In-Depth Notes on Heart Function and Electrocardiography

Overview of Heart Function

Primary Function: Pumping blood through two main circuits:

• Systemic Circuit:

  • Delivers oxygen (O2) and nutrients to tissues.
  • Removes carbon dioxide (CO2) and metabolites.

• Pulmonary Circuit:

  • Exchanges O2 and CO2 in the lungs.

Electrocardiography (ECG)

Definition: A method to assess the electrical activity of the heart.

Rhythmic Contraction: Heart function is represented by rhythmic contractions controlled by:

• Pacemaker Cells:

  • Located in the sinoatrial node (SA), initiating the heartbeat.

• Heart Valves:

  • Maintain proper blood flow direction through the heart and vessels.

Heart Activity Regulation

• Sympathetic Activation:

  • Increases heart rate (e.g., during inhalation).

• Parasympathetic Activation:

  • Decreases heart rate (e.g., during exhalation), leading to Respiratory Sinus Arrhythmia (RSA), which is an indicator of parasympathetic system impact.

• Chemical Influences:

  • O2 and CO2 levels in blood affect heart activity.

Conduction Pathway of Heart

Electrical Signal Pathway:

• SA Node ➔ Internodal Fibers ➔ Atrioventricular Node ➔ Bundle of His (Tawara branches) ➔ Purkinje Fibers.

Delay in Contraction: Ventricular contraction follows atrial contraction by approximately 200 ms.

Depolarization & Repolarization:

• Systole: Contraction due to depolarization.

• Diastole: Relaxation due to repolarization.

Stroke Volume and Pulse Measurement

• Stroke Volume: Volume of blood ejected from the heart during systole; creates a pressure wave detectable in major arteries and fingertips.

• Pulse Plethysmography:

  • Technique: Measures blood volume changes using near-infrared light.
  • Types:
  • Reflection Plethysmography: Measures light reflected from blood capillaries.
  • Transmission Plethysmography: Measures light passing through tissues.

ECG Measurements and Lead Patterns

• Electrocardiogram (ECG) Measurements: Detected via electrodes on the skin.

• Lead II Positioning:

  • Positive electrode on the left ankle.
  • Negative on the right wrist.
  • Ground on the right ankle.

• Modified Lead II: Reduces movement artifact for clearer readings.

• Key Components of ECG:

  • P Wave: Represents atrial depolarization.
  • QRS Complex: Represents ventricular depolarization.
  • T Wave: Represents ventricular repolarization.

• Timing:

  • R-peak precedes peak blood pressure by about 150 ms, and pulse wave propagation varies.

Heart Rate and Variability

• Heart Rate (HR): Number of heart contractions per minute; normal range is typically 60-80 bpm at rest.

• Interbeat Interval (IBI): Average interval between heartbeats, typically 750-1000 ms.

• Heart Rate Variability (HRV): Temporal variation between heartbeats; indicates vagal influence on the heart.

• Measured using various HRV metrics (e.g., HF, RMSSD).

Factors Affecting Heart Rate

• Physical Exercise: Increases HR over 200 bpm; recovery leads to a gradual decrease back to resting levels.

• Psychological Factors: Stress, attention, novel situations, and emotions impact HR positively or negatively.

• Vagal Component of HRV: Correlates with stress-related conditions and cognitive performance.

• Cardioceptive Accuracy: Awareness of heartbeats correlates with emotions and mental experience assessment.

Practical Application

• Demonstration Goal: Basics of cardiac activity recording using Biopac Student Lab.