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Cardiac Cycle and Conduction Pathway

Jul 11, 2024

Cardiac Cycle and Conduction Pathway

Overview

  • Topics Covered: Cardiac cycle, conduction pathway, techniques to monitor heart function at rest and during stress.
  • Importance: Essential for understanding heart function, especially in clinical populations.

Autoc Conducted Contraction

  • Definition: Built-in electrical pathway in the heart, allows heart to contract without external stimuli.
  • Pathway: Signal starts in atriums → travels through ventricles → returns through ventricular walls → muscle contraction.
  • Intrinsic Rate: 60 to 100 BPM (ideal: 60-80 BPM).

Cardiac Conduction System

Key Components

  1. SA Node (Sinoatrial Node)
    • Located in atriums
    • Initiates signal
    • Known as the pacemaker
  2. AV Node (Atrioventricular Node)
    • Holds signal briefly to allow atrium blood to fill ventricles.
  3. Bundle of His
    • Transmits signal from AV node to right/left bundle branches and Purkinje fibers.
  4. Purkinje Fibers
    • Ensure all ventricular walls receive the signal for contraction.

Alternative Pathways

  • If SA Node Fails: AV node picks up signal (40-60 BPM).
  • If AV Node Fails: Ventricular cells take over (20-40 BPM).
  • Functional Syncytium: Heart functions as a mass of interconnected cells.

Cardiac Cycle

  • Definition: Comprised of systole (contraction) and diastole (relaxation).
  • Resting State: ~2/3 time in diastole (0.3s systole, 0.5s diastole at 75 BPM).
  • Exercise State: ~2/3 time in systole (at 180 BPM).
    • Importance of Stroke Volume: Higher stroke volume and lower blood viscosity aid faster blood emptying and better metabolic supply during high heart rates.

Autonomic Regulation

  • Resting: Balanced parasympathetic (mainly) and sympathetic control.
  • Exercise: Reduced parasympathetic tone, increased sympathetic activity.
  • Nervous System Influence:
    • Parasympathetic (Vagus Nerve): Releases acetylcholine → hyperpolarizes heart tissue → lowers heart rate.
    • Sympathetic (Norepinephrine): Increases heart rate and contractility via beta receptors.

Electrocardiogram (EKG/ECG)

Purpose

  • Measures electrical activity of the heart at rest and stress states.

Key Waves and Intervals

  1. P-Wave: Atrial depolarization.
  2. QRS Complex: Ventricular depolarization.
  3. T-Wave: Ventricular repolarization.
  4. PR Interval: SA node to AV node signal travel.
  5. QRS Interval: Intraventricular conduction.
  6. ST Interval: Ventricular repolarization time.
  • Isoelectric Line: No electrical activity; flatline indicates no heart activity.

12-Lead EKG Setup

  • Electrodes: Right/left arm, right/left leg (hips), chest leads (V1-V6).
  • Lead Placement: Specific anatomical locations to capture heart’s electrical activity.
  • EKG Paper: 25 mm/sec feed rate, 1 mm = 0.04 s, 1 large square = 0.2 s.
  • Heart Rate Calculation: Count large/small boxes between R-waves, divide 300/1500 by number of boxes.

Common EKG Abnormalities

  • Sinus Rhythm: Normal rhythm.
  • PVC (Premature Ventricular Complex): No P-wave, premature QRS.
  • Ventricular Fibrillation: Chaotic, fluttering ventricular activity.
  • Torsade de Pointes: Twisting around the isoelectric line, leads to quick but often self-correcting flutter.
  • Bundle Branch Block: Double-peaked QRS complex (bunny ears).
  • Heart Blocks: Different degrees (first, second (Type I/II), third) indicating delays/failures in AV node signal transmission.

Concluding Notes

  • EKG is crucial for diagnosing heart function issues in clinical settings.
  • Upcoming lecture: Respiratory system, pressures, adaptations, and effects of altitude training.

Full transcript