle: 1-2 milliseconds, ensuring longer contractions for effective blood ejection.

Summary of ECG Features

  • Typical lead placement forms a 12-lead ECG, capturing electrical activity from different angles. Key intervals include:

    • PR Interval: ~0.16 seconds

    • ST Segment: Indicates the plateau phase of ventricles

    • QT Interval: Measures the total time for depolarization and repolarization of the ventricles.

Closing Thoughts

  • Understanding heart rhythms, conduction pathways, and the interpretation of ECG readings is crucial for diagnosing cardiac issues.

  • Students are encouraged to engage with visual and practical resources to grasp concepts more effectively.