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Key USMLE Cardiac Physiology Concepts

Apr 23, 2025

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USMLE Cardiac Physiology Webinar Notes

Introduction

  • Session overview: High-yield topics for USMLE exam
  • Focus: Cardiac cycle, pressure-volume loops, cardiac action potentials
  • Goal: Learn, integrate, apply material actively

Cardiac Cycle

Wiggers Diagram

  • Historical Context: Developed by Dr. Carl Wigger in 1915
  • Purpose: Visualize electrical and mechanical heart activities

Main Phases

  1. Systole

    • Isovolumetric Ventricular Contraction
      • Ventricular pressure increases, no change in volume
      • All valves closed
      • Related to QRS complex; S1 heart sound (mitral and tricuspid valve closure)
    • Ventricular Ejection
      • Blood flows from ventricle to aorta
      • Aortic pressure increases, ventricular volume decreases
      • Related to ST segment; systolic murmurs like aortic stenosis

  2. Diastole

    • Isovolumetric Ventricular Relaxation
      • Ventricular pressure decreases, volume constant
      • All valves closed; S2 heart sound (aortic and pulmonic valve closure)
    • Rapid Ventricular Filling
      • Blood flows from atrium to ventricle
      • Increase in ventricular volume
      • Associated with S3 heart sound in pathologies

Jugular Venous Pressure (JVP) Tracing

  • A Wave: Atrial contraction
  • C Wave: Bulging of tricuspid valve
  • X Descent: Atrial relaxation
  • V Wave: Venous filling
  • Y Descent: Atrial emptying

Pressure-Volume Loops

  • Key Points
    • Mitral Valve Closure: Beginning of isovolumetric contraction
    • Aortic Valve Opening: Start of ejection
    • End Diastolic/Systolic Volumes: Key for calculating stroke volume and ejection fraction

Alterations

  • Increased Contractility: Decrease in end systolic volume, increased stroke volume
  • Increased Preload: Higher end diastolic volume, increased stroke volume
  • Increased Afterload: Higher end systolic volume, decreased stroke volume

Cardiac Action Potentials

Ventricular Action Potential

  • Phase 0: Sodium influx
  • Phase 2: Calcium influx, plateau phase
  • Phase 3: Potassium efflux, repolarization

Nodal Action Potential

  • Phase 0: Calcium influx
  • Phase 4: Pacemaker potential, mediated by sodium "funny" channels

Clinical Correlations

  • Murmurs and Heart Sounds
    • S3: Dilated cardiomyopathy, heart failure
    • S4: Left ventricular hypertrophy, hypertension
  • Pathologies
    • Cardiac Tamponade: Pulsus paradoxus, decreased systolic BP with inspiration
    • Aortic Regurgitation: Diastolic murmur

Study Resources

  • UWorld Notes: Consolidated explanations
  • Rapid Review Course: High-yield concepts for pre-exam preparation
  • Integration with Study Tools: Pathoma, First Aid, metabolic pathways, and ethical concepts

Conclusion

  • Opportunities for Questions
  • Further Resources: Links to courses, study guides, and website for additional support

These notes provide a detailed overview of the key concepts covered in the cardiac physiology webinar, highlighting the essential points that are likely to be tested on the USMLE exam.

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