Cardiac Cycle Lecture Notes

Introduction

• Topic: Cardiac cycle
• Focus: Electrical and mechanical events in the heart
• Conduction system precedes contraction

ECG Basics

• ECG Trace: lead 2 focuses on electrical events in the heart
• ECG Peaks and Troughs: Represents electrical activity

Electrical Conduction in the Heart

• SA Node: Initiates depolarization
• Atrial Depolarization:
  • Direction of depolarization affects ECG trace
  • P Wave: Represents atrial depolarization
• Electrical Signal Pathway:
  • Moves through the AV node (Flatline phase)
  • Proceeds through bundle branches (Q Wave: septal depolarization)
  • Moves through Purkinje fibers towards the lead (R Wave: ventricular depolarization)
  • Completes with S Wave: end of ventricular depolarization
• QRS Complex: Collective representation of ventricular depolarization
• Repolarization:
  • Follows depolarization
  • T Wave: Represents ventricular repolarization

Mechanical Function of the Heart

• Diastole (Relaxation):
  • Ventricular Filling: Passive filling of ventricles
  • Atrial Contraction: Fills remaining blood into ventricles
• Systole (Contraction):
  • Ventricular Contraction: Isovolumetric contraction (AV and SL valves closed)
  • Ventricular Ejection: Blood ejected through open SL valves
• Isovolumetric Relaxation: Prepares heart for next cycle of filling

Heart Sounds

• S1 (First Heart Sound): Closing of AV valves (heard during isovolumetric contraction)
• S2 (Second Heart Sound): Closing of SL valves (heard during isovolumetric relaxation)

Correlation with ECG

• Electrical events precede mechanical events
• Atrial Contraction:
  • Begins midway through P Wave
• Ventricular Contraction:
  • Begins midway through QRS Complex
  • Continues until midway through T Wave
• Heart sounds correlate with specific points in the ECG trace

Conclusion

• Understanding the cardiac cycle involves linking electrical activity with mechanical contraction and valve function
• The ECG provides a visual representation of these events, crucial for diagnosing cardiac function issues.